Ubuntu Manpage: sam_overview - Overview of the Simple Availability Manager

Provided by: corosync_2.3.3-1ubuntu4_amd64

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
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  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.

Quorum integration

       SAM  has  special  policies (SAM_RECOVERY_POLICY_QUIT and SAM_RECOVERY_POLICY_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 instances.  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_POLICY_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 checking.

       Confdb integration with corosync wathdog can be used in implicit and explicit 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 usefull 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.