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https://corosync.github.io/corosync/

The Corosync Cluster Engine
https://corosync.github.io/corosync/

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The Corosync Cluster Engine

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README 

        
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

my_sync_state may have the values SYNC_CHECKPOINT, SYNC_REFCOUNT

my_current_iteration_state contains any data used to iterate the checkpoints

	and sections.

checkpoint data

	refcount_set 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 process_checkpoints_enter



pseudocode executed by a processor when the synchronization service calls

the process event in the SYNC_CHECKPOINT state

	if lowest processor identifier of old ring in new ring

		transmit checkpoints or sections starting from my_current_iteration_state

	if all checkpoints and sections could be queued

		call sync_refcounts_enter

	else

		record my_current_iteration_state



	require process to continue



pseudocode executed by a processor when the synchronization service calls

the process event in the SYNC_REFCOUNT 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 my_current_iteration_state for checkpoint reference counts



sync_checkpoints_enter:

	my_sync_state = SYNC_CHECKPOINT

	my_current_iteration_state set to start of checkpoint list



sync_refcounts_enter:

	my_sync_state = 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 my_saved_ring_id to my_old_ring_id



pseudocode called when the synchronization service calls the abort event:

	free all temporary checkpoints and temporary sections