Patent Publication Number: US-7590098-B2

Title: Publish/subscribe model in a wireless sensor network

Description:
CROSS REFERENCE TO RELATED CASES 
   This application is related to the following applications filed on even date herewith, all of which are hereby incorporated herein by reference: 
   U.S. patent application Ser. No. 10/974,216, entitled “EVENT-BASED FORMALISM FOR DATA MANAGEMENT IN A WIRELESS SENSOR NETWORK.” 
   U.S. patent application Ser. No. 10/974,073, entitled “LAYERED ARCHITECTURE FOR DATA MANAGEMENT IN A WIRELESS SENSOR NETWORK.” 
   U.S. patent application Ser. No. 10/974,173, entitled “DISCRETE EVENT OPERATORS FOR EVENT MANAGEMENT IN A WIRELESS SENSOR NETWORK.” 
   U.S. patent application Ser. No. 10/974,198, entitled “MACHINE ARCHITECTURE FOR EVENT MANAGEMENT IN A WIRELESS SENSOR NETWORK.” 
   TECHNICAL FIELD 
   The following description relates to wireless sensor networks in general and to data management in a wireless sensor network in particular. 
   BACKGROUND 
   Systems often include some type of functionality for providing data management. Data management is concerned with providing a logical view of the data that is available in a system. Such a logical view is also referred to here as the “data model” for the system. Data management is also concerned with the underlying physical organization of the data in the system and the transformation between the logical view of the data and the underlying physical organization. In addition, data management is typically concerned with a query mechanism for retrieving data from the system, a frame structure for the data, and the optimization of queries based on various parameters. 
   One type of system is a wireless sensor network. A wireless sensor network typically include several nodes that communicate with one another over wireless communication links (for example, over radio frequency communication links). One or more of the nodes in the wireless sensor network incorporate (or are otherwise coupled to) a sensor. Such nodes are also referred to here as “wireless sensor nodes” or “sensor nodes.” Each sensor is capable of generating a value that, at any given point in time, is indicative of some physical attribute of interest. In one configuration, the sensor nodes are battery powered and have limited computational resources (for example, limited memory and processing capability). 
   One approach to providing data management in a sensor network employs techniques used in relational database management systems (RDBMS). In such an approach, sensor data generated by sensor nodes in the network are logically organized into tables. Relational algebra is used for specifying the behavior of the logical view of the sensor data. Such an RDBMS approach, however, may not be suitable in a wireless sensor network that makes use of sensor nodes that have limited resources (for example, power, memory, or processing capability). 
   SUMMARY 
   In one embodiment, a wireless sensor network comprises a plurality of nodes that communicate over wireless communication links. At least one of the plurality of nodes receives sensor data from a sensor. A subscription for an event of interest occurring in the wireless sensor network is installed in the wireless sensor network. A publisher node included in the plurality of nodes determines when the event of interest occurs and, when the event of interest occurs, publishes data related to the event of interest for a subscriber node included in the plurality of nodes. 
   In another embodiment, a wireless sensor node comprises a wireless transceiver to communicate over a wireless communication link and a sensor interface to receive sensor data from a sensor. When the wireless sensor node receives, from the wireless communication link, a request comprising an event filter associated with an event of interest, the wireless sensor node filters the sensor data in order to determine when the event of interest occurs. When the event of interest occurs, the wireless sensor node transmits event data related to the event of interest to a requesting node over the wireless communication link. 
   Another embodiment is a method of processing a query for data from a wireless sensor network comprising a plurality of nodes that communicate over wireless communication links, at least one of which receives sensor data from a sensor. The query specifies a set of events of interest, at least one of which is related to sensor data sensed by the sensor. The method comprises, for each event included in the set of events of interest, identifying, at a producer node included in the plurality of nodes, when that event occurs, and when that event occurs, communicating event data related to that event from the producer node to a consumer node included in the plurality of nodes over a wireless communication link. 
   The details of various embodiments of the claimed invention are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. 

   
     DRAWINGS 
       FIG. 1  is a block diagram of one exemplary embodiment of a wireless sensor network. 
       FIG. 2  is a block diagram of one embodiment of a wireless sensor node. 
       FIG. 3  is a block diagram of one embodiment of a logical source entity that generates an event of interest in a wireless sensor network. 
       FIG. 4  is a high-level flow diagram of one embodiment of a method of generating an event of interest using the source entity of  FIG. 3 . 
       FIG. 5  illustrates one example of a subscription request in the wireless sensor network of  FIG. 1 . 
       FIG. 6  illustrates, generally, one example of a recursive subscription request in the wireless sensor network. 
       FIG. 7  is block diagram of one embodiment of a data management stack for providing data management functionality in a wireless sensor network. 
       FIG. 8  is a Backus-Naur Form of one example of a high-level query language. 
       FIG. 9  is a flow diagram of one embodiment of a method of compiling a source form of a query in order to generate the binary form of that query. 
       FIG. 10  is a block diagram illustrating one exemplary instruction format for use with the method of  FIG. 9 . 
       FIG. 11  illustrates one example of an instruction set. 
       FIG. 12  illustrates one example of a query expressed in source form using the grammar set forth above in  FIG. 8 . 
       FIG. 13  illustrates a graph generated from the query that is expressed in source form in  FIG. 12 . 
       FIG. 14  is a table having a row for each node in the graph shown in  FIG. 13 . 
       FIG. 15  is a binary form of the query set forth in  FIG. 12 . 
       FIG. 16  illustrates one example of a set of recursive subscriptions that can result from the query of  FIG. 12 . 
       FIG. 17  is a block diagram of one embodiment of a virtual machine for use in a wireless sensor network. 
       FIG. 18  is a flow chart illustrating the processing of an event program by the embodiment of the virtual machine shown in  FIG. 17 . 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of one exemplary embodiment of a wireless sensor network  100 . The wireless sensor network  100  includes multiple wireless sensor nodes  102  that communicate with one another and/or a base station  104  using wireless communication links. The nodes of the wireless sensor network  100 , in some embodiments, are distributed over a large geographical area. In one embodiment of the wireless sensor network  100 , wireless sensor nodes  102  are distributed over an environment that is to be monitored. Each wireless sensor node  102  includes (or is otherwise coupled to) a sensor that is capable of generating a value that, at any given point in time, is indicative of some physical attribute of interest. Each wireless sensor node  102  receives sensor data from a respective sensor. 
   In one embodiment, the wireless sensor nodes  102  and the base station  104  communicate with one another using radio frequency (RF) communication links. In other embodiments, other wireless communication links (for example, infrared wireless communication links) are used instead of or in addition to RF wireless communication links. In one embodiment, the wireless sensor network  100  is implemented as an ad-hoc, peer-to-peer network. In such an embodiment, the nodes of the wireless sensor network  100  communicate with each other wirelessly using a multi-hop protocol. Such a multi-hop protocol provides a mechanism for a packet (or other unit of data) to be transmitted by a source node to a destination node outside of the wireless transmission range of the source node by transmitting the packet to an intermediate node within the source node&#39;s wireless transmission range. The intermediate node then forwards the packet onto the destination node (if the destination node is within the intermediate node&#39;s wireless transmission range) or onto another intermediate node within the first intermediate node&#39;s wireless transmission range. This forwarding process is repeated until the packet reaches the destination node. In another embodiment, the wireless sensor network  100  is implemented using a different wireless networking approach (for example, using an infrastructure wireless network in which wireless communications are routed through an access point). 
   The base station  104  provides a static point from which queries can be injected into the wireless sensor network  100  and from which data that is retrieved by such queries can be received. In one embodiment, a user communicates a query to the base station  104 . The base station  104  receives the query and injects the query into the wireless sensor network  100 . The query propagates to appropriate sensor nodes  102 , which communicate data back to the base station  104  (via one or more intermediate nodes) as specified in the query. In one implementation, the base station  104  also acts as a gateway to another network or device not otherwise included in the wireless sensor network  100  from which queries are received and/or to which data retrieved from the wireless sensor network  100  is communicated. 
   The wireless sensor network  100  can also include other types of nodes. For example, as shown in  FIG. 1 , a personal digital assistant (PDA)  106  is included in the network  100 . The PDA  106  includes a wireless transceiver that enables the PDA  106  to communicate with other nodes in the wireless sensor network  100  over one or more wireless communication links. In one usage scenario, a user uses the PDA  106  to input a query for data from the wireless sensor network  100 . The PDA  106  communicates the query to the base station  104  (via one or more intermediate nodes, if necessary). The base station  104  receives the query and injects the query into the wireless sensor network  100  and communicates back to the PDA  106  any data received from the wireless sensor network  100  in response to the query. 
   In the exemplary embodiment shown in  FIG. 1 , at least a portion of the nodes in the network  100  are logically arranged into regions  108 . A region  108 , in such an embodiment, defines a geographic area. Each region  108  is considered to include those nodes that are physically located within the geographic area of that region  108 . For example, as shown in  FIG. 1 , the wireless sensor nodes  102  and the PDA  106  are arranged into four regions  108  (specifically, identified in  FIG. 1  as region A, region B, region C, and region D). In such an embodiment, a user or application that retrieves data from the network  100  using the regions  108 , if appropriate for the needs of that user or application. In other embodiments, such logical regions  108  are not used. 
     FIG. 2  is a block diagram of one embodiment of a wireless sensor node  102 . The wireless sensor node  102  shown in  FIG. 2  is suitable for use in the embodiment of a wireless sensor network  100  shown in  FIG. 1 . The embodiment of a wireless sensor node  102  shown in  FIG. 2  comprises a sensor interface  202  that couples a sensor  204  to the wireless sensor node  102 . In the particular embodiment shown in  FIG. 2 , the sensor  204  is integrated into the wireless sensor node  102  (for example, by enclosing the sensor  204  within a housing that encloses the sensor  204  along with the other components of the wireless sensor node  102 ). In another embodiment, the sensor  204  is not integrated into the wireless sensor node  102  but is otherwise communicatively coupled to the other components of the wireless sensor node  102  via the sensor interface  202 . 
   The sensor  204  is capable of generating a value that, at any given point in time, is indicative of some physical attribute of interest. Examples of sensors  204  include devices that generate a value indicative of temperature, light, magnetic field, air flow, acceleration, vibration, sound, or power. The sensor interface  202  comprises appropriate interface hardware or software for communicatively coupling the sensor  204  to the other components of the wireless sensor node  102 . For example, in one embodiment, the software interface  202  includes, for example, an analog-to-digital converter and/or a software driver for the sensor  204 . 
