8. Ack Messages

AMPS provides information to the application and AMPS client about the status of commands using acknowledgement, or ack, messages.

A command to AMPS must explicitly request acknowledgement to receive a response from AMPS. By default, the AMPS client libraries request acknowledgements as needed to detect failures: see the AMPS User Guide chapter on acknowledgements and the Developer Guide for each client language for details.

In some cases, an application may want additional information about the state of a command, and may request an ack explicitly. For example, an application may want to know how many records would be returned by a particular query. In that case, a common technique is to request a stats (statistics) ack, while setting the number of data messages to be returned to 0 (using the top_n=0 option).

This section describes the ack messages returned by AMPS.

ack message

The ack message returns status information from AMPS.

AMPS does not create ack messages unless an acknowledgment is specifically requested. The exact meaning and content of ack messages depends on the command the requests the message. AMPS supports the following types of ack messages with the general semantics described below.

ack Type	Meaning
completed	An operation has completed. For example, subscriptions that replay from the transaction log can produce a completed acknowledgment to indicate when transaction log replay has finished and further messages for the subscription are the result of new publishes.
persisted	Data has been persisted.
processed	AMPS has processed the command. Notice that, depending on the command, AMPS may not have executed the command when this acknowledgment is produced.
received	AMPS has received the command, but has not yet processed it.
stats	Statistics for the command. This acknowledgment is typically produced after the command has fully completed.

Types of ack message

Common Header Fields For Ack Messages

Field	Description
Command	Type of message. Always ack, as encoded by the protocol.
AckType	The type of acknowledgment. One of completed, persisted, processed, received or stats.
CommandId	The CommandId that this ack refers to. Clients can use this field to correlate the ack returned with the command being acknowledged.
Status	The status of the command.
Reason	The reason for a failure status.

Header fields provided in ack

Additional fields for logon

When the ack message is produced in response to a logon command, the following additional header fields may be set:

Field	Description
ClientName	The name of the client provided with the command.
SequenceId	The last SequenceId persisted to the transaction log for this client, as identified by the ClientName.
Bookmark	The last bookmark from this client.
UserId	UserId to use when the status is retry.
Password	Password to use when the status is retry.
Version	The version of the AMPS server.

Additional ack headers for logon

Additional fields for publish and delta_publish

When the ack message is produced in response to a publish or delta_publish command, the following additional header fields may be set:

Field	Description
SequenceId	The last SequenceId persisted for this client.
Bookmark	The last Bookmark persisted for this client.

Additional ack headers for publish or delta_publish

Additional fields for subscribe and delta_subscribe

When the ack message is produced in response to a subscribe or delta_subscribe command, the following additional header fields may be set:

Field	Description
SubId	The SubId sent with the command, or the SubId generated by AMPS if no SubId was provided. This field is not returned in processed acks.
Options	Returned when the command is a subscribe to a queue. Contains the following options: max_backlog indicates the effective maximum backlog that the server has assigned for this subscription.
Bookmark	For a completed acknowledgement on a bookmark subscription, this indicates the point in the transaction log where the acknowledgement message was generated. Notice that the message that corresponds to this bookmark need not be a message matched by the subscription.

Additional ack headers for subscribe or delta_subscribe

Additional fields for unsubscribe

When the ack message is produced in response to a unsubscribe, AMPS does not provide additional header fields.

Additional fields for sow, sow_and_subscribe, sow_and_delta_subscribe

When the ack message is produced in response to a sow, sow_and_subscribe, or sow_and_delta_subscribe, the following additional header fields may be set:

Field	Description
SubId	The SubId sent with the sow command.
QueryId	The QueryId sent with the sow command.
RecordsReturned	The number of records returned by a SOW query. This header field is present on stats acknowledgments.
TopicMatches	The total number of records compared across all matching SOW topics. This header field is present on stats acknowledgments.
Matches	The number of records that match the topic regular expression and content filter. This header field is present on stats acknowledgments.

Additional ack headers for sow, sow_and_subscribe, sow_and_delta_subscribe

Additional fields for sow_delete

When the ack message is produced in response to a sow_delete the following additional header fields may be set:

Field	Description
QueryId	The QueryId sent with the sow_delete command.
RecordsDeleted	The number of records deleted by the command. This header field is present on stats acknowledgments.
TopicMatches	The total number of records compared across all matching SOW topics. This header field is present on stats acknowledgments.
Matches	The number of records that match the topic regular expression and content filter. This header field is present on stats acknowledgments.

Additional ack headers for sow_delete