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Channel Data Model
This is a high-level overview of how channel and access information is stored in the _sync metadata.
Functionally, a channel defines read-side security for documents. Documents are assigned to zero or more channels, and users are granted access to zero or more channels. In order for a user to read a document through the Sync Gateway REST API (and by extension, via client replication), the document needs to be assigned to a channel that the user has access to.
Channels are associated with a particular revision of a document. All channel information for a document is stored in Sync Gateway's private _sync metadata block in the document itself. The _sync metadata isn't accessible to clients for read or write - it's managed by Sync Gateway, and is stripped from documents before they are replicated.
Sync Gateway stored channel assignment in the revision tree for the document. For each revision in the rev tree, we store the list of channels that are assigned to that revision of the document.
In addition to the information in the rev tree, we also store channel history for the active revision independently. The channels property in the _sync metadata lists of all channels the document has ever been assigned. For channels that aren't assigned to the active revision, the channels property includes the revision when the document was removed from that channel.
This property simplifies tracking of document channel assignment without requiring a full rebuild of the rev tree - particularly for use by views and on performance-sensitive Sync Gateway processing.
Sample data:
"channels": {
"channel_B": null,
"channel_A": {
"seq": 22,
"rev": "2-f38e39675f20d50803192f14232f2226"
}
},
In the above example, the current (active) revision of the document belongs to channel channel_B. It previously was assigned to channel_A, but that assignment was removed at revision 2-f38e39675f20d50803192f14232f2226.
If a document is added/removed from a channel multiple times, only the most recent removal is tracked in the _channels metadata.
The channels property has the following use cases:
- Source data for the "channels" MR view - used to generate the complete set of documents in a given channel
- Used to identify the sequence at which a document has been removed from a channel, to trigger removal notification during replication
- Used during Couchbase Server mutation feed processing by Sync Gateway, to build the in-memory cache of recent channel history (used during replication)