Abstract:
A covert channel is established between a network service and one or more service monitors in a service group. The covert channel minimizes overhead by providing an indication of the status of the service through use of operating system utilities rather than conventional remote procedure calls (RPCs) or posting methods. The covert channel relies on one or more communication files established and updated by the service and having attributes which are in proportion to the workload of the service. By monitoring these attributes, the service monitor is able to determine the status of the service, including its workload and availability, without incurring costly operational overhead.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to network service groups, and more particularly, to a method and apparatus for monitoring the status of services performed by such groups. 
     2. Description of Related Art 
     In a client-server networking environment, a network service is typically provided by an application running on a server machine that processes requests sent from many client machines via the network. The primary challenge is to provide a scalable and reliable platform for a network service to process an increasing volume of client requests. U.S. patent application Ser. no. 08/763,289 entitled “Load Balancing and Failover of Network Services,” to Swee Boon Lim, and filed Dec. 9, 1996, now U.S. Pat. No. 5,938,732, describes a system having scalable and reliable architecture that uses a group of server machines, each running the same application, to cooperatively provide a network service. To a client machine, the group of server machines, hereafter referred to as a service group, appears as a single server machine. 
     In such applications, one server machine may provide multiple network services and may belong to multiple service groups. Each server machine that is a member of a service group has a service monitor that monitors the workload and determines the availability status of the service on the server machine. Service monitors of a service group communicate with each other in the service group through a network. One of the service monitors is designated as the service group leader, which periodically collects the workload and availability status of each member of the service group. The service group leader uses the information for load balance and recovery of service crashes. 
     There are three methods that a service monitor can use to obtain workload and availability status of a service. These are the direct method of remote procedure calls (RPCs), the direct method of posting, and the indirect method of using operating system services. Under the first, remote procedure call (RPC) method, the service monitor implements the client side of the RPC, and the service being monitored by the service monitor implements the server side of the RPC. The service monitor periodically makes a remote procedure call to the service in order to obtain workload information. If the service monitor does not receive an RPC reply from the service and a retry fails after a certain interval, the service monitor can safely assume that the service is dead. 
     While the RPC approach is straightforward, programming RPCs and setting up the RPC client and server is a complex and expensive procedure, making other options, such as the posting method, attractive. Under the posting method, a service periodically publishes separately its workload and availability status (for example, as the current time) using operating systems services such as shared memory or IP multi-cast or broadcast. A service monitor retrieves the published information from the service that it monitors periodically, which may be at a different interval than that in which publication by the service occurred, and determines the status of the service therefrom. The service monitor can thus detect that the service that it monitors is not available if the time published by the service that represents service availability does not change after a certain interval. Again, however, a drawback of the posting method is the complex programming technique needed to program a service and its service monitor. 
     Under the indirect method of using operating system services, a service monitor can take advantage of utilities provided by the operating system to determine for example whether a computing process exists, or to obtain the overall CPU (Central Processing Unit) utilization of the system, without directly communicating with the service it monitors. This is particularly useful since service monitors and applications running on a server machine are typically implemented as background computing processes in a server machine and thus lend themselves to such interaction. One operating system that provides such utilities is Solaris™ of Sun Microsystems. 
     As an example, a service monitor may rely on the CPU utilization of a server machine as a representation of the current workload of a service running on the server machine. There is a direct relationship between the CPU utilization of a system and the workload of a service running in the system. This is because if the CPU utilization of a server machine is high, there will not be enough CPU cycles left over for the service in the server machine to process client requests, i.e. high workload. 
     Similarly, a service monitor can periodically check the existence of the computing process that implements the service. If the computing process of the service does not exist, the service is not available. 
     The indirect method is efficient since the cost of obtaining workload or availability status is merely the cost of using the system calls provided by the host operating system. In addition, the service monitor can obtain the workload and service availability information any time without waiting for the service to respond. 
     A drawback of the indirect method is that it cannot obtain workload information specific to a service. Also, the indirect method cannot really be sure that a service is available: it cannot distinguish a hung computing process that cannot process client requests from a normal computing process that can process client requests. 
