16. Command Acknowledgement

AMPS command processing is designed to be asynchronous. The design of the server makes it possible for an application to send a command to AMPS, and receive the results of that command at a later time. Acknowledgement of commands is always optional: the server makes no requirement that an application request acknowledgement. The AMPS client libraries automatically request the acknowledgements required to maintain the guarantees the client API provides.

The status and results of a command are returned to a client in the form of an acknowledgment, or ack, message. AMPS can return status updates at various checkpoints throughout the command processing sequence.

For many applications, it may not be necessary for the application to request message acknowledgements explicitly. The AMPS clients request a set of acknowledgements by default that balance performance with error detection.

AMPS supports a variety of ack types, and allows you to request multiple ack types on each command. For example, the received ack type requests that AMPS acknowledge when the command is received, while the completed ack type requests that AMPS acknowledge when it has completed the command (or the portion of the command that runs immediately). Each AMPS command supports a different set of types, and the precise meaning of the ack returned depends on the command that AMPS is acknowledging.

AMPS commands are inherently asynchronous, and AMPS does not provide acknowledgement messages by default. A client must both explicitly request an acknowledgement and then receive and process that acknowledgement to know the results of a command. It is normal for time to elapse between the request and the acknowledgement, and so AMPS acknowledgements provide ways to correlate the acknowledgement with the command that produced it. This is typically done with an identifier that the client assigns to a command, which is then returned in the acknowledgement for the command.

AMPS supports the acknowledgement types listed in the following table:

Acknowledgment Type	General Description
completed	The command (or a portion of the command) has completed.
persisted	The results of the command have been persisted to durable storage.
processed	AMPS has processed the command.
received	AMPS has received the command.
stats	AMPS returns statistics associated with the command.

Table 16.1: AMPS acknowledgement messages

Not all commands support all acknowledgement types, and the meaning of each acknowledgement may differ depending on the command submitted. See the AMPS Command Reference for details.

Acknowledgements for different commands may not arrive in the order that commands were submitted to AMPS. For example, a publish command to a topic that uses synchronous replication will not return a persisted acknowledgment until the synchronous replication destinations have persisted the message. If the client issues a subscribe command in the meantime, the processed acknowledgement for the subscribe command – indicating that AMPS has processed the subscription request – may well return before the persisted acknowledgement.

Requesting Acknowledgements

Acknowledgements from the AMPS server are always optional. The AMPS clients will request acknowledgements as necessary for processing and error reporting. In addition, an application can request acknowledgements as necessary to meet the requirements of an application.

The AMPS Command Guide contains information on the acknowledgement types available for each command and the meaning of those acknowledgements.

Receiving Acknowledgements

Acknowledgements for a specific command or subscription are delivered as a part of the message stream for that command. In an AMPS client, this means that whatever method an application is using to receive responses from AMPS will include acknowledgement messages.

Acknowledgements from AMPS have a command type of ack. This command type is reserved for responses from AMPS to a client. The CommandId of the acknowledgement message contains the CommandId provided by the client on the command that AMPS is responding to. The client uses this CommandId to route the acknowledgement message to the correct message processor in the application.

Notice that when acknowledgements are _conflated_ – that is, the acknowledgement refers to a number of publish commands – AMPS does not include a command ID in the acknowledgement message. Further details are provided at Acknowledgement Conflation and Publish Acknowledgements.

Bookmark Subscriptions and Completed Acknowledgements

When AMPS is processing a bookmark subscription, a completed acknowledgement indicates that the subscription has completed replay from the transaction log. Messages delivered on the subscription after the completed acknowledgement are from new publishes.

Acknowledgement Conflation and Publish Acknowledgements

For some commands, AMPS will conflate acknowledgements and return acknowledgements for multiple commands at one time. When AMPS conflates acknowledgements, AMPS provides an identifier other than the command identifier that describes which commands the acknowledgement applies to.

For example, when a transaction log is configured, AMPS conflates persisted acknowledgements in response to publish commands and sow_delete commands. These conflated acknowledgements contain the last client sequence number that the acknowledgement applies to rather than the command identifiers or sequence numbers for all messages being acknowledged. For example, if an application publishes messages with sequence numbers 1, 2, 3, 4, and 5, and message 3 fails due to entitlement restrictions, AMPS will return an ack indicating success for message 2, an ack indicating failure for message 3, and an ack indicating success for message 5.

AMPS produces conflated acknowledgements for a given connection approximately once a second.

To see more information about the different commands and their supported acknowledgment types, please refer to the AMPS Command Reference, provided with 4.0 and greater versions of the AMPS clients and available on the 60East web site.