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My takeaways from this are: 1] MySQL supports more replication options, whereas PostgreSQL focuses on the ones that 95% of users will care about and does not i

by dstorrs 16y ago

My takeaways from this are: 1] MySQL supports more replication options, whereas PostgreSQL focuses on the ones that 95% of users will care about and does not implement the rest. 2] MySQL's replication has several failure modes that can lead to data loss or non-synchronization. For example, it is statement based by default, so any statement which includes a call to NOW() will return different results on master and slave. PostgreSQL also has failure modes, but the defaults are designed to minimize them -- e.g. it always uses WAL and data file rewrites to ensure that data is properly synced and consistent. 3] Due to points 1 & 2, if you aren't an expert and don't need the extra features offered by MySQL, you are probably better off with PostgreSQL. 4] 3rd party tools exist for both servers which close many of the feature gaps between them. Could someone who is more knowledgeable than me tell me if this is a fair reading?

wiredfool 16y ago
Pretty reasonable, from someone who has been following postgres for 10 years. I'm not sure I'd say that postgres covers 95% of use cases, as it really only covers single master. That is probably the most common case, but I'd have a hard time putting numbers on it. Extra features cut both ways, some apps are better with postgres, some with MySql. Postgres was 'late' to the replication party, and only in this last release have they had hot slave machines. (with core functionality, 3rd party has a range of alternatives). Currently, it is single master, multi slave replication. Previously, it was been a warm spare arrangement, up to 1 WAL log behind, for durability, and not for load balancing. There were ways of making it less laggy, but it was not trivial to restore a log, bring the db up, and then be in a position to restore another log.
bmurphy 16y ago
Close. Postgres has traditionally focused on letting the community provide replication solutions, but we've been screaming so long for one that "just works" that they're finally getting around to building it. I wouldn't say it covers 95% of the use cases. In fact, I find it very limiting, but if all you need is to make sure your database is backed up it's a great step in the right direction. That being said, the topology limitations suck. If you lose your master server, you are running in a degraded environment until you can rebuild ALL slaves from scratch. This really sucks if you have a very large database that takes a long time to rebuild. Postgres 9.0 replication is a great first step, but it's only a step. There's still a lot of work to be done.
- wiredfool 16y ago
  If you lose your master server, you are running in a degraded environment until you can rebuild ALL slaves from scratch. This really sucks if you have a very large database that takes a long time to rebuild. I just tested this, and it's not quite true. Assume you have 1 master, and 2 slaves(1=new master/2=additional slave) each replicating directly from the master. If they're in sync, then if you: * shutdown the master * shutdown slave1/new master * remove slave1's recovery.conf file * assign slave1 old master's ip address * start slave1 as new master then slave2 will follow and you'll still have a replicated set. If, instead of doing the shutdown/remove recovery.conf/startup dance, you touch the trigger file, a new timeline is created and the slave2 isn't going to follow that timeline. I haven't found a way to make a slave follow to a new timeline yet.
  - bmurphy 16y ago
    Yes, that's exactly what I was referring to, you're getting a new timeline. You need to rebase the slaves and you have no real-time backup for a few hours. Sucks. You have to be very careful, and there are all sorts of ways you can break it. It's not something I'd want to rely on right now when under fire.
    - wiredfool 16y ago
      Looks like there's at least one person who's looking at doing something about the timeline issue: http://blog.tapoueh.org/articles/blog/_Back_from_PgCon2010.html http://blog.tapoueh.org/articles/blog/_Back_from_PgCon2010.h... . I'd bet that there are some pretty basic failure modes in the timeline preserving case where the slaves could be out of sync when one was promoted, and then that out of syncness could be propagated. I suspect that a missing transaction on the new master would trigger some sort of duplicate transaction oid issue, but a missing transaction on the slave would be more likely to go unnoticed. I've just tested a case where I killed the master when I was running transactions against it, and it seems to work as well as the shutdown case. Bringing up the slave is where the timeline gets incremented. I suppose I could throw one of the replication connections through a ip delay and intentionally introduce replication lag.. edit: Looks like http://www.postgresql.org/docs/9.0/static/warm-standby.html#STREAMING-REPLICATION http://www.postgresql.org/docs/9.0/static/warm-standby.html#... , section on monitoring gives the pg_current_xlog_location on the primary and the pg_last_xlog_receive_location on the slaves, which would at least tell one if the slaves are in sync, and if not, which one is the farthest ahead. So, the process could be: * master dies. either gracefully or not. * all slaves are queried * most advanced one gets shutdown, promoted by ip and removing recovery.conf then started * all the other slaves should track those changes and bring themselves up to date. * Resync the old master as a new slave.
spudlyo 16y ago
1. I don't pretend to know what 95% of users will care about. 2. MySQL writes the timestamp for NOW() into the binlog so provided your master and slave are in the same timezone, it's replication safe. Functions like UUID() and RAND() will behave as you describe. 3. I disagree. MySQL's replication is more mature, widely understood, and has more documentation than PostgreSQL's.
- moe 16y ago
  I disagree strongly with "more mature", "widely understood" and "more documentation". 1. Mature? I have seen MySQL replication blow up in so many awful ways, it's not funny. And yes, I've seen it happen on 5.* deployments. The worst part is that MySQL hardly ever detects a problem, and you hardly ever know why it went wrong - it just silently corrupts. The only way to detect the corruption is through regular checksum runs which can be costly on large databases (there's a reason why the checksum-utility from maatkit has fairly sophisticated options for incremental checksumming). 2. Widely understood? I challenge you to prove your understanding and explain only a small subset of the functionality: Please enumerate all failure modes that can lead to corruption in statement based replication, and how to avoid them. 3. More documentation? That has to be a joke. The MySQL documentation is a mess. It is poorly structured, poorly written and full of conflicting and misleading bits. Please show me the MySQL document about replication that comes remotely close to http://www.postgresql.org/docs/9.0/static/hot-standby.html http://www.postgresql.org/docs/9.0/static/hot-standby.html in terms of clarity and exhaustiveness. Notice the long section about "query conflicts" - where is the MySQL equivalent?
  - spudlyo 16y ago
    1. This is the first release of baked in replication for PostgreSQL. MySQL has had years to evolve their approach and to fix bugs. That's not to say either system is perfect, but their relative maturity seems obvious. 2. No. It's widely understood because MySQL has a much larger mindshare than PostgreSQL. Statement based replication has its faults, but complexity isn't one of them. 3. It's not a joke. There is more documentation. There is the official documentation, but if you don't like that there are plenty of blog posts, books, and recorded talks on the subject.

