sam_overview(8)

1SAM_OVERVIEW(8)   Corosync Cluster Engine Programmer's Manual  SAM_OVERVIEW(8)
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NAME

6       sam_overview - Overview of the Simple Availability Manager
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OVERVIEW

10       The  SAM  library provide a tool to check the health of an application.
11       The main purpose of SAM is to restart a local process when it fails  to
12       respond to a healthcheck request in a configured time interval.
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15       During  sam_initialize(3),  a  duplicate copy of the process is created
16       using the fork(3) system call.  This duplicate  process  copy  contains
17       the  logic for executing the SAM server.  The SAM server is responsible
18       for requesting healthchecks from the active  process,  and  controlling
19       the  lifecycle  of  the  active  process  when it fails.  If the active
20       process fails to respond to the healthcheck request  sent  by  the  SAM
21       server, it will be sent a user configurable signal (default SIGTERM) to
22       request shutdown of the application.  After a configured time interval,
23       the  process  will  be  forcibly killed by being sent a SIGKILL signal.
24       Once the active process terminates, the SAM server will  create  a  new
25       active process.
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28       The Simple Availability Manager is meant to be used in conjunction with
29       the cpg service.  Used together,  it  is  possible  to  restart  a  cpg
30       process that fails healthchecking during operation.
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33       The main features of SAM include:
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36              ·  A configurable recovery policy.
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38              ·  A configurable time interval for health check operations.
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40              ·  A notification via signal before recovery action is taken.
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42              ·  A  mechanism  to  indicate  to  the application the number of
43                 times an active process has been created by the SAM server.
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45              ·  Both application driven  health  checking  and  event  driven
46                 health checking.
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Initializing SAM

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

56       User  configurable  signal (default SIGTERM) is sent to the application
57       when a recovery action is planned.  The application can  use  the  sig‐
58       nal(3) system call to monitor for this signal.
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61       There  are  no  special constraints on what SAM apis may be called in a
62       warning callback.  After time_interval expires,  a  SIGKILL  signal  is
63       sent to the active process to force its termination.
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Registering the active process

67       The  active  process is registered with SAM by calling sam_register(3).
68       This function should only be called one time in  a  process.   After  a
69       recovery  action  is taken, the new active process will begin execution
70       at the next line of code in a user process after sam_register(3).
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Enabling event driven healthchecking

74       Two types of healthchecking are available to the user.  The first model
75       is one where the user application healthchecks during its normal opera‐
76       tion.  It is never requested to healtcheck, and if the  active  process
77       doesn't   respond  within  the  time  interval,  the  process  will  be
78       restarted.
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81       A more useful mechanism for healthchecking is event driven healthcheck‐
82       ing.  Because this model is directed by the SAM server, It isn't neces‐
83       sary to guess  or  add  timers  to  the  active  process  to  signal  a
84       healthcheck  operation is successful.  To use event driven healthcheck‐
85       ing, the sam_hc_callback_register(3) function should be executed.
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NAME

OVERVIEW

Initializing SAM

Setting warning callback

Registering the active process

Enabling event driven healthchecking

BUGS

SEE ALSO