Re: Server Redundancy for WO Apps
Re: Server Redundancy for WO Apps
- Subject: Re: Server Redundancy for WO Apps
- From: Dev WO <email@hidden>
- Date: Thu, 11 Mar 2010 13:21:59 +0100
Hello Gino,
If you'd like to be able to failover either a service or a server, you will have to go through the data synchronization issue.
I think synchronizing things are really hard, like web server resources, databases etc. So the first step would be to get the data storage out of the Xserve.
Here's what we have done, it may not be exactly what you are looking for, but it should give you some clues:
-We invested in a Promise Vtrack and Xsan to get a clustered filesystem, so every xsan clients (which are only xserves) can access the external storage all the time
-We symlinked the database, the webserver folder and the webobjects folder into the Xsan storage, that way, everything is already external to each xserve
-You could even go as far as symlinked your config files for Apache, wotaskd, etc but it means you won't be able to load balance between servers (at least not with "global" config files) which may not be an issue
You would still get downtimes with this setup mainly because of DNS, as it may take some time to update the records to the failover xserve, but the failover xserve could do other services, it will only be a little more loaded if the first one fail.
If you want zero downtime, you could setup something like this, you would need:
-3 xserves
-1 Promise vtrack
-Xsan
Xserve A would be the meta-data controller for Xsan and also an Open Directory replica
Xserve B would be your main server with all the services, but every data and config files would be symlinked onto the Xsan volume (it would be good if it is also a secondary metadata controller)
Xserve C would be setup as a failover server, so it would take the IP of Xserve B in case of failure, and as all the data are on the Xsan volume, it should work smoothly
Xserve C can't do anything, it will only be able to get B services if it fails, and do nothing if everything is working on B.
The biggest psychological issue I add to go through when we setup our Xsan grid was to accept to have an Xserve dedicated to nothing (just providing the clustered metadata to the clients)... and in the setup above, you would even get only one Xserve out of the 3 which is actually doing something.
But you would get a great deal of uptime availability.
Xsan, with the fiber switch and the complete setup is expensive, but it may be worth it for the targeted availability.
Anyway, one of the first step for high availability is getting every data external so another server could access them.
I hope it gives you some pointers,
Xavier
> Hi there..
>
> Maybe this is a little off list as its not a development question....
> But I have a main Xserve running about 10 Apps for clients and there has been various outages caused by DNS, Hard Drive or Power failures occurring infrequently but enough to cause concern.
>
> Does anyone have any advice how to set up a redundant server/service and what are the implications?
> ie Files needed instant transfer to redundant server instantly (or periodically) and how to switch between them in cause of failure.
>
> Any help or being pointed in the right direction appreciated.
>
> Gino
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