   The wireless sensor node  102  shown in  FIG. 2  further comprises a programmable processor  206 . The programmable processor  206  is programmed with appropriate program instructions to perform at least a portion of the processing described here as being performed by the wireless sensor node  102 . The wireless sensor node  102  shown in  FIG. 2  includes memory  208  in which such program instructions and any data structures used by the program instruction are stored. The memory  208  includes any appropriate type of memory now known or later developed including without limitation, read-only memory (ROM), random access memory (RAM), and a set of registers included within the processor  206 . 
   The wireless sensor node  102  shown in  FIG. 2  also comprises a wireless transceiver  216  that transmits and receives data over one or more wireless communication links. In one embodiment, the wireless transceiver  216  comprises a RF transceiver that sends and receives data over one or more RF communication links. In other embodiments, the wireless transceiver  216  comprises other types of wireless transceivers for sending and receiving data over other types of wireless communication links (for example, an infrared transceiver for sending and receiving data over infrared communication links) instead of or in addition to an RF transceiver. 
   The wireless sensor node  102  also comprises a power source  218 . In the embodiment shown in  FIG. 2 , the power source  218  includes a battery  220 . In other embodiments, the power source  218  comprises, in addition to or instead of a battery  220 , an interface for coupling the wireless sensor node  102  to an external power source such as a source of alternating current (AC) power. The wireless sensor node  102  also comprises one or more hardware timers  222  that are used generating interrupts based on timing-related events. 
   In one implementation of the embodiment shown in  FIG. 2 , the wireless sensor node  102  is implemented using a CHIPCON CC1010 integrated circuit that includes an 8-bit micro-controller, 32 kilobytes of flash memory, and 2 kilobytes of RAM. 
   In the embodiment shown in  FIGS. 1 and 2 , an event-based data management model is used to implement data management functionality in the wireless sensor network  100 . Each of the nodes in the wireless sensor network  100  includes a data management interface  110  that implements at least a portion of such data management functionality. The data management interface  110 , in one implementation, comprises software that executes on a programmable processor included in each node. 
   In such an embodiment, the wireless sensor network  100  is logically viewed as a set of discrete events and a set of logical entities that “generate” the discrete events. The wireless sensor network  100  is queried, in such an embodiment, by specifying a set of events of interest. With such an event-based data management model, a discrete event operator algebra can be used as a formalism to specify the behavior of such a logical system and to verify the correctness and completeness of the specification. 
   Each event of interest is logically viewed as having a logical entity that is the source of that event. This source entity is also referred to here as the “producer” of that event. Also, each event of interest is logically viewed as having one or more logical entities that are sinks of that event (and/or data related to that event). Each of these sink entities is also referred to here as a “consumer” of that event or event-related data. The data management model used in such an embodiment, in other words, makes use of a “producer/consumer model.” For each logical entity, there is a corresponding node in the network  100  that physically implements the processing for that logical entity. The underlying node that implements a given source entity is also referred to here as a “data source” and the underlying node that implements a given sink entity is also referred to here as a “data sink.” For example, where an event of interest is a function of sensor data provided by a particular sensor  204 , the source entity for that event is implemented on a wireless sensor node  102  that is coupled to that sensor  204  (that is, on the wireless sensor node  102  that is the data source for the desired sensor data). It may be the case, that a particular node in the wireless sensor network  100  implements both the source entity and the sink entity for a given event. 
     FIG. 3  is a block diagram of one embodiment of a logical source entity  300  that generates an event of interest  302  in a wireless sensor network. The source entity  300  makes use of an event filter  304  to identify an occurrence of the event of interest  302  for which the entity  300  is the sink entity. The event filter  302  comprises a condition  306  and, when the condition  306  is true, the event of interest  302  is considered to have occurred. An event filter&#39;s condition  306  can be specified as a function of continuous data  308  (for example, sensor data generated by a sensor) and/or other events  310  generated by other entities (for example, an event for which the source entity  300  is also a sink entity). 
     FIG. 4  is a high-level flow diagram of one embodiment of a method  400  of generating an event of interest using the source entity of  FIG. 3 . Method  400  is used to generate each of a set of events of interest specified in a query that is received at the wireless sensor network  100 . A user or application that desires to retrieve information from the wireless sensor network  100  formulates the query and injects the query into the network  100 . The query specifies a set of events of interest about which the user or application wishes to receive information. In one embodiment, the query is converted into a form suitable for communication to the nodes in the wireless sensor network  100 . In one implementation (for example, as described below in connection with  FIGS. 9-15 ), the query is converted into a binary form of the query that is executed by one or more nodes. When a node receives the query information, the node determines whether that node is able to serve as a source entity for any of the set of events of interest specified in the query. If that node is able to serve as a source entity for an event of interest specified in the query, that node performs the processing of method  400  for that event. 
   For a particular event of interest specified in a query, an event filter is created at the source entity for that event (block  402 ). The query includes information that is used by the source entity to create the event filter for that event. For example, in one embodiment, the query specifies a condition, for each event of interest set forth in the query, that is used to identify each occurrence of that event. Once created, the event filter is used to identify when the particular event of interest has occurred. When the event filter determines that the particular event of interest has occurred (block  404 ), the source node communicates data related to that event to the sink entity specified for that event (block  406 ). That is, the physical node on which the source entity is implemented communicates the event-related data to the physical node on which the sink entity is implemented over one or more wireless communication links provided in the wireless sensor network  100 . 
   The event-based data management model, in one such embodiment, makes use of a combined producer/consumer and publish/subscribe model. In such a model, from a logical point of view, a sink entity that wishes to receive data related to a particular event informs the wireless sensor network  100  of that entity&#39;s interest in that event. The sink entity&#39;s interest in that event is then communicated to an entity that is able to serve as a source entity for that event. The sink entity indicates that it is interested in a particular event of interest by “subscribing” to that event. A subscription is formulated and is communicated to a source entity for the event of interest. The subscription identifies the event of interest (for example, by specifying a condition for use in an event filter that identifies that event) and the sink entity to which data related to the event should be sent when the event occurs. The source entity receives the subscription and creates an event filter for that event. The source entity “publishes” the event of interest when the event occurs. That is, when the event of interest specified in the subscription occurs, the source entity sends data related to that event to the specified sink entity. In this way, the nodes in the wireless sensor network  100  only monitor (and process and communicate data about) those events that are of interest to some entity in the network  100  (that is, those events to which a sink entity has subscribed). 
   In such an embodiment, each subscription is installed at the physical node that implements the logical source entity for the event specified by that subscription. Each subscription can be installed, deferred, paused, resumed, dropped or updated (for example, by a user or application) at anytime during the lifetime of the query. Each subscription has a specified activation time when the source entity is to begin checking for and publishing occurrences of the event of interest. Each subscription, in such an embodiment, also has as a specified lifetime after which the subscription ceases to exist (that is, the source entity no longer checks for and publishes the event of interest). Also, in such an embodiment, each subscription can specify how often the event filter should be evaluated in order to check for occurrences of the event of interest (also referred to here as the “event rate”). 
     FIG. 5  illustrates one example of a subscription request in the wireless sensor network  100 . In this example, a sink entity wishes to receive information about an event that is associated with a particular sensor. The sink entity is implemented on the base station node  104  and the source entity is implemented on the wireless sensor node  102  that includes the sensor that the sink entity is interested in. The data management interface  110  on the base station  104  communicates the subscription request to the wireless sensor node  102  on which the source entity is implemented (also referred to here as the “source wireless sensor node  102 ). In communicating the request to the wireless sensor node  102 , the data management interface  110  of the base station  104  interacts with appropriate underlying layers of the networking stack to route the subscription request to the wireless sensor node  102 . 
   When the source wireless sensor node  102  receives the subscription request, the source wireless sensor node  102  installs the event filter associated with that subscription request using the event-filter information specified in the subscription request. After the subscription has been activated and during the lifetime of the subscription, when the source wireless sensor node  102  determines that the event of interest has occurred, the source wireless sensor node  102  publishes the event to the sink entity implemented on the base station  104 . In publishing the event, the data management interface  110  on the source wireless sensor node  102  interacts with appropriate underlying layers of the networking stack to route data related to the event that occurred to the base station  104 . 
   In such an embodiment, a subscription can be formulated by a user of the wireless sensor network  100  as a query. The user formulates the query in a formal and verifiable query language. The query language is implemented as a high-level, human-readable language, which provides an interface to access data that resides in the wireless sensor network  100  and provides an interface to specify the intent of the user for the data. The user, in such an implementation, supplies the query, specified in the query language, to a node in the wireless sensor network  100  (for example, the base station  104  or the PDA  106 ). The node that receives such a query parses the query, checks the query for any syntactic or semantic errors and converts the query into a set of subscriptions. The conversion of the human-readable query (also referred to here as the “source” form of the query) into a set of subscriptions is also referred to here as “compiling” the query. In one embodiment, the set of subscriptions is expressed in a binary form that is designed for convenient execution by the nodes in the wireless sensor network  100 . In one implementation of such an embodiment, the data management interface  110  of the node that receives the query parses, checks, and compiles the received query and injects the query into the wireless sensor network  100 . 
   In such an embodiment, subscriptions can also be formulated by an application that resides in or interacts with the wireless sensor network  100 . For example, in one implementation, the application formulates the subscriptions in source form using the high-level query language. In another implementation, the application formulates the subscriptions directly in binary form (thereby avoiding the need to compile the query). 
   In such an embodiment, an optimization process that operates on the source form of the query and/or an optimization process that operates on the binary form of the query can be performed. Examples of various optimizations that can be performed in such an implementation are described below. 
   In such an embodiment, recursive subscriptions are supported. Each subscription comprises a condition that identifies when the event of interest has occurred. The event of interest associated with the subscription can be a “simple” event that is not expressed in terms of any other events in the wireless sensor network  100  (for example, where the event is a function of continuous data from a sensor or a timer). That is, a simple event comprises a condition that is not a function of any other event in the wireless sensor network  100 . Alternatively, the event of interest associated with a particular subscription can be a “complex” or “parent” event that is expressed in terms of one or more other events (also referred to here as “child events”) in the wireless sensor network  100 . That is, such a parent event comprises a “parent” condition that is a function of one or more child events. Likewise, each child event can itself be a simple event or a parent event that is expressed in terms of one or more child events. 
