==== Overview ====
Write ahead log or WAL is analog of many other databases transactional logs:
  * Oracle - Archive log
  * DB2 - Transactional Log
  * Microsoft - Transactional Log
  * and so on.

The basic idea is to keep little files with information about our changes. In postgreSQL we have 3 levels of WAL settings:

  - Minimal
  - Archive
  - Replica
  - Logical

If you depend on WAL for recovery, REPLICA is good option.

====Enabling====
In order to enable WAL, you have edit the postgresql.conf file and change the following settings:

  * wal_level (replica or logical)
  * wal_max_senders (should be >= 1)
  * archive_mode (should be on)
  * archive_command (depends on your environment but here is an example: 'test ! -f /var/lib/pgsql/9.6/archive/%f && cp %p /var/lib/pgsql/9.6/archive/%f'

Once all is done, we have to restart the service as follows:


<sxh bash>
[root@localhost data]# service postgresql-9.6 restart
Redirecting to /bin/systemctl restart postgresql-9.6.service
[root@localhost data]# service postgresql-9.6 status
Redirecting to /bin/systemctl status postgresql-9.6.service
● postgresql-9.6.service - PostgreSQL 9.6 database server
   Loaded: loaded (/usr/lib/systemd/system/postgresql-9.6.service; enabled; vendor preset: disabled)
   Active: active (running) since Mon 2019-05-20 03:44:44 EDT; 9s ago
     Docs: https://www.postgresql.org/docs/9.6/static/
  Process: 3748 ExecStartPre=/usr/pgsql-9.6/bin/postgresql96-check-db-dir ${PGDATA} (code=exited, status=0/SUCCESS)
 Main PID: 3753 (postmaster)
   CGroup: /system.slice/postgresql-9.6.service
           ├─3753 /usr/pgsql-9.6/bin/postmaster -D /var/lib/pgsql/9.6/data/
           ├─3755 postgres: logger process
           ├─3757 postgres: checkpointer process
           ├─3758 postgres: writer process
           ├─3759 postgres: wal writer process
           ├─3760 postgres: autovacuum launcher process
           ├─3761 postgres: archiver process
           └─3762 postgres: stats collector process

May 20 03:44:43 localhost.localdomain systemd[1]: Starting PostgreSQL 9.6 database server...
May 20 03:44:44 localhost.localdomain postmaster[3753]: < 2019-05-20 03:44:44.132 EDT > LOG:  redirecting log output to logging collector process
May 20 03:44:44 localhost.localdomain postmaster[3753]: < 2019-05-20 03:44:44.132 EDT > HINT:  Future log output will appear in directory "pg_log".
May 20 03:44:44 localhost.localdomain systemd[1]: Started PostgreSQL 9.6 database server.
</sxh>

====Verify====
Write Ahead Log (WAL) Enables us to make online base backup, so let's verify if the WAL works firstly:

<sxh bash>
[root@localhost archive]# ls -lart
total 81924
drwx------. 5 postgres postgres       66 May 19 08:35 ..
-rw-------. 1 postgres postgres 16777216 May 19 08:43 000000010000000000000001
-rw-------. 1 postgres postgres 16777216 May 19 08:48 000000010000000000000002
-rw-------. 1 postgres postgres 16777216 May 19 08:48 000000010000000000000003
-rw-------. 1 postgres postgres 16777216 May 19 08:48 000000010000000000000004
-rw-------. 1 postgres postgres      302 May 19 08:48 000000010000000000000004.00000028.backup
drwxr-xr-x. 2 postgres postgres      214 May 19 08:50 .
-rw-------. 1 postgres postgres 16777216 May 19 08:50 000000010000000000000005
</sxh>


Seems all is fine, now we can make a base backup, explained further in the wiki.