README.recovery

   1 SYNCHRONIZATION ALGORITHM:
   2 -------------------------
   3 The synchronization algorithm is used for every service in corosync to
   4 synchronize state of the system.
   5
   6 There are 4 events of the synchronization algorithm.  These events are in fact
   7 functions that are registered in the service handler data structure.  They
   8 are called by the synchronization system whenever a network partitions or
   9 merges.
  10
  11 init:
  12 Within the init event a service handler should record temporary state variables
  13 used by the process event.
  14
  15 process:
  16 The process event is responsible for executing synchronization.  This event
  17 will return a state as to whether it has completed or not.  This allows for
  18 synchronization to be interrupted and recontinue when the message queue buffer
  19 is full.  The process event will be called again by the synchronization service
  20 if requested to do so by the return variable returned in process.
  21
  22 abort:
  23 The abort event occurs when during synchronization a processor failure occurs.
  24
  25 activate:
  26 The activate event occurs when process has returned no more processing is
  27 necessary for any node in the cluster and all messages originated by process
  28 have completed.
  29
  30 CHECKPOINT SYNCHRONIZATION ALGORITHM:
  31 ------------------------------------
  32 The purpose of the checkpoint synchronization algorithm is to synchronize
  33 checkpoints after a partition or merge of two or more partitions.  The
  34 secondary purpose of the algorithm is to determine the cluster-wide reference
  35 count for every checkpoint.
  36
  37 Every cluster contains a group of checkpoints.  Each checkpoint has a
  38 checkpoint name and checkpoint number.  The number is used to uniquely reference
  39 an unlinked but still open checkpoint in the cluster.
  40
  41 Every checkpoint contains a reference count which is used to determine when
  42 that checkpoint may be released.  The algorithm rebuilds the reference count
  43 information each time a partition or merge occurs.
  44
  45 local variables
  46 my_sync_state may have the values SYNC_CHECKPOINT, SYNC_REFCOUNT
  47 my_current_iteration_state contains any data used to iterate the checkpoints
  48         and sections.
  49 checkpoint data
  50         refcount_set contains reference count for every node consisting of
  51         number of opened connections to checkpoint and node identifier
  52         refcount contains a summation of every reference count in the refcount_set
  53
  54 pseudocode executed by a processor when the synchronization service calls
  55 the init event
  56         call process_checkpoints_enter
  57
  58 pseudocode executed by a processor when the synchronization service calls
  59 the process event in the SYNC_CHECKPOINT state
  60         if lowest processor identifier of old ring in new ring
  61                 transmit checkpoints or sections starting from my_current_iteration_state
  62         if all checkpoints and sections could be queued
  63                 call sync_refcounts_enter
  64         else
  65                 record my_current_iteration_state
  66
  67         require process to continue
  68
  69 pseudocode executed by a processor when the synchronization service calls
  70 the process event in the SYNC_REFCOUNT state
  71         if lowest processor identifier of old ring in new ring
  72                 transmit checkpoint reference counts
  73         if all checkpoint reference counts could be queued
  74                 require process to not continue
  75         else
  76                 record my_current_iteration_state for checkpoint reference counts
  77
  78 sync_checkpoints_enter:
  79         my_sync_state = SYNC_CHECKPOINT
  80         my_current_iteration_state set to start of checkpoint list
  81
  82 sync_refcounts_enter:
  83         my_sync_state = SYNC_REFCOUNT
  84
  85 on event receipt of foreign ring id message
  86         ignore message
  87
  88 pseudocode executed on event receipt of checkpoint update
  89         if checkpoint exists in temporary storage
  90                 ignore message
  91         else
  92                 create checkpoint
  93                 reset checkpoint refcount array
  94
  95 pseudocode executed on event receipt of checkpoint section update
  96         if checkpoint section exists in temporary storage
  97                 ignore message
  98         else
  99                 create checkpoint section
 100
 101 pseudocode executed on event receipt of reference count update
 102         update temporary checkpoint data storage reference count set by adding
 103         any reference counts in the temporary message set to those from the
 104         event
 105         update that checkpoint's reference count
 106         set the global checkpoint id to the current checkpoint id + 1 if it
 107         would increase the global checkpoint id
 108
 109 pseudocode called when the synchronization service calls the activate event:
 110 for all checkpoints
 111         free all previously committed checkpoints and sections
 112         convert temporary checkpoints and sections to regular sections
 113 copy my_saved_ring_id to my_old_ring_id
 114
 115 pseudocode called when the synchronization service calls the abort event:
 116         free all temporary checkpoints and temporary sections