[users@httpd] Web Server Farm Architecture Help

[Sailesh Krishnamurthy <sa...@gmail.com>]

Hi,

We currently have two web servers in an active-passive failover
configuration, servicing some pretty
high profile sites. If we want to move towards a Web server farm
architecture, Where can
I find some information about what would be the advantages of that, the
design consideration etc...

Appreciate the ehlp, did a bit of googlig but initial results were not
exactly what I was looking for.

Rgds
Sailesh

Re: [users@httpd] Web Server Farm Architecture Help

[Graeme Fowler <gr...@graemef.net>]

Hi

On Sun, 2008-02-10 at 11:23 -0500, Sailesh Krishnamurthy wrote:
> Thanks for that input, what I am looking for is more along the lines
> of what would be the benefit of a web server farm, what would be the
> design considerations and so on...

How long have you got?

The design considerations of a "farm" approach (or cluster, or pool, or
whatever terminology you choose to use) are myriad, and depend entirely
on what you're setting out to achieve.

The principle benefit of a "farm" is one of resilience - failure of a
single node does not render your service unavailable. Wedded to that is
scalability - if you know you have a period of high load coming, you put
in more servers. If you start to deliver a resource-hungry app, you put
in more servers. If you want to do maintenance on part of your farm but
not affect your service, you put in more servers; if you want to deliver
a completely new service, guess what? You see where this is going :)

And so on, and so forth.

In the simplest sense, you start out with a "farm" of one machine
running a simple webserver with no complex applications. If the server
or machine (or network) fails (for whatever reason), the sites hosted by
the server are inaccessible.

Then you move to two servers - one serves all requests in normal
operation, and when it fails you manually configure the other machine to
take over. That gives a little bit more continuity and makes your system
a bit more robust but requires human intervention to, for example, keep
all the files up-to-date on the spare machine.

Time passes, friend...

Your company ends up with many (tens, hundreds, thousands, tens or
hundreds of thousands, millions even) of customers who have many and
varied requirements - Apache, CGI, PHP, Java servlets, .NET
environments, IIS, Websphere, NGINX, lighttpd, MySQL, PostgreSQL, MS SQL
Server, hell even IBM's DB2 or Oracle - suddenly you're several orders
of magnitude away from your initial "farm" idea.

You need a load balancer (software or hardware). In order to ensure you
don't lose service when that needs patching (or breaks), you need two.
They have to failover transparently between themselves, with state
sharing so sessions don't get broken on failover. In order that a
network outage doesn't cause a service delivery failure, you need
another two on a physically different network. So a local power outage
doesn't cause a failure, you need that network to be in a different
location.

The load balancers need to be application aware, or have an additional
system which monitors the other systems which make up the "farm" and
stop sending requests to them if they fail. They also need to notify a
human being if they do fail (although the threshold for this is
dependent on your available resources). To make sure you don't stop
doing that, you need more than one machine doing the same task; with
multiple locations you end with multiple machines.

On top of that, because all your customers' data needs to be accessible
from all the servers in the farm in all those different locations, you
need replicated storage which does real-time, block-level replication
with minimal latency. You therefore need a decent inter-location
infrastructure, multiple paths between the two (three, four, five)
locations; you need a way to validate the data integrity at each
location so you don't end up serving stale data.

I could go on, but I won't. The possibilities are quite literally
endless and depend entirely on what you need to achieve.

In your case - active/passive - assuming they're Linux boxes, you have
load balancing built in to the kernel already. Google for IPVS or LVS,
keepalived, heartbeat, ldirectord; look at the load balancing module(s)
available for Apache. You can move quite quickly to active/active from
your current position without too much pain; much of the pain will come
from your backend storage.

Graeme


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Re: [users@httpd] Web Server Farm Architecture Help

[Sailesh Krishnamurthy <sa...@gmail.com>]

Hi Hans,

Thanks for that input, what I am looking for is more along the lines of what
would be the benefit of a web server farm, what would be the design
considerations and so on...

Thanks



On Feb 8, 2008 11:10 PM, Hans <ha...@ezpear.com> wrote:

> Sailesh Krishnamurthy wrote:
> > Hi,
> >
> > We currently have two web servers in an active-passive failover
> > configuration, servicing some pretty
> > high profile sites. If we want to move towards a Web server farm
> > architecture, Where can
> > I find some information about what would be the advantages of that,
> > the design consideration etc...
> >
> > Appreciate the ehlp, did a bit of googlig but initial results were not
> > exactly what I was looking for.
> >
> > Rgds
> > Sailesh
> Hi!
> My company is using Load Balancing (pfsense) web servers (apache,php)
> with  remote  mysql server (2 servers master/master)  and nfs (two
> machines drbd/lvm).
>
> Regards,
> Hans
>
> ---------------------------------------------------------------------
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>

Re: [users@httpd] Web Server Farm Architecture Help

[Hans <ha...@ezpear.com>]

Sailesh Krishnamurthy wrote:
> Hi,
>  
> We currently have two web servers in an active-passive failover 
> configuration, servicing some pretty
> high profile sites. If we want to move towards a Web server farm 
> architecture, Where can
> I find some information about what would be the advantages of that, 
> the design consideration etc...
>  
> Appreciate the ehlp, did a bit of googlig but initial results were not 
> exactly what I was looking for.
>  
> Rgds
> Sailesh
Hi!
My company is using Load Balancing (pfsense) web servers (apache,php) 
with  remote  mysql server (2 servers master/master)  and nfs (two 
machines drbd/lvm).

Regards,
Hans

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