# Patent

## Title
Suspending a result set and continuing from a suspended result set for transparent session migration

## Abstract

A system, method, computer program and system for suspending a result set and continuing from a suspended result set for transparent session migration is disclosed. The rows from a row source are suspended into storage, so that after session migration, the client fetches can transparently continue from the suspended result set.

## Background
CROSS REFERENCED AND RELATED APPLICATIONS
This application claims benefit of Provisional Application Ser. No. 60/601,346; filed on Aug. 12, 2004, which is hereby incorporated by reference as if fully set forth herein.
This application is related to the following other applications: (a) Ser. No. 10/144,665, filed May 10, 2002, entitled “Method and System for Scrollable Cursors”; (b) Ser. No. 10/144,666, filed May 10, 2002, entitled “Method and Mechanism for Reducing Client-Side Memory Footprint of Data for Scrollable Cursors”; (c) Ser. No. 10/917,953 filed Aug. 12, 2004, entitled “Transparent Session Migration Across Servers”, (d) Ser. No. 11/097,065 filed on even date herewith entitled “Suspending a Result Set and Continuing from a Suspended Result Set for Scrollable Cursors”, based on Provisional Application Ser. No. 60/601,259; (e) Ser. No. 11/097,422 filed on even date herewith entitled “Suspending a Result Set and Continuing from a Suspended Result Set”, based on Provisional Application Ser. No. 60/601,250; and (f) U.S. Provisional Application Serial No. 60/495,368 filed Aug. 14, 2003, entitled “computer Resource Provisioning”. Each of the above disclosures is hereby incorporated by reference as if fully set forth herein.


BACKGROUND
The invention is related to computer systems, and more particularly to query processing.
Many computer systems utilize servers, such as “database servers” to store and maintain information. In a client-server computer system model (or a multi-tiered computer architecture), users that wish to access or modify information at the server are often located at a “client.” To facilitate the explanation the terms “database server” and “database client” may be used in this document in place of “server” and “client;” however, the embodiments are not limited in applicability to database systems, and indeed, can be utilized in many other types of computer systems.
In client-server systems, users at a database client submit commands to the database server to store, modify, or retrieve data. In response to the user commands, data manipulation or query activities are performed at the database server, with data results returned back to the database client. In networked environments, the database server often performs data manipulation or query commands submitted by remotely located clients. The client may establish a direct connection to the database server over the network, or may establish a connection through one or more intervening system components, such as an application server or transaction processing monitor. In either case, the database server processes the user commands and generates appropriate data outputs to be returned to the client.
A common database function, for example, is to perform data queries using a database query language such as SQL. The database server receives each query and generates a query result set that satisfies the criteria defined by a particular query. The query result set is subsequently transferred to the database client from which the query originated.
A cursor is a handle to a query execution area, e.g., an area in memory in which a parsed structured query language (“SQL”) statement and other information for processing the SQL statement is stored. In many database systems, a cursor is an available resource for accessing or parsing SQL statements embedded within an application. Cursors enable a SQL statement to retrieve rows from a database table into a given result set, which can then be accessed, displayed, or operated upon.
Each cursor has a client side cursor state and a server side cursor state. In server processing there are circumstances when the server side cursor state will become unavailable. For example, when performing load balancing a server will migrate, restart, and/or redirect a session (e.g., a client and/or server session) from one server to another. This can cause an interruption in result set availability. It may be necessary to have access to a result set for the life of a client side cursor.
SUMMARY
The embodiments described herein are directed to a method, system, and computer program product for suspending a result set and continuing from a suspended result set for session migration. The rows from a row source are suspended into storage so that when a fetch is requested the data can be sent (continued) from storage and does not depend on the original row source or session to be available. In embodiment(s), suspending a result set is used to migrate a server session. A method for suspending a result set for transparently migrating a session includes receiving notification that the first session is selected for migration, preparing to migrate by suspending the result set for the first session, determining that the first session is ready to migrate, and migrating the first session.
Further details of the embodiments are described below in the detailed description, drawings, and claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a flowchart 150 of an example of a generic server processing a client fetch query.
FIG. 1B is a flowchart 100, a generic example of a server processing a client fetch of a result set with an option of suspending the result set.
FIG. 2A is a representation of a process 200, an example implementation of suspending a row of a server result set.
FIG. 2B is a representation of process 240, a generic example of a server suspending a server row source.
FIG. 2C is a representation of a row mapping index.
FIG. 2D is a representation of process 270, the suspend all cursors process.
FIG. 3 is a representation of process 300, an example of fetching a row of a result set while the result set is being suspended.
FIG. 4A is a representation of a generic data flow for suspending and fetching operations.
FIG. 4B is a representation of the data flow in one example implementation of suspending and fetching operations.
FIG. 5A is a representation process 520, a generic example of continuing client fetches from a suspended result set after suspension.
FIG. 5B is a representation of process 500, an example embodiment of continuing client fetches from a suspended result set after suspension.
FIG. 6A is a flow chart of process 650, an example embodiment of transparently migrating a session utilizing suspending a result set and continuing from a suspended result set.
FIG. 6B is a flow chart of process 600, an example of transparently migrating a session.
FIG. 7 is a graphical representation of server and client sessions during the suspending and continuing of a suspended result set for session migration.
FIG. 8 is a representation of a system on which suspending a result set and continuing a suspended result set would take place.



