postgresql-11 master-slave replication (stream replication) deployment

Master-slave introduction
- PostgreSQL streaming replication is asynchronous by default. If the primary server crashes, some of the committed transactions may not have been replicated to the backup server, which can cause data loss. The amount of data lost is proportional to the replication delay during failover.
- Synchronous replication can ensure that all modifications of a transaction can be transmitted to one or more synchronous backup servers. This expands the standard persistence level provided by a transaction commit. This level of protection is called 2-safe replication in computer science theory. When synchronous_commitis set remote_write, then a group-1-safe (group- safe and 1-safe).
- When requesting synchronous replication, each commit of a write transaction will wait until it receives a confirmation that the commit has been written to the write-ahead log on disk on both the primary and backup servers. The only possibility that data will be lost is that both the primary server and the backup server crash at the same time. This can provide a higher level of persistence, although only the system administrator is responsible for the placement and management of the two servers. Waiting for confirmation improves the user's confidence that the modification will not be lost, but it also unnecessarily increases the response time to the requested transaction. The minimum waiting time is the round-trip time between the primary server and the backup server.
- Read-only transactions and transaction rollbacks do not need to wait for the reply from the standby server. Sub-transaction commits do not need to wait for the response from the backup server, only top-level commits need to wait. Long-running actions (such as data loading or index building) will not wait for the final commit message. All two-phase submission actions require submission waiting, including preparation and submission.
- The synchronization backup can be a physical replication backup or a logical replication subscriber. It can also be a consumer of any other physical or logical WAL replication stream, and it knows how to send appropriate feedback messages. In addition to the built-in physical and logical replication system, comprising a further pg_receivewaland pg_recvlogicalspecial procedures and the like, as well as some third-party copy system and custom programs. For details of synchronous replication support, please refer to the corresponding documentation.

Master-slave machine distribution

IP address	DB version	Master-slave relationship
192.168.63.134	11.6	the Lord
192.168.63.141	11.6	From

Install postgresql

yum install https://download.postgresql.org/pub/repos/yum/reporpms/EL-7-x86_64/pgdg-redhat-repo-latest.noarch.rpm -y
yum install postgresql11 -y
yum install postgresql11-server -y

Main library configuration

Main library initialization

/usr/pgsql-11/bin/postgresql-11-setup initdb
systemctl start postgresql-11

Create replication users for master-slave synchronization

[root@localhost data]# sudo -i -u postgres
-bash-4.2$ psql
psql (11.6)
Enter "help" to get help information.

postgres=# create role repl login replication encrypted password '123456';
CREATE ROLE

Configure the slave library to use the repl account on the master library

vim /var/lib/pgsql/11/data/pg_hba.conf

#Only need to add the following two lines to the station, repl is used for backup users, and the following 192.168.63.0/24 is the IP address in the network
host    replication     repl            192.168.63.0/24         md5
host    all             repl            192.168.63.0/24         trust

vim /var/lib/pgsql/11/data/postgresql.conf 

listen_addresses = '*'            # what IP address(es) to listen on;
port = 5432                # (change requires restart)
max_connections = 512            # (change requires restart)
shared_buffers = 128MB            # min 128kB
dynamic_shared_memory_type = posix    # the default is the first option
wal_level = hot_standby        # minimal, replica, or logical
archive_mode = on        # enables archiving; off, on, or always
archive_command = 'cp %p /var/lib/pgsql/11/data/pg_archive/%f'        # command to use to archive a logfile segment
max_wal_senders = 6        # max number of walsender processes
wal_keep_segments = 10240    # in logfile segments, 16MB each; 0 disables
wal_sender_timeout = 60s    # in milliseconds; 0 disables
log_directory = 'log'    # directory where log files are written

After the modification, you need to create some directory structures you just configured:

mkdir /var/lib/pgsql/11/data/pg_archive/
chown -R postgres.postgres /var/lib/pgsql/11/data

Restart the main library service
```
systemctl restart postgresql-11
```

Slave configuration

After the installation from the library is complete, do not initialize, if it has been initialized, delete its data directory

#Copy all data files of the master node
[root@localhost ~]# pg_basebackup -h 192.168.63.134 -U repl -D /var/lib/pgsql/11/data/ -X stream -P
Password: 
25312/25312 kB (100%), 1/1 table space
[root@localhost ~]# ls /var/lib/pgsql/12/data/
backup_label  current_logfiles  log         pg_commit_ts  pg_hba.conf    pg_logical    pg_notify    pg_serial     pg_stat      pg_subtrans  pg_twophase  pg_wal   postgresql.auto.conf
base          global            pg_archive  pg_dynshmem   pg_ident.conf  pg_multixact  pg_replslot  pg_snapshots  pg_stat_tmp  pg_tblspc    PG_VERSION   pg_xact  postgresql.conf

From the library configuration file, modify according to the following configuration.

[root@localhost ~]# vim /var/lib/pgsql/11/data/postgresql.conf
listen_addresses = '*'            # what IP address(es) to listen on;
port = 5432                # (change requires restart)
max_connections = 1000            # (change requires restart)
shared_buffers = 128MB            # min 128kB
dynamic_shared_memory_type = posix    # the default is the first option
wal_level = replica        # minimal, replica, or logical
archive_mode = on        # enables archiving; off, on, or always
archive_command = 'cp %p /var/lib/pgsql/12/data/pg_archive/%f'        # command to use to archive a logfile segment
wal_sender_timeout = 60s    # in milliseconds; 0 disables
hot_standby = on            # "on" allows queries during recovery
max_standby_streaming_delay = 30s    # max delay before canceling queries
wal_receiver_status_interval = 10s    # send replies at least this often
hot_standby_feedback = on        # send info from standby to prevent
log_directory = 'log'    # directory where log files are written,

Create a recovery file recovery.conf.

[root@localhost 11]# cp /usr/pgsql-11/share/recovery.conf.sample /var/lib/pgsql/11/data/recovery.conf
[root@localhost 11]# vim /var/lib/pgsql/11/data/recovery.conf

# Adjustment parameters:
recovery_target_timeline = 'latest' #Sync to the latest data
standby_mode = on #Specify the identity of the slave
trigger_file = 'failover.now'
primary_conninfo = 'host = 192.168.63.134 port = 5432 user = repl password = 123456' #connect to the main library information

Start slave
```
systemctl start postgresql-11
```

Verify master-slave configuration

Run the following command on the main library

postgres=# select client_addr,sync_state from pg_stat_replication;
  client_addr   | sync_state 
----------------+------------
 192.168.63.141 | async
(1 line record)

You can create a database to verify yourself

Please refer to https://www.cnblogs.com/miclis/p/10480979.html for details of master-slave switching

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