   The parent condition for such a parent event can combine the condition for a child event (referred to here as a “child condition”) with child conditions for zero, one, or more other child events and/or with zero, one, or more time-based conditions that specify when an event should be published, specify a duration for the subscription, specify an event-rate for the subscription, and/or specify when the subscription should be activated. These child conditions and/or time-based conditions can be combined together, for example, using logical operators (such as AND, OR, or XOR). In one implementation of such an embodiment, the time-based conditions are also implemented as events. 
     FIG. 6  illustrates, generally, one example of a recursive subscription request in the wireless sensor network  100 . In this example, a recursive subscription indicates that an entity (referred to here as the “parent sink entity”) wishes to receive information about an event that is associated with a first sensor when data from a second sensor meets a certain condition. In this example, the recursive subscription comprises a parent event that has a condition that indicates that the value of the first sensor is to be published when the value of the second sensor is greater than 20. In this example, the parent sink entity is implemented on the base station  104  and the source entity (also referred to here as the “parent source entity”) is implemented on the wireless sensor node  102  that includes the first sensor. This wireless sensor node  102  is also referred to here as the “first” wireless sensor node  102 . The data management interface  110  on the base station  104  communicates the original subscription request to the first wireless sensor node  102 , which install an event filter based on the condition specified in the original subscription request. 
   The parent source entity also “splices” the original subscription request in order to generate a second subscription, based on the original subscription received from the parent sink entity. The second subscription indicates that the parent source entity wishes to be informed as to when the second sensor is greater than 20. In other words, this second subscription identifies a child event that is of interest to the parent source entity. 
   For this second subscription, the parent source entity is the sink entity. The source entity (also referred to here as the “child source entity”) for this child event is the wireless sensor node  102  that includes the second sensor (also referred to here as the “second” wireless sensor node  102 ). The data management interface  110  of the first wireless sensor node  102  communicates the second subscription request to the second wireless sensor node  102 , which installs an event filter based on the condition specified in the second subscription request. 
   When the second wireless sensor node  102  determines that the child event has occurred (that is, the value of the second sensor is greater than 20), the second wireless sensor node  102  publishes the child event to the child sink entity, which is implemented on the first wireless sensor node  102 . In this example, the child sink entity is also the parent source entity. When the parent source entity on the first wireless sensor node  102  learns that the value of the second sensor node is greater than 20, the first wireless sensor node  102  publishes the parent event to the parent sink entity, which is implemented on the base station  104 . That is, the first wireless sensor node  102  provides the value of the first sensor to the parent sink entity implemented on the base station  104 . 
     FIG. 7  is block diagram of one embodiment of a data management stack  700  for providing data management functionality in a wireless sensor network. The embodiment of the data management stack  700  shown in  FIG. 7  is described here as being implemented using the wireless sensor network  100  and the wireless sensor node  102  of  FIGS. 1 and 2 , respectively. Other embodiments are implemented in other ways. One or more of the various layers described here are implemented on each node in the wireless sensor network  100 . 
   In the embodiment shown in  FIG. 7 , the data management stack  700  comprises six layers. The data management stack  700  includes a query formalism layer  702  that provides the formal framework and language used for querying data from the wireless sensor network  100 . For example, the query formalism layer  702  provides the functionality for compiling the source form of a query into a binary form and for injecting the query into the wireless sensor network  100 . In such an embodiment, the query formalism layer  702  implements one or more of the following features:
         the ability to subscribe for an event.   the ability to publish events to all subscribers.   the ability to uniquely identify each subscription that is made.   the ability to uniquely identify each event and the subscription that generates that event.   the ability to install a subscription, defer installation of a subscription, pause a subscription, resume a subscription, drop a subscription, update or modify a subscription, and activate a subscription.   the ability to specify a lifetime for a subscription.   the ability to specify an activation time for a subscription.   the ability to specify aggregation operations for a subscription, including a singe-source aggregation operation or a multiple-source aggregation.   the ability to specify a subscription that executes only once (that is, only one event is published for that subscription).   the ability to specify a priority for a subscription.   the ability to specify a subscription to one or more simple data sources and/or one or more complex data sources.   the ability to specify a condition used in an event filter for a subscription.   the ability to specify one or more parameter for a subscription.   the ability to specify one or more parameter for a publication.   the ability to specify during initialization a classification of a data source as a producer or a consumer of a particular item of data.   the ability to specify constraints on each subscription and corresponding publications for that subscription, such as a minimum power at a node to install that subscription at that node, a minimum power to generate lower power publication, and a minimum amount of time that a value should be stable before publishing any event that is based on that value.       

   A Backus-Naur Form (BNF) of one example of a high-level query language that implements these features is shown in  FIG. 8 . 
   A description of various statements and clauses shown in  FIG. 8  is given below.
         SUBSCRIBE EVENT Statement: This statement is used to specify a subscription request. A subscription request is set by an entity when an event of interest is to be monitored.   OF Clause: The OF clause is a part of a SUBSCRIBE EVENT statement that is used to associate a parameter with a region  108  of the wireless sensor network  100 .   ACTIVATE Clause: The ACTIVATE clause is a part of a SUBSCRIBE EVENT statement that enables specification of the time when the subscription should be activated by the source entity. If the ACTIVATE clause is not specified the default value of IMMEDIATE (explained below) is used for activation of the subscription. The time parameter for this clause is specified in HH:MM:SS format.   IMMEDIATE Clause: The IMMEDIATE clause is an optional clause of a SUBSCRIBE EVENT statement. When used, this clause specifies that the subscription should be activated immediately (that is, without any delays).   WHEN Clause: The WHEN clause is an optional clause of a SUBSCRIBE EVENT statement. This clause is used to specify a condition for an event filter that is evaluated to identify when the event of interest occurs. The event of interest is published only when the condition is evaluated and found to be true.   LIFETIME Clause: The LIFETIME clause is an optional clause of a SUBSCRIBE EVENT statement. When specified, this clause defines a duration for which the SUBSCRIBE EVENT statement is to be kept alive at the node where the subscription is installed. After the duration specified in this clause has elapsed, this subscription is “removed” from that node. The time parameter for this clause is specified in HH:MM:SS format.   INFINITE Clause: The INFINITE clause is an optional clause of a SUBSCRIBE EVENT statement. When used, it must be used along with a LIFETIME clause in the SUBSCRIBE EVENT statement. This clause defines the lifetime of that subscription as infinite, which means that the subscription will be active while the node at which the subscription is installed remains alive.   ONCE Clause: The ONCE clause is an optional clause of a SUBSCRIBE EVENT statement. When specified, this clause should be specified along with a LIFETIME clause. When this clause is specified with the LIFETIME clause, the subscription is evaluated only once and then removed from the node at which the subscription is installed. This is the default value used if a LIFETIME clause is not specified in the SUBSCRIBE EVENT statement.   PUBLISH EVENT Statement: The PUBLISH EVENT statement is used to publish an event of interest that has been subscribed to in a corresponding SUBSCRIBE EVENT statement. A PUBLISH EVENT statement is generated by a source entity and communicated to one or more sink entities specified in a corresponding SUBSCRIBE EVENT statement when the conditions specified in the SUBSCRIBE EVENT statement are evaluated and are true.   VALUES Clause: The VALUES clause is used in a PUBLISH EVENT statement to communicate the current values of one or more parameters. This clause associates a current value for a parameter with a parameter identifier for that parameter.   INSTALL EVENT Statement: The INSTALL EVENT statement is used to install an event filter at a particular node on which a source entity is implemented for a particular subscription. The event filter is, for example, installed into the program memory of that node and prepared for evaluation by that node. The subscription to be installed is specified by the event-id parameter of the INSTALL EVENT statement.   UNINSTALL EVENT Statement: The UNINSTALL EVENT statement is used to remove a subscription from the wireless sensor network  100 . This statement causes the event specified by an event identifier to no longer be checked for by the node on which the corresponding subscription has been installed. The subscription is cached for a limited period of time after being uninstalled for use in optimization processing.   SUSPEND EVENT Statement: The SUSPEND EVENT statement is used to suspend a subscription. The subscription is specified using an event identifier and the state of the subscription is set as “suspended.”   DROP EVENT Statement: The DROP EVENT statement is used to remove a subscription from the wireless sensor network  100 . This statement causes the event specified by an event identifier to no longer be checked for by the node on which the corresponding subscription has been installed. Unlike with the UNINSTALL EVENT statement, the subscription is not cached for use in optimization processing.   MODIFY EVENT Statement: The MODIFY EVENT statement is used to modify an existing subscription at runtime.   SET LIFETIME Clause: The SET LIFETIME clause is a part of a MODIFY EVENT statement to modify the lifetime of an existing subscription.   ACTIVATE EVENT Statement: The ACTIVATE EVENT statement is used activate a subscription that has previously been suspended via the SUSPEND EVENT statement. The state of the specified subscription is changed from “suspended” to “active.”   DEFER EVENT Statement: The DEFER EVENT Statement is used to defer the evaluation of the specified subscription for the specified duration.   MIN|MAX|AVERAGE|SUM|MEDIAN Clauses: These clauses are used to specify that a particular application-specific aggregation operation should be performed as a part of a subscription. The MIN clause indicates that a minimum value of a group of events should be computed, the MAX clause indicates that a maximum value of a group of events should be computed, the AVERAGE clause indicates that an average value of a group of events should be computed, the SUM clause indicates that a sum of a group of events should be computed, and the MEDIAN clause indicates that a median value of a group of events should be computed.       

   The data management stack  700 , in the embodiment shown in  FIG. 7 , also comprises a discrete event view (DEV) layer  704 . The DEV layer  704 , in such an embodiment, is scalable so that the functions performed by the DEV layer  704  can be implemented in high-end systems and low-end systems. For example, the source and sink functionality described below is required in both high-end systems and low-end systems, but is scaled appropriately. In such an embodiment, the DEV layer  704  also decides on the actions to be performed for any constraints that are specified in a query. It may be the case, however, that such constraint decisions cannot, during operation, be evaluated and made in the DEV layer  704  due to limitations of resources and information. 