DETAILED DESCRIPTION OF INVENTION
A method and system of suspending a result set, and continuing from a suspended result set for transparent session migration is described. In the following description, for the purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the presented embodiments. It will be apparent, however, to one skilled in the art that the embodiments may be practiced without these specific details.
Result Set
The result set is the output of executing a query. The embodiments may be applied to any type of data in the result set, including relational data. To facilitate the present explanation, the following description is made in the context of relational data. It is noted, however, that the embodiments may be applied to non-relational data.
Typically, a defined sequence of actions is performed to execute a query statement, e.g., a SQL query statement. The SQL query is typically processed using a parse, execute, fetch, and close cycle of operation. A cursor, or query execution area, is created, either in conjunction with the SQL query or independently. The cursor may be automatically handled by the database system, or manually controlled by a programmer through an application programming interface; The SQL statement is thereafter parsed, optimized, and executed to produce a set of rows called the result set or row source. The rows in the result set can be extracted from the cursor (“fetched”) either a row at a time or in groups. A row source can be located in the SQL execution cursor area on the server for a database application.
For example, process 150 shown in FIG. 1A represents one way for a server to handle fetch operations. The server waits for a client call in process action (152). Process action (154) processes a fetch request for a client. A row from the result set produced by the cursor is sent from the cursor to the client in process action (156). Process action (158) determines if there are more rows to be fetched. If there are more rows, process 150 returns to process action (156) to return another row. If there are no more rows process action (160) determines if the cursor should be closed. If the cursor should be closed, the cursor is closed in process action (162). After the cursor is closed or if the cursor should not be closed, process 150 returns to process action (152) and waits for the next call.
The database server produces a result set composed of a quantity of data that can be sent to a database client. The user may initially place an explicit request to transmit a first portion of that result set from the database server to the client. At a later time, the client may request a second portion of the result set. This process may proceed until the entire result set is sent, or until the client stops requesting additional portions of the result set. This process results in one or more roundtrips between the database server and the client.
A portion of the result set can be transmitted prior to the client request for that portion. Transmitting information prior to it being immediately requested by the client is referred to as pre-fetching. Pre-fetching can be based on predictive selection of data anticipated to be requested by a client in the future. A determination is made of whether there exists any need or advantage to pre-fetch. The determination is made in a predictive manner based upon different factors. Some example factors include, but are not limited to, data compression ratios, likelihood of the client requesting the data, and roundtrip delay.
While the cursor is active it has a client side cursor state and a server side cursor state. The cursor is used to access the result set. In certain instances the server side cursor is eliminated, disrupting access to the result set. However, it may be necessary to have access to a result set for the life of a client side cursor. Not having access could be a correctness issue. One option is to reissue the query on a new server session. However, even having to reissue the client query to obtain the next set of rows can be a correctness issue. For example re-execution of a query may not guarantee the same results or the same order of results. Access can be maintained to the result set by suspending the result set, i.e. storing the result set such that access is preserved even in the event that the cursor is unavailable. Process 240 in FIG. 2B shows an example suspend process. This process is explained in the SUSPENDING section.
Storage
As mentioned above, when the cursor is active, operations can be performed against the specific row or group of rows at which the cursor is pointing. In some cases, the result set can only be accessed by the specific cursor (e.g., a specific client cursor), and only while the cursor is active. It may also be desirous, in one embodiment, to access and operate on the result set after the server side cursor has been closed. Another embodiment accesses the result set from another client session.
One method of accommodating the various embodiments, is to suspend the result set into the client. However, the client may not have a large enough storage to store the entire result set or have the desire to use the storage for the result set. Further, a client storage may only be available to that client. The solution to make a result set available after a cursor is closed and not consume much of the client storage space, as presented in one embodiment, is to pre-fetch and store the cursor result set.
Storage for a suspended result set can take many forms. Example storage forms include but are not limited to: server cache, server workspace, server persistent storage, server memory shared across nodes, server persistent storage shared across nodes, session cache, session shared memory, session private memory, client cache, client persistent storage, other client memory, other server memory, persistent storage that does not incur transactional overhead such as redo or undo logging or minimizes the use of such transactional logging, or a combination of any of the foregoing. For example, some embodiments depicted in FIG. 2-5 show the suspended result set using a combination of persistent and memory storage. The type of storage used to suspend the result set is selected by the application, or chosen internally, and depends on the intended use of the result set and availability of system resources. The result set storage, in one embodiment, is contiguous space. Another embodiment uses non-contiguous space. In one embodiment the type of storage may be automatically determined based on statistics collected over a period of time. For example, suppose an application was using a result set located on a server with limited available memory. If this result set is suspended, the storage location(s) chosen would be persistent storage or a combination including persistent storage so as minimize consumption of additional memory. However, if the current server memory usage is low, the storage location(s) chosen for a suspended result set could be server memory.
If persistent storage is chosen as the form of storage, one embodiment uses Large Objects (LOB) for the persistent storage, however any storage that is accessible between database nodes is acceptable. The following are two example approaches to creating an LOB. A temporary, or non-sharable, LOB can be created that is visible only in the database node in which it is created. A permanent, or sharable, LOB can be created that is more persistent. That is, the sharable LOB is able to exist for the lifetime of a database and is sharable across database nodes. Hence, a non-sharable LOB can be created in one database node that is not valid in another database node, and a sharable LOB can be created in one database node that is valid in another database node. The persistent storage can be located on the server or on the client.
In one embodiment, the suspended result set storage can be in the memory of a first node, and transmitted to a second node that is continuing the fetches. This may minimize costs (e.g., I/O costs) of accessing persistent storage. Example of methods that may be employed to accomplish this include distributed SQL, inter-process communications, or fast interconnects.
Packing
The suspended result set, in one embodiment, is self-describing storage. That is, the storage has a specific layout and includes information describing that layout. Self-describing storage can include data descriptions, or metadata, within the storage so that entities accessing the storage know what to expect upon reading the storage. Self-describing storage can be helpful, for example, when the server-side cursor is eliminated and the result set still needs to be accessible.
The metadata of the suspended result set can be packed using one packing logic and the values of the suspended result set can be packed using another packing logic. Another embodiment uses compression techniques in addition to the packing layout. In another embodiment, the packing layout includes a version to allow compatibility with older and future versions. Another embodiment uses both version and compression techniques. Further details regarding compression techniques that can be applied are disclosed in application Ser. No. 10/144,666, filed May 10, 2002, entitled “Method and Mechanism for Reducing Client-Side Memory Footprint of Data for Scrollable Cursors,” and/or application Ser. No. 09/515,674, filed Feb. 29, 2000, entitled “Method and Mechanism for Reducing Client-Side Memory Footprint of Transmitted Data,” which are hereby incorporated by reference in their entirety.
The area of the suspended result set containing the metadata is called the header. Table 1 shows one embodiment of the metadata content. This content can include header size, storage version, header checksum, and number of columns. The header size details the number of bytes the metadata consumes. The storage version specifies how to pack and unpack the result set. The header checksum is the checksum of all the header bytes. The number of columns details the number of columns that the result set contains.
In one example embodiment, as shown in Table 1, the header area related to individual column metadata contains two metadata values: data type and max length. The metadata for each column follows one after the other. The data type details what data type the result set column contains. The maximum length is the maximum length that the data type will consume and can be used to allocated the space for the receiving the row from the suspended result set.







TABLE 1





Example Suspended result set storage header (metadata) layout







Metadata Content


Header size


storage version


Header checksum


Number of columns


Column 1: data type


Column 1: max length


Column n: data type


Column n: max length


. . .









The data types can be simple scalar types, such as a NUMBER or CHAR, or complex non-scalar, such as a collector type or object type. In one embodiment, such data types are packed as a linear image of the original object and the header can include additional type metadata information, such as for example, object (or collector) ID length and object (or collector) ID bytes. In one embodiment this additional metadata information can be used by the client to unpack the stored linear image into its original collector type data form.
One embodiment for the row packing format is shown in Table 2. The row packing format can include: row size, and column specific information for each column such as actual length, column flags and column value. The row size can be used as a checksum to ensure all the bytes were packed and unpacked correctly. In another embodiment, the row size and column size can be stored with the row mapping index (see FIG. 2C and related text), as opposed to being stored in the suspended result set. Note that data in the suspended result set can be packed using actual length to minimize storage size. Another embodiment combines the row packing layout with data compression techniques to save space while suspending and continuing from a suspended result set.







TABLE 2





Example Suspended Result Set Packed Row (data) format

















Row Content



Rowsize



Column 1: actual length



Column 1: flags



Column 1: value



Column n: . . .