     Thus, it is desirable for a service monitor to obtain workload information and availability status directly from the service that it monitors since such an approach would yield more accurate results. Since direct methods conventionally involve making changes to the services so that workload information specific to the services could be communicated to the service monitors, it is desirable that this information be communicated without using complex programming techniques such as shared memory, remote procedure calls or networking programming. Finally, it is desirable to reduce the cost of obtaining this information in order to maximize service throughput. 
     SUMMARY OF THE INVENTION 
     The invention overcomes the deficiencies of the prior art by providing a covert channel to allow communication between a service monitor and the service that it monitors without incurring excessive overhead for monitoring and updating or passing messages indicating certain information. In the preferred embodiment, the covert channel is a communication file whose size corresponds to the workload of the service being monitored, such that the service monitor can determine the workload of the service by merely examining the communication file size attribute. The communication file is also constantly updated by the service in order to provide a “heartbeat” to the service monitor indicating that service is available. 
     In a second embodiment in accordance with the invention, the “heartbeat” is provided by a second communication file provided for that service. A separate communication file is especially desirable in systems which do not provide a time stamp, or last modification date of a file, in which case the second communication file, by being continuously modified in size by the service, provides the indication that the service is available. 
     The service or process running on a server in a multi-server environment thus periodically updates information about the communication file to indicate the status and availability of the process. The file is typically a “holey” file, that is, one that occupies no file system memory. The running process updates the size of the file to indicate, for example, the workload of the running process, and the date-last-modified to indicate the availability of the running process. Any other running process and/or monitor, and even ones on other servers, need only examine the file attributes to determine the running process&#39; status and availability. Thus, a covert channel is established between the running process and the monitoring process, bypassing all normal message processing overhead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
     FIG. 1 is schematic diagram showing operation of a covert channel; 
     FIG. 2 is a schematic diagram showing a service monitoring system in accordance with the preferred embodiment of the invention; and 
     FIG. 3 is a schematic diagram showing a service monitoring system in accordance with a second embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A covert channel is a communication channel that allows a computing process to transfer information by exploiting a mechanism not intended to be used for communication. If one computing process can change the state of some characteristics that another process can sense, then a covert channel exists between the two processes. A covert storage channel uses shared system variables as the means of information transfer. FIG. 1 illustrates a covert channel C established between Processes A and B, designated as  30  and  32 , respectively. The covert channel C arises with Process A&#39;s interaction with any device or process  34  and Process B&#39;s separate interaction or access to that same device or process  34 . Any manipulations of device or process  34  by Process A can be used to effectively communicate information about Process A to Process B, and vice versa if the channel is bidirectional. For more information on covert channels, see “Information Security, an integrated collection of essays”, edited by Marshall D. Abrams, Sushil Jajodia Harold J Podell, IEEE Computer Society Press, 1995, p. 117. A list of covert channel examples can be found in the paper McIlroy,-nd Reeds, J A, “Multilevel security with fewer fetters”, Proceeding UNIX Security Workshop (pp. 24-31). 
     Historically, the objective of a covert channel is to break the multi-level security of a multi-level security enabled computing system. A multi-level secure system contains information with different sensitivities that simultaneously permit access by users with different security clearances and needs to know, but prevents users from obtaining access to information for which they lack authorization. (See Information Security, an integrated collection of essays, edited by Marshall D. Abrams, Sushil Jaodia, Harold J Podell, IEEE Computer Society Press, 1995, pp. 330-349.) 
     The communication efficiency of a covert channel usually is not of crucial importance, and each information transfer between the cooperating processes via the covert channel may incur expensive overhead. 
