SUBSCRIBE_UPDATE

A subscriber sends a SUBSCRIBE_UPDATE to a publisher to modify an existing subscription.

When updating the Subscription Filter (see Section 5.1.2), the Start Location MUST not decrease, as an attempt to to do so could fail. If Objects with Locations smaller than the current subscription's Largest Location are required, FETCH can be used to retrieve them. A publisher MUST terminate the session with a PROTOCOL_VIOLATION if the SUBSCRIBE_UPDATE violates this rule.

When a subscriber narrows their subscription (increase the Start Location and/or decrease the End Group), it might still receive objects outside the new range if the publisher sent them before the update was processed. When a subscriber increases the End Location, the Largest Object at the publisher might already be larger than the previous End Location. This will create a gap in the subscription. The REQUEST_OK in response to the SUBSCRIBE_UPDATE will include the LARGEST_OBJECT parameter, and the subscriber can issue a FETCH to retrieve the omitted Objects, if any.

The receiver of a SUBSCRIBE_UPDATE MUST respond with exactly one REQUEST_OK or REQUEST_ERROR message indicating if the update was successful. When an update is unsuccessful, the publisher MUST also terminate the subscription with PUBLISH_DONE with error code UPDATE_FAILED.

Like SUBSCRIBE, End Group MUST be greater than or equal to the Group specified in Start.

If a parameter previously set on the subscription in SUBSCRIBE, PUBLISH_OK or SUBSCRIBE_UPDATE is not present in SUBSCRIBE_UPDATE, its value remains unchanged.

There is no mechanism to remove a parameter from a subscription.

The format of SUBSCRIBE_UPDATE is as follows:

SUBSCRIBE_UPDATE Message {
  Type (i) = 0x2,
  Length (16),
  Request ID (i),
  Subscription Request ID (i),
  Number of Parameters (i),
  Parameters (..) ...
}

• Request ID: See Section 9.1.

• Subscription Request ID: The Request ID of the SUBSCRIBE (Section 9.9) this message is updating. This MUST match an existing Request ID. The publisher MUST close the session with ` PROTOCOL_VIOLATION if the subscriber specifies an invalid Subscription Request ID.

• Parameters: The parameters are defined in Section 9.2.1.