by dstorrs 16y ago

wiredfool 16y ago

Pretty reasonable, from someone who has been following postgres for 10 years. I'm not sure I'd say that postgres covers 95% of use cases, as it really only covers single master. That is probably the most common case, but I'd have a hard time putting numbers on it. Extra features cut both ways, some apps are better with postgres, some with MySql. Postgres was 'late' to the replication party, and only in this last release have they had hot slave machines. (with core functionality, 3rd party has a range of alternatives). Currently, it is single master, multi slave replication. Previously, it was been a warm spare arrangement, up to 1 WAL log behind, for durability, and not for load balancing. There were ways of making it less laggy, but it was not trivial to restore a log, bring the db up, and then be in a position to restore another log.

bmurphy 16y ago

Close. Postgres has traditionally focused on letting the community provide replication solutions, but we've been screaming so long for one that "just works" that they're finally getting around to building it. I wouldn't say it covers 95% of the use cases. In fact, I find it very limiting, but if all you need is to make sure your database is backed up it's a great step in the right direction. That being said, the topology limitations suck. If you lose your master server, you are running in a degraded environment until you can rebuild ALL slaves from scratch. This really sucks if you have a very large database that takes a long time to rebuild. Postgres 9.0 replication is a great first step, but it's only a step. There's still a lot of work to be done.

