Fujitsu PostgreSQL Blog Logical Replication Tablesync Workers Peter Smith | September 2, 2021

The Fujitsu OSS team, in collaboration with other OSS community members, has been contributing code to enhance Logical Replication for PostgreSQL.

Much of what we have been doing in this area is not user-facing; it is necessary to give some background information so that our changes can be described in context.

The CREATE SUBSCRIPTION command launches a single Apply Worker. This Apply Worker then launches N x Tablesync Workers (one for each table that the SUBSCRIPTION subscribes to).

The Apply Worker process is long-lived - it exists (in the absence of errors) until the SUBSCRIPTION is dropped. The Tablesync Worker processes are transient - each Tablesync Worker only exists for the synchronization phase of the associated table.

The primary purpose of the Tablesync Worker is to initialize the replicated table by doing a COPY of all rows of the published table (at the time of the CREATE SUBSCRIPTION).

The Tablesync Workers and Apply Worker are each capable of receiving and handling pending replication protocol messages – e.g., messages for replicating the CRUD (Create, Read, Update, Delete) operations at the subscriber side.

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