Tag Archives: PostgreSQL architecture

WAL Commit Process

PostgreSQLACID Transactions, archive_command, archive_mode, br8dba, Checkpoint Process, Crash Recovery, Database Durability, Database Internals, Database Recovery, Dirty Pages, Enterprise PostgreSQL, fsync, Log Sequence Number, LSN, Open Source Database, pg_switch_wal, pg_wal, PostgreSQL, PostgreSQL 16, PostgreSQL 17, PostgreSQL administration, PostgreSQL architecture, PostgreSQL checkpoint, PostgreSQL DBA, PostgreSQL Guide, PostgreSQL internals, PostgreSQL Monitoring, PostgreSQL Performance, PostgreSQL performance tuning, PostgreSQL recovery, PostgreSQL training, PostgreSQL Tutorial, PostgreSQL WAL, Shared Buffers, synchronous_commit, Transaction Commit, WAL archiving, WAL Buffer, WAL Commit Process, WAL files, WAL Flush, WAL Lifecycle, WAL Record, WAL Segment, WAL Writer, write-ahead logging

PostgreSQL WAL Commit Process

1. The Modification (In-Memory)

When a user executes a data-modifying query (like INSERT, UPDATE, or DELETE):

The change is made to the table or index data inside the Shared Buffers (RAM). The page in memory is now marked as “dirty.”
Simultaneously, a record of this exact change is constructed and written sequentially into the WAL Buffers (also in RAM).

2. The `COMMIT` Command Issued

When the client sends the COMMIT command, PostgreSQL must guarantee that this change will survive a sudden power outage or crash before it can tell the user “Success.”

3. Flushing to Disk (`XLogFlush`)

To ensure durability without the massive overhead of writing entire data pages to disk immediately, Postgres uses the Write-Ahead Logging protocol:

The internal function XLogFlush() is called.
It identifies the exact position (Log Sequence Number, or LSN) of the commit record in the WAL Buffer.
It issues a synchronous write to flush all WAL buffers up to that LSN out of RAM and into the current 16MB WAL segment file on permanent storage.
An fsync() system call is issued to ensure the OS cache actually commits the data to physical disk platters or flash memory.

4. Acknowledgment to the Client

Once the operating system confirms that the WAL record is safely written to the physical storage, the transaction status is updated to “committed” in the commit log (CLOG), and PostgreSQL sends a success acknowledgment back to the client application.

Crucial Architectural Concepts

Write-Ahead Rule: The core rule of WAL is that changes to data pages must not be written to permanent database files on disk until the log records describing those changes have been flushed to stable storage. If the server crashes, Postgres reads the WAL from the last checkpoint forward and reapplies the changes (“redoes” them).
Asynchronous Commit Alternative: If you set the configuration parameter synchronous_commit = off, Postgres will acknowledge the client’s COMMIT before the WAL buffer is flushed to disk (relying on the WAL Writer background process to flush it within roughly 3 times wal_writer_delay). This massively increases write throughput but introduces a risk of losing up to a split-second of recent transactions if the server suddenly loses power.

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Thank you

Rajasekhar Amudala

Email: br8dba@gmail.com
LinkedIn: linkedin.com/in/rajasekhar-amudala

Types of Shutdown in PostgreSQL

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Types of Shutdown in PostgreSQL

PostgreSQL Shutdown Type	Oracle	Behavior
Smart (SIGTERM)	SHUTDOWN NORMAL	The server stops accepting new connections but allows existing sessions to continue running normally. PostgreSQL shuts down only after all active sessions disconnect and their transactions complete successfully.
Fast (SIGINT – Default)	SHUTDOWN IMMEDIATE	The server stops accepting new connections and immediately terminates all active sessions. Any uncommitted transactions are rolled back, and PostgreSQL performs a clean shutdown gracefully.
Immediate (SIGQUIT)	SHUTDOWN ABORT	The server terminates all processes instantly without performing a proper shutdown. No cleanup or checkpoint occurs, so PostgreSQL performs crash recovery by replaying WAL files during the next startup.

