How PostgreSQL WAL Actually Works (And Why Everything Depends On It)

Every change in PostgreSQL is written twice. First to WAL . Then later to the actual table files . This mechanism is called Write-Ahead Logging (WAL) and it’s one of the most important parts of PostgreSQL’s architecture. Without WAL, PostgreSQL would not be able to safely recover from crashes, replicate data, or support tools like CDC pipelines and backup systems. In this Post we’ll break down: how PostgreSQL stores data internally what WAL is and why it exists what WAL files actually contain how PostgreSQL recovers from crashes why replication, CDC, and backups depend on WAL How Data Is Stored in PostgreSQL (Quick Glance) Before understanding WAL, it helps to know how PostgreSQL actually stores data on disk. When you create a table in PostgreSQL: CREATE TABLE users ( id SERIAL PRIMARY KEY , name TEXT ); PostgreSQL creates a table file on disk inside its data directory. But rows are not stored randomly inside that file. Instead, PostgreSQL organizes table storage into fixed-size pages