pg_rewind in PostgreSQL 9.5

http://hlinnaka.iki.fi/2015/03/23/pg_rewind-in-postgresql-9-5/

by willlll • 11 years ago

243 60 11 years ago

hlinnaka.iki.fi

hobs2• 11 years ago

Nice, definitely something that is a pain for mirroring in SQL Server, though if you have a good runbook its just time consuming and boring.

sqlcook2• 11 years ago

? MSSQL mirroring failover is very easy and painless( arguably one of best compared to other rdbms), just make sure not to do automatic failover as that can cause false failover with spotty network between nodes and witness.

hobs2• 11 years ago

Sorry about that, I was referencing failing back over to the primary after failing over to the secondary. Specifically from the post:

- Cool. And how do I fail back to the old master?

- Umm, well, you have to take a new base backup from the new master, and re-build the node from scratch..

iradik• 11 years ago

how does mysql failover compare in this regard?

morgo• 11 years ago

This looks similar* to replication w/GTIDs + transactional replication (both MySQL 5.6 features).

* MySQL replication works a little different by using binary logs.

barrkel• 11 years ago

MySQL in master/master mode is fairly painless. Set the secondary site to readonly mode, and when failing over, ensure the primary site is definitely down before enabling writes on the secondary site.

When failing back, bring the primary site back up in readonly mode, give it some time to catch up, make secondary site readonly again, verify if you want to be sure (Percona tools help here), and finally make primary site read-write.

The initial setup in MySQL is a bit fiddly, but overall it's been fairly problem-free as far as I've seen. It's not automatic, but the cost of manual intervention is far lower than the headache of split brain and write divergence.

(Disclaimer: I far prefer postgres as a development target, but hassle-free failover and failback is a hard requirement where I work, and our business model includes giving every customer (banks etc.) a completely separate database instance, including multiple VMs on a separate vlan; per-CPU costs don't work out, it's pretty much MySQL or nothing, until postgres makes it just as pain-free.)

keypusher• 11 years ago

One pain point we have run into is the inability to fail over to a previous master that only has access to other WAL logs. That is, we have nodes that can see each other's WAL logs, but not each other's full database. Last master can come back alone, a previous slave can come back alone, but a node that went down as master and came back after having missed transactions apparently cannot.

rdtsc• 11 years ago

ActorDB is an interesting project that operates on distributed SQLite database. It tries to provide clustering between instances and it does it by continuously replicating the WAL between nodes.

I am not affiliated with the project, but just saw it the other day and thought it was a pretty cool pattern:

http://www.actordb.com/

Here is the excerpt from their description page:

---

Actors are replicated using the Raft distributed consensus protocol. The way we have integrated Raft with the database engine is by replicating the WAL (write-ahead log). Every write to the database is an append to WAL. For every append we send that data to the entire cluster to be replicated. Pages are simply inserted to WAL on all nodes. This means the master executes the SQL, but the slaves just append to WAL.

If a server is very stale or is added new, ActorDB will first send the base actor file to the new server, then it will send the WAL pages (if there are any).

We use a combined WAL file for all actors. This means even with potentially thousands of actors doing writes at the same time, the server will not be appending to thousands of files at once. All writes are appends to the same file and that performs very well.

---

Would this work for PG replication as well I wonder?

amitlan• 11 years ago

I hope something like BDR project evolves in a UX-centric (too!) direction.

[0] http://blog.2ndquadrant.com/dynamic-sql-level-configuration-...

[1] https://wiki.postgresql.org/wiki/BDR_User_Guide

moe13• 11 years ago

This looks like a great tool, but it's also a sour reminder that replication still feels a lot like open heart surgery on postgresql.

Why can't we just type "enslave 10.0.0.2" into psql and have the computer do the hard work? The machinery is "almost there" for a half a decade now. Who do we have to bribe (wink wink, nudge) to bring the UX into a state where crutches like pg_rewind are not needed?

Erwin• 11 years ago

I asked a few people that at the excellent pgnordic conference and got some hand-waving about how repmgr fixes everything: https://github.com/2ndQuadrant/repmgr/blob/master/QUICKSTART...

Having tried that out, I didn't find it particular user friendly compared to the various fancy NoSQL database where it's just something like... database-server --connect the_master:12324 -- and you've got your cluster even with automatic replication of data depending on your sharding rules.

I suppose that ACID-SQL makes it harder to set this up reliably.

Is there one of the commercial things like EnterpriseDB that fixes that? Effortless, reliably clustering with a nice status that says: slave2 is 95% sync'ed with master1 ETA 2 hours.

afarrell2• 11 years ago

enslave "10.0.0.2"

I know it is functionally immaterial, but boy howdy do I ever wish we'd chosen a better convention for how to refer to the relationship between these system components.

scott_karana• 11 years ago

simon says 10.0.0.2

twerquie• 11 years ago

Let's just agree to use better words starting now.

Primary/Replica?

moe13• 11 years ago

I know it is functionally immaterial

There's nothing wrong with Master/slave. We also read man pages, shove male plugs into female ports, fork and kill children, and use dozens of other odd words to describe perfectly harmless things.

sanderjd• 11 years ago

Heroku uses "follower", which is much less jarring.

neilk• 11 years ago

My proposal: muse and scribe. It's evocative and if you know the old terminology the initials are the same.

olefoo• 11 years ago

I try to use 'primary' and 'replica'; but depending on who I'm talking to I find myself slipping back to the Master/Slave terminology.