Shutdown Type	Signal	Command Example	Behavior
Smart	SIGTERM	`pg_ctl stop -D /pgData -m smart`	Stops new connections, waits for existing sessions to finish normally, then shuts down.
Fast (Default)	SIGINT	`pg_ctl stop -D /pgdata -m fast`	Stops new connections, terminates active sessions, rolls back uncommitted transactions, and shuts down cleanly.
Immediate	SIGQUIT	`pg_ctl stop -D /pgdata -m immediate`	Stops immediately without proper shutdown. Recovery from WAL files happens during next startup.

Difference Between Reload and Restart

When we change server configuration parameters, PostgreSQL needs to read those changes before they become active.
Reload applies the new configuration without stopping or restarting the database service. Existing connections and database activity continue normally.
Some configuration parameter changes cannot be applied through reload and will only take effect after a restart.
Restart stops the PostgreSQL server gracefully, ends all running activities, releases resources, closes open files, and then starts the server again with the new configuration.

Smart Shutdown (SIGTERM):

[postgres@pgdb02 ~]$ pg_ctl stop -D /pgData/pgsql17/data -m smart
waiting for server to shut down.... done
server stopped
[postgres@pgdb02 ~]$

2026-07-03 14:26:59.741 +08 [3476] LOG:  received smart shutdown request
2026-07-03 14:26:59.752 +08 [3476] LOG:  background worker "logical replication launcher" (PID 3483) exited with exit code 1
2026-07-03 14:26:59.753 +08 [3478] LOG:  shutting down
2026-07-03 14:26:59.755 +08 [3478] LOG:  checkpoint starting: shutdown immediate
2026-07-03 14:26:59.765 +08 [3478] LOG:  checkpoint complete: wrote 3 buffers (0.0%); 0 WAL file(s) added, 0 removed, 0 recycled; write=0.002 s, sync=0.003 s, total=0.012 s; sync files=2, longest=0.003 s, average=0.002 s; distance=0 kB, estimate=0 kB; lsn=1/105465F0, redo lsn=1/105465F0
2026-07-03 14:26:59.771 +08 [3476] LOG:  database system is shut down


[postgres@pgdb02 ~]$ pg_ctl start -D /pgData/pgsql17/data
waiting for server to start....2026-07-03 14:27:18.394 +08 [3487] LOG:  redirecting log output to logging collector process
2026-07-03 14:27:18.394 +08 [3487] HINT:  Future log output will appear in directory "log".
 done
server started
[postgres@pgdb02 ~]$

2026-07-03 14:27:18.394 +08 [3487] LOG:  starting PostgreSQL 17.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 11.5.0 20240719 (Red Hat 11.5.0-14), 64-bit
2026-07-03 14:27:18.396 +08 [3487] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2026-07-03 14:27:18.396 +08 [3487] LOG:  listening on IPv6 address "::", port 5432
2026-07-03 14:27:18.399 +08 [3487] LOG:  listening on Unix socket "/run/postgresql/.s.PGSQL.5432"
2026-07-03 14:27:18.404 +08 [3487] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-07-03 14:27:18.413 +08 [3491] LOG:  database system was shut down at 2026-07-03 14:26:59 +08
2026-07-03 14:27:18.425 +08 [3487] LOG:  database system is ready to accept connections

Fast Shutdown (SIGINT – Default):

[postgres@pgdb02 ~]$ pg_ctl stop -D /pgData/pgsql17/data -m fast
waiting for server to shut down.... done
server stopped
[postgres@pgdb02 ~]$

2026-07-03 14:28:26.839 +08 [3487] LOG:  received fast shutdown request
2026-07-03 14:28:26.844 +08 [3487] LOG:  aborting any active transactions
2026-07-03 14:28:26.854 +08 [3487] LOG:  background worker "logical replication launcher" (PID 3494) exited with exit code 1
2026-07-03 14:28:26.860 +08 [3489] LOG:  shutting down
2026-07-03 14:28:26.862 +08 [3489] LOG:  checkpoint starting: shutdown immediate
2026-07-03 14:28:26.874 +08 [3489] LOG:  checkpoint complete: wrote 3 buffers (0.0%); 0 WAL file(s) added, 0 removed, 0 recycled; write=0.002 s, sync=0.005 s, total=0.015 s; sync files=2, longest=0.004 s, average=0.003 s; distance=0 kB, estimate=0 kB; lsn=1/105466A0, redo lsn=1/105466A0
2026-07-03 14:28:26.882 +08 [3487] LOG:  database system is shut down

