Background

[tcapturesupport]

Write-Ahead Log (WAL)

• Ensuring data integrity.
• Changes to data files must be written only after those changes have been logged
• After log records describing the changes have been flushed to permanent storage.
• No need to flush data pages to disk on every transaction commit

Logical Decoding

• ● Introduced in 9.4
• ● Plugin infrastructure (Extensible, Adaptable)
• ● The process of extracting all persistent changes to a databases tables into
   ○ Coherent
   ○ easy to understand format
   ○ interpreted without detailed knowledge of the database's internal state.
• ● Implemented by decoding the contents of the write-ahead log
   ○ into an application-specific form such as a stream of tuples or SQL statements
• ● Relies on Replication Slots

Replication Slots

• ● In the context of logical replication
   ○ Stream of changes
   ○ Can be replayed to a client in the order they were made on the origin server
   ○ Each slot streams a sequence of changes from a single database.
• ● Each has an identifier that is unique across all databases in a cluster
• ● Persisted independently of the connection
• ● Crash-safe
• ●Each change is emitted only once
• ○ Current position of each slot is persisted only at checkpoint
• ○ In case of a crash, the slot returns to an earlier LSN
• ○ Changes will be resent on server restart
• ● Up to logical decoding clients to handle same message more than once
• ○ May record the last LSN they saw

Logical Decoding Plugins

• ● The format in which those changes are streamed is determined by the output plugin used
• ● An example plugin is provided in the PostgreSQL distribution
• ● Additional plugins can be written to extend the choice of available formats without modifying any core code
• ● Every output plugin has access to each individual
• ○ new row produced by INSERT
• ○ old new row version created by UPDATE
• ○ The id and old version of a row removed with DELETE
• ● Changes can be consumed
• ○ using the streaming replication protocol
• ○ Or by calling functions via SQL
• ● It is the responsibility of the plugin to produce the desired output the consumer expects and to filter out unnecessary changes