Chapter 6. Reconnect and failover

Messaging systems use message acknowledgment to track if the goal of sending a message is truly accomplished.

When a message is sent, there’s a period of time after the message is sent and before it is acknowledged (the message is "in flight"). If the network connection is lost during that time, the status of the message delivery is unknown, and the delivery might require special handling in application code to ensure its completion.

The sections below describe the conditions for message delivery when connections fail.

Non-transacted producer with an unacknowledged delivery

If a message is in flight, it is sent again after reconnect, provided a send timeout is not set and has not elapsed.

No user action is required.

Transacted producer with an uncommitted transaction

If a message is in flight, it is sent again after reconnect. If the send is the first in a new transaction, then sending continues as normal after reconnect. If there are previous sends in the transaction, then the transaction is considered failed, and any subsequent commit operation throws a TransactionRolledBackException.

To ensure delivery, the user must resend any messages belonging to a failed transaction.

Transacted producer with a pending commit

If a commit is in flight, then the transaction is considered failed, and any subsequent commit operation throws a TransactionRolledBackException.

To ensure delivery, the user must resend any messages belonging to a failed transaction.

Non-transacted consumer with an unacknowledged delivery

If a message is received but not yet acknowledged, then acknowledging the message produces no error but results in no action by the client.

Because the received message is not acknowledged, the producer might resend it. To avoid duplicates, the user must filter out duplicate messages by message ID.

Transacted consumer with an uncommitted transaction

If an active transaction is not yet committed, it is considered failed, and any pending acknowledgments are dropped. Any subsequent commit operation throws a TransactionRolledBackException.

The producer might resend the messages belonging to the transaction. To avoid duplicates, the user must filter out duplicate messages by message ID.

Transacted consumer with a pending commit

If a commit is in flight, then the transaction is considered failed. Any subsequent commit operation throws a TransactionRolledBackException.

The producer might resend the messages belonging to the transaction. To avoid duplicates, the user must filter out duplicate messages by message ID.