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SAM_OVERVIEW(3)    Corosync Cluster Engine Programmer's Manual	 SAM_OVERVIEW(3)

NAME
     sam_overview - Overview of the Simple Availability Manager

OVERVIEW
     The  SAM library provide a tool to check the health of an application.  The
     main purpose of SAM is to restart a local process when it fails to  respond
     to a healthcheck request in a configured time interval.

     During  sam_initialize(3), a duplicate copy of the process is created using
     the fork(3) system call.  This duplicate process copy  contains  the  logic
     for executing the SAM server.  The SAM server is responsible for requesting
     healthchecks  from the active process, and controlling the lifecycle of the
     active process when it fails.  If the active process fails  to  respond  to
     the healthcheck request sent by the SAM server, it will be sent a user con-
     figurable	signal (default SIGTERM) to request shutdown of the application.
     After a configured time interval, the process will be  forcibly  killed  by
     being  sent  a SIGKILL signal.  Once the active process terminates, the SAM
     server will create a new active process.

     The Simple Availability Manager is meant to be used in conjunction with the
     cpg service.  Used together, it is possible to restart a cpg  process  that
     fails healthchecking during operation.

     The main features of SAM include:

	    *  A configurable recovery policy.

	    *  A configurable time interval for health check operations.

	    *  A notification via signal before recovery action is taken.

	    *  A mechanism to indicate to the application the number of times an
	       active process has been created by the SAM server.

	    *  Both  application  driven health checking and event driven health
	       checking.

Initializing SAM
     The SAM library is initialized by sam_initialize(3).  sam_initalize(3)  may
     only  be  called once per process.  Calling it more then once has undefined
     results and is not recommended or tested.

Setting warning callback
     User configurable signal (default SIGTERM) is sent to the application  when
     a recovery action is planned.  The application can use the signal(3) system
     call to monitor for this signal.

     There  are no special constraints on what SAM apis may be called in a warn-
     ing callback.  After time_interval expires, a SIGKILL signal is sent to the
     active process to force its termination.

Registering the active process
     The active process is registered with SAM by calling sam_register(3).  This
     function should only be called one time in a process.  After a recovery ac-
     tion is taken, the new active process will begin execution at the next line
     of code in a user process after sam_register(3).

Enabling event driven healthchecking
     Two types of healthchecking are available to the user.  The first model  is
     one  where  the  user application healthchecks during its normal operation.
     It is never requested to healtcheck, and if the active process doesn't  re-
     spond within the time interval, the process will be restarted.

     A	more useful mechanism for healthchecking is event driven healthchecking.
     Because this model is directed by the SAM server,	It  isn't  necessary  to
     guess or add timers to the active process to signal a healthcheck operation
     is  successful.   To  use	event  driven  healthchecking,	the sam_hc_call-
     back_register(3) function should be executed.

Quorum integration
     SAM has special policies  (SAM_RECOVERY_POLICY_QUIT  and  SAM_RECOVERY_POL-
     ICY_RESTART) for integration with quorum service. This policies changes SAM
     behaviour in two aspects.

	    *  Call of sam_start(3) blocks until corosync becomes quorate

	    *  User  selected recovery action is taken immediately after lost of
	       quorum.

Storing user data
     Sometimes there is need to store some  data,  which  survives  between  in-
     stances.  One can in such case use files, databases, ... or much simpler in
     memory  solution  presented  by  sam_data_store(3), sam_data_restore(3) and
     sam_data_getsize(3) functions.

Confdb integration
     SAM has policy flag used for confdb system  integration  (SAM_RECOVERY_POL-
     ICY_CONFDB).   If	process  is registered with this flag, new confdb object
     PROCESS_NAME:PID is created with following keys:

	    *  recovery - will be quit or restart depending on policy

	    *  poll_period - period of health checking in milliseconds

	    *  last_updated - Timestamp (in  nanoseconds)  of  the  last  health
	       check.

	    *  state  -  state	of  process  (can be one of registered, started,
	       failed, waiting for quorum)

     Object is automatically deleted if process exits with stopped health check-
     ing.

     Confdb integration with corosync watchdog can be used in implicit	and  ex-
     plicit way.

     Implicit way is achieved by setting recovery policy to QUIT and let process
     exit with started health checking.  If this happened, object is not deleted
     and corosync watchdog will take required action.

     Explicit  way  is	useful for situations, when developer can deal with some
     non-fatal fall of application.  This mode is achieved by setting policy  to
     RESTART  and using SAM same as without Confdb integration.  If real fail is
     needed (like too many restarts at all, per/sec, ...), it's possible to  use
     sam_mark_failed(3) and let corosync watchdog take required action.

BUGS
SEE ALSO
     sam_initialize(3), 	sam_data_getsize(3),	    sam_data_restore(3),
     sam_data_store(3),   sam_finalize(3),   sam_mark_failed(3),   sam_start(3),
     sam_stop(3),   sam_register(3),   sam_warn_signal_set(3),	 sam_hc_send(3),
     sam_hc_callback_register(3)

corosync Man Page		   21/05/2010			 SAM_OVERVIEW(3)

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