Overview
Visual Studio has its own integrated ASP.NET engine which is used to run the ASP.NET Web
application from Visual Studio. ASP.NET Development Server is responsible for executing all the
requests and responses from the client. Now after the end of development, when you want to host
the site on some server to allow other people to access, concept of web servers comes in between. A
web server is responsible for providing the response for all the requests that are coming from clients.
The below diagram shows the typical deployment structure of an ASP.NET Web application with a
single IIS.
Clients request for resources and IIS process the request and send back to clients. If you want to
know more details on How IIS Processes the request, please read one of my articles about “How IIS
Process ASP.NET Request?”.
Web Farm
This is the case where you have only one web server and multiple clients requesting for resources
from the same server. But when are is huge amount of incoming traffic for your web sites, one
standalone server is not sufficient to process the request. You may need to use multiple servers to
host the application and divide the traffic among them. This is called “Web Farm”. So when you are
hosting your single web site on multiple web servers over load balancer is called “Web Farm”. The
below diagram shows the overall representation of Web Farms.
In general web farm architecture, a single application is hosted on multiple IIS Server and those are
connected with the VIP (Virtual IP) with Load Balancer. Load Balancer IPs are exposed to external
world to access. So whenever some request will come to server from clients, it will first hit the Load
Balancer, then based on the traffic on each server, LB distributes the request to the corresponding
web server. These web servers may share the same DB server or may be they can use a replicated
server in the back end.
So, in a single statement, when we host a web application over multiple web servers to distribute the
load among them, it is called Web Farm.
Web Garden
Now, let’s have a look at what is Web Garden? Both the terms sound the same, but they are totally
different from each other. Before starting with Web Garden, I hope you have a fundamental idea of
what an Application Pool is and what a Worker Process is. If you have already read the article, “How
IIS Processes ASP.NET Request ?”, then I can expect that you now have a good idea about both of
them.
Just to recall, when we are talking about requesting processing within IIS, Worker Process (w3wp.exe)
takes care of all of these. Worker Process runs the ASP.NET application in IIS. All the ASP.NET
functionality inside IIS runs under the scope of worker process. Worker Process is responsible for
handling all kinds of request, response, session data, cache data. Application Pool is the container of
worker process. Application pool is used to separate sets of IIS worker processes and enables a
better security, reliability, and availability for any web application.
Now, by default, each and every Application pool contains a single worker process. Application which
contains the multiple worker process is called “Web Garden”. Below is the typical diagram for a web
garden application.
In the above diagram, you can see one of the applications containing the multiple worker processes,
which is now a web garden.
So, a Web application hosted on multiple servers and access based on the load on servers is called
Web Farms and when a single application pool contains multiple Worker processes, it is called a
web garden.
Create Web Garden in IIS 6 and IIS 7
Now, I am going to show how you can change the Number of Worker processes in both IIS 6 and IIS
7. For IIS 6, Right Click on Application Pool > Properties > Goto Performance Tab.
In the “Performance Tab” section, you would have one option called “Web Garden” where worker
process sets to “1”, you can set the number of worker processes that you required.
For IIS 7, Right Click on Application Pool > Go To Advance Settings > In Process Model section, you
will have “Maximum Worker Processes”. You can change it more than 1 to make it as a web garden.
In the above image, you can also check the definition of Web Garden.
You can find one of my previous articles on the basics of the same over here.
Advantages of Web Farm and Web Garden
Now, let’s have a look into the advantages of both the Web Farms and Web Gardens.
Advantages of Web Farm
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It provides high availability. If any of the servers in the farm goes down, Load balancer can redirect
the requests to other servers.
Provides high performance response for client requests.
Provides better scalability of the web application and reduces the failure of the application.
Session and other resources can be stored in a centralized location to access by all the servers.
Advantages of Web Garden
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Provides better application availability by sharing requests between multiple worker process.
Web garden uses processor affinity where application can be swapped out based on preference and
tag setting.
Less consumption of physical space for web garden configuration.
How to Manage Session in Web Farm Mode?
While using session, requests are distributed among different servers. By default, session mode is set
to In Proc where session data is stored inside worker process memory. But, in Web farm mode, we
can share the session among all the servers using a single session store location by making it Out
proc (State Server or SQL Server Mode). So, if some of the servers go down and request is transferred
to the other server by the Load balancer, session data should be available for that request.
In the above diagram, you can see that we can both the IIS server sharing the same session data
which is stored in out of worker process. You can read one of my previous articles “Exploring Session
in ASP.NET” where I have explained how you can configure session mode for Out Process mode.
How to Manage Session in Web Garden
Mode?
When we are using Web garden where request is being taken care of by different worker process,
we have to make the session mode as out process session mode as described earlier. For Web
Garden, we have to configure the out process within the same server but for different worker
process.
While using Web garden with your application, you need make a couple of configuration settings in
web.config in <process Model> section where you need to set certain properties like cpuMask,
RequestLimit, webGarden, ClientConnectCheck, etc.
Summary
When we host a web application over multiple web servers to distribute the load among them, it is
called Web Farm and when one application has multiple worker processes, it is called a Web
garden.
In this blog post, I have explained the very basics of what a Web Farm is and what a Web Garden is.
As this blog post contains the basic information to understand the fundamentals of web farms and
web garden concept, I will be posting a separate article with details configuration setting for web
garden and web farm. You can read the following articles for more information:
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Boosting performance using an IIS web garden
How-To Configure Session State in a Windows Web Farm
