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CP Sessions

For CP data structures that involve resource ownership management, such as Locks or Semaphores, sessions are required to keep track of liveliness of callers. In this context, caller means an entity that uses CP Subsystem APIs. It can be either a Hazelcast member or a client. A caller initially creates a session before sending its very first session based request to the CP group, such as a Lock / Semaphore acquire. After creating a session on the CP group, the caller stores its session ID locally and sends it alongside its session-based operations. A single session is used for all lock and semaphore proxies of the caller. When a CP group receives a session-based operation, it checks the validity of the session using the session ID information available in the operation. A session is valid if it is still open in the CP group. An operation with a valid session ID is accepted as a new session heartbeat. While a caller is idle, in other words, it does not send any session based operation to the CP group for a while, it commits periodic heartbeats to the CP group in the background in order to keep its session alive. This interval is specified in CPSubsystemConfig.getSessionHeartbeatIntervalSeconds().

A session is closed when the caller does not touch the session during a predefined duration. In this case, the caller is assumed to be crashed and all its resources are released automatically. This duration is specified in CPSubsystemConfig.getSessionTimeToLiveSeconds(). See the CP Subsystem Configuration section for recommendations to choose a reasonable session time-to-live duration.

Sessions offer a trade-off between liveliness and safety. If you set a very small value using CPSubsystemConfig.setSessionTimeToLiveSeconds(int), then a session owner could be considered crashed very quickly and its resources can be released prematurely. On the other hand, if you set a large value, a session could be kept alive for an unnecessarily long duration even if its owner actually crashes. However, it is a safer approach to not to use a small session time-to-live duration. If a session owner is known to be crashed, its session could be closed manually via CPSessionManagementService.forceCloseSession(String, long).

See the CP Subsystem Configuration section for more details.