[postgres@pgdb02 ~]$ pg_ctl start -D /pgData/pgsql17/data
waiting for server to start....2026-07-03 14:29:00.574 +08 [3505] LOG:  redirecting log output to logging collector process
2026-07-03 14:29:00.574 +08 [3505] HINT:  Future log output will appear in directory "log".
 done
server started
[postgres@pgdb02 ~]$

2026-07-03 14:29:00.575 +08 [3505] LOG:  starting PostgreSQL 17.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 11.5.0 20240719 (Red Hat 11.5.0-14), 64-bit
2026-07-03 14:29:00.579 +08 [3505] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2026-07-03 14:29:00.580 +08 [3505] LOG:  listening on IPv6 address "::", port 5432
2026-07-03 14:29:00.582 +08 [3505] LOG:  listening on Unix socket "/run/postgresql/.s.PGSQL.5432"
2026-07-03 14:29:00.587 +08 [3505] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-07-03 14:29:00.595 +08 [3509] LOG:  database system was shut down at 2026-07-03 14:28:26 +08
2026-07-03 14:29:00.607 +08 [3505] LOG:  database system is ready to accept connections

Immediate Shutdown (SIGQUIT):

[postgres@pgdb02 ~]$ pg_ctl stop -D /pgData/pgsql17/data -m immediate
waiting for server to shut down.... done
server stopped
[postgres@pgdb02 ~]$

2026-07-03 14:29:47.471 +08 [3505] LOG:  received immediate shutdown request
2026-07-03 14:29:47.485 +08 [3505] LOG:  database system is shut down


[postgres@pgdb02 ~]$ pg_ctl start -D /pgData/pgsql17/data
waiting for server to start....2026-07-03 14:30:17.965 +08 [3520] LOG:  redirecting log output to logging collector process
2026-07-03 14:30:17.965 +08 [3520] HINT:  Future log output will appear in directory "log".
. done
server started
[postgres@pgdb02 ~]$

2026-07-03 14:30:17.966 +08 [3520] LOG:  starting PostgreSQL 17.10 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 11.5.0 20240719 (Red Hat 11.5.0-14), 64-bit
2026-07-03 14:30:17.966 +08 [3520] LOG:  listening on IPv4 address "0.0.0.0", port 5432
2026-07-03 14:30:17.966 +08 [3520] LOG:  listening on IPv6 address "::", port 5432
2026-07-03 14:30:17.970 +08 [3520] LOG:  listening on Unix socket "/run/postgresql/.s.PGSQL.5432"
2026-07-03 14:30:17.974 +08 [3520] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-07-03 14:30:17.983 +08 [3524] LOG:  database system was interrupted; last known up at 2026-07-03 14:29:00 +08
2026-07-03 14:30:19.211 +08 [3524] LOG:  database system was not properly shut down; automatic recovery in progress
2026-07-03 14:30:19.216 +08 [3524] LOG:  redo starts at 1/10546718
2026-07-03 14:30:19.216 +08 [3524] LOG:  invalid record length at 1/10546750: expected at least 24, got 0
2026-07-03 14:30:19.217 +08 [3524] LOG:  redo done at 1/10546718 system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2026-07-03 14:30:19.221 +08 [3522] LOG:  checkpoint starting: end-of-recovery immediate wait
2026-07-03 14:30:19.231 +08 [3522] LOG:  checkpoint complete: wrote 3 buffers (0.0%); 0 WAL file(s) added, 0 removed, 0 recycled; write=0.002 s, sync=0.003 s, total=0.012 s; sync files=2, longest=0.002 s, average=0.002 s; distance=0 kB, estimate=0 kB; lsn=1/10546750, redo lsn=1/10546750
2026-07-03 14:30:19.239 +08 [3520] LOG:  database system is ready to accept connections

Thank you,
Rajasekhar Amudala
Email: br8dba@gmail.com
Linkedin: https://www.linkedin.com/in/rajasekhar-amudala/

How SELECT, INSERT, UPDATE, and DELETE Work Internally in PostgreSQL

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How SELECT, INSERT, UPDATE, and DELETE work internally in PostgreSQL?