     In accordance with the invention, use of the general purpose but complex and high overhead interprocess communication services provided by the operating system for publishing workload and availability status of a service is avoided. Instead, one or two simple covert channels are relied upon for conveying information regarding workload and availability status of a service. More specifically, the covert channel used in this invention is the size of a predetermined file. For lowest communication overhead, if a system supports holey files and a memory based file system, the covert channel used is the size of a holey file in a memory based file system. Relying on the covert channel, a service monitor thus acquires information indicative of the status of a service and uses this information to inter alia apprise other monitors of this status 
     The file that is agreed upon by a service and its service monitor to be used as the covert channel is hereinafter referred to as a communication file. As can be seen from FIG. 2 illustrating an embodiment in accordance with the invention, a service  36  has associated therewith a communication file  38 . The workload of service  36  is represented as an integer number X corresponding to the percent of full capacity at which the service  36  is utilized. The service  36  publishes its workload number periodically by setting the size of the communication file  38  to be the same as the workload number X. The service monitor  40  of service  38  periodically and independently reads the size of the communication file  38  to obtain the workload number X. 
     Communication file  38  can also be used to indicate the availability status of service  36  if the operating system of the server machine (not shown) provides a utility to obtain the last modification date of a file. The last modification date of a file changes if the contents or the attributes of the file changes. In such a system, service  36  continuously updates the workload number, even if the workload number does not change, enabling its service monitor  40  to rely on the last modification date of the communication file  38  to infer the availability status of the service  36 . If the last modification date does not change after a certain interval, the service monitor  40  can safely assume that the service  36  that it monitors is not available. 
     Alternatively, a service and a service monitor can designate another, separate, communication file specifically for communicating service availability status. This embodiment of the invention is illustrated in FIG. 3, with service  42  being monitored by service monitor  44  via communication files  46  and  48 . Communication file  46  operates as above, providing information about the percentage of utilization of the service  42 . Communication file  48  is dedicated to providing information about the availability status of service  42 . This is accomplished by selecting the size of the communication file  48  to be a monotonic increasing integer starting from 0. Service  42  periodically increments the size of the communication file  48  by one, thereby providing the equivalent of a “heartbeat” indicating that the service  42  is “alive,” or available. If the size of the communication file  48  does not increase after a certain interval as sensed by service monitor  44 , the service monitor may assume that the service  42  is not available. 
     In most operating system environments, updating the size of a communication file may require allocating disk storage for the file and may incur a disk I/O (input/output) operation each time the file size is updated. However, these costs can be eliminated for certain systems that support memory based file system and holey file. A hole is a region in a file that does not occupy storage space and is treated as if all the data stored in the region are bytes having a value of zero. A holey file is a file that contains at least one hole. 
     A memory based file system is a file system that uses computer random access memory (RAM) to store the contents of data files and the directory hierarchy of the file system. The advantage of a memory based file system is speed since accessing files in a memory based file system has the same speed as accessing RAM. The major drawback of a memory based file system is that the contents of a file of a memory based file system do not persist across system reboot. Solaris™ operating system of Sun Microsystems supports a memory based file system called tmpfs. The ufs file system in the Solaris™ operating system supports holey files. 
     If a holey file in a memory based file system is designated as the communication file  38  by service  36  and its service monitor  40 , updating the size of the communication file  38  does not incur a costly disk I/O (input/output) operation. Furthermore, if the communication file  38  does not contain any data (that is, it is a holey file containing one big hole), then no RAM allocation is needed for the file  38 . Thus, accessing the communication file  38  has the same speed as accessing RAM, and the cost of setting or retrieving the size of the holey communication file  38  in a memory based file system is about the same cost as a simple system call. 
     Utilization of file system services in the form of a covert channel in accordance with the invention is much simpler than writing RPC or networking programs. Furthermore, for systems such as Sun Microsystems&#39; Solaris™ operating system that support memory based file systems and holey files, using a covert channel as described above to communicate workload information and service availability has very low communication overhead and is extremely efficient. It should be noted, however, that other file attributes may also be used as a covert channel for publishing workload and availability of a service such as service  36 . However, among the available file attributes that can be used as a covert channel, file size is the simplest to use. 
     The above are exemplary modes of carrying out the invention and are not intended to be limiting. It will be apparent to those skilled in the art that modifications thereto can be made without departure from the spirit and scope of the invention as set forth in the following claims.