This section describes the logical replication protocol, which is the message
flow started by the START_REPLICATION
SLOT slot_name
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 versions 1 and
2 are supported. The version 2
is supported only for server version 14 and above, and it allows
streaming of large in-progress transactions.
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 53.9.
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. It also sends changes of large in-progress transactions between a pair of Stream Start and Stream Stop messages. The last stream of such a transaction contains Stream Commit or Stream Abort message.
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 indicates 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.