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corosync
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The Corosync Cluster Engine

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SYNCHRONIZATION ALGORITHM:

The synchronization algorithm is used for every service in corosync to synchronize state of the system.

There are 4 events of the synchronization algorithm. These events are in fact functions that are registered in the service handler data structure. They are called by the synchronization system whenever a network partitions or merges.

init: Within the init event a service handler should record temporary state variables used by the process event.

process: The process event is responsible for executing synchronization. This event will return a state as to whether it has completed or not. This allows for synchronization to be interrupted and recontinue when the message queue buffer is full. The process event will be called again by the synchronization service if requested to do so by the return variable returned in process.

abort: The abort event occurs when during synchronization a processor failure occurs.

activate: The activate event occurs when process has returned no more processing is necessary for any node in the cluster and all messages originated by process have completed.

CHECKPOINT SYNCHRONIZATION ALGORITHM:

The purpose of the checkpoint synchronization algorithm is to synchronize checkpoints after a partition or merge of two or more partitions. The secondary purpose of the algorithm is to determine the cluster-wide reference count for every checkpoint.

Every cluster contains a group of checkpoints. Each checkpoint has a checkpoint name and checkpoint number. The number is used to uniquely reference an unlinked but still open checkpoint in the cluster.

Every checkpoint contains a reference count which is used to determine when that checkpoint may be released. The algorithm rebuilds the reference count information each time a partition or merge occurs.

local variables mysyncstate may have the values SYNCCHECKPOINT, SYNCREFCOUNT mycurrentiterationstate contains any data used to iterate the checkpoints and sections. checkpoint data refcountset contains reference count for every node consisting of number of opened connections to checkpoint and node identifier refcount contains a summation of every reference count in the refcount_set

pseudocode executed by a processor when the synchronization service calls the init event call processcheckpointsenter

pseudocode executed by a processor when the synchronization service calls the process event in the SYNCCHECKPOINT state if lowest processor identifier of old ring in new ring transmit checkpoints or sections starting from mycurrentiterationstate if all checkpoints and sections could be queued call syncrefcountsenter else record mycurrentiteration_state

require process to continue

pseudocode executed by a processor when the synchronization service calls the process event in the SYNCREFCOUNT state if lowest processor identifier of old ring in new ring transmit checkpoint reference counts if all checkpoint reference counts could be queued require process to not continue else record mycurrentiterationstate for checkpoint reference counts

synccheckpointsenter: mysyncstate = SYNCCHECKPOINT mycurrentiterationstate set to start of checkpoint list

syncrefcountsenter: mysyncstate = SYNC_REFCOUNT

on event receipt of foreign ring id message ignore message

pseudocode executed on event receipt of checkpoint update if checkpoint exists in temporary storage ignore message else create checkpoint reset checkpoint refcount array

pseudocode executed on event receipt of checkpoint section update if checkpoint section exists in temporary storage ignore message else create checkpoint section

pseudocode executed on event receipt of reference count update update temporary checkpoint data storage reference count set by adding any reference counts in the temporary message set to those from the event update that checkpoint's reference count set the global checkpoint id to the current checkpoint id + 1 if it would increase the global checkpoint id

pseudocode called when the synchronization service calls the activate event: for all checkpoints free all previously committed checkpoints and sections convert temporary checkpoints and sections to regular sections copy mysavedringid to myoldringid

pseudocode called when the synchronization service calls the abort event: free all temporary checkpoints and temporary sections

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