sow_and_subscribe

A sow_and_subscribe command is used to combine the functionality of sow and a subscribe command in a single command.

The sow_and_subscribe command is used: (a) to query the contents of a SOW topic (this is the sow command); and (b) to place a subscription such that any messages matching the subscribed SOW topic and query filter will be published to the AMPS client (this is the subscribe command). As with the subscribe command, publish messages representing updates to SOW records are provided for records that match the filter when there is an update published to that record.

Header Fields

The following table contains the header fields supported by a sow_and_subscribe command.

Returns

A sow_and_subscribe command returns the following command types:

AMPS will send acknowledgment messages for the following AckType fields: received, processed along with a populated Status header field describing the acknowledgment message.

If the sow_and_subscribe command is successful, AMPS will return a group_begin message to notify the client that a group of messages is being returned as part of the sow portion of the command.

For more information about SOW topic query behavior, see the chapter on Querying the State of the World in the AMPS User Guide.

The following table contains the AckType messages that can be returned by a sow_and_subscribe.

The stats acknowledgment message includes three values in the header: Matches, TopicMatches and the RecordsReturned. These are defined below:

TopicMatches

The total number of records compared across all matching SOW topics.

Matches

The number of records returned that match the topic regular expression and the content filter. This value can be greater than RecordsReturned in the case where the number of returned records is limited by TopN.

RecordsReturned

The total number of records returned to the client, which can be limited by the TopN header value.

Options Field

The following table contains a list of the Options available and their definitions when used in the AMPS sow_and_subscribe command.

Errors

Errors for a sow_and_subscribe query are returned in the Status field if an AckType has been defined. Errors will also be logged in the errors and events log (subject to logging configuration).

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