SUBSCRIBE initiates a subscription to a track. The associated parameters determine the range and mechanism of object delivery; the Location Filter (see Section 3.3.1) selects which Objects are sent, and a FILL_PARAMETERS parameter (see Section 9.20.15) additionally retrieves the fill range on a fill fetch stream (see Section 3.4).
The format of SUBSCRIBE is as follows:
SUBSCRIBE Message {
Type (vi64) = 0x3,
Length (16),
Request ID (vi64),
Track Namespace (..),
Track Name Length (vi64),
Track Name (..),
Number of Parameters (vi64),
Parameters (..) ...
}
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Request ID: See Section 6.4.2.1.
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Track Namespace: Identifies the namespace of the track as defined in (Section 8.7).
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Track Name: Identifies the track name as defined in (Section 2.4.1).
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Parameters: The parameters are defined in Section 9.20. The parameters that can appear in a SUBSCRIBE are OBJECT_DELIVERY_TIMEOUT, AUTHORIZATION_TOKEN, RENDEZVOUS_TIMEOUT, SUBGROUP_DELIVERY_TIMEOUT, FORWARD, SUBSCRIBER_PRIORITY, LOCATION_FILTER, GROUP_ORDER, FILL_PARAMETERS, SUBGROUP_FILTER, OBJECTID_FILTER, PRIORITY_FILTER, OBJECT_PROPERTY_FILTER, NEW_GROUP_REQUEST and INCLUDE_PROPERTIES.
On successful subscription, the publisher MUST reply with a SUBSCRIBE_OK, allowing the subscriber to determine the start group/object when not explicitly specified, and start sending objects.