When PostgreSQL starts, it doesn't just quietly listen for queries; it launches a series of essential background processes that keep the database healthy, consistent, and performant.
In this blog, we'll explore:
1. Core background processes that run automatically
2. Additional background workers that activate based on features
3. Real examples of how to enable and monitor them
What Are Background Processes in PostgreSQL?
PostgreSQL uses background processes (sometimes called postmaster subprocesses) to handle critical system operations like:
* Data writes
* WAL logging
* Auto-vacuuming
* Stats collection
* Replication and more
You can see them using the ps aux command:
ps aux | grep postgres
Core Background Processes (Always Running)
These processes are launched by default as soon as the PostgreSQL server starts:
1. Checkpointer
Purpose:
The checkpointer is responsible for flushing "dirty" pages—modified blocks of data—from PostgreSQL’s shared memory (buffers) to disk at regular intervals. This process helps maintain consistency between what’s in memory and what’s persisted on disk. It minimizes the amount of data that needs to be recovered in case of a crash by ensuring frequent updates are stored safely.
Why it matters:
Without the checkpointer, PostgreSQL would risk losing recent changes if the system crashes before data is written to disk. By continuously syncing memory with storage, it reduces the recovery time and effort needed after unexpected shutdowns and helps keep the database resilient and recoverable.
Monitored as:
postgres 1052 0.0 0.0 221092 6116 ? Ss 08:27 0:00 postgres: 14/main: checkpointer
2. Background Writer
Purpose:
The background writer continuously scans the shared buffer pool and writes dirty buffers (modified data pages) to disk before the checkpointer needs to act. By handling routine writes proactively, it ensures that fewer dirty pages accumulate in memory. This reduces the burden on the checkpointer process and spreads out disk I/O over time.
Why it matters:
By offloading some of the writing workload from the checkpointer, the background writer helps prevent sudden performance drops during checkpoints. It ensures smoother, more predictable disk write operations, improving overall database responsiveness, especially under heavy write loads.
Monitored as:
postgres 1053 0.0 0.0 220980 4580 ? Ss 08:27 0:00 postgres: 14/main: background writer
3. WAL Writer
Purpose:
The WAL (Write-Ahead Log) Writer is responsible for flushing WAL buffers—records of changes made to the database—to disk at regular intervals. This ensures that all transactional changes are securely stored in the log before the actual data is written to disk. It operates continuously to maintain a consistent and recoverable transaction log.
Why it matters:
In the event of a crash or power failure, PostgreSQL uses WAL to restore the database to a consistent state. The WAL Writer plays a key role in ensuring those logs are always up-to-date, enabling reliable crash recovery and protecting against data loss in transactional systems.
Monitored as:
postgres 1054 0.0 0.0 220980 4980 ? Ss 08:27 0:00 postgres: 14/main: walwriter
4. Autovacuum Launcher
Purpose:
The Autovacuum Launcher is responsible for initiating background workers that perform vacuum operations on tables. These operations clean up dead tuples—obsolete row versions created by UPDATE or DELETE operations—to reclaim storage. It activates intelligently based on table activity, ensuring that database health is maintained automatically.
Why it matters:
Without regular vacuuming, dead tuples accumulate and lead to table bloat, slowing down queries and increasing disk usage. The Autovacuum Launcher ensures that these issues are addressed continuously, maintaining optimal performance and storage efficiency without requiring manual intervention.
Monitored as:
postgres 1055 0.0 0.0 221548 5988 ? Ss 08:27 0:00 postgres: 14/main: autovacuum launcher
5. Stats Collector
Purpose:
The Stats Collector used to be a dedicated background process that gathered real-time statistics about database activity, such as table and index usage, row reads, and query patterns. This data helped the query planner make informed decisions for efficient execution.
Now:
As of PostgreSQL 14+, this functionality has been moved into shared memory, reducing inter-process communication overhead and improving performance. Stats are still vital, but they’re now accessed more efficiently without relying on a separate collector process.
Why it matters:
Accurate statistics are essential for the planner to choose the best execution strategy. With shared memory handling the task, PostgreSQL reduces latency and boosts planning efficiency, especially in high-load environments.
Monitored as:
postgres 1057 0.0 0.0 75572 4708 ? Ss 08:27 0:00 postgres: 14/main: stats collector
6. Logical Replication Launcher
Purpose:
The Logical Replication Launcher activates and manages replication workers that stream row-level changes (inserts, updates, deletes) from a publication on the source database to its subscribers. It ensures each subscriber stays synchronized with the changes occurring in real-time.
When it appears:
This process only starts when at least one active subscription exists. Without any logical subscriptions configured, the launcher does not run, keeping PostgreSQL lightweight by default.
Why it matters:
This process is critical for supporting modern use cases like real-time data integration, zero-downtime migrations, and distributed systems. It provides fine-grained control over what data gets replicated and how.
Monitored as:
postgres 1059 0.0 0.0 221396 5220 ? Ss 08:27 0:00 postgres: 14/main: logical replication launcher
Additional Background Workers (Conditional)
These workers start only when specific features are enabled. Let’s look at some real-world use cases.
1. Logical Replication Launcher
* Purpose: Coordinates delivery of data to subscribers.
* Enable it with:
Publisher (source database):
CREATE PUBLICATION my_pub FOR TABLE my_table;
Subscriber (target database):
CREATE SUBSCRIPTION my_sub
CONNECTION 'host=localhost dbname=target user=replicator'
PUBLICATION my_pub;
Monitored as
ps aux | grep logical
SELECT * FROM pg_stat_replication;
2. WAL Sender Process
* Purpose: Sends WAL logs from primary to standby.
* Enable it with:
# postgresql.conf
wal_level = replica
max_wal_senders = 3
Monitored as:
SELECT * FROM pg_stat_replication;
3. WAL Receiver Process
* Purpose: Receives WAL logs on the standby server.
* Enable it with:
# On standby
primary_conninfo = 'host=primary port=5432 user=replicator'
Monitored as:
SELECT * FROM pg_stat_wal_receiver;
4. Archiver Process
* Purpose: Archives completed WAL segments for backup and PITR.
* Enable it with:
archive_mode = on
archive_command = 'cp %p /path/to/archive/%f'
Monitored as:
ps aux | grep archiver
5. Parallel Workers
* Purpose: Used by the planner to execute queries in parallel (e.g., joins, scans).
* Enable it with:
max_parallel_workers = 8
max_parallel_workers_per_gather = 4
Run a query:
EXPLAIN ANALYZE SELECT COUNT(*) FROM large_table;
Monitored as:
ps aux | grep parallel
Conclusion
PostgreSQL's background workers are the silent engines that keep the database running smoothly. From handling routine maintenance tasks to enabling advanced features like replication, parallelism, and crash recovery, these processes play a pivotal role behind the scenes.
Understanding how they work isn’t just for DBAs — it's crucial for developers, performance tuners, and anyone working deeply with PostgreSQL internals. In short, learning about PostgreSQL’s background processes isn’t just technical knowledge — it’s the foundation for becoming a true PostgreSQL power user. Discover essential strategies for optimizing your database's concurrency and preventing common issues like deadlocks. This guide, "How to Understand & Manage Locks in PostgreSQL Tables," provides a clear overview of different lock types and practical methods for effective lock management to ensure smooth operations.