When persistent storage is used, one embodiment organizes the writes and reads into a particular size called a chunk. The actual size of the column value can vary so the readwrite sizes can be converted into chunks at run-time. The conversion to chunks is aided by the layout or packing logic. The packing logic provides a map so that flags can act as place holders and identify suspended actions. Memory chunks are sized to optimize persistent storage access instead of accessing persistent storage columns or rows at a time.
The memory buffer could be exhausted at any point of writing these bytes. The state is maintained so that next call (and next chunk) can continue from where the packing-phase was last suspended. For example, assume only a portion of the bytes of the size field (at a particular column) could be written when the end of the chunk was reached. The state and offset information is stored. In the next call, the saved state and offset value are used to write the next byte(s).
Suspending
The suspending operation captures rows of the row source into storage so that if something were to happen to the row source, to the server cursor, or to the session, the result set would still be available. This allows the result set to be available for the life of the client cursor, not just the life of the server cursor. The suspending operation can handle distributed SQL queries and queries involving client pre-fetching, or both. In addition, suspending during an already running fetch does not require any application changes. The suspension operation occurs transparently with minimal impact to client fetch requests. That is, the suspension operation is performed in a manner that minimizes client fetch request delay so that the client remains unaware of the suspension. In one embodiment, the suspending operation of a cursor result set can be completely independent from fetch operations and/or other continuing-from-suspended-result-set operations that may be performed upon that result set. For example, in one embodiment, the suspending can occur after every round-trip from the client even though the client request may be for a DML or for a different cursor. In one embodiment suspending of the result set on the cursor that has been parsed and executed on the server can occur before any rows are sent to the client. In another embodiment, suspending can occur after rows are sent to the client, i.e., in the middle of fetching. Suspending allows the pending result set to be retained without retaining the server cursor area or the server session. In one example, the pending result set for sequential cursors would be the rows not yet seen by the client. In another example, the pending result set for scrollable cursors can potentially be the entire result set.
One way to suspend a result set is by process 240, a generic example of a server suspending a row source, shown in FIG. 2B. Process 240 is a reentrant process, i.e., it can be stopped and invoked later to suspend additional rows. The following description of FIG. 2B will reference FIG. 4B. Process 240 starts at process action (242) when a call is made to suspend a server result set, e.g., by another process. Process action (244) determines if there are more rows in the row source 422. The row source is the SQL execution cursor area on the server for a database application. If there are more rows in the row source 422, the row is suspended in process action (246) by storing the row in the suspended result set storage 424. Then client 426 can access the suspended result set in storage 424 even after the cursor has become unavailable.
Process action (248) determines if any of a set of constraints have been violated. Constraints may be based upon any factors/parameters that can be set to control whether, how, and when, the present suspending process can be employed, e.g., the constraint conditions may be related to time and space and set to minimize the impact that the suspending process 200 has on other server operations (e.g., client fetches). For example, the constraint may include or relate to factors that prevent the process from affecting client latency or system performance. This may relate to, for example, server resource utilization, time-based factors, number of rows, consumption of I/O resources, etc. The constraints may be based upon, for example, each cursor, all cursors of a session, all sessions in a node, and/or all nodes of a cluster. The suspend process will suspend as many rows in the result set row source (e.g., action 244 of FIG. 2B) until it is limited by one or more (or even all) of the constraints.
In one embodiment, the timeout constraint is set by padding the average cursor roundtrip time with an absolute delay value or a percent delay factor. This prevents delay of any roundtrip that uses the whole (average roundtrip time+delay value) time. It also allows the suspend operation to occur, and adds only a small delay to any roundtrip that uses less than the average roundtrip time. The timeout limit can be set per cursor or per session, for all the cursors as a group. A space limit is set for each cursor's active memory capacity of for all cursors as a group. Another example of a space limit is for limiting the amount of persistent storage occupied by the suspended result set(s). These memory and/or persistent storage constraints can be set for each cursor, for all cursors in a session, all sessions in a node, and/or all nodes in a cluster.
If any constraints have been violated, control returns to the process that invoked process 240. If the constraints have not been violated, process 240 returns to process action (244) to determine if there are more rows in the row source to suspend. If there are no more rows in the row source, control returns to the invoking process (such as process 270 in FIG. 2D).
Another way to suspend a row of the result set is by the embodiment of the suspend a row process 200 shown in FIG. 2A. Process 200 is a reentrant process, i.e., it can be stopped and invoked later to suspend additional rows. As mentioned above, the storage chosen by the suspend process can be any of several types. This embodiment shows the use of persistent storage as the result set repository. However, the entire suspended set could be stored in other locations, e.g., in memory if available. Process 200 also uses a write buffer, the row source, and a read buffer. The write buffer is used to hold row bytes of the suspended result set being suspended to optimize access to persistent storage, and minimize any performance degradation to the client application. The write buffer receives the result set in sequential order (although there may be gaps in the sequence) in each call. Gaps may occur because all the rows need not be suspended. The read buffer is holding a portion of the result set for the rows that are being set to the client in order to optimize access to persistent storage.
The following description of FIG. 2A will reference FIG. 4A, which is a representation of the dataflow of the suspending and continuing processes. Process action (202) begins the suspend a row process and creates a Result Set ID.
The Result Set ID serves as a pointer to the suspended result set that is either passed to the client, stored in a database for later extraction, and/or used for deleting the suspended result set (and freeing all its resources) from the database. In one embodiment, the Result Set ID include a logical session identifier for the client session and a cursor identifier for the client cursor. The logical session identifier can be used to locate the result set when the server state of the client session changes (e.g., during transparent session migration). The contents of a Result Set ID need not be interpreted by the entity that invokes the suspend/continue logic. Further information regarding session identifiers can be found in co-pending application Ser. No. 11/071,275, entitled “Global Session Identifiers in a Multi-Node System” filed Mar. 25, 2005, which is hereby incorporated by reference as if fully set forth herein.
In one embodiment, the invoking entity calls the suspend logic with a cursor identifier, number of rows to suspend, and a set of constraints. After suspension, the Result Set ID is returned to the invoking entity. The invoking entity passes the Result Set ID to the continue logic along with the number of rows to be fetched.
It is determined, in process action (204), if there are more rows in the row source to be stored. One embodiment suspends all the rows of a result set 402, while another embodiment suspends a selected subset of the result set 402. If there are no more rows in result set 402 to be stored, process 200 returns control to the invoking process. If there are more rows in the row source 402 that need to be suspended, process action (206) determines if the entire row has been suspended. If the entire row has not been suspended, process action (208) suspends the remaining row bytes from row source 402 to the write buffer 404 and process action (210) determines if cursor write buffer 404 is full. If the entire row has been suspended or if the remaining row bytes are suspended to the write buffer and the writer buffer is not full, process action (214) determines if the constraint values are violated. If process action (210) determines that the write buffer is full, the data is flushed into persistent storage 406 in process action (212).
Process action (220) determines if the last fetch obtained data from (points to) the write buffer. If the last fetch points to the write buffer, process action (222) updates the read buffer 408 from persistent storage 406. This is done because the full write buffer was just flushed into persistent storage, therefore, by updating the read buffer from persistent storage, the data the client is likely to request next is already in the read buffer. If the fetch does not point to the write buffer or after the read buffer has been updated, process 200 returns to process action (206) to determine if the entire row is suspended and the process continues from process action 206 as detailed above.
