Availability of payload replication

Payload replication constitutes creating a copy of the message payload that is uploaded to one data center, in all other data centers.

In case of immediate (synchronous) replication, for message processing to complete, a copy of the message payload must be created in at least one another data center in the Global Mailbox topology. Following tasks are involved in payload replication:
  1. If size of the payload is larger than the inline payload threshold, segments are created in shared disk.
  2. If size of the payload is less than the inline payload threshold, payload is stored in Cassandra.
  3. Payload segments from shared disk are replicated to another data center. Replication is triggered by the Global Mailbox management node in the other data center. Payload is transferred by the replication server in the receiving data center.
  4. Payload blobs are replicated to another data center by Cassandra.

Availability or unavailability of some Global Mailbox components impact the availability of the payload replication. The following sections explain the same in detail:

How Cassandra nodes impact availability of payload replication

For inline payloads, replication is done by Cassandra. Payload does not appear in another data center unless the Cassandra nodes are up in that data center. If the Cassandra nodes are down when the file was uploaded, you must run a repair to replicate the payload to the nodes that were down.

For non-inline payloads, after payload is received by a data center, the Global Mailbox management nodes in other data centers detect the new payload and pull them into their resident data centers. The Global Mailbox management nodes query the Cassandra database looking for new messages that have non-inline payload. 50% + 1 of the nodes in the local data center must be up for the Global Mailbox management node to replicate payload to the local data center.

How ZooKeeper nodes impact availability of payload replication

During normal functioning of the Global Mailbox, a minimum of 2 ZooKeeper servers must be operational across the data centers for payload replication to complete. The ZooKeeper watchdog process must also be running on all ZooKeeper nodes so that ZooKeeper failures can be detected, and recovery with a reduced ensemble can be initiated. While the watchdog is setting up the reduced ensemble, there is a window of time when replication fails. Payload replication proceeds after the reduced ensemble is started. If the number of running ZooKeeper servers reduces to 1 or 0, payload replication fails until the number of ZooKeeper servers comes back to 2 or more.

How shared disk impacts availability of payload replication

The shared disk is not used to store inline payload, therefore the replication of these payloads is not dependent on the shared disk.

Non-inline payloads are stored in shared disk. The shared disk must be available for the Global Mailbox management node to pull the payload from another data center to the local data center.

How replication server impacts payload replication

For inline payloads, there is no dependency on replication server, because inline payloads are replicated by Cassandra.

For non-inline payloads, the Global Mailbox management node pulls payloads from another data center to the local data center. To do so, at least one replication server must be operational in the local and the other data center.

How WebSphere MQ impacts payload replication

Payload replication is not dependent on WebSphere® MQ.