Primary & secondary server with same db?

[Anon]

hi,

I am implementing a campus intranet project with RH Linux 7.1 and Apache, PHP and MySQL.

Now, all nodes (currently around 35) will be making requests to the primary linux server. I want to keep a secondary backup server also connected to the network such that when the primary fails for any reason, the secondary should take in the traffic automatically.

1. can this be done with software or is it only possible with specialised hardware?
2. can someone please guide me to any documentation on the above setup on the net?

thanks for any info
steve

[[deleted]]

Google works wonders when you are looking for information.

What exactly are you planning to do, create a complete mirror of the entire server (www+db) and make that failsafe or just the database server?

This is not really PHP related, but this is the basic idea:

Create a primary and backup server, and use some sync-tool like rsync to replicate the data files from the primary server to the backup server.

The primary and backup server send eachother small messages and expect to get a reply, a 'heartbeat' if the primary server fails to reply to the backup-servers requests or fails to send the heartbeat to the backup, the backup server assumes the primary server has died and will take over by configuring itself as primary server.

Drawback: if the primary can't send the heartbeat because of a software problem, the backup will also become primary, and doodie starts to fly.

You can take it one step further and use the heartbeat to notify the backup server that the primary server is back only, and make the backup server go to it's "backup" state again automatically.

[Anon]

thats was a great help to me. what i was planning to do was create a mirror(www+db)such that the secondary will take over automatically. can you please inform me whether this "synching" is going to choke the server performance?

also it would be reeaallly helpful if you could in brief explain to me how the stuff of making the secondary "take over" is to be done. my networking knowledge is somewhere near zero.

thanks for any info,
steve

as for google, i am still lost as to what i should search for ie what keywords i shuold use?

[[deleted]]

"can you please inform me whether this "synching" is going to choke the server performance?"

that depends entirely on how much data changes over a certain time, and how often you want to sync.

an Rsync process simply copies files from one machine to another over the network, so if you have a 100mbit link between the machines, you can send around 7mbyte/second, more than enough.

As for the database, most databases have their own replication scheme, you'll have to look into that yourself.

The simplest version of 'taking over' is to make the backup serve assume the identity of the primary server when the primary server goes down. Just make it so that the IP of the backup server changes to that of the primary server.
Problem: if the primary doesn't go all the way down, ie: it's IP service remains active, then you'd have two machines at the same IP, which is bad.
In those cases you'll need some extra hardware (can be a third server) that acts as a gateway. Then the outsideworld always talks to the gateway, and the gateway reroutes the requests to the primary server. If the primary server goes down, the gateway notices (through hearbeats) and redirects all requests to the backup server instead.
That way both the primary and backup can be 'alive' all the time, and never have to assume eachothers identity.
New SPOF: the gateway, but that is such a simple device that it's very unlikely to crash (or at least, much less likely than the busy webservers behind it)

On google, saerch for words like redundancy, backup, loadbalancing (which is basically sort of the same thing) heartbeat, or even just the question you asked here, google is clever like that :-)