If process action (214) determines that the constraint values are not exceeded, process 200 returns to process action (204) to determine if there are more rows in the row source. If constraints are violated then process action (216) determines if cursor write buffer 404 needs to be flushed to store the remaining data prior to stopping. If the write buffer needs to be flushed, process action (218) flushes cursor write buffer 404 into persistent storage 406. Process action (230) determines if the last fetch obtained data from, or points to, the write buffer. If the last fetch points to the write buffer, process action (232) updates the read buffer 408 from persistent storage 406. This is done because the full write buffer was just flushed into persistent storage, and by updating the read buffer from persistent storage, the data the client is likely to request next is already in the read buffer. In process 200, if the fetch does not point to the write buffer, or after the read buffer has been updated, process 200 returns control to the invoking program. The process will return control to the invoking program if a constraint condition has been exceeded but there is no need to flush the write buffer as in when the write buffer has already been flushed.
Process 270, the suspend all cursors process is shown in FIG. 2D. It is used for suspending all the cursors in a session. For example, a server may have idle time and suspend cursor result sets as background operations. The server would invoke process 270 to accomplish the cursor suspension. In one embodiment, once the server receives a request from the client, it can halt suspension for subsequent rows as a constraint violation.
Process action (272) starts the suspend all cursors process. Process action (274) determines if there are more cursors to be suspended. If there are no more cursors to be suspended, process 270 returns control to the invoking process (e.g., sever background operations). If there are more cursors to suspend, process action (276) computes the constraint values for the next cursor. Process action (278) determines if any constraint values are violated. If the constraint values are violated, process 270 return control to the invoking process. If the constraint are not violated, process action (280) invokes the suspend a server result set process (e.g., process 240, shown in FIG. 2B). Process 270 loops back to process action (274) to determine if there are more cursors to suspend.
According to one embodiment, the system can remove or age out one or more of the suspended result sets. If the client cursor or session corresponding to a result set has ended, then that result set is eligible for removal. Any suitable approach may be used to determine which eligible result set should be removed or aged out. For example, a least recently used approach may be employed to remove one or more eligible result sets. Alternatively, a clean-up procedure may be employed to periodically remove some or all of the result sets eligible for removal.
During the suspending of the row source, a row mapping index is created and maintained. This row mapping index, correlates the result set row to a specific storage location and offset. For example, FIG. 2C is a representation of one embodiment of such a mapping index. Another embodiment has a mapping index for each storage area. In one embodiment, the row mapping index only stores the location index for the next row (e.g., for sequential cursors). In FIG. 2C the result set row number is located in column 258, and the row storage information is located in column 268. For each row number (elements 250-254) in column 258, there exists a corresponding row storage area (elements 260-264) in column 268 that identifies its location. For example, row 1 element 250, is shown in row storage area 260 as located partially in persistent storage at the persistent storage offset and partially at the read buffer at the read buffer offset. Row 3, element 254, is shown in row storage area 264 as located in write buffer at the write buffer offset.
The row mapping index is updated each time any row in the result set is moved. Some examples of when the row mapping is updated are when the rows are suspended into the write buffer, and when the read buffer is updated with a chunk from persistent storage. The row mapping index can be stored in one or more of any of the storage mechanisms mentioned in this disclosure, including as part of the suspended result set.
The process may be optimized with respect to handling of a distributed query. A distributed query is one in which the query may access underlying base data from multiple network locations. In one embodiment, the suspending logic is transparent with respect to the node(s) at which the data is located. In an alternate embodiment, suspending a result set related to a distributed query may be optimized by first transferring data responsive to the query from one node to another, and then performing the suspend operation at the node which now contains all the data.
Suspending a result set has many applications. One example application of a suspended result set is to be able to continue from the suspended result set, which is described in detail below. In one embodiment, suspending and continuing from a suspended result set can be transparent to the client application. In another embodiment, the suspending and the continuing operations can be explicitly invoked by the client application. Other examples include: if the system is busy the result set can be suspended to allow use at a later time and yet free up resources now, if the system wants to retain access to the data after the cursor is closed and/or session has ended.
Continuing During Suspending
Continuing during suspending is the term used for the operation of accessing a result set while the result set is being suspended. The client is allowed to maintain fetch operations while the server is suspending the result set. This provides the client with the desired data while it allows the server to prepare for the eventual elimination of the server cursor.
FIG. 1B is a flowchart 100, a generic example of a server processing a client fetch of a result set with an option of suspending the result set. The discussion of FIG. 1B will reference FIG. 4B. In process action (101) the client executes a fetch query and the server processes the query. Process action (102) determines if result set suspension has been enabled. If result set suspension has not been enabled, the result set is simply fetched from the row source 422 in process action (104). However, if result set suspension has been enabled a Result Set ID is created and, process action (106) determines if the precise row requested has been suspended, i.e., if the precise row exists in storage location 424 used for the suspension process. If the row has been suspended, process action (108) fetches the row from the suspended result set and the fetched row is returned to client 426 in process action (114). If the row has not been suspended, process action (110) determines if the row should be suspended. If the row should be suspended, process action (112) suspends the row (e.g., into the suspended result set) (See FIGS. 2A and 2B). If the row is not to be suspended, process action (104) returns the row to client 426 from the cursor row source 422. Process action (116) determines if there are more rows in the result set. If there are more rows in the result set, process 100 returns to process action (102) and repeats the process. If there are no more rows in the result set, process action (118) determines if the cursor should be closed. If the cursor should be closed, process action (120) closes the cursor. If the cursor is not to be closed, process action (122) waits for the next client call and returns to process action (101).
An advantage of the embodiments is that the suspending operation is transparent to the client as the client is still receiving the fetched data without noticeable delay. The suspending operation causes minimal impact on the client fetch operation, so that the client is unaware that the suspending operation is taking place. The client is receiving the data while the server is suspending the result for other uses. Also, note in process 100 that the suspension process can be called at any time to add more rows to the suspended result set.
FIG. 3 is a flowchart representing another embodiment of a server processing a client fetch of a result set while the result set is being suspended, the cursor fetch from a suspended result set process 300. The following description of FIG. 3 will reference FIG. 4A. Process action (302) begins the fetch row from the suspended result set process. Process action (304) determines if the result set suspension is enabled. If the result set suspension is not enabled, the fetch sends the row from cursor row source 402 to client 410 in process action (306). If the result set suspension is enabled the Result Set ID is obtained and, process action (305) determines if the specific row has been suspended.
If the row has not been suspended, a determination is made whether the row should be suspended (331). If it is determined that the row should be suspended, then the row is sent from the row source (333) and is then suspended (335). In another embodiment, one can first suspend the row and then extract the row starting from action 308 in FIG. 3. Alternatively, and with respect to FIG. 1B, the row is suspended first (112), then fetched (extracted) from the suspended result set (108), then sent to the client (114). One embodiment of suspending a row is described in FIG. 2A. If the row is not to be suspended based upon the determination of 331, then the process returns to action 306 to send the row from the row source.
In one embodiment, for sequential cursors, it is not necessary to suspend rows that are being sent to the client. Therefore, action 331 would be not to suspend the row. For scrollable cursors, action 331 would be to suspend the row since the client fetches can potentially scroll backwards.
If the row has been suspended based upon the action of 305, then process action (308) determines if the entire row has been sent to the client. If the entire row has been sent to the client, the process returns to the invoking process.