   The DEV layer  704  performs a semantic check of each subscription by checking the event-filter condition specified in each subscription to check that valid source and sink entities have been specified. The DEV layer  704 , as a part of such processing, checks the validity of any time-based conditions (for example, conditions specifying when the corresponding event should be published, the lifetime of the subscription, an event rate for the subscription, and/or when the subscription should be activated). The DEV layer  704 , as a part of such processing, checks the validity of any parameters used in the event-filter condition. In an alternative embodiment, such semantic checking is not performed by the DEV layer  704 . 
   In the embodiment shown in  FIG. 7 , the DEV layer  704  identifies the logical source entity and one or more logical sink entities for each subscription and for each corresponding publication. The DEV layer  704  determines and keeps track of which physical node in the wireless sensor node  100  implements each such logical entity and buffers data for these logical entities so as to provide an interface for users and applications to interact with the logical entities. The DEV layer  704 , in such an embodiment, also handle recursive subscriptions and the corresponding publications. For example, the DEV layer  704  maps each publication generated by a source entity to the corresponding subscription so that the “chain” specified in a recursive subscription can be maintained. 
   In the embodiment shown in  FIG. 7 , the DEV layer  704  maintains information about the subscriptions that exist in the wireless sensor network  100  at any given point in time. Such information is used by the DEV layer  704  to optimize, at least partially, new queries and/or existing queries. In one embodiment, the optimization processing performed by the DEV layer  704  includes optimizing recursive subscriptions that are received by the wireless sensor network  100 . Such optimization processing is also referred to here as “recursive-subscription optimization.” The DEV layer  704 , when a recursive subscription is received, recursively converts the original, recursive subscription into a set of smaller and more efficient subscriptions. The DEV layer  704  also maintains the proper linkage of the set of subscriptions to main the logical relationship specified in the original, recursive subscription. 
   In one embodiment, the optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account the organization of the logical entities in the wireless sensor network  100 . Such optimization processing is also referred to here as “data-source organization optimization” or “data-source optimization.” As noted above, logically, the wireless sensor network  100  can be viewed as a set of entities, each of which is able to send and/or receive data related discrete events of interest. In the embodiment shown in  FIG. 1 , the entities are organized geographically into regions  108 . A region  108  is an abstract entity that has an associated geographical area defined by a user of the network  100 . Each region  108  is considered to include all the logical entities that are implemented on a physical node that is physically located within the geographical area associated with that region  108 . The DEV layer  704  can optimize a query using heuristics that are based on the organization of the logical entities involved in the query. For example, such heuristics can be based on the physical distance between nodes used to implement logical entities involved in a given query. 
   In one embodiment, the optimization processing performed by the DEV layer  704  includes processing that is based on or that optimizes the use of available resources in the wireless sensor network  100  (for example, based on the availability of memory, power, and network bandwidth). Such optimization processing is also referred to here as “resource-influenced optimization.” 
   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on or that alters the status of one or more subscriptions. Such optimization processing is also referred to here as “subscription-status influenced optimization” or “subscription-status optimization.” As noted above, at any given point in time, many subscriptions typically exist in the wireless sensor network  100 . Each subscription that exists in the wireless sensor network  100  has a current state, such as “active,” “suspended,” or “uninstalled.” Optimization processing performed by the DEV layer  704  includes, for example, changing the state of a subscription in order to optimize some attribute of the network  100  (for example, network bandwidth used by the node on which a subscription is implemented). In one implementation of such an embodiment, a finite state machine representation of the status of each subscription is used in the DEV layer  704 , among other things, to perform such subscription-status optimization. Examples of heuristics that can be used in such an embodiment include the following:
         if a new subscription is received by the DEV layer  704  that specifies the same event that is specified in an existing subscription but with an extended lifetime, the DEV layer  704  causes the node on which the existing subscription is installed to modify the existing subscription by extending the lifetime of the existing subscription and to publish events to both the sink entity specified in the existing subscription and the sink entity specified in the new subscription.   if a new subscription is received by the DEV layer  704  that is the same as an existing subscription but with a different event rate, the DEV layer  704 , if possible, causes the node on which the existing subscription is installed to modify the existing subscription by adjusting the event rate of the existing subscription to satisfy both the existing subscription and the new subscription and to publish events to both the sink entity specified in the existing subscription and the sink entity specified in the new subscription.   if a new subscription is received by the DEV layer  704  that specifies parameters similar to parameters specified in an existing subscription, the DEV layer  704  causes the node on which the existing subscription is installed to copy the existing subscription information and to modify the copy in order to install the new subscription on that node.   if a new subscription is received by the DEV layer  704  that is exactly the same as an existing subscription except for specifying a different sink entity, the DEV layer  704  causes the node on which the existing subscription is installed to publish events to both the sink entity specified in the existing subscription and the sink entity specified in the new subscription.   if a new subscription is received by the DEV layer  704  that specifies a first event generated by a first source entity but a second entity in the network  100  is also able generate a second event that is similar to the first event, the DEV layer  704  is able to install the subscription on the second source entity or modify an existing subscription that is already exists on the second source entity to publish events to the sink entity specified in the new subscription.   when the lifetime of a subscription has elapsed, the node on which that subscription is installed can be instructed to save the subscription (for example, by changing the status of the subscription from “installed” to “uninstalled”) for a predetermined period of time after uninstalling the subscription, thereby making the subscription available to the DEV layer  704  (for example, for one or more of the optimizations described above) for an additional period of time.       

   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account a multi-source aggregation operation specified in a subscription. Such optimization processing is also referred to here as “multi-source aggregation optimization” or “multi-source optimization.” In a multi-source aggregation operation, more than one source entity is specified for a subscription and any corresponding publications. An example of such a multi-source aggregation operation is a subscription in which one event generated by a first source entity influences an aggregation event that is generated by a second source entity. Examples of heuristics that can be used in such an embodiment include performing localized data-source optimizations before performing multi-source optimization, performing subscription-status optimization, resource-influenced optimization, or other kinds of optimization processing before performing multi-source optimization, and/or performing event-sequencing optimization so that events from disparate sources are ordered optimally. 
   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account a single-source aggregation operation specified in a subscription. Such optimization processing is also referred to here as “single-source aggregation optimization” or “single-source optimization.” In a single-source aggregation operation, a single source entity is specified for a subscription and any corresponding publications. In such single-source aggregation optimization, information related to the single source entity is used in performing optimization processing. Examples of heuristics that can be used in such optimization processing include implementing the source entity for such an aggregation operation on a physical node that has higher resource availability and/or that results in the lowest (or lower) amount of power being expended to communicate with that physical node. 
   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account the ordering of subscriptions and/or publications (for example, within a given subscription, region, and/or the network  100  as a whole). Such optimization processing is also referred to here as “event sequencing optimization.” In such optimization processing, dependencies between the various events and parameters are used in finding an optimal (or improved) sequencing of subscriptions and/or publications. In one implementation, a Petri net-based model is maintained in the DEV layer  704  for use in such optimization processing. 
   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account a “when” clause specified in a subscription. The when clause for a given subscription is used to specify a condition that is used in the event filter for that subscription. Such optimization processing is also referred to here as “when-clause optimization.” The when-clause optimization processing breaks down the when clause into unit/atomic conditions that are optimized using the optimization processing described above. In one implementation of such an embodiment, techniques similar to those used in optimizing “where” clauses in structured query language (SQL) queries are used. For example, in one such implementation, a subscription/publication parse tree is assembled from the query and evaluated to find the cost of each condition-expression given in the when clause and appropriate optimizations are performed. 
   In one embodiment, optimization processing performed by the DEV layer  704  includes processing that is based on, that alters, or that otherwise takes into account one or more constraints specified in a subscription. Constraints are specified for a given subscription in order to specify the boundaries of the subscription. The constraints can be used to eliminate some of the methods or approaches used to manage the subscription and any corresponding publications. 
   In one embodiment, the DEV layer  704  uses one or more finite state machine models to implement at least a portion of the functionality provided by the DEV layer  704 . In such an embodiment, a finite state machine-based model is used to model those aspects of the network  100  that involve specific states and state transitions. Examples of where a finite state machine-based model is used include:
         tracking the status of subscriptions; each subscription transitions through one or more of the various states mentioned above (for example, “installed,” “active,” “suspended,” “resumed,” and “waiting”).   tracking data source dependencies; global dependencies between those entities in the network  100  that wait on other entities in the network  100  for data or events are used to identify those entities at which additional load can be scheduled (for example, because those entities are in a wait state).   tracking the current status of entities in the network; a given entity transitions through various states such as “initialization,” “fault,” and diagnostics.” The states are required during subscription scheduling optimization. For example, when an entity is in a “diagnostic” state, operations should not be scheduled on that entity.   tracking the current health status of an entity; the health of an entity can be modeled using states and transitions between the states. For example, the power and memory available at a given entity can be modeled using various states and transitions between those states can be used to represent the health of the entity.       

   In one embodiment, the DEV layer  704  uses one or more Petri net models to implement at least a portion of the functionality provided by DEV layer  704 . In such an embodiment, a Petri net model is used in event scheduling. The events are scheduled based on dependencies that exist between the events. The tokens of the Petri net are used to schedule and sequence the various subscriptions and publications that exist in the wireless sensor network  100 . 
   In the embodiment shown in  FIG. 7 , the data management stack  700  further comprises a logical layer  706 . The logical layer  706  implements an abstract view of the data in the wireless sensor network  100  using the event-based data model described above in connection with  FIGS. 3 through 6  in which the wireless sensor network  100  is logically viewed as a set of logical entities that generate discrete events. Such an event-based data model makes use of a producer/consumer model in which a given entity can be classified as a consumer of certain data and a producer of other data. The use of the producer/consumer model, among other things, supports sensing, actuation, and control in sensor network  100 . The event-based data model also makes use of a publish/subscribe model so that the nodes in the wireless sensor network  100  only monitor (and process and communicate data about) those events that are of interest to some entity in the network  100 . 
   The data management stack  700 , in the embodiment shown in  FIG. 7 , also comprises an extended logical layer  708  in which application scenario-specific extensions to the logical layer  706  are made. In one implementation, the extended logical layer  708  includes extensions that maintain information related to event dependencies, data dependencies (for example, dependencies between events that are generated by various data sources), aggregation dependencies, replication dependencies, control dependencies (for example, dependencies that exist between various data sources for performing a control operation), actuation dependencies, and availability management (for example, information pertaining to availability of data to indicates, for example, that data should be stored in an intermediary data source for ensuring a desired level of data availability). In one implementation, the extended logical layer  708  is implemented at the base station  104  and at each data source in the network  100 . 
