Thank you, Liu, for your suggestion.
I might be missing something (I am new to postgres), but it seems that your suggestion will not help much in my case. Since the number of db requests will grow with time (more users will come into the system), the current server will become incapable of serving all the requests quickly enough.
The idea is to increase overall calculation capacity of the system by adding more servers to it. Throwing more hardware to a single main server is not an option in my case. Creating multiple replicas (slaves) is also not an good option – it would be way more efficient to have a group of db servers, each serving only some subset of users and hosting data for those users only. Buying new servers in advance is not an option too.
What I am looking for is switching some of the users to another db server when the capacity of the existing server(s) is not enough. The point is to do it without interrupting the users’ work (so they do not see that horrible “Sorry! This site is under maintenance…”).
Londiste is very good at replicating a database. It's normally used for real-time backup or to load-balance read-only applications, but it can also be used for the situation you describe. At some point when you decide it's necessary to split your database, you would create a new database on a second server, create an empty duplicate schema (using pg_dump's schema-only feature) then install Londiste. Londiste would replicate your database in real time up to the point where you were ready to make the actual switch.
To switch, you'd simply stop Londiste and simultaneously reconfigure your load-balancing system so that a subset of your users were directed to the new database. Then you could uninstall Londiste, and clean out each database by removing the user data that is for users on the other server.
You might also consider partitioning. If you know in advance that you're going to be discarding large subsets of the data, it will be much more efficient if you partition it at the outset. When you split your system in two, the cleanup process will be nothing more than dropping partitions. You won't be plagued by bloated indexes and files.
Craig
If I missed something it would be very kind of you to point this out.
Thank you once again,
-igorS
From: Haifeng Liu [mailto:liuhaifeng@xxxxxxxx]
Sent: May-29-12 9:13 PM
To: Igor Shmain
Cc: pgsql-admin@xxxxxxxxxxxxxx
Subject: Re: Data split -- Creating a copy of database without outage
Why not use a partitioned table? You can write a trigger to control which partition the coming data should be inserted.
On May 30, 2012, at 4:13 AM, Igor Shmain wrote:
Can you please help with advice?
I need to design a solution for a database which will grow and will require a horizontal split at some moment.
Here is how I am planning to do it: Every record in every table has a shard number. After the database becomes too large (gets too many requests), the tables need to be horizontally split. It means that for every table all records with some specific shard numbers need to be moved to a new database.
My plan is to (1) create a copy of the database on a new server (backup/restore?), (2) synchronize the databases (how?), and then (3) forward all requests for the moved shards to the new database. (I will get rid of the unneeded shards later). An important requirement: there should be no outage for the users.
What are the ways to achieve it?
Thank you in advance,
-igorS
