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Architecture Components

Write-Ahead Log (WAL)

Logical Decoding

Replication Slots

Logical Decoding Plugins

Queue

• Essentially a set of tables in a PostgreSQL database.
• Default is to have 3 tables per queue, these are rotated to efficiently purge discarded events.
• Event is discarded when all the consumers have processed it.
• There can be multiple queues in one database.
• Any number of producers and consumers to the queue.

Producer

• application module that pushes transactions into queue.

Consumer

• application that reads transactions from queue.

Architecture Pillar

Features

• Transactional. Events are created transactionally, can be coupled with surrounding business logic.
• Efficient. Events are processed in batches which gives low per event overhead.
• Flexible. No limits on the number of producers or consumers. Custom event formats.
• Reliable. Events are stored in PostgreSQL database – this adds the benefit of write ahead logging and crash recovery.
• Easy to use. Simple SQL interface, API-s for several languages.
• Open Source. No licensing fees, but occasionally you’ll have to get your hands dirty

Replication Metadata (publications and subscriptions) hosted and managed outside of database

Benefits of queuing

• client can send a request to an unavailable server
• server can send a reply to an unavailable client
• can have many servers feed off a single queue