If the entire row has not been sent to the client, the process action (310) determines if the row bytes are in the read buffer 408. If the row mapping index indicates that the subject row bytes are located in read buffer 408, process action (312) locates the row bytes at the indexed offset in the read buffer and sends the remaining row byes from read buffer 408 to the client 410. If the row mapping index indicates that subject row bytes are not located in read buffer 408, process action (314) determines if the row bytes are located in persistent storage 406 (placed there when the write buffer is flushed to persistent storage in the suspension processes). If the row mapping index indicates that subject row bytes are located in persistent storage 406, the row byte offset from the row mapping index is used to locate the row bytes in persistent storage and read buffer 408 is updated with a chunk of row byte data from persistent storage 406 in process action (316). The row bytes are sent from read buffer 408 to client 410 in process action (312). If the row mapping index indicates that subject row bytes are not located in persistent storage 406, process action (318) determines if the subject row bytes are in write buffer 404. If the row mapping index indicates that subject row bytes are located in write buffer 404, process action (320) locates the row bytes using the offset from the row mapping index in write buffer 404 and sends the row bytes from write buffer 404 to client 410. If the row mapping index indicates that subject row bytes are not in write buffer 404 (i.e., not in any of the storage locations), the fetch is in error and is handled by the error processing system.
If the data was sent from the read buffer in process action (312), or the write buffer in process action (320), process 300 returns to process action (308) to determine of the entire row was sent to the client or if more row bytes are required. If the entire row has not yet been sent to the client the process precedes to process action (310) as described above. If the entire row has been sent to the client, the process returns to the invoking operation. Using memory buffers as mentioned above help reduce I/O costs to persistent storage, however, they are not a required element. Any form of storage can be used.
Continuing After Suspending
Continuing after suspending is the term used for the operation of accessing a result set that has previously been suspended. One example of when continuing from a suspended result set may be required is when a server cursor is no longer available, perhaps due to the migration of the client to another server session. Continuing from the suspended result set allows the client cursor access to the result set even though the server cursor is no longer available.
Fetch process 520 shown in FIG. 5A is a generic all purpose example of a server processing a fetch from a client. Discussion for FIG. 5A will reference FIG. 4B. Process 520 begins with process action (522) after being invoked by client fetch query. Process action (524) determines if the fetch is continuing from a suspended result set. If the result set is not suspended, process action (538) sends the row from row source 422 to client 426. If the result set is suspended, process action (526) fetches the row from the suspended result set storage 424. Process action (528) returns the row to client 426. Process action (530) determines if there are more rows to be fetched. If there are more rows, process 520 loops back to process action (524) to process the additional rows. If there are no more rows in the client query request to be fetched, process action (532) determines if the cursor should be closed. If the cursor should be closed, process action (534) closes the cursor. If the cursor should not be closed, process action (536) waits for the next client call.
Fetch process 500 of FIG. 5B is another embodiment of the cursor fetch process after suspending. This process can be used either after the suspending process is complete or when no result set suspending is necessary. The discussion for this figure will refer to the FIG. 4A dataflow representation. Note that fetch process 500 does not utilize the write buffer 404. The data from the write buffer, after suspending, has been stored in the persistent storage 406. In another embodiment, the write buffer 404 can be maintained, for example, in order to optimize the number of disk I/Os.
Fetch process 500 begins with process action (502). Process action (504) determines if the fetch is continuing from a suspended result set. If the fetch is not continuing from a suspended result set, process action (506) sends the row to the client 410 from the row source 402. If the fetch is continuing from a suspended result set, the Result Set ID is obtained and, process action (508) determines if the entire row has been sent to the client. If the entire row has been sent to the client, the process 500 returns control to the invoking operation (e.g., client query). If the entire row has not been sent to the client, process action (510) determines if the subject row bytes are in the cursor read buffer 408. If the row mapping index indicates that the subject row bytes are in the read buffer, process action (512) uses the row mapping index offset to locate the row bytes and sends the remaining row bytes from the read buffer 408 to the client 410. Process action (512) loops back to process action (508) until the entire row has been sent to the client. If the row mapping index indicates that the subject row bytes are not in the read buffer, then process action (514) determines if the subject row bytes are located in persistent storage 406. If the row mapping index indicates that the subject row bytes are located in persistent storage, process action (516) uses the row mapping index offset to locate the row bytes and sends a chunk of data to update the read buffer 408. Process action (512) sends the row bytes from the updated read buffer 408 to the client 410. If the row mapping index indicates that the subject row bytes are not located in the cursor persistent storage 406, the fetch is in error and is handled by the error processing system.
Note that the movement of result set data (e.g., update of the read buffer with data from persistent storage) would cause an update to the row mapping index.
Storage compaction can be performed after fetching rows from the suspended result set, e.g., for sequential cursors. Results sets can potentially be very large and compaction can therefore minimize the resources that are used to suspend the result set.
In one embodiment, the size of the read buffer and/or write buffer can be predetermined. In an alternate embodiment, the sizes of the read and/or write buffers can be tuned dynamically, e.g., based upon system resources. In an embodiment the read buffer could be a read cache with multiple memory chunks. In another embodiment, the write buffer could be a write cache with multiple memory chunks. The memory chunks need not be contiguous memory.
As described in the earlier sections, there is a read buffer to optimize access to persistent storage, and minimize any performance degradation to the client application. In an embodiment, there need not be a read buffer or read cache. Continuing fetches from the suspended result set can be done directly from persistent storage.
Alternatively, there need not be a write buffer or write cache. The suspending of the result set can be done directly using the persistent storage.
In one embodiment, it is not necessary to re-create the server cursor before continuing from the suspended result set. In another embodiment, one may re-create the server cursor before continuing from the suspended result set.
In one embodiment, the constraint violations are checked after each row is suspended. In another embodiment the constraint violations may be checked in the middle of suspending a row, for example, after suspending each column.
The suspend and continuing processes described above can be applied to numerous specific applications. One example application is for scrollable cursors. Another example application of the suspend and continuing processes is for using or migrating a result set between multiple client sessions. Another example application is when the server is busy (e.g., due to server resource constraints or otherwise), the server may choose to suspend some or all of its cursors in fetch. Once the cursors are suspended, the server can release these cursor areas and/or session to reclaim resources. Later when the system is less busy or otherwise, the server can continue fetches from the suspended result set, in perhaps another (server) session.
For example, consider a result set that is generated for a browser session which connects to a database server. The browser session accesses the results of a query across one or more web pages through a client session. Further consider if the browser session is separated or cut off from its associated client session, e.g., because of resource constraints or timeout policies. Each client session will have a corresponding session state at the database server. The present approach can be used to suspend the browser result set when the browser session is detached from the client session. When the browser session is reconnected, it may be associated with a different client session. Indeed, the original client session may itself no longer exist. Instead of re-executing the query, the browser session can directly continue fetching from the suspended result set through a different client session.
Cursor Scrolling
One example of continuing a suspended result set is for cursor scrolling. A recent advance in cursor fetching is to allow cursor accesses to a result set in an arbitrary order. In this approach the cursor can move in any direction or jump anywhere in the result set to access a row of data, even for data that has already been fetched. One way to fetch data from anywhere in the result set is to convert the cursor request into a row number.
Non-scrollable cursors are limited to fetching rows in forward sequential manner. According to an embodiment, a scrollable cursor fetch provides support for forward and backward access to a result set from a current position, using absolute or relative row number offsets into the result set. One may also use keywords such as FIRST, LAST, PRIOR (or PREVIOUS), CURRENT, NEXT to specify the scrollable fetch position.
