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
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
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
See the CP Subsystem Configuration section for more details.