Replication Slot Does Not Exist

| 29-05-2021
[RAND-1-100] Comments

As a complement to the accepted answer, I'd like to mention that following command will not fail in case the slot does not exist (this was useful for me because I scripted that). Select pgdropreplicationslot(slotname) from pgreplicationslots where slotname = 'bottledwater'. If this fails, either because the remote host is not reachable or because the remote replication slot cannot be dropped or does not exist or never existed, the DROP SUBSCRIPTION command will fail. To proceed in this situation, disassociate the subscription from the replication slot by executing ALTER SUBSCRIPTION. SET (slotname = NONE).

The first beta release of PostgreSQL 10 was announced a couple of days ago.This release brings with it the much-awaited logical replication feature.

We decided to take it for a spin and see how it works.

Streaming Replication

The existing replication feature built into PostgreSQL is called StreamingReplication. Slot tool present sensor lacking. If you aren’t familiar with it, check outour popular blog post about it.

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Essentially, you can use streaming replication to maintain multiple, up-to-date,read-only replicas of a single PostgreSQL server (technically, a singlePostgreSQL cluster). This contains databases, roles (users),tablespaces and more.

With streaming replication, you get all or nothing. The replicas (standbys) arebyte-for-byte copies of the main server’s set of on-disk files. Streamingreplication is perfect for maintaining a hot standby server, to which you canfailover in case the primary server fails. They are also good for maintainingread-only replicas for analytics-related use cases.

What streaming replication cannot do is to replicate a subset of the data in themain server. If you want to have an off-server copy of, say a single table, itwon’t be possible with streaming replication.

You also can’t modify the data in the replica with streaming replication. Let’ssay you use the replica for analytics, and need to delete the data once yourdaily batch jobs are done processing it. The deletion won’t work because thereplica is strictly read-only with streaming replication.

Streaming replication also does not work across PostgreSQL versions. You can’tupgrade PostgreSQL versions without downtime by trying to bring up a standbywith the next PostgreSQL version then failing over to it.

Typically, you’d use one of the tools listed hereto implement such requirements. But now with logical replication, it’s possibleto do most of these without external tools. At the very least, it is a powerful toolon which to further build customized solutions.

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Logical Replication

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Logical Replication can replay, logically, the changes (as in insert, update,delete of rows) happening to one or more persistent tables in a database.

The source server must create a named object called a Publication. Apublication serves as an endpoint from which a log of changes can be fetched bya Subscription. The subscription is created on another, a destination, server.The subscription includes a standard connection string that tells how to connectto the source server.

Let’s see how it works. First, we have a server that has a database dbsrc,which has a table t1:

We’ll also need a user with replication privileges:

Unlike streaming replication, the replication user needs read access to thetables themselves.

Let’s insert some rows into t1 before we start replication:

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OK, here comes the first logical replication related command. We’ll create apublication pub1 that will serve as an endpoint for replicating changeshappening to the table t1:

Now let’s connect to another server, with another database called dbdst.Unlike streaming replication, we need to create the table t1 here first. If wetry to create a subscription without that:

it doesn’t work. So let’s create the table:

Ha! We snuck another column in there. Let’s create the subscription and see whathappens:

It’s replicated! We can see the initial set of rows in the destination table.Further changes get replicated as well:

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Here they are on the destination:

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Things work as expected. Let’s try some modifications on the destination and seewhat happens:

Let’s also do some modifications on the source:

And here’s what we get at the end of it all:

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Interesting! We see that:

  • the row we deleted at the destination stays deleted
  • the row we updated only at the destination stays the same
  • the row we updated first at the destination and then at the source got updatedentirely – the entire tuple has been replaced, and our changes to “val2”column have been lost
  • the row we inserted at the source was replicated

Let’s try something else now. What happens if we delete the table t1 at thedestination and then recreate it?

Oops, we broke the magic! To get the sync back on, we need to refresh thesubscription:

Ah, we’re back in business! Refreshing is needed in other cases too, like if thepublication was altered to include another table.

Going From Here

There are a few more things that logical replication can do that we didn’t coverhere:

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  • publishing only insertions, updates or deletions to tables
  • pulling in changes from multiple sources into one destination
  • adding more tables into an existing publication
  • transactional consistency – destinations see only complete and committedtransactions
  • streaming replication – streaming and logical replication features workindependently without interfering with each other

If you’re planning to get your hands dirty trying out logical replication foryourself, here are somethings that’ll help:

  • In the source server’s postgresql.conf, set wal_level = logical,max_replication_slots = 10.
  • Also ensure pg_hba.conf is updated for the replication user to connect.
  • In the destinaton server’s postgresql.conf, set max_replication_slots =10, max_logical_replication_workers = 4 and max_sync_workers_per_subscription = 2.
  • We had to compile the binaries from the source tarball available here.

And here are links to relevant documentation:

Replication

Let us know how it goes!

Monitoring PostgreSQL With OpsDash

With our own product,OpsDash, you can quickly start monitoring yourPostgreSQL servers, and get instant insight into key performance and healthmetrics including replication stats.

Here’s a default, pre-configured dashboard for PostgreSQL.

OpsDash strives to save you the tedious work of setting up a useful dashboard.The metrics you see here were carefully chosen to represent the most relevanthealth and performance indicators for a typical PostgreSQL instance.

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OpsDash understands the streaming replicationfeature of PostgreSQL and displays per-slave replication status on the master:

Replication Slot Does Not Existing

The replication lag (as a length of time) is also visible on a slave’s dashboard.In the graph below (part of the dashboard of a slave), we can see that the slavecould not catch up to the master’s changes for a while.

Error Relation Pg_replication_slots Does Not Exist

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