wiredfool 16y ago
If you lose your master server, you are running in a degraded environment until you can rebuild ALL slaves from scratch. This really sucks if you have a very large database that takes a long time to rebuild. I just tested this, and it's not quite true. Assume you have 1 master, and 2 slaves(1=new master/2=additional slave) each replicating directly from the master. If they're in sync, then if you: * shutdown the master * shutdown slave1/new master * remove slave1's recovery.conf file * assign slave1 old master's ip address * start slave1 as new master then slave2 will follow and you'll still have a replicated set. If, instead of doing the shutdown/remove recovery.conf/startup dance, you touch the trigger file, a new timeline is created and the slave2 isn't going to follow that timeline. I haven't found a way to make a slave follow to a new timeline yet.
- bmurphy 16y ago
  Yes, that's exactly what I was referring to, you're getting a new timeline. You need to rebase the slaves and you have no real-time backup for a few hours. Sucks. You have to be very careful, and there are all sorts of ways you can break it. It's not something I'd want to rely on right now when under fire.
  - wiredfool 16y ago
    Looks like there's at least one person who's looking at doing something about the timeline issue: http://blog.tapoueh.org/articles/blog/_Back_from_PgCon2010.html http://blog.tapoueh.org/articles/blog/_Back_from_PgCon2010.h... . I'd bet that there are some pretty basic failure modes in the timeline preserving case where the slaves could be out of sync when one was promoted, and then that out of syncness could be propagated. I suspect that a missing transaction on the new master would trigger some sort of duplicate transaction oid issue, but a missing transaction on the slave would be more likely to go unnoticed. I've just tested a case where I killed the master when I was running transactions against it, and it seems to work as well as the shutdown case. Bringing up the slave is where the timeline gets incremented. I suppose I could throw one of the replication connections through a ip delay and intentionally introduce replication lag.. edit: Looks like http://www.postgresql.org/docs/9.0/static/warm-standby.html#STREAMING-REPLICATION http://www.postgresql.org/docs/9.0/static/warm-standby.html#... , section on monitoring gives the pg_current_xlog_location on the primary and the pg_last_xlog_receive_location on the slaves, which would at least tell one if the slaves are in sync, and if not, which one is the farthest ahead. So, the process could be: * master dies. either gracefully or not. * all slaves are queried * most advanced one gets shutdown, promoted by ip and removing recovery.conf then started * all the other slaves should track those changes and bring themselves up to date. * Resync the old master as a new slave.

spudlyo 16y ago

1. I don't pretend to know what 95% of users will care about. 2. MySQL writes the timestamp for NOW() into the binlog so provided your master and slave are in the same timezone, it's replication safe. Functions like UUID() and RAND() will behave as you describe. 3. I disagree. MySQL's replication is more mature, widely understood, and has more documentation than PostgreSQL's.

moe 16y ago
I disagree strongly with "more mature", "widely understood" and "more documentation". 1. Mature? I have seen MySQL replication blow up in so many awful ways, it's not funny. And yes, I've seen it happen on 5.* deployments. The worst part is that MySQL hardly ever detects a problem, and you hardly ever know why it went wrong - it just silently corrupts. The only way to detect the corruption is through regular checksum runs which can be costly on large databases (there's a reason why the checksum-utility from maatkit has fairly sophisticated options for incremental checksumming). 2. Widely understood? I challenge you to prove your understanding and explain only a small subset of the functionality: Please enumerate all failure modes that can lead to corruption in statement based replication, and how to avoid them. 3. More documentation? That has to be a joke. The MySQL documentation is a mess. It is poorly structured, poorly written and full of conflicting and misleading bits. Please show me the MySQL document about replication that comes remotely close to http://www.postgresql.org/docs/9.0/static/hot-standby.html http://www.postgresql.org/docs/9.0/static/hot-standby.html in terms of clarity and exhaustiveness. Notice the long section about "query conflicts" - where is the MySQL equivalent?
- spudlyo 16y ago
  1. This is the first release of baked in replication for PostgreSQL. MySQL has had years to evolve their approach and to fix bugs. That's not to say either system is perfect, but their relative maturity seems obvious. 2. No. It's widely understood because MySQL has a much larger mindshare than PostgreSQL. Statement based replication has its faults, but complexity isn't one of them. 3. It's not a joke. There is more documentation. There is the official documentation, but if you don't like that there are plenty of blog posts, books, and recorded talks on the subject.