PostgreSQL uses MVCC (Multi-Version Concurrency Control), which means every modification creates a new version of the row instead of overwriting the old one. This is the core concept behind all DML operations.

1. Common Foundation (Applies to All Operations)

Heap: Data is stored in heap files (tables). Each row is a tuple.
Tuple Header: Every row contains:
- xmin: Transaction ID that created the row
- xmax: Transaction ID that deleted/updated the row (null if active)
- Other visibility information
WAL (Write-Ahead Log): All changes are first written to WAL for durability before being applied to data files.
Shared Buffers: Data pages are read into memory before being modified.
Visibility Rules: A row is visible to a transaction based on its snapshot (which transactions it can see).

2. How SELECT Works

Visibility Check: For each row, PostgreSQL checks:

- The row’s xmin must be committed and visible to the current transaction.
- The row’s xmax must be either null or not visible to the current transaction.

3. How INSERT Works

Transaction Starts: A new transaction ID (xid) is assigned.
Buffer Allocation: PostgreSQL finds a free slot in a heap page (or allocates a new page).
Tuple Creation: A new row is created with:
- xmin = current transaction ID
- xmax = null
WAL Logging: The change is written to the WAL buffer first.
Page Modification: The new tuple is inserted into the shared buffer.
Commit: On commit, the WAL is flushed to disk (fsync), making the change durable.

4. How UPDATE Works

UPDATE in PostgreSQL does not modify the existing row. It creates a new version of the row.

Find the row using the same visibility rules as SELECT.
Mark old row as dead: Set xmax = current transaction ID on the old tuple.
Insert new row: Create a completely new tuple with:
- xmin = current transaction ID
- xmax = null
- Updated column values
Update indexes (if needed): New index entries point to the new tuple.
WAL Logging: Both the old row’s xmax change and the new tuple are logged in WAL.
Commit: Changes become visible to other transactions after commit.

Important: The old row version remains in the table until VACUUM cleans it up.

5. How DELETE Works

DELETE also uses MVCC — it doesn’t remove the row immediately.

Locate the row using visibility rules.
Mark as deleted: Set xmax = current transaction ID on the tuple.
WAL Logging: The change is logged.
Commit: The row becomes invisible to new transactions.
Physical Removal: The row is not physically deleted from disk until VACUUM runs.

Thank you,
Rajasekhar Amudala
Email: br8dba@gmail.com
Linkedin: https://www.linkedin.com/in/rajasekhar-amudala/

PostgreSQL 17 DBA Free Training Roadmap

PostgreSQLdatabase administration, Database Backup and Recovery, Database Course, Database Security, DBA Training, DevOps, Failover, Free Training, High Availability, IT training, Linux for DBA, Online Training, Open Source Database, pg_basebackup, pg_dump, PITR, PostgreSQL, PostgreSQL 17, PostgreSQL architecture, PostgreSQL DBA, PostgreSQL for beginners, PostgreSQL installation, PostgreSQL performance tuning, PostgreSQL replication, PostgreSQL training, PostgreSQL Tutorial, PostgreSQL Upgrade, RDBMS, SQL, streaming replication, Telugu Tutorial

PostgreSQL DBA Free Coaching

DBA Training Program

// 5-week structured training · 19 sessions · hands-on labs

Weeks

Sessions

100+

Topics

∞

Future Free

Week 01 Lab Setup, Linux & PostgreSQL Fundamentals

Session 01 Lab Setup & Storage Configuration ▾

Pre-requisites

Install VirtualBox on Windows (Do your self)
Install PgAdmin4 on Windows (Do your self)
Configure Oracle Linux 9.6 VM (will share link)

Linux VM Setup

Create Oracle Linux 9.6 VM
VM Hardware Sizing for PostgreSQL
Add Multiple Virtual Disks

