SAM_OVERVIEW(8)            Corosync Cluster Engine Programmer's Manual            SAM_OVERVIEW(8)



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
       configurable 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 sig‐
       nal.  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 warning 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 action 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  respond  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_callback_register(3) function should be executed.


BUGS
SEE ALSO
       sam_initialize(3),   sam_finalize(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                           12/01/2009                            SAM_OVERVIEW(8)