How to Replicate PostgreSQL Database

1. Service Platform Architect
Brandon Kang
sangjinn@gmail.com
https://tech.brandonkang.net
July 2020
How to Replicate
PostgreSQL Database
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2. Replication Overall
Primary Server
• A server for both READ and WRITE
Standby Server = Slave server, Backup server, Replica, etc.
• A server where the data is kept in sync with the master continuously
• A warm standby server can’t be connected until it is promoted to primary
• A hot standby server accepts connections and serves read-only queries
• Data is written to the master server and propagated to the slave servers
• When primary server has an issue, a standby server will take over and
continue to take ‘WRITE” process

3. WAL Shipping Replication
WAL(Write-Ahead Logging)
• A log file where all the modifications to the database
• WAL is used for recovery after a database crash, ensuring data integrity
• WAL is used in database systems to achieve atomicity and durability
WAL based Replication ­ Streaming WAL Records
• Write-ahead log records are used to keep the data in sync
• WAL record chunks are streamed by database servers to synchronize
• Standby server connects to the master to receive the WAL chunks
• WAL records are streamed as they are generated
• The streaming of WAL records need not wait for the WAL file to be filled

4. Streaming Replication
PRIMARY
Server
Standby
Server
WAL
Sender
WAL
Receiver
WAL Record
READ ONLYREAD/WRITE
pg_wal
directory
WAL
File
pg_wal
directory
WAL
File
Synchronize
- WAL Record1
- WAL Record2
- WAL Record3
……

5. Replication Configuration
Primary Node ­ pg_hba.conf
/PG_HOME/DATA/pg_hba.conf
# Allow replication connections from localhost,
# by a user with the replication privilege.
# TYPE DATABASE USER ADDRESS METHOD
host replication repl_user IPaddress(CIDR) md5
Restart PostgreSQL service on the primary node
Internal IP ranges
Firewall Open for this.

6. Replication Configuration
Primary Node ­/PG_HOME/DATA/postgres.conf
# Possible values are replica|minimal|logical
wal_level = hot_standby
archive_mode = on #Syncronize with Primany node using wal archive
archive_command = ‘cp %p /PG_TBS/backup/archive/%f’ #psql_superuser archive backup
# required for pg_rewind capability when standby goes out of sync with master
wal_log_hints = on
# sets the maximum number of concurrent connections from the standby servers.
max_wal_senders = 2
# The below parameter is used to tell the master to keep the minimum number of
# segments of WAL logs so that they are not deleted before standby consumes them.
# each segment is 16MB
wal_keep_segments = 16

7. Replication Configuration
Secondary Node ­/PG_HOME/DATA/postgres.conf
hot_standby = on
Secondary Node ­/PG_HOME/DATA/recovery.conf
standby_mode = ‘on’
restore_command = ‘cp %p /PG_TBS/backup/archive/%f “%p”’
Primary_conninfo = ‘host=… ports=… user=postgres password= application_name=slave1’
# Explaining a few options used for pg_basebackup utility
# -X option is used to include the required transaction log files (WAL files) in the
# backup. When you specify stream, this will open a second connection to the server
# and start streaming the transaction log at the same time as running the backup.
# -c is the checkpoint option. Setting it to fast will force the checkpoint to be
# created soon.
# -W forces pg_basebackup to prompt for a password before connecting
# to a database.
pg_basebackup -D <data_directory> -h <master_host> -X stream -c fast -U
repl_user -W

8. Replication Configuration
Secondary Node ­ pg_basebackup utility
# Explaining a few options used for pg_basebackup utility
# -X option is used to include the required transaction log files (WAL files) in the
# backup. When you specify stream, this will open a second connection to the server
# and start streaming the transaction log at the same time as running the backup.
# -c is the checkpoint option. Setting it to fast will force the checkpoint to be
# created soon.
# -W forces pg_basebackup to prompt for a password before connecting
# to a database.
pg_basebackup -D <data_directory> -h <master_host> -X stream -c fast -U repl_user -W
Example)
$pg_basebackup ­h localhost ­p 5170 -u postgres ­D /PG_TBS/cluster -checkpoint=fast --progress

9. Replication Configuration
Secondary Node ­ Replication configuration file
# Specifies whether to start the server as a standby. In streaming replication,
# this parameter must be set to on.
standby_mode = ‘on’
# Specifies a connection string which is used for the standby server to connect
# with the primary/master.
primary_conninfo = ‘host=<master_host> port=<postgres_port> user=<replication_user>
password=<password> application_name=”host_name”’
# Specifies recovering to a particular timeline. The default is to recover along the
# same timeline that was current when the base backup was taken.
# Setting this to latest recovers to the latest timeline found
# in the archive, which is useful in a standby server.
recovery_target_timeline = ‘latest’
Start PostgreSQL service on the Standby node

10. Replication Testing
postgres=# select * from pg_stat_replication ;
-[ RECORD 1 ]----+---------------------------------
pid | 1114
usesysid | 16384
usename | repuser
application_name | walreceiver
client_addr | 127.0.0.1
client_hostname |
client_port | 52444
backend_start | 08-MAR-20 19:54:05.535695 -04:00
state | streaming
..
sync_priority | 0
sync_state | async
ps ­ef | grep postgres
[Master] wal sender process check
[Standby] wal receiver & startup process check
SELECT * FROM pg_stat_replication; Query execution on the Primary/Standby nodes

11. Replication Testing
pg_is_in_recovery(): A function to check if standby server is still in recovery mode or not
postgres=# select pg_is_in_recovery();
pg_is_in_recovery
-------------------
t
(1 row)
pg_last_xact_replay_timestamp(): A function shows time stamp of last replica transaction
postgres=# select pg_last_xact_replay_timestamp();
pg_last_xact_replay_timestamp
----------------------------------
08-MAR-20 20:54:27.635591 -04:00 (1 row)
SELECT CASE WHEN pg_last_xlog_receive_location() = pg_last_xlog_replay_location()
THEN 0
ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp())
END AS log_delay;
Calculating logs in seconds

12. Scaling PostgreSQL
PRIMARY
Server
Standby
Server
WAL
Sender
WAL
Receiver
WAL Record
WRITE
Synchronize
HAProxy /
NGINX / F5 READ
WRITE
READ
pgBouncer
primary_db
standby_db
Application

13. - Thank You. -
Service Platform Architect
Brandon Kang
sangjinn@gmail.com
https://tech.brandonkang.net