   The data management stack  700 , in the embodiment shown in  FIG. 7 , also comprises an execution layer  710 . The execution layer  710  provides an abstract view and snap-shot, both static and dynamic, of the execution state of the wireless sensor network  100 . The execution layer  710  maintains information pertaining to aggregation points in the network  100 , adaptive query operations, query execution points, and replication points. The information maintained by the execution layer  710  is updated in response to every occurrence of an event of interest (that is, every event that is subscribed to) in the network  100 . 
   The data management stack  700 , in the embodiment shown in  FIG. 7 , also comprises a physical layer  712  in which network-specific dependencies are managed. The topology, routing and base-band issues are managed by the physical layer  712 . The execution layer  710  interacts with and uses the services provided by the physical layer  712  to publish events, identify aggregation points and optimize data management functionality. 
   As noted above, an event-based formalism is used for data management in the wireless sensor network  100 . The formalism includes the grammar used for specifying the subscriptions and resulting publications of data from the sensor network. In one embodiment, the event-based formalism is extended to include a set of operators (also referred to here as “discrete event operators”) that are used for forming an execution plan that can be subjected to analysis and optimization using the properties of the operators. The discrete event operators are defined from an algebraic formalism. This algebraic formalism is also referred to here as a “discrete event process algebra.” The discrete event operators in this algebra are extended and modified appropriately for execution plan creation. 
   The discrete event process algebra is used by the data management stack  700  of the wireless sensor network  100  to formulate “processes” for use in the execution planning phase performed by the query formalism layer  700 . The following describes one embodiment of a discrete event process algebra. In this embodiment, a “trajectory” is defined as the sequence of events that are accepted and processed by a given process until termination of that process. The symbol “ε” is defined as the string whose symbols are the events from the event set. The symbol “Σ” is defined as the set of all the events that are applicable for a process, including both the events that are accepted and rejected by the process. The symbol “Xi” is defined as the string of events that are rejected by a process before an event is successfully accepted by the process. The symbol “σi” is defined as the event that is accepted by a process. The symbol “Σext” is defined as the event set Σ that is extended with events Termination, Divergence, Positive Response and Feedback Response. 
   In one embodiment, any decision point along the wireless sensor network  100  is defined as a process. Formally, a process P in the wireless sensor network  100  is a subset P ⊂ O td :=(2exp(Σ ext )X Σ)*X 2exp(Σ ext ) satisfying the following conditions:
         Condition 1: (ε, Φ) ∈P; Null trajectory is in every process. This is the idle state of the process.   Condition 2: ((X 0 , σ 1 )(X 1 , σ 2 ) . . . (X k1 −, σ k ), X k ) ∈P &amp; ∃j: 0≦j≦k−1; σ j+1 ∈X j   ((X 0 , σ 1 ) . . . (X j−1 , σ j ), X j  ∪ ∈P; All trajectories of a non-divergent process must be valid.   Condition 3: Termination symbol ( ) is a standalone symbol.   Condition 4: A process terminates with the termination event ( ) and accepts no further events.   Condition 5: A process generates event (/^) in response a set of accepted events.       

   In such an embodiment of a discrete event process algebra, an event set Σ is augmented using /^ and ^\ symbols. In such an embodiment, the following discrete event operators are defined:
         (a) Prefix Operator (→.): The prefix operator in the data management stack  700  is used for sequencing the operations in the wireless sensor network  100 . The prefix operator Q:=σ→P specifies that the process Q starts at an initial state and then moves to (transitions to) process P on event σ. An example of use of prefix operator is the ACTIVATE clause of the data model (describe above in  FIG. 8 ). One example of a subscription that includes this operator is:   SUBSCRIBE EVENT (‘124’, ‘125’, ‘126’) OF ‘902’ WHEN (‘512’ OF ‘345’&gt;‘903’ OF ‘567’) ACTIVATE ‘10:32:12” LIFETIME ‘12:30:00’ EVERY ‘00:00;10’;       

   In this example, P and Q are defined as:
         Q=Idle process Δ that is waiting for an event.   P=(‘512’ OF ‘345’&gt;‘903’ OF ‘567’)   σ=Timer event ‘10:32:12’   Δ:=“10:32:12”--------------------→(‘512’ OF ‘345’&gt;‘903’ OF ‘567’)   (b) Controlled Alternative Operator (+): This operator enables transition to two different processes that depend on two different and mutually exclusive events. Given Q 1 =σ 1 →P 1  and Q 2 =σ 2 →P 2  the operator + enables transition from the start state of process Q to either P 1  or P 2  depending upon either the σ 1  or σ 2  event respectively. Q=(σ 1 +P 1 )+(σ 2 +P 2 ). One example of a subscription that includes this operator is:   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’&gt;50 AND ‘602’ OF ‘831’&gt;70) OR (‘602’ OF ‘831’&lt;20 AND ‘129’ OF ‘513’&lt;90)) ACTIVATE IMMEDIATE;       

   In this example, P 1 , P 2 , σ 1 , σ 2 , and Q are defined as follows:
         P 1 =(‘512’ OF ‘345’&gt;50 AND ‘602’ OF ‘831’&gt;70)   P 2 =(‘602’ OF ‘831’&lt;20 AND ‘129’ OF ‘513’&lt;90)   σ 1 =(‘602’ OF ‘831’&gt;70)   σ 2 =(‘602’ OF ‘831’&lt;20)   Q=Δ (which is referred to here as the “idle process”) and is defined as:   Δ=((‘602’ OF ‘831’&gt;70)→(‘512’ OF ‘345’&gt;50 AND ‘602’ OF ‘831’&gt;70)+((‘602’ OF ‘831’&lt;20)→(‘602’ OF ‘831’&lt;20 AND ‘129’ OF ‘513’&lt;90))   (c) Uncontrolled Alternative Operator (⊕): This operator is not used in this embodiment of a discrete event process algebra, as the wireless sensor network  100  in this embodiment is deterministic and no non-deterministic operators are possible.   (d) Event-Internalization (\σ): This operator is used to remove the occurrences of a given event from external view. Given P=(a→b→Δ)+(c→Δ), P\a is given by (b→Δ)+(c→Δ). An example of one application of this operator is during the LIFETIME and EVERY clauses of the grammar defined in  FIG. 8 . The event for LIFETIME clause is internalized with that of EVERY clause so that LIFETIME events are made ‘invisible’ from the external view.   (e) Parallel Composition without Sync. (.∥Φ.): This operator enables two processes to operate in parallel completely independent of each other. This operator is used in the data management stack  700  for performing operations that need not be synchronized by timer events. One class of operation for which this operator is applicable is the ‘Decomposable aggregation’ operation.   (f) Parallel Composition with full Synchronization (.∥Σ.): This operator enforces a rule that all the events in the event set Σ be completed and fully synchronized. This operator is used in the data management stack  700  to execute events that are conjoined by the AND logical operator in the WHEN clause of the grammar set forth above in  FIG. 8 . One example of a query that uses this operator is:   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’&gt;50 AND ‘602’ OF ‘831’&gt;70) OR (‘602’ OF ‘831’&lt;20 AND ‘129’ OF ‘513’&lt;90)) ACTIVATE IMMEDIATE;       

   In this example, the synchronization set is:
         Σ 1 ={(‘512’ OF ‘345’&gt;50), (‘602’ OF ‘831’&gt;70)}   Σ 2 ={(‘602’ OF ‘831’&lt;20), (‘129’ OF ‘513’&lt;90)}       

   In this example, the event-sets Σ 1  and Σ 2  are synchronized using the “.∥Σ.” operator.
         (g) Parallel Composition on Synchronization Set (.∥A.): This operator is similar in functionality to the .∥Σ. operator except that A⊂Σ. This restricted set is applicable in the data management stack  700  when the WHEN clause of a SUBSCRIBE statement contains an OR logical operator A=Σ(AND) event−Σ(OR) events.   (h) Prioritized Synchronization Composition (.A∥B.): The prioritized sets are defined on the processes and synchronization is enforced on these events only. This operator is used in the data management stack  700  to enforce priority on the events of a data source. Only those events that have priority above a certain threshold are included in the priority event sets A and B.   (i) Termination ( ): A process terminates if after processing a set of events the process refuses any further events. A termination operator is used in the data management stack  700  to denote the completion of the SUBSCRIPTION. A variation of the   operator is the instance termination operator (↓), which terminates the current instance of SUBSCRIPTION and initializes the subscription for a new execution.   (j) Divergence ( ): The divergence operator models the catastrophic behavior of the process and it is applicable when the process reaches a chaotic state. In the data management stack  700 , this operator is used for modeling the behavior of the wireless sensor network  100  when either the node dies or the node gets disconnected from the network.   (k) Positive Response Operator (/^): This operator generates events from the process in response to the processing function&#39;s output. This is a positive response as it contains values that are generated. An example of this kind of operator would be used when values are PUBLISHED from the data source in response to an aggregation operation.   (l) Feedback Response Operator (^\): The feedback response operator is used to signal the completion of some task. In the data management stack  700 , this operator is used to start the operation of parameter assimilation specified in a SUBSCRIPTION/PUBLICATION list. One example of a subscription query that uses this operator is:   SUBSCRIBE EVENT (‘124’) OF ‘902’ WHEN ((‘512’ OF ‘345’&gt;50 AND ‘602’ OF ‘831’&gt;70) OR (‘602’ OF ‘831’&lt;20 AND ‘129’ OF ‘513’&lt;90)) ACTIVATE IMMEDIATE;       

     FIG. 9  is a flow diagram of one embodiment of a method  900  of compiling a source form of a query in order to generate the binary form of that query. The embodiment of method  900  shown in  FIG. 9  is described here as being implemented using the wireless sensor network  100 , wireless sensor node  102 , and data management stack  700  of  FIGS. 1 ,  2 , and  7 , respectively. Other embodiments are implemented in other ways. 
   Method  900  includes receiving a query in source form (block  902 ). For example, in one usage scenario, a user of the wireless sensor network  100  formulates a query using the query language described above in connection with  FIG. 8  and inputs (or otherwise communicates) the query to the base station  104 . In one implementation, the query formalism layer  702  implements the functionality that receives the query in source form. 
