Re: Justifying a PG over MySQL approach to a project

One concern I have about these trigger based replication systems is that I fear it may ping the slave for each and every DML statement separately in time and in a transaction. My slave will literally be 1400 miles away and all replication communications will be over the net. If I have a transaction which has 1000 DML statements in it, is this thing going to update the slave 1000 times separately over the net ? (I may not live long enough to see it finish) Or will it be smart enough to wait until I "commit" then send over a single bundle of 1000 DML? The time diff will be more than significant.

Thanks for all the great input on this!

-----Original Message-----
From: pgsql-general-owner(at)postgresql(dot)org [mailto:pgsql-general-owner(at)postgresql(dot)org] On Behalf Of Greg Sabino Mullane
Sent: Thursday, December 17, 2009 11:58 AM
To: pgsql-general(at)postgresql(dot)org
Subject: Re: [GENERAL] Justifying a PG over MySQL approach to a project

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> How difficult is it to switch the master's hat from one DB instance
> to another? Let's say the master in a master-slave scenario goes
> down but the slave is fine. Can I designate the slave as being the
> new master, use it for read/write, and then just call the broken
> master the new slave once it comes back to life (something like that)?

Sure. Bucardo slaves are not changed at all, so they are already
read/write and don't need anything special done to "unslave" them.

One possible way to handle the scenario is:

Assuming three servers:
* A (master) sends changes to B, receives read/write queries
* B (slave) has transaction_read_only set to true, receives read queries
* C has the Bucardo database and daemon

Box A goes down suddenly.

* Stop Bucardo on box C
* Flip the boxes around in the bucardo.db table
* Do a 'bucardo_ctl validate sync all'
(This will create the needed triggers on B)
* Set B's transaction_read_only to false
* Point your apps at B instead of A for read/write queries

When A comes back up:

* DROP SCHEMA bucardo CASCADE; (drops all triggers)
* Set transaction_read_only to true
* Start Bucardo on C
* Once caught up, point read-only queries to A

If you are in a rush, you point things to B immediately after A fails,
but you'll have to recopy the entire table data to the slave, as the
triggers won't be in place yet.

- --
Greg Sabino Mullane greg(at)turnstep(dot)com
End Point Corporation http://www.endpoint.com/
PGP Key: 0x14964AC8 200912171153
http://biglumber.com/x/web?pk=2529DF6AB8F79407E94445B4BC9B906714964AC8
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