Storage Configuration

Create Partitions
Create Filesystems
Mount Filesystems
Configure /etc/fstab

PostgreSQL Directory Layout

Create Mount Points: /pgData, /pgWal, /pgArch, /pgBackup, /pgLog

Session 02 Linux Administration for DBAs ▾

Basic Linux Commands
File and Directory Management
User and Group Management
Permissions and Ownership
Process Management
Service Management using systemctl
Network Commands
Disk and Memory Monitoring

Session 03 PostgreSQL 17 Installation ▾

Overview

PostgreSQL Overview and Versions

Installation Methods

RPM Installation
DNF/YUM Installation
Source Code Installation

Database Initialization

initdb
Custom WAL Location
PostgreSQL Service Configuration

Validation

Start PostgreSQL
Connect using psql
Connect using PgAdmin4

Session 04 PostgreSQL Architecture ▾

Memory Architecture

Shared Buffers
WAL Buffers
Work Memory
Maintenance Work Memory

Process Architecture

Postmaster
Backend Processes
Checkpointer
Background Writer
WAL Writer
Autovacuum

Physical Storage Layout

Base Directory
Global Directory
WAL Directory
Tablespaces

Session 05 PostgreSQL Configuration Files ▾

postgresql.conf
postgresql.auto.conf
pg_hba.conf
pg_ident.conf
Reload vs Restart

Week 02 Administration & Security

Session 06 Startup, Shutdown ▾

PostgreSQL Startup Process
Smart Shutdown
Fast Shutdown (default)
Immediate Shutdown

Session 07 Database Administration ▾

Creating Databases
Creating Schemas
Schema Search Path
Roles and Users
Access Control

Session 08 Tablespaces Management ▾

pg_default
pg_global
Custom Tablespaces
Move Objects Between Tablespaces
Tablespace Monitoring

Session 09 Security ▾

pg_hba.conf
Authentication Methods
pg_ident.conf
.pgpass
SSL/TLS Setup

Session 10 Vacuum and Analyze ▾

MVCC Concepts
Dead Tuples
VACUUM
VACUUM FULL
ANALYZE
Autovacuum

Week 03 Backup & Recovery

Session 11 WAL Archiving ▾

WAL Fundamentals
Archive Mode
Archive Command

Session 12 Logical Backup and Restore ▾

pg_dump
pg_dumpall
Backup Formats
pg_restore
Restore using psql

Session 13 Physical Backup and Restore ▾

pg_basebackup
Full Cluster Backup
Database Refresh on New Server

Session 14 PITR — Point-in-Time Recovery ▾

PITR Concepts and Architecture
PITR Demonstration (hands-on)

Session 15 Full Recovery & Disaster Recovery ▾

Full Database Recovery
Disaster Recovery Scenarios

Week 04 Replication, Failover

Session 16 Streaming Replication ▾

Primary Configuration
Replica Configuration
Base Backup for Replica
Verify Replication

Session 17 Replication Administration ▾

Replication Slots
Monitoring Replication
Async Replication
Sync Replication
Convert from ASYNC to SYNC

Session 18 Failover and pg_rewind ▾

Manual Failover
Promote Replica
pg_rewind
Rejoin Old Primary as Replica

Week 05 Upgrades

Session 19 PostgreSQL Upgrades & Final Lab ▾

Minor Version Upgrade
Major Version Upgrades
pg_upgrade
Upgrade using pg_dump/pg_restore
Upgrade Validation
Final End-to-End Lab

Future Batch Advanced Topics — Free Training

// Topics reserved for the next advanced batch · access is free

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PostgreSQL DBA Step by Step Learning

