Logical replication is a method that we can use to replicate data based on the concept of identity replication. REPLICA IDENTITY is a parameter present in table management commands (such as CREATE TABLE and ALTER TABLE); this parameter is used by PostgreSQL to obtain additional information within WAL segments to recognize which tuples have been eliminated and which tuples have been updated. The REPLICA IDENTITY parameter can take four values:
- DEFAULT
- USING INDEX index_name
- FULL
- NOTHING
The concept behind logical replication is to pass the logic of the commands executed on the master machine to the server and not the exact copy of the blocks to be replicated byte by byte. At the heart of logical replication, there is a reverse engineering process that, starting from the WAL segments and using a logical decoding process, is able to extrapolate the original SQL commands and pass them on to the replication machine using a logical decoding process.
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