This section describes the logical replication protocol, which is the message flow started by the
LOGICAL replication command.
The logical streaming replication protocol builds on the primitives of the physical streaming replication protocol.
The logical replication
START_REPLICATION command accepts following parameters:
Protocol version. Currently only version
1 is supported.
Comma separated list of publication names for which to subscribe (receive changes). The individual publication names are treated as standard objects names and can be quoted the same as needed.
The individual protocol messages are discussed in the following subsections. Individual messages are described in Section 52.9.
All top-level protocol messages begin with a message type byte. While represented in code as a character, this is a signed byte with no associated encoding.
Since the streaming replication protocol supplies a message length there is no need for top-level protocol messages to embed a length in their header.
With the exception of the
START_REPLICATION command and the replay progress messages, all information flows only from the backend to the frontend.
The logical replication protocol sends individual transactions one by one. This means that all messages between a pair of Begin and Commit messages belong to the same transaction.
Every sent transaction contains zero or more DML messages (Insert, Update, Delete). In case of a cascaded setup it can also contain Origin messages. The origin message indicated that the transaction originated on different replication node. Since a replication node in the scope of logical replication protocol can be pretty much anything, the only identifier is the origin name. It's downstream's responsibility to handle this as needed (if needed). The Origin message is always sent before any DML messages in the transaction.
Every DML message contains an arbitrary relation ID, which can be mapped to an ID in the Relation messages. The Relation messages describe the schema of the given relation. The Relation message is sent for a given relation either because it is the first time we send a DML message for given relation in the current session or because the relation definition has changed since the last Relation message was sent for it. The protocol assumes that the client is capable of caching the metadata for as many relations as needed.