Note that when a scrollable cursor fetch call, all the rows from the last row suspended through to all the rows in this client request may be suspended. In another embodiment, the server may suspend more rows that what the client has requested. In another example, if the scrollable cursor requests the last row in the result set, the last row is returned to the client and all the rows from last row suspended from the row source through the last row in the result set are suspended.
In one embodiment, the scrollable cursor fetch is converted into an absolute row number in the result set. For example, assume that a client scrollable cursor current position starts at row number 0. Further assume the next fetch uses relative positioning with offset +5 and the number of rows equals to 10. In this example, the fetch request is converted to absolute row number current position +5, i.e., 0+5=5. The server fetch logic is requested to fetch row numbers 5 through 14. Then, the row mapping index is used to determine the location of the rows in the suspended result set.
For sequential cursors the fetching is in sequential order. For example, using FIG. 1B, the client executes a query in process action (101). Process action (102) determines if the query result set suspension is enabled. Process 100 fetches the rows from the cursor row source in process action (104) without suspending the result set. More specifically, as process action (104) calls fetch process 300 in FIG. 3 or fetch process 500 in FIG. 5B, process action (306) and (506) both return the result set directly from the row source starting with the first row and proceeding sequentially.
For scrollable cursors the client fetch does not have to be in any order as stated in the example above. That is, the fetching processes, process 300 in FIG. 3 and process 500 in FIG. 5B, must be able to return a row from anywhere in the result in any order.
In one embodiment the implementation of scrollable cursors can be to directly suspend each row it obtains from the row source without requiring an additional scrollable cursor workspace. In another embodiment, rows are converted from the scrollable workspace into the suspended storage. The scrollable workspace format may be different from the suspended storage format.
In another embodiment, the scrollable workspace would have the rows stored in the same format as needed by the suspend/continuing logic. Now the copying of rows from scrollable workspace into suspended storage is more efficient.
In one embodiment, action 204 in FIG. 2A would first suspend the rows from the scrollable workspace and then continue suspending rows from the cursor row source.
In another embodiment, action 204 in FIG. 2A is implemented such that the cursor row source itself is scrollable so it would return the row to be suspended without requiring an additional scrollable workspace. For sequential cursors the suspending of rows may only be for those rows not yet sent to the client. For scrollable cursors, since the client can ask for any rows(s), including those already seen, the entire result set may need to be suspended.
Using FIG. 1B, the client executes a scrollable cursor in process action (101). Process action (102) determines if the result set suspension is enabled. Process action (106) determines if the requested row has been suspended in the enabled result set suspension process. If the row has been suspended, process 100 initiates a fetch (extract) operation in process action (108). That is, process action (108) calls fetch process 300 in FIG. 3 (or fetch process 500 in FIG. 5B if the suspending is complete). In one embodiment, when a scrollable cursor requests a row in a result set, process action (310) in FIG. 3 or process action 510 in FIG. 5b (if suspending is complete) determines the absolute row number, uses the row mapping index to find the storage and offset in storage of the requested row, updates the read buffer if necessary, and returns the row to the client.
If the row requested has not yet been suspended (as in the case of jumping ahead with the scrollable cursor), process action (110) determines if the row should be suspended. For a scrollable cursor, process action 110 would determine that the row should be suspended. For a partial scrollable cursor, it is possible that process action 110 would determine not to suspend all rows. If process action (110) determines that the row should not be suspended, a fetch process is called to return the row from the cursor row source. If the row should be suspended, process action (112) calls process 200 in FIG. 2A to suspend the row.
Referring to FIG. 3, in one embodiment, if it is determined to suspend a row (331), then the row is sent from the row source (333) and then suspended (335). Alternatively, and with reference to FIG. 1B, the row is suspended first (112), then fetched from the suspended result set (108) and then sent to the client (114).
Transparent Session Migration
One example of suspending a result set and continuing fetches from a suspended result set is for session migration. A session is known as the active connection between two computers, e.g., between a client and a database server. A session is created when an application connects to the database, and is destroyed when the application logs off. Example session information includes client properties such as user identity, user security roles, application name, platform, connection type, etc. A session can have multiple cursors executing simultaneously, and therefore, multiple fetches.
One embodiment uses suspended result sets and continuing from suspended result sets during transparent session migration in which migration of server session state occur in a manner that is transparent to the client. In an alternate embodiment, the server session migration may occur in a manner that is not transparent to the client,. e.g., by allowing the client to control aspects of the server migration, such as the specific server to which a session will be migrated.
In one embodiment, there can be multiple session migrations. For example, a client session can be migrated multiple times, e.g., in a cascaded manner. A cascaded migration refers to a sequence in which a client session migrates multiple times. For example, the client session migrates from a first server session to a second server session, and then from the second server session to a third server session.
One reason to migrate a session is for load balancing purposes. For example, if one processor is heavily loaded while another is not, it would be advantageous to, increase database throughput, and reduce response time to each client, to move one or more sessions from the heavily loaded processor to the processor that is less loaded. However, a session processing a fetch operation cannot be migrated, until the fetch is completed. This causes delays in load balancing operations.
Another reason to migrate an application session is when a server session becomes unavailable. For example, consider a result set that is generated for a browser application session which connects to a database server. The browser session accesses the results of a query across one or more web pages through a client session. Further consider if the browser session is separated or cut off from its associated client session, e.g., because of resource constraints or timeout policies. Each client session will have a corresponding session state at the database server. The present approach can be used to suspend the browser result set when the browser session is detached from the client session. When the browser session is reconnected, it may be associated with a different client session. Indeed, the original client session may itself no longer exist. Instead of re-executing the query, the browser session can directly continue fetching from the suspended result set through a different client session. It is noted that the browser access can be of any kind, including either sequential or scrollable.
Transparent session migration may be used for load balancing and dynamic CPU provisioning, and therefore cannot afford to have errors in the result-set migration. A solution, in one embodiment, is to store the entire pending result set in persistent storage on the server so fetches for an application can continue during and after session migration, in exactly the same order with same column values. For sequential cursors, the pending result set includes rows not yet seen by the client. For scrollable cursors, the pending result set can potentially include the entire result set. As mentioned above, the storage type is dependent on the application and required use of the result set once suspended. In another transparent session migration embodiment, the result set is stored in shared memory or persistent shared memory.
FIG. 6B is a top level flowchart representing a process 600 for executing a client call and handling transparent session migration. Process action (602) waits for a client call. Process action (604) processes the client call. If the client call is a fetch, the appropriate fetch process: e.g., fetch during suspending process 300 shown in FIG. 3 or fetch after suspending process 500 shown in FIG. 5B is called. Process 600 determines if the session has been selected for migration in process action (606). Although the flowchart shows the determination of migration selection after the client call, the order can be reversed. If the session has not been selected for migration the process returns to process action (602) and waits for a client call. If the session has been selected for migration, process action (608) invokes the migrate session process 650.
Note that if process actions (606) and (608) are isolated from the rest of the diagram, they represent a example of an server execution of a client call where transparent session migration is not available.
