IBM WebSphere Adapters User Manual page 100

Callback event processing for event delivery with XA transaction
To provide data integrity and to make sure events are not delivered more than
once, which would cause errors in the downstream system in the integration
scenario, the invention provides a mechanism to achieve once-and-only delivery
and the same is accomplished using XA transaction.
When assured delivery is required, the basic flow described in Callback event
processing for basic delivery becomes more complex. For more complex scenarios,
CallbackEventSender creates an instance of XA to bring the delivery under a new
transaction. The beforeDevliery() call to endpoint is the starting point of XA
transaction and it will last till afterDelivery(). If problems occur within this
transaction scope, a proper rollback mechanism will ensure data integrity is
maintained.
XA transaction will be active and supported only when the adapter is configured
with one EndpointFactory.
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WebSphere Adapters: WebSphere Adapter Toolkit User Guide