#	PostgreSQL DBA Topics
1	How to Install PostgreSQL ON Linux?
2	How to Install PostgreSQL on Linux 7 using source code?
3	How to START/STOP PostgreSQL ON Linux?
4	How to Create Database in PostgreSQL?
5	PostgreSQL User Management
6	PostgreSQL pg_hba.conf Guide
7	PostgreSQL Change Data Directory
8	Understanding WAL Files in PostgreSQL – For Oracle DBAs
9	Change PostgreSQL WAL Directory Path (pg_wal)
10	Enable Archive Mode in PostgreSQL 17
11	How to Disable ARCHIVELOG Mode
12	PostgreSQL Tablespace Management
13	PostgreSQL pg_dump and pg_restore Guide
14	PostgreSQL Backup and Restore Using pg_dumpall and psql
15	pg_basebackup – Backup, Restore, and Recovery
16	Backup & Restore PostgreSQL DB Cluster to Another Host (No Archive Mode)
17	Backup & Restore PostgreSQL DB Cluster on Same Host
18	Restore PostgreSQL to New Host using pg_basebackup + WAL Archives
19	PostgreSQL PITR – Point in Time Recovery
20	Configure Streaming Replication in PostgreSQL
21	Manual Failover in PostgreSQL Streaming Replication
22	Convert Asynchronous Replication to Synchronous Replication

Thank you,
Rajasekhar Amudala
Email: br8dba@gmail.com
Linkedin: https://www.linkedin.com/in/rajasekhar-amudala/

PSQL

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PostgreSQL DBA Step by Step Learning

#	PostgreSQL DBA Topics
1	How to Install PostgreSQL ON Linux?
2	How to Install PostgreSQL on Linux 7 using source code?
3	How to START/STOP PostgreSQL ON Linux?
4	How to Create Database in PostgreSQL?
5	PostgreSQL User Management
6	PostgreSQL pg_hba.conf Guide
7	PostgreSQL Change Data Directory
8	Understanding WAL Files in PostgreSQL – For Oracle DBAs
9	Change PostgreSQL WAL Directory Path (pg_wal)
10	Enable Archive Mode in PostgreSQL 17
11	How to Disable ARCHIVELOG Mode
12	PostgreSQL Tablespace Management
13	PostgreSQL pg_dump and pg_restore Guide
14	PostgreSQL Backup and Restore Using pg_dumpall and psql
15	pg_basebackup – Backup, Restore, and Recovery
16	Backup & Restore PostgreSQL DB Cluster to Another Host (No Archive Mode)
17	Backup & Restore PostgreSQL DB Cluster on Same Host
18	Restore PostgreSQL to New Host using pg_basebackup + WAL Archives
19	PostgreSQL PITR – Point in Time Recovery
20	Configure Streaming Replication in PostgreSQL
21	Manual Failover in PostgreSQL Streaming Replication
22	Convert Asynchronous Replication to Synchronous Replication

Thank you,
Rajasekhar Amudala
Email: br8dba@gmail.com
Linkedin: https://www.linkedin.com/in/rajasekhar-amudala/

BRIGHT DBA

ORACLE DBA – Tips and Techniques

Tag Archives: PostgreSQL architecture

WAL Commit Process

PostgreSQL WAL Commit Process

1. The Modification (In-Memory)

2. The `COMMIT` Command Issued

3. Flushing to Disk (`XLogFlush`)

4. Acknowledgment to the Client

Crucial Architectural Concepts

Types of Shutdown in PostgreSQL

Types of Shutdown in PostgreSQL

How SELECT, INSERT, UPDATE, and DELETE Work Internally in PostgreSQL

How SELECT, INSERT, UPDATE, and DELETE work internally in PostgreSQL?

1. Common Foundation (Applies to All Operations)

2. How SELECT Works

3. How INSERT Works

4. How UPDATE Works

5. How DELETE Works

PostgreSQL 17 DBA Free Training Roadmap

PostgreSQL DBA Free Coaching

PostgreSQL

PostgreSQL DBA Step by Step Learning

PSQL

PostgreSQL DBA Step by Step Learning

1. The Modification (In-Memory)

2. The COMMIT Command Issued

3. Flushing to Disk (XLogFlush)

4. Acknowledgment to the Client

Crucial Architectural Concepts

Types of Shutdown in PostgreSQL

How SELECT, INSERT, UPDATE, and DELETE work internally in PostgreSQL?

1. Common Foundation (Applies to All Operations)

2. How SELECT Works

3. How INSERT Works

4. How UPDATE Works

5. How DELETE Works

PostgreSQL DBA Step by Step Learning

PostgreSQL DBA Step by Step Learning

2. The `COMMIT` Command Issued

3. Flushing to Disk (`XLogFlush`)