FIG. 6A is a flowchart of a process 650 for suspend and continuing for transparent session migration according to one embodiment. The following description of FIG. 6A will reference FIG. 7, the data flow diagram for session migration. In process action (652) the session is selected for migration and process 650 has been invoked. The client 702 is notified by the server 714 of the selection in process action (654). Both the server and the client prepare to migrate in process action (656). During 656, the session state information is prepared for migration. Another activity that occurs during migration preparation is suspending the cursor result sets (e.g., cursor 708, and cursor 710) by invoking process 270 shown in FIG. 2D. This suspending of all cursors is independent of the specifics of any client call that may be active. However, the work to suspend is performed after each client call as described above in the SUSPEND section. Process action (658) determines if the client 702 and the server 714 are prepared to migrate the session 716. If the client and the server are not prepared to migrate, flow returns to process action (656) to continue migration preparations. If the client 702 and the server 714 are prepared to migrate session 714, process action (660) opens a new session 726 on server 724, copies the session state information into the new session 726, and creates a connection from the client 702 to new session 726 on server 724. The client 702 makes calls to the new session 726 in process action (662) and the result set stored in persistent storage 706 of database 704 is returned to the client 702 in process action (664).
FIG. 7 is a representation of a client continuing a suspended result set. Persistent storage 706 is shareable across nodes and is used as one example, but other storage types may be used without detraction from the embodiment such as shared memory 722. Client 702 has an 2 client side cursors 708 and 710 communicating with server cursor 718 and 720 via connection 719 to server session 716 of Server 714. Client cursors 708 and 710 are fetching result set data from server cursors 718 and 720 respectively using fetching process 300, shown in FIG. 3. The result sets of these cursors are suspended in database 704 in shared persistent storage 706 using the suspend process 200 shown in FIG. 2A, and the client 702 is unaware of the activity of the suspend operation. Once session 726 is available on server 724, client 702 connects to it via connection 721 to access the cursor result sets. Server 724 can access the suspended result set from shared persistent storage 706. In another embodiment, server 724 can access the suspended result set from shared memory 722. In another embodiment, server 724 car receive a copy of the suspended result set for cursor 718 and 720 to be stored on server 724, in suspended result set 728 and 730, respectively. Once the connection 721 to the new server session 726 is established, connection 719 to the original server 714 is severed.
In one embodiment, once the entire result set is suspended, client 702 and server 714 synchronize a session migration. In another embodiment, the entire result set does not have to be suspended to proceed with session migration. Only the rows which the client can potentially fetch need to be suspended to proceed with session migration. The second session, session 726 on server 724 is opened and made available to client 702. Session 726 has access to persistent storage 710 and can honor any data request for data in the result set. The data requests from client 702 to session 726 are not satisfied from the original server cursor execution area. Another embodiment allows for cascaded session migration.
Client 732 is another client that can now access the result sets of cursors 718 and 720 since they are located in persistent storage 710. In one embodiment, Client 732 accesses the persistent storage using data file requests. In another embodiment, the persistent storage data file may be transferred to client 732 as one file allowing client 732 to access the result set on its own station and not have to use network resources. In another embodiment, client 732 uses SQL or programmatic fetch APIs to access the suspended result set.
System Architecture Overview
The execution of the sequences of instructions required may be performed in some embodiments by a computer system 1400 as shown in FIG. 8. In an embodiment, execution of the sequences of instructions required is performed by a single computer system 1400. According to other embodiments, two or more computer systems 1400 coupled by a communication link 1415 may perform the sequence of instructions required in coordination with one another. In order to avoid needlessly obscuring the explanation, a description of only one computer system 1400 will be presented below; however, it should be understood that any number of computer systems 1400 may be employed.
A computer system 1400 according to an embodiment will now be described with reference to FIG. 8, which is a block diagram of the functional components of a computer system 1400 according to an embodiment. As used herein, the term computer system 1400 is broadly used to describe any computing device that can store and independently run one or more programs.
Each computer system 1400 may include a communication interface 1414 coupled to the bus 1406. The communication interface 1414 provides two-way communication between computer systems 1400. The communication interface 1414 of a respective computer system 1400 transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. A communication link 1415 links one computer system 1400 with another computer system 1400. For example, the communication link 1415 may be a LAN, in which case the communication interface 1414 may be a LAN card, or the communication link 1415 may be a PSTN, in which case the communication interface 1414 may be an integrated services digital network (ISDN) card or a modem, or the communication link 1415 may be the Internet, in which case the communicate interface 1414 may be a wireless, cable, or dial-up modem.
A computer system 1400 may transmit and receive messages, data, and instructions, including program, i.e., application, code, through its respective communication link 1415 and communication interface 1414. Received program code may be executed by the respective processor(s) 1407 as it is received, and/or stored in the storage device 1410, or other associated non-volatile media, for later execution.
In an embodiment, the computer system 1400 operates in conjunction with a data storage system 1431, e.g., a data storage system 1431 that contains a database 1432 that is readily accessible by the computer system 1400. The computer system 1400 communicates with the data storage system 1431 through a data interface 1433. A data interface 1433, which is coupled to the bus 1406, transmits and receives electrical, electromagnetic or optical signals, that include data streams representing various types of signal information, e.g., instructions, messages and data. In embodiments, the functions of the data interface 1433 may be performed by the communication interface 1414.
Computer system 1400 includes a bus 1406 or other communication mechanism for communicating instructions, messages and data, collectively, information, and one or more processors 1407 coupled with the bus 1406 for processing information. Computer system 1400 also includes a main memory 1408, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1406 for storing dynamic data and instructions to be executed by the processor(s) 1407. The main memory 1408 also may be used for storing temporary data, i.e., variables, or other intermediate information during execution of instructions by the processor(s) 1407.
The computer system 1400 may further include a read only memory (ROM) 1409 or other static storage device coupled to the bus 1406 for storing static data and instructions for the processor(s) 1407. A storage device 1410, such as a magnetic disk or optical disk, may also be provided and coupled to the bus 1406 for storing data and instructions for the processor(s) 1407.
A computer system 1400 may be coupled via the bus 1406 to a display device 1411, such as, but not limited to, a cathode ray tube (CRT), for displaying information to a user. An input device 1412, e.g., alphanumeric and other keys or a pointing device such as a mouse, is coupled to the bus 1406 for communicating information and command selections to the processor(s) 1407.
According to one embodiment, an individual computer system 1400 performs specific operations by their respective processor(s) 1407 executing one or more sequences of one or more instructions contained in the main memory 1408. Such instructions may be read into the main memory 1408 from another computer-usable medium, such as the ROM 1409 or the storage device 1410. Execution of the sequences of instructions contained in the main memory 1408 causes the processor(s) 1407 to perform the processes described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, embodiments are not limited to any specific combination of hardware circuitry and/or software. In one embodiment, the term “logic” shall mean any combination of software or hardware that is used to implement some or all of the embodiments.
The term “computer-usable medium,” as used herein, refers to any medium that provides information or is usable by the processor(s) 1407. Such a medium may take many forms, including, but not limited to, non-volatile, volatile and transmission media. Non-volatile media, i.e., media that can retain information in the absence of power, includes the ROM 1409, CD ROM, magnetic tape, and magnetic discs. Volatile media, i.e., media that can not retain information in the absence of power, includes the main memory 1408. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise the bus 1406. Transmission media can also take the form of carrier waves; i.e., electromagnetic waves that can be modulated, as in frequency, amplitude or phase, to transmit information signals. Additionally, transmission media can take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications.
In the foregoing specification, the embodiments have been described with references to specific elements. However, it will be evident that various modifications can changes my be made thereto without departing from the broader spirit and scope. For example, the reader is to understand that the specific ordering and combination of process actions shown in the process flow diagrams herein is merely illustrative, and the embodiments can be performed using different or additional process actions, or a different combination or ordering of process actions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.