And to the OP, replication is a complex process fraught with security risks. Take about 10 minutes thinking about just roles and authentication in the context of your proposed syntax and what you would need to set up ahead of time to make it work.

The current method of starting with a pg_basebackup on the replica is nearly as simple. Even if it does require that you do some minor configuration on the master. It's not nearly open heart surgery.

lmm2• 11 years ago

I find it's a useful heuristic for knowing whether someone can think clearly and abstractly. Kind of like the "all green birds have two heads" test.

rosser4• 11 years ago

A thing people consistently seem not to get when bitching about PostgreSQL (streaming) replication is that, by design, it's slave pulls instead of master pushes. There are Very Good Reasons for that.

And it's really not that hard. MVPRC (Minimum Viable PostgreSQL Replication Configuration):

  Master:
    postgresql.conf:
      wal_level = 'hot_standby'
      max_wal_senders = N
    pg_hba.conf:
      host replication all A.B.C.D/E md5
  Slave:
    postgresql.conf:
      hot_standby = 'on'
    recovery.conf
      standby_mode = 'on'
      primary_conninfo = 'host=W port=X user=Y password=Z'

Literally six lines of "directive = value". (Well, five plus the HBA rule.)

Then you use pg_basebackup, and start the slave.

EDIT: Decided to be specific instead of handwavy.

moe13• 11 years ago

Literally six lines

Which is six lines too many for what is inherently a runtime configuration that shouldn't be mentioned in any config file at all.

Then you use pg_basebackup, and start the slave.

I take it you don't care for failover and monitoring then?

Otherwise you would have mentioned either repmgr and pgpool here, or your conglomerate of rather complex homegrown scripts to mimic their functionality.

jeffdavis2• 11 years ago

I'd summarize the replication story of postgres to be: "harder to use but easier to understand".

PostgreSQL generally builds things from the bottom up -- first working really hard to get the fundamental infrastructure right, and then the utility starts falling into place.

This has a few advantages:

* The fundamental infrastructure tends to be more general, so many different functional improvements come out quickly after the infrastructure is in place.

* It forces everyone involved to think through the edge cases first, leading to a more robust feature and a better-tested feature.

* When things go wrong, all of the parts make more sense individually, so it's easier to understand what happened and fix it.

* Eventually, the usability issues are addressed in a way that's not dependent on a lot of black magic.

Anecdotally, when things go wrong in $MAGICALLY_REPLICATING_DB, they tend to go very wrong, very fast. I am skeptical by nature, so I generally assume that $MAGICALLY_REPLICATING_DB glosses over a lot of the finer points, which don't get exercised in a normal testing environment. I haven't presented any facts here, so you may disagree.

A lot of people share my skepticism, and are willing to put up with a few rough edges in the usability as long as the pieces make sense and they feel they can understand and trust them. I was at a presentation by Instagram, and replication was one of the primary reasons they went with postgres. They used a tiny engineering team (one and a half engineers working on the database) to scale to tens of millions of active users (also using sharding; another thing that is harder to use but easier to understand in postgres).

nierman• 11 years ago

If you are performing a planned failover then the old master can be turned into a slave without extra tools or steps. Simply shut down the master first. As part of this process it waits until the slave has the necessary wal.

pg_rewind will be great for remastering under other scenarios (unexpected failovers, etc.)

mappu• 11 years ago

Can anyone comment on how this compares to postgres-BDR?

I'm in the market for an asynchronous multi-master RDBMS to cope with a dozen masters and huge (~800ms) latencies - i think my best bets are either BDR or maybe the Cassandra storage engine for MariaDB.

bradhe• 11 years ago

every step forward in modern relational DBs for reliability is a step backwards for operations and the simplicity of the model. If you're steeped in the ecosystem and know how things "used to be" you don't see how insane things actually are. Forrest, trees, etc.

jeffdavis2• 11 years ago

Can you please expand?

dijit• 11 years ago

he's scared of the increasing complexity of individual systems.

personally, I see where he's coming from with things like systemd.

complexity != usability.

simple things work well and work reliably.

however, I like the increasing features surrounding postgresql- and I'm playing devils advocate. (and anyway, it's not like it's part of the core build, the postgresql binary is untouched!)

snuxoll• 11 years ago

It's nice to see a lot of work being put into mirroring replication in the PostgreSQL 9.0 line, but until better admin tools are there I'll probably just keep using corosync and pacemaker with a shared fiber channel volume for clustering. Sure, it's cold standby, but it only takes a couple seconds for a standby node to come up and I just have to keep my WAL backups like normal for recovery.

andyidsinga• 11 years ago

have to say - "pg_rewind" is nice and meta here on HN.

need to convince a postgres contributor to add easter egg "pg_essay".

corford• 11 years ago

I don't get it. Why is it a problem taking a pg_basebackup of the new master to re-seed the old master (which is now a slave) and then promoting it to master again?

Or does pg_rewind offer a way to do this that doesn't require taking the current master offline when you promote a slave?

pg_rewind in PostgreSQL 9.5

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