How to use streaming replication with PostgreSQL

In this post, I want to explain how to set up streaming replication for PostgreSQL. We want to be able to do (read-only) reporting on live data, but reduce load on the main database server.

Some background: We offer Metabase to many of our customers for data analysis and reporting. This means we don’t have to hand-code everything into our application. It even allows savvy customers to write their own reports. We set up a read-only PostgreSQL user on the production server. Metabase connects with the database as this user over an SSH-tunnelled PostgreSQL port.

For most of our systems, this setup is just fine. A few of our clients have so many users that the databases are in constant use. Some reports can be quite heavy, but they should not affect operations. So, we decided to replicate these databases to our Metabase servers. This allows us to query the local copy of the database, reducing system load on production. We currently rely on pg_dump for backups, which is getting a bit slow too. We are considering replication for backups, too. When the database becomes even more unwieldy we could use this for load-balancing, too.

Documentation overload

In general, the PostgreSQL documentation is to the point, clear and even usable as a tutorial. For replication though, I found the docs overwhelming and even a little confusing.

There are two chapters of interest: The first is under “backup”, called Continuous Archiving and Point-in-Time Recovery (PITR). The next chapter is High Availability, Load Balancing and Replication. Both chapters are a bit heavy on terminology, but for the most part, I was just overwhelmed by all the options.

The backup chapter starts out by explaining manual WAL archiving. You generally don’t want that. Streaming replication with replication slots was added later to the docs, as the feature is newer. This is what we’re going to use.

Untangling the confusion

Let’s start by demystifying some terminology. First of all, we want to replicate the database. The definition of replication is: “copy, reproduction”. That’s exactly what this is: we run a second server which will be an identical copy of the original database server. As you can see from the documentation, there are multiple solutions to perform replication. I won’t go into all of them. The one we’re concerned with is Write-Ahead Log Shipping.

Postgres uses the Write-Ahead Log (or WAL) internally to guarantee consistency and prevent data loss. For a detailed explanation, see the chapter on reliability. Normally, Postgres keeps only a few WAL “segments”, enough to replay the log after a power failure or crash. It uses these to restore consistency, much like logged file systems such as ext3 do. If the database kept the entire log since first boot, you could recreate the database from scratch up to the current state. WAL replication is based on this idea.

Log shipping can be done in two ways: the oldest one is to set up hooks in Postgres to copy (“ship” or “archive”) the WAL log files to another server using, for example, rsync or scp. This is what the WAL Archiving section in the manual is all about. To me, this feels somewhat fiddly, like a duct tape solution.

The original database server is called the master or primary. The one that runs the copy is called the standby or secondary. If you use streaming replication, the standby server uses the streaming replication protocol inside Postgres instead of manual file copying, which makes things simpler. However, with this mechanism you may miss WAL segments if the primary starts deleting the corresponding files.

To prevent this deletion of logs, Postgres offers replication slots. A replication slot is a record in the database with a name and a “last seen WAL segment” (plus more, but none of that matters to us). Once all of the replication slots have seen a WAL segment, Postgres can delete it.

How to set it up

Once you know what to do, it is straightforward to set up replication. Let’s start by creating a read-only user on the primary (but please, pick a good password!):

primary$ sudo -u postgres psql -c "CREATE USER readonly WITH password 'test123'"

Now, we can create a replication cluster on the secondary. For streaming replication, the versions of PostgreSQL must match between primary and secondary. The examples assume Postgres 11; change the number if you are on a different version:

secondary$ sudo -u postgres pg_createcluster 11 replica

.... output ....

Success. You can now start the database server using:

    /usr/lib/postgresql/11/bin/pg_ctl -D /var/lib/postgresql/11/replica -l logfile start

Warning: systemd does not know about the new cluster yet. Operations like "service postgresql start" will not handle it. To fix, run:
  sudo systemctl daemon-reload
Ver Cluster Port Status Owner    Data directory                 Log file
11  replica 5434 down   postgres /var/lib/postgresql/11/replica /var/log/postgresql/postgresql-11-replica.log

secondary$ sudo systemctl daemon-reload

Now, we’re going to do something somewhat strange; we’ll delete the database files in this cluster (which is currently not running):

secondary$ rm -rf /var/lib/postgresql/11/replica

Use whatever directory the pg_createcluster command printed if you don’t have it in /var/lib/postgresql.

After deleting the files, we start by mirroring the existing database from the primary using pg_basebackup:

secondary$ sudo -u postgres pg_basebackup -D /var/lib/postgresql/11/replica -U readonly -h primary -PRCS my_replication_slot
 2211288/13584399 kB (16%), 0/1 tablespace

The -C -S my_replication_slot options tell it to create and use the replication slot called my_replication_slot. This means that no matter how long it takes us to get the secondary server up and running, the primary will hold on to WAL segments for us in the mean time.

Also, be sure to supply the correct connection settings: -h primary indicates the hostname, you probably need to change at least that part. Now wait until the counter reaches 100%.

After this, we can check out the configuration file which pg_basebackup created (this is what the -R option does):

secondary$ sudo cat /var/lib/postgresql/11/replica/recovery.conf
primary_conninfo = 'user=readonly password=''test123'' host=primary port=5432 sslmode=prefer sslcompression=0 krbsrvname=postgres target_session_attrs=any'
primary_slot_name = 'my_replication_slot'

Looks good? Then, let’s start the secondary replication cluster!

secondary$ sudo systemctl start postgresql@11-replica.service

Operational notes

The simplest way to check if the secondary is correctly processing WAL events from the primary is to check that changes you made after setting everything up are reflected in the secondary (ie, checking MAX(id) from a table or something).

You can also the built-in pg_replication_slots view in Postgres to check the status of a replication slot:

primary$ psql -c "SELECT slot_name, slot_type, active, restart_lsn FROM pg_replication_slots;"
      slot_name      | slot_type | active | restart_lsn
 my_replication_slot | physical  | t      | 10D/DA000000
(1 row)

The restart_lsn column shows the oldest Log Sequence Number which the secondary has seen. To verify that the secondary is currently connected, check that active is true. To check that it is processing the WAL entries, check that restart_lsn is changing.

To verify the LSN values, you can check the primary’s current log location like so:

primary$ sudo -u postgres psql -c "select pg_current_wal_lsn()"
(1 row)

To get the latest LSN the secondary received (should always be the same as restart_lsn):

secondary$ sudo -u postgres psql -c "SELECT pg_last_wal_receive_lsn()"
(1 row)

To check the replication lag (in bytes) between the primary and secondary, use pg_wal_lsn_diff:

primary$ sudo -u postgres psql -c "select pg_size_pretty(pg_wal_lsn_diff('10D/E697D450', '10D/E68501F0'))"
 1205 kB
(1 row)

On the primary, you can also check the size of the /var/lib/postgresql/11/main/pg_wal directory. This is where WAL segments are stored until they can be deleted.

If something does not work, always check the standard log file. It will contain error messages about missing WAL log files or when it can’t connect to the primary.

Remember (and this is important), when you take a secondary out of commission, also delete its replication slot. This slot blocks deletion of WAL logs, so eventually your disk will fill up! To delete it:

primary$ sudo -u postgres psql -c "SELECT pg_drop_replication_slot('my_replication_slot')"

That’s it. Enjoy your new replication server!