Re: PostgreSQL HA config recommendations - Mailing list pgsql-general
| From | William Dunn |
|---|---|
| Subject | Re: PostgreSQL HA config recommendations |
| Date | |
| Msg-id | CAEva=V=JBapOuT32ex5mWbphNRty_Eq6pj5eMuO8TZJr3HF6ig@mail.gmail.com Whole thread Raw |
| In response to | Re: PostgreSQL HA config recommendations (Fabio Ugo Venchiarutti <fabio@vuole.me>) |
| Responses |
Re: PostgreSQL HA config recommendations
|
| List | pgsql-general |
Alex,
Note that you should be weary of suggestions to make your replication synchronous. Synchronous replication is rarely used for this kind of use case (Cisco Jabber) where the most complete durability of the standby is not of the utmost concern (as it would be in a banking application). Not only will it decrease performance, but since you expect to have only one local standby it could actually decrease your availability because if your standby went down no transactions would be able to commit on the master. See the Synchronous Replication section of the docs for more details (http://www.postgresql.org/docs/devel/static/warm-standby.html).
Also note that the suggestion provided by Fabio that you should not have your application commit more than one transaction per user operation is only applicable in synchronous replication (though since this is for a Cisco Jabber, where you neither have control over nor much information regarding the number of commits sent by the transaction per user operation, that suggestion is not applicable anyway...). In the case of asynchronous master-slave replication the typical issue with streaming replication latency is that you have transactions going to the master and then the application sends a read only transaction to the slave before the slave receives the transaction. So long as you don't have the application consider the user operation completed before all the transactions are committed I don't think having multiple transactions would make your replication latency issue any less.
For example, if you had a calendar application where a user enters event details and creates an event for the calendar. The application may be set up to execute 2 transactions, 1) Add the event and details to the calendar events table and 2) once the event creation transaction returns add the current user as an attendee for that event. In this case both transactions would be going against the master, so how far the slave is behind wouldn't be a factor. Of course it would be faster overall to send the inserts as a single database procedure, but that all goes against the master database so the streaming replication is not a factor in that consideration.
William J. Dunn
William J. Dunn
On Thu, Apr 30, 2015 at 9:02 AM, Fabio Ugo Venchiarutti <fabio@vuole.me> wrote:
> WAN delays can cause problems for any replication system; you just have
> to be aware of that and not push things too hard (or try and violate the
> laws of physics). For example, streaming replication set to be
> synchronous crossing the planet is something you'd probably be rather
> unhappy with. :)
In my experience streaming replication fits most use cases due to inherent its simplicity and robustness, but you might need to adjust your software design to get the best out of it.
More specifically, latency issues can be heavily mitigated by having application software commit no more than one transaction per user operation, provided 1 x "master<->sync_slave round trip time" is acceptable delay when they submit forms or the like.
It can get much worse if the application server is on a different geographical node than the DB master. In such case it is realistically beneficial to batch multiple write operations in a single STATEMENT instead.
If the replication synchronous slave is on yet another node, the best case (single statement) scenario would be 2 x round trip time. This configuration is more common than you might think as some setups feature remote app servers reading off synchronous slaves at their own physical location but committing against a master that is somewhere else.
Cheers
On 30/04/15 11:06, Jim Nasby wrote:On 4/29/15 1:13 PM, Alex Gregory wrote:I was thinking that I could use Slony but then I read that it does not
like WAN replication. I have also read about streaming replication
native to Postgres but was not sure how that would work over the WAN.
Bucardo seems better for Data Warehousing or multimaster situations
which this is not. That leaves pgpool ii which seems like it would
add an extra layer of complexity.
WAN delays can cause problems for any replication system; you just have
to be aware of that and not push things too hard (or try and violate the
laws of physics). For example, streaming replication set to be
synchronous crossing the planet is something you'd probably be rather
unhappy with. :)
I haven't played with Slony in forever, but when I did it loved to lock
things. That would not play well with high latency.
I have run londiste between sites within the same city, and that worked
well.
Bucardo and pg_pool are both based on the idea of replaying SQL
statements instead of replicating actual data. They have their uses, but
I personally distrust that idea, especially for DR.When it comes down to to there are so many choices I am not sure if I
need one or a combination of two. Any help you could provide could
be greatly appreciated.
If you want to replicate within a data center then streaming replication
is pretty nice, and as a bonus you might be able to do synchronous as
well. The downside to streaming rep is that it's binary, so if you ever
suffer data corruption you're practically guaranteed that corruption
will end up on the replica. Logical replication like londiste or Slony
are much more robust against that. You also can't use temporary tables
with streaming rep, and you have to replicate the details of ALL
activity, including maintenance like VACUUM. In some environments that
might be slower than logical replication.
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