   The query is checked for syntactic and semantic errors (block  904 ). If there are any errors, the user is informed of the error (block  906 ) and processing of the received query is terminated. If there are no errors, a graph is generated from the source form of the query (block  908 ). The graph that is generated expresses the query in an instruction set that is used in the wireless sensor network  100 . The source form of the query is parsed and the corresponding graph is generated based on the contents of the query. Each node in the graph that is generated is associated with a particular instruction from the instruction set. In this embodiment, the instruction set comprises a set of discrete event operators from a discrete event process algebra (also referred to here as “DEO instructions”) and a set of operators that are used to define and/or retrieve a value for a simple event (also referred to here as “simple-event instructions”). A discrete event operator instruction is associated with each parent node in the graph (that is, each node that includes one or more child nodes). The discrete event operator instruction defines a relationship (for example, sequencing, ordering and/or synchronization) of the subject defined by and under each child node of that parent node. The DEO instruction associated with each parent node is also referred to here as the “parent instruction” for that parent node. Each child node can itself be a parent node (having an associated DEO instruction and one or more child nodes) or a leaf node. Each leaf node is associated with a single, simple event and a simple-event instruction is associated with each leaf node. 
   Method  900  further comprises attempting to optimize the query as expressed in the generated graph (block  910 ). For example, one or more of the optimization techniques described above are performed by the DEV layer  704  of the data management stack  700 . A binary form of the query is then generated from the optimized graph (block  912 ). The graph generated for the query is traversed and the instruction associated with each node is generated, properly populated, and added to the end of the binary form of the query. In this way, the binary form of the query is generated. 
     FIG. 10  is a block diagram illustrating one exemplary instruction format  1000  for use with the embodiment of method  900  shown in  FIG. 9 . Each instruction includes an operation code (also referred to here as an “opcode”) field  1002  that identifies the particular operation for that instruction. Each instruction also includes an operand size field  1004  that contains the total size of the operands (if any) included in that instruction. In the embodiment shown in  FIG. 10 , the opcode field  1002  and the operand size field  1004  are each 4 bits wide and are located in the first byte of the instruction (labeled “Byte  0 ” in  FIG. 10 ). Each instruction also includes a parent identifier field  1006  that contains an identifier that identifies the parent instruction for that instruction (which corresponds to a particular parent node in a corresponding graph). The parent identifier field  1006 , in the embodiment shown in  FIG. 10 , is one byte wide and is located in the second byte of the instruction (labeled “Byte  1 ” in  FIG. 10 ). Each instruction includes none, one, or more operands depending on the particular opcode specified for that instruction. In the embodiment shown in  FIG. 10 , each operand follows the parent identifier field  1006  in the instruction. 
     FIG. 11  illustrates one example of an instruction set  1100 . The instruction set  1100  includes a set of DEO instructions  1102 , each of which corresponds to an operator in the discrete event process algebra described above. The instruction set  1100  also includes a set of simple-event instruction  1104  that are used to define and retrieve a simple event. 
     FIG. 12  illustrates one example of a query  1200  expressed in source form using the grammar set forth above in  FIG. 8 . The query  1200  shown in  FIG. 12  specifies a subscription request for getting data from a particular region of interest. In the query  1200  shown in  FIG. 12 , the particular region of interest for the subscription has a region identifier of  99 . This subscription subscribes to three events that related to the parameters identified by the parameter identifiers of  124 ,  125 , and  126 . For example, in one implementation, these three parameters relate to temperature, airflow and light intensity, respectively. In the query shown in  FIG. 12 , a lifetime of one minute and thirty seconds is specified for the subscription. The query  1200  specifies that the subscription be activated after five seconds and be evaluated every 30 minutes. Additional conditions are specified for the subscription request in a “WHEN” clause. These additional conditions specify when the parameters of interest should be fetched and published as events at the specified points of time and when the condition specified in the WHEN clause, as a whole, is valid. 
     FIG. 13  illustrates a graph  1300  generated from the query  1200  that is expressed in source form in  FIG. 12 . Each parent node of the graph  1300  is displayed in  FIG. 13  using the discrete event process algebra symbol that represents the discrete event operator associated with that parent node. Also, the discrete event operator associated with each parent node can also be viewed as a complex event, each occurrence of which is determined as a function of the child nodes of that parent node. Each leaf node in the graph  1300  is displayed in  FIG. 13  using a description of the particular simple event associated with that leaf node. Also, each node of the graph  1300  has an associated index number (referred to here as an “event index”). The event index for each node in the graph  1300  shown in  FIG. 13  is set forth in parentheses to the left of that node. Each node is also refereed to here using the event index for that node (for example, the node having an event index of 1 is referred to here as “node  1 ” or “event  1 ” and the node having an event index of 2 is referred to here as “node  2 ” or “event  2 ”). Each node shown in  FIG. 13  also has an associated identifier number (referred to here as an “event identifier”) that is set forth to the right of or below that node. 
     FIG. 14  is a table  1400  having a row for each node in the graph  1300  shown in  FIG. 13 . The table  1400  also includes a first column that identifies the event index of the node of each row, a second column that identifies the event identifier for each row, a third column that identifies the event index of the parent event for each row, a fourth column that identifies the event identifier for the parent event for each row in decimal form, and a fifth column that sets forth the event identifier of the parent event for each row in hexadecimal form. 
   The binary form of the query  1200  set forth in  FIG. 12  is shown in  FIG. 15 . The binary form of the query  1200  is shown in  FIG. 15  in a grid in which the first byte of the binary form of the query  1200  (that is, “00”) is shown in the upper left corner of the grid (that is, at the cell at row 0 and column 0). Each instruction is surrounded by a box that is referenced in  FIG. 15  with the event index for the node (and the complex event represented by that node) to which that instruction corresponds. For example, the first instruction shown in  FIG. 15  is the instruction that corresponds to the node  1  shown in  FIG. 14 . The first instruction is “00 FF”, where the first 4 bits of the first byte (“0”) is the opcode for a SUBSCRIBE DEO instruction, the second 4 bits of the first byte (“0)” is the size of any operands for that opcode (which is zero because there are no operands for that opcode), and the second byte is the event identifier for that instruction&#39;s parent event (which is “FF” for the first instruction). The eleventh instruction shown in  FIG. 15 , for example, is the instruction that corresponds to node  11  shown in  FIG. 14 . The eleventh instruction is “E5 15 2D 0B 63 01 32”, where the first 4 bits of the first byte (“E”) is the opcode for the EVENT external-event operator shown in  FIG. 11 , the second 4 bits of the first byte (“5”) is the number of operands, in bytes, for this opcode, and the second byte (“15”) is the event identifier, in hexadecimal notation, for that instruction&#39;s parent event. The next five bytes (that is, the first three operands for this instruction) specify a particular event: when parameter “45” (“2D” in hexadecimal notation) associated with (opcode “B”) region “99” (“63” in hexadecimal notation) is greater than (opcode “01”) “50” (“32” in hexadecimal notation). 
   In this example, the binary form of the query  1200  is injected into the wireless sensor network  100 . As the query  1200  propagates among the nodes of the wireless sensor network  100 , each node that receives binary form of the query  1200  from a peer node parses the binary form of the query  1200  and determines if that receiving node is able to act as a source entity for any of the simple events (specified by simple-event instructions  1104 ) specified in the binary form of the query  1200 . If the receiving node is able to act as a source entity for such a simple event, the receiving node informs the peer node from which the receiving node received the binary form. As this process plays out throughout the network  100 , a series of recursive subscriptions are formed, as managed by the DEV layer  704  of the data management stack  700 .  FIG. 16  illustrates one example of a set of recursive subscriptions that can result from the query  1200  of  FIG. 12 . As shown in  FIG. 16 , a set of eight subscriptions (labeled S 1  through S 8  in  FIG. 16 ) are generated in order to satisfy the query  1200 . In this example, the query  1200  is injected into the wireless sensor network  100  from the base station  104  and the base station  104 , in this example, is the sink entity for the original subscription generated from the query  1200  (which is subscription S 1  in  FIG. 16 ). The source entity for the subscription S 1  is an intermediary node (labeled node IM 1  in  FIG. 16 ). The node IM 1  publishes events related to parameters  124 ,  125 , and  126  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of the subscription S 1  are all true. In this example, the node IM 1  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of the subscription S 1 . The node IM 1  is a sink entity for a subscription S 2  for an event related to parameter  124 , a sink entity for a subscription S 3  for an event related to parameter  125 , and a sink entity for subscription S 4  for an event related to parameter  126 . 
   In this example, the source entity for subscription S 2  is the node in the wireless sensor network  100  that is the data source for parameter  124  (labeled node  124  in  FIG. 16 ). Node  124  publishes events related to parameter  124  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 2  are all true. In this example, the node  124  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of the subscription S 2 . The node  124  is a sink entity for a subscription S 5  for an event related to parameter  45  of region  99 , a sink entity for a subscription S 6  for an event related to parameter  54  of region  99 , a sink entity for subscription S 7  for an event related to parameter  81  of region  99 , and a sink entity for a subscription S 8  for an event related to parameter  90  of region  99 . 
   In this example, the source entity for subscription S 3  is the node in the wireless sensor network  100  that is the data source for parameter  125  (labeled node  125  in  FIG. 16 ). Node  125  publishes events related to parameter  125  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 3  are all true. In this example, the node  125  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 3 . The node  125  is also a sink entity for subscription S 5 , a sink entity for subscription S 6 , a sink entity for subscription S 7 , and a sink entity for subscription S 8 . 
   In this example, the source entity for subscription S 4  is the node in the wireless sensor network  100  that is the data source for parameter  126  (labeled node  126  in  FIG. 16 ). Node  126  publishes events related to parameter  126  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 4  are all true. In this example, the node  126  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 4 . The node  126  is also a sink entity for subscription S 5 , a sink entity for subscription S 6 , a sink entity for subscription S 7 , and a sink entity for subscription S 8 . 
   In this example, the source entity for subscription S 5  is the node in the wireless sensor network  100  that is the data source for parameter  45  of region  99  (labeled node  45  in  FIG. 16 ). Node  45  publishes events related to parameter  45  of region  99  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 5  are all true. In this example, the node  45  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 5  and the data event specified in the WHEN clause of subscription S 5 . 