## Claims

1. A computer-implemented method for suspending a cursor result set for transparently migrating a first session, the method comprising:
receiving notification the first session at a first server is selected for migration;
preparing to migrate, preparing including suspending a cursor result set for the first session;
suspending the cursor result set by storing the cursor result set in a storage area of a computer-usable medium for later access without re-execution of a query;
determining if the first session is ready to migrate; and
migrating the first session at the first server to a second server, the migrating comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

2. The method of claim 1 wherein migrating comprises:
opening the second session;
copying the suspended cursor result set to the second session;
connecting a client to the second session; and
breaking the client connection to the first session.

3. The method of claim 2, further comprising executing a fetch request on the suspended cursor result set from the second session.

4. The method of claim 3, wherein executing the fetch request on the suspended cursor result set comprises:
determining a location of a subject row of the cursor result set; and
returning the subject row from the location.

5. The method of claim 2, wherein executing a call on the suspended result set via the second session is preceded by the same cursor executing a call on the result set via the first session.

6. The method of claim 5, wherein the storage area comprises memory, persistent storage, or a combination of persistent storage and memory.

7. The method of claim 5, wherein the storage area includes all the rows in cursor result set.

8. The method of claim 2, wherein the client is unaware that a session is migrating or of a switch from the first session to the second session.

9. The method of claim 1, wherein suspending cursor result sets comprises storing the rows of each cursor, each cursor's rows stored into the storage area associated with a corresponding cursor.

10. The method of claim 1 in which a client accesses session data from the first session before migration has completed.

11. The method of claim 1 in which a client accesses session data from the first session after migration has completed.

12. A computer-implemented method for suspending a cursor result set and continuing a suspended cursor result set for transparent session migration, the method comprising:
executing a query on a first session on a first server;
receiving notification the first session was selected for migration;
preparing to migrate the first session;
suspending a cursor result set by storing the cursor result set in a computer-usable storage medium for later access without re-execution of a query;
determining if ready to migrate the first session; and
migrating the first session at the first server to a second server and accessing the cursor result set from the suspended cursor result set, the migrating comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

13. The method of claim 12, wherein preparing to migrate comprises:
determining the cursor result set is being fetched;
computing a set of constraint values for a cursor; and
suspending the result set of the cursor.

14. The method of claim 13, wherein preparing to migrate further comprising sending notification to the client of the selection for migration.

15. The method of claim 13, the suspending further comprising:
a) caching a row into a storage location;
b) determining if any of the set of constraint values have been exceeded;
c) determining if there are more rows to be suspended if the constraint values have not been exceeded; and
d) repeating a)-c) if there are more rows to be suspended.

16. The method of claim 12, wherein migrating comprises:
opening the second session on a second server;
copying the first session's state information to the second session;
connecting a client to the second session; and
breaking connection between the client and the first session.

17. The method of claim 16, further comprising executing a call on the suspended result set data via a second session.

18. The method of claim 17, wherein the continuing the suspended cursor result set further comprises:
e) determining that the result set is suspended;
f) determining the location of a subject row of the result set;
g) repeating e)-g) until all the required rows have been fetched.

19. The method of claim 17, wherein executing a call on the suspended result set via a second session is preceded by a cursor executing a call on the result set via the first session.

20. The method of claim 16, wherein suspending a result set comprises:
determining the cursor result set is to be suspended;
storing a first row of the cursor result set in a storage area; and
repeating the storing until all the required rows have been suspended.

21. The method of claim 20, wherein the suspending is performed in a manner so that the performance degradation of the returning rows to the client caused by the suspending is unnoticeable to a user.

22. The method of claim 12, wherein a user is unaware of switching from the first session to the second session.

23. The method of claim 12, wherein the suspended result set is transmitted from one node to another node.

24. The method of claim 12, wherein the executing query is a distributed sequential query language (SQL) query.

25. The method of claim 12 wherein the query does not have to adjust any application parameters due to the migration.

26. The method of claim 12, wherein the transparent session migration is a cascaded transparent session migration.

27. A computer program product comprising a computer usable storage medium having executable code to execute a process for suspending a cursor result set and continuing a suspended cursor result set for transparent session migration, the process comprising:
executing a query on a first session on a first server;
receiving notification the session was selected for migration;
preparing to migrate, preparing including suspending a cursor result set for the first session;
suspending the cursor result set by storing the cursor result set for later access without re-execution of a query;
determining if ready to migrate the first session; and
migrating the first session at the first server to a second server, when ready and accessing the cursor result set from the suspended cursor result set, the migrating comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

28. The computer program product of 27, the process further comprising: executing a fetch request on the suspended cursor result sets from the second session.

29. The computer program product of 27, wherein a client is unaware that a session is migrating or of a switch from the first session to the second session.

30. A system comprising a computer usable storage medium for suspending a cursor result set and continuing a suspended cursor result set for transparent session migration, comprising:
means for executing a query on a first session on a first server;
means for receiving notification the first session was selected for migration;
means for preparing to migrate the first session;
means for suspending a cursor result set by storing in the computer usable storage medium the cursor result set for later access without re-execution of a query;
means for determining if ready to migrate the first session; and
means for migrating the first session at the first server to a second server and accessing the cursor result set from the suspended cursor result set, the migrating comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

31. The system of claim 30, further comprising:
means for executing a fetch request on the suspended cursor result sets from a second session.

32. The system of claim 30, wherein a client is unaware that a session is migrating or of a switch from the first session to the second session.

33. A computer program product comprising a computer usable storage medium having executable code to execute a process for suspending a cursor result set for transparently migrating a first session, the process comprising:
executing a query on a first session on a first server;
receiving notification the first session was selected for migration;
preparing to migrate the first session;
suspending a cursor result set by storing the cursor result set for later access without re-execution of a query;
determining if ready to migrate the first session; and
migrating the first session at the first server to a second server, the migrating comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

34. The computer program product of 33, the process further comprising: executing a fetch request on a suspended cursor result sets from the second session.

35. The computer program product of 33, wherein a client is unaware that a session is migrating or of a switch from the first session to the second session.

36. A system comprising a computer usable storage medium for suspending a cursor result set for transparently migrating a first session, the method comprising:
means for executing a query on a first session on a first server;
means for receiving notification the first session is selected for migration;
means for preparing to migrate, preparing including suspending a cursor result set for the first session;
means for suspending the cursor result set by storing in the computer usable storage medium the cursor result set for later access without re-execution of a query;
means for determining if the first session is ready to migrate; and
means for migrating the first session at the first server to a second server, the migrating
comprising:
opening a second session on the second server;
copying session state of the first session into the second session; and
creating a connection to the second session on the second server, wherein the connection to the second session on the second server allows for continued execution of the query from the suspended cursor result set without re-execution of the query.

37. The system of claim 36, further comprising:
means for executing a fetch request on a suspended cursor result sets from the second session.

38. The system of claim 36, wherein a client is unaware that a session is migrating or of a switch from the first session to the second session.