   In this example, the source entity for subscription S 6  is the node in the wireless sensor network  100  that is the data source for parameter  54  of region  99  (labeled node  54  in  FIG. 16 ). Node  54  publishes events related to parameter  54  of region  99  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 6  are all true. In this example, the node  54  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 6  and the data event specified in the WHEN clause of subscription S 6 . 
   In this example, the source entity for subscription S 7  is the node in the wireless sensor network  100  that is the data source for parameter  81  of region  99  (labeled node  81  in  FIG. 16 ). Node  81  publishes events related to parameter  81  of region  99  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 7  are all true. In this example, the node  81  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 7  and the data event specified in the WHEN clause of subscription S 7 . 
   In this example, the source entity for subscription S 8  is the node in the wireless sensor network  100  that is the data source for parameter  90  of region  99  (labeled node  90  in  FIG. 16 ). Node  90  publishes events related to parameter  90  of region  99  when the WHEN clause and the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 8  are all true. In this example, the node  90  is able to act as a source of the time events specified in the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 8  and the data event specified in the WHEN clause of subscription S 8 . 
   When nodes  45  determines that the LIFETIME, EVERY, ACTIVATE, and WHEN clauses of subscription S 5  are all true, node  45  (which implements the source entity for subscription S 5 ) publishes an event related to parameter  45  of region  99  to the sink entities of subscription S 5 . When node  54  determines that the LIFETIME, EVERY, ACTIVATE and WHEN clauses of subscription S 6  are all true, node  54  (which implements the source entity for subscription S 6 ) publishes an event related to parameter  54  of region  99  to the sink entities of subscription S 6 . When node  81  determines that the LIFETIME, EVERY, ACTIVATE and WHEN clauses of subscription S 7  are all true, node  81  (which implements the source entity for subscription S 7 ) publishes an event related to parameter  81  of region  99  to the sink entities of subscription S 7 . When node  90  determines that the LIFETIME, EVERY, ACTIVATE and WHEN clauses of subscription S 8  are all true, node  90  (which implements the source entity for subscription S 8 ) publishes an event related to parameter  90  of region  99  to the sink entities of subscription S 8 . The sink entities for subscriptions S 5 , S 6 , S 7 , and S 8  are implemented on node  124 , node  125 , and node  126 , respectively. 
   When node  124  determines that the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 2  are all true and receives information from node  45  and node  54  or from node  81  and node  90  indicating that the WHEN clause of subscription S 2  is true, node  124  (which implements the source entity for subscription S 2 ) publishes an event related to parameter  124  to the sink entity of subscription S 2  (which is implemented on node IM 1 ). 
   When node  125  determines that the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 3  are all true and receives information from node  45  and node  54  or from node  81  and node  90  indicating that the WHEN clause of subscription S 3  is true, node  125  (which implements the source entity for subscription S 3 ) publishes an event related to parameter  125  to the sink entity of subscription S 3  (which is implemented on node IM 1 ). 
   When node  126  determines that the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 4  are all true and receives information from node  45  and node  54  or from node  81  and node  90  indicating that the WHEN clause of subscription S 4  is true, node  126  (which implements the source entity for subscription S 4 ) publishes an event related to parameter  126  to the sink entity of subscription S 4  (which is implemented on node IM 1 ). 
   When node IM 1  determines that the LIFETIME, EVERY, and ACTIVATE clauses of subscription S 1  are all true and receives information from node  124 , node  125 , or node  126  indicating that the WHEN clause of subscription S 1  is true (the WHEN clause of subscription S 1  is the same as the WHEN clause in subscriptions S 2 , S 3 , and S 4 ), node IM 1  (which implements the source entity for subscription S 1 ) publishes an event related to parameter  124  (if node IM 1  has received a publication for subscription S 2 ), parameter  125  (if node IM 1  has received a publication for subscription S 2 ), and/or parameter  126  (if node IM 1  has received a publication for subscription S 2 ) to the sink entity of subscription S 1  (which is implemented on the base station  104 ). 
     FIG. 17  is a block diagram of one embodiment of a virtual machine  1700  for use in a wireless sensor network. The embodiment of virtual machine  1700  shown in  FIG. 17  is described here as being implemented using the wireless sensor network  100  and wireless sensor node  102  of  FIGS. 1 and 2 , respectively. Other embodiments are implemented in other ways. The virtual machine  1700  shown in  FIG. 17  executes one or more “event programs” as described below. 
   In such an embodiment, each event program is the binary form of a subscription that has been installed on the particular node on which the virtual machine  1700  is implemented (also referred to here as the “target” node) and comprises a set of instructions (for example, from the instruction set  1100  shown in  FIG. 11 ). Each event program identifies one or more simple events (for example, identified using the simple-event instructions  1104  described above). In the embodiment shown in  FIG. 17 , the occurrence of each simple event is signaled to the virtual machine  1700  by an interrupt generated by the underlying hardware of the target node. As described above, each discrete event operator instruction  1102  is also viewed, in the context of the virtual machine  1700 , as a complex event. When each discrete event operator instruction  1104  is executed, particular processing is performed by the target node. This processing that is performed when each discrete event operator instruction  1104  is executed is also referred to here as the “event handler” for the internal event associated with that discrete event operator instruction  1104 . 
   The embodiment of a virtual machine  1700  shown in  FIG. 17  employs techniques used to implement Petri nets. Petri nets are typically used to model systems with interacting concurrent components where the transfer of information or materials from one component to another component requires that the activities of the involved components be synchronized during the interaction. In such a system, it is often the case that one component must wait for another component in order for the two components to remain synchronized. The timing of actions by different components may be very complex and the resulting interactions between components difficult to describe and synchronize. Petri nets provide a useful mathematical foundation to manage these kinds of systems. 
   A Petri net can be represented as a graph with two types of nodes—“places” and “transitions.” Each transition has a set of input places and a set of output places. In the graph for a Petri net, each input place is shown as being connected to the transition by an arc that is directed from the input place to the transition. Similarly, each output place is shown as being connected to the transition by an arc that is directed from the transition to the output place. Places can be “marked” by “tokens.” Each place can hold an integer number of tokens. Transitions whose input places are all marked by at least one token are said to be “enabled” and are fired. When a transition is fired, a token is placed in each of the output places for that transition. 
   In the embodiment of virtual machine  1700  shown in  FIG. 17 , each event (including each simple event and each complex event) in the event program is modeled as a “place.” The event handler for each complex event (that is, each discrete event operator instruction  1104 ) is modeled as a “transition.” Each transition associated with a particular complex event has a set of input places comprising the child events of that complex event. Each transition associated with a particular complex event also comprises an output place that corresponds to the parent event of that complex event. The event handler associated with each transition is executed when the transition is enabled. Each transition is enabled when a logical function associated with that transition evaluates to “true.” This logical function is defined, in this embodiment, by a logical operator (for example, AND, OR, or XOR) that is used to logically combine all the input places for the transition, where each input place has a logical value of “true” if the input place is marked and has a logical value of “false” if the input place is not marked. Each such logical function, in other words, implements the event filter condition for the associated complex event. 
   In such an embodiment, each place (that is, each event) is allocated a data structure that is used to determine if that place has been marked. This data structure is referred to here as a “token data structure” or just a “token.” A place is marked by storing a value corresponding to logical true (for example, a value “1”) in the token for that place. A place is not marked when the token for that place contains a value corresponding to logical false (for example, a value of “0”). The set of tokens allocated to a set of input places for a given transition (that is, a given event handler) is also referred to here as the set of “input tokens” for that transition. Likewise, the token allocated to the output place of a given transition is also referred to here as the “output token” for that transition. 
   The virtual machine  1700  comprises a token management subsystem  1702  for allocating and managing the token data structures used by the virtual machine  1700 . The number of tokens that can be allocated in the virtual machine  1700  at any time is limited. The token management subsystem  1702  includes a token scheduler unit  1704  and a token register bank  1706 . The token register bank  1706  comprises a set of bit-addressable token registers  1708  in which the tokens that are allocated by the token management subsystem  1702  are stored. In the embodiment shown in  FIG. 17 , the token register bank  1706  includes five token registers  1708 , though the token register bank  1706  includes a different number of token registers  1708  in other embodiments. The bits contained within each of the token registers  1708  are allocated by the token scheduler unit  1704  based on considerations such as runtime management of tokens that have been allocated, the allocation of tokens to events such that each token resolves to a unique address, and/or the scheduling of tokens for new events that are generated at run time. 
   The token management subsystem  1702  also allocates and manages a data structure (referred to here as the “event-token map”  1710 ) that maps each event in the event program to the location within the token register bank  1706  of the token allocated for that event. The token management subsystem  1702  also allocates and manages a data structure (referred to here as the “token-association list”  1712 ) that is used in determining when a particular event handler should be executed. 
   In the embodiment shown in  FIG. 17 , the token-association list  1712  is implemented as a table in which each event handler in the event program is assigned a row in the table. As noted above, each event handler is modeled as a transition that has a set of input places and an output place. Each row contains a handle (or other reference) to the program instructions that comprise the event handler associated with that row. Each row also specifies the address (or other identifier) of the input tokens for the event associated with that row (labeled “Token List” in  FIG. 17 ). Each row also specifies the logical operator (for example, AND, OR, or XOR) that is used to logically combine the input places for the transition associated with that row&#39;s event (labeled “Association” in  FIG. 17 ). The input places are logically combined to determine if the transition associated with each row is enabled and, as a result, the event handler associated with that transition should be executed. This operation is also referred to here as “evaluating” that row of the token-association list  1712 . If the result of logically combining the input tokens is “true”, then that event handler is executed. When the event handler is executed for a particular complex event, the event handler, among other things, clears (that is, unmarks) the input tokens of that complex event and marks the output token of that complex event. 
   Every row in the token-association list  1712  is evaluated when any token within the token register bank  1706  has been marked. The token management subsystem  1702  comprises an event handling unit  1714  that performs such processing using the token-association list  1712 . That is, the event handler  1714 , when any token in the token register bank  1706  is marked, evaluates each row in the token-association list  1712 . If a row evaluates to true, the event handling unit  1714  determines the address for the event handler associated with that row using the handle (or other reference) contained in that row and passes the address to the event-execution management subsystem  1716  (described below) to schedule the event handler for execution thereby. 
   The virtual machine  1700  also comprises an interrupt management subsystem  1718  that allocates and handles interrupts and timers that are assigned to simple events specified in an event program. The number of interrupts and timers that are available on the target node is limited. The interrupt management subsystem  1718  comprises an interrupt multiplexer register bank  1720 . The interrupt multiplexer register bank  1720  comprises a set of interrupt multiplexer registers  1722 . In the embodiment shown in  FIG. 17 , the interrupt multiplexer register bank  1720  includes five interrupt multiplexer registers  1722 , though the interrupt multiplexer register bank  1720  includes a different number of interrupt multiplexer registers  1722  in other embodiments. 
   In the embodiment shown in  FIG. 17 , the interrupt management subsystem  1718  comprises an interrupt multiplexer unit  1726  that allocates the physical interrupts and timers to the simple events specified in each event program. For example, the interrupt multiplexer unit  1726  performs such processing when each event program is installed on the target node. The interrupt multiplexer unit  1726  identifies all simple events that are data events (as opposed to timer events) and allocates a physical interrupt to each data event. The interrupt multiplexer unit  1726  also configures the underlying hardware of the target node (for example, the programmable processor  206 ) to generate that physical interrupt only when the condition specified for that data event is true. The interrupt multiplexer unit  1726  also identifies all the simple events that are timer events (as opposed to data events) and allocates a hardware timer to that timer event. For timer events, the interrupt multiplexer unit  1726  also allocates physical interrupt to the timer event and configures the underlying hardware of the target node (for example, the programmable processor  206 ) to generate the physical interrupt at the appropriate time as specified in the timer event. 
   The interrupt multiplexer unit  1726  allocates and maintains a data structure (referred to here as the “event-interrupt map”  1724 ) that maps each simple event specified in the event program to the physical interrupt allocated to that event. Each time a physical interrupt occurs, the interrupt multiplexer unit  1726  determines if that physical interrupt has been allocated to any event. If the physical interrupt has been allocated to one or more of the events, the interrupt multiplexer unit  1726  “routes” the interrupt to an interrupt handling unit (IHU)  1728  included in the interrupt management subsystem  1710 . The interrupt handling unit  1728  executes a generic interrupt service routine (ISR) that evaluates each entry in the event-interrupt map  1724 . If the physical interrupt that occurred is contained in a particular entry in the event-interrupt map  1724 , the generic ISR marks the token allocated to the event specified in that entry of the event-interrupt map  1724 . The generic ISR uses the event-token map  1710  to determine which token has been allocated to the event specified in that entry of the event-interrupt map  1724 . As noted above, whenever a token is marked, the event handling unit  1714  evaluates each row in the token-association list  1712  to determine if any event handlers should be scheduled for execution by the event-execution management subsystem  1716 . 
   The virtual machine  1700 , in the embodiment shown in  FIG. 17 , further comprises an event-execution management subsystem  1716 . The event-execution management subsystem  1716  allocates and manages memory in which the program logic that implements each event handler installed on the target node is stored. The event-execution management subsystem  1716  also receives requests from the token management subsystem  1702  to schedule a particular event handler for execution. The event-execution management subsystem  1716  comprises an event scheduler  1730  that performs the scheduling of event handlers. The event scheduler  1730  schedules event handlers for execution based on factors such as priority, time to execute and system resources. The event-execution management subsystem  1716  comprises an event-execution unit  1732  that interacts with the underlying hardware (for example, the programmable processor  206 ) of the target node to execute the program logic of the event handlers in accordance with the schedule established by the event scheduler  1730 . The event-execution management subsystem  1716 , in the embodiment shown in  FIG. 17 , comprises an event queue  1734  that is used by the event-execution scheduler  1730  and the event-execution unit  1732  to schedule and execute the program logic of the event handlers, respectively. 
   In the embodiment shown in  FIG. 17 , the virtual machine  1700  comprises an arithmetic and logic unit  1736  that the event-execution management subsystem  1716  uses to interact with the underlying programmable processor  206  of the target node in order to execute the program logic of the various event handlers installed on the target node. The virtual machine  1700  further comprises a hardware abstraction unit  1738  that is used to access the other parts of the underlying hardware of the target node. For example, in the embodiment shown in  FIG. 17 , the interrupt management subsystem  1710  uses the hardware abstraction unit  1738  to communicate with the underlying hardware of the target node about the physical interrupts and hardware timers used by the virtual machine  1700 . 
   In the embodiment shown in  FIG. 17 , the virtual machine  1700  also comprises an event parser  1740  that parses each event program that is installed on the target node. During parsing, the event program is stored in event program memory  1742 . In one embodiment, the event program memory  1742  is located in a data structure allocated in the memory  208  of the target node. In the embodiment shown in  FIG. 17 , each event program is parsed in two steps. During the first step, the event parser  1740  identifies each discrete event operator instruction  1102  in the event program. For each discrete event operator instruction  1102  that is identified, the event parser  1740  causes the token scheduler unit  1704  to allocate a token for that complex event and causes the event-execution management subsystem  1716  to store the program logic for that event handler. Also, during the first step, the event parser  1740  identifies each simple-event instruction  1104  in the event program. For each simple-event instruction  1104  that is identified, the event parser  1740  causes the token scheduler unit  1704  to allocate a token for that simple event and causes the interrupt management subsystem  1718  to allocate a physical interrupt (and a hardware timer, if the simple event is a timer event). After all of the tokens have been allocated for the event program, the event parser  1740  performs the second step of the parsing process. In the second step, the event parser orders and sequences the tokens that have been allocated by causing the token scheduler unit  1704  to populate a row in the token-association list  1712  for each discrete event operator instruction  1102 . For each discrete event operator instruction  1102 , a row in the token-association list  1712  is populated with the set of input tokens for that discrete event operator instruction  1102 , the logical operator for combining the set of input tokens to determine if the associated event handler should be executed, and a handle (or other reference) to the program logic for that event handler. 
   In one implementation, the virtual machine  1700  is implemented using the “TinyOS” operating system developed at the University of California at Berkeley. The event scheduler  1730 , in such an implementation, is implemented using the event scheduler component of the TinyOS operating system. In such an implementation, the round-robin scheduling logic of the TinyOS scheduler is extended to implement a priority-based scheduling algorithm for use with the virtual machine  1700  of  FIG. 1700 . The hardware abstraction unit  1738  of the virtual machine  1700  is implemented, in such an implementation, using the hardware-abstraction-layer of the TinyOS operating system. 
     FIG. 18  is a flow chart illustrating the processing of an event program by the embodiment of the virtual machine  1700  shown in  FIG. 17 . The processing described here as being performed by the virtual machine  1700  is carried out by the underlying hardware of the target node (for example, by the programmable processor  206 ). The underlying target node, in one implementation, includes a scheduling mechanism that is able to, among other things, allocate processing resources between the processing performed in method  1800 , the program logic for any event handlers that are executed by the event execution unit  1732 , and any processing performed in order to generate interrupts and management hardware timers. 
   Method  1800  comprises receiving the event program (that is, the binary form of a subscription) at the virtual machine (block  1802 ). The event program, in this embodiment, is expressed as a set of instructions, comprising one or more discrete event operator instructions  1102  and simple-event instructions  1104  of the type shown in  FIG. 11 . The event program is received by the target node using the underlying networking and routing layer implemented on the target node and included in the physical layer  712  of the data management stack  700 . The event program is parsed by the event parser  1740  of the virtual machine  1700  (block  1804 ). In this embodiment, the event program is parsed using the two-step process described above. In the first step, the tokens for each event of the event program (that is, for each instruction) are allocated, the event handler for each discrete event operator instruction  1102  is allocated, and the interrupts and hardware timers are allocated for the simple events specified in the event program. In the second step, the tokens are ordered and sequenced by populating the rows of the token-association list  1712  with the set of input tokens for the complex event associated with that row, the logical operator for combining the set of input tokens, and the handle (or other reference) to the program logic for the event handler for that event. 
   After parsing the event program, the subsystems of the virtual machine  1700  are initialized (block  1806 ). For example, the token registers  1708 , the interrupt multiplexer registers  1722 , and the event scheduler  1730  are initialized along with the underlying hardware of the target node (for example, the processor  206  that generates interrupts and the hardware timers  222 ). 
   After the parsing and initialization is complete, the event program is executed by the virtual machine  1700  (block  1808 ). In the embodiment shown in  FIG. 18 , the event program is not executed until the ACTIVATE event specified in the event program has occurred (checked in block  1810 ). In this embodiment, the ACTIVATE event is allocated an interrupt and hardware timer by the interrupt management subsystem  1718  and the underlying hardware of the target node is initialized to generate this interrupt when the condition specified in the ACTIVATE instruction of the event program is true. 
   Executing the event program comprises determining when an interrupt has been generated (block  1812 ). When an interrupt is generated, the interrupt is checked to determine if that type of interrupt (indicated by an interrupt identifier) has been allocated to an event in the event program (block  1814 ). In this embodiment, the interrupt multiplexer unit  1726  makes this determination. If that type of interrupt has not been allocated to an event in the event program, the interrupt is not processed (returning back to block  1812 ). If that type of interrupt has been allocated, the token associated with the event allocated to that interrupt is marked (block  1816 ). In this embodiment, if that type of interrupt has been allocated, the interrupt multiplexer unit  1726  routes the interrupt to the interrupt handler unit  1728 . The interrupt handler unit  1728  executes the generic interrupt service routine, which uses the event-interrupt map  1724  to determine which event is associated with that interrupt. The generic interrupt service then marks the token allocated to that event, which is stored in the token register bank  1706 . 
   After the token is marked, each row in the token-association list  1712  is evaluated (block  1818 ) and if the row evaluates to true (checked in block  1820 ), the event handler specified by that row is scheduled for execution (block  1822 ). When all rows of the token-association list  1712  have been evaluated (checked in block  1824 ), method  1800  waits for the next interrupt (returning to block  1812 ). This is done until the event-program is terminated, for example, by an event handler (for example, by an event handler associated with a timer event). 
   The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, or in combinations of them. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). 
   A number of embodiments of the invention defined by the following claims have been described. Nevertheless, it will be understood that various modifications to the described embodiments may be made without departing from the spirit and scope of the claimed invention. Accordingly, other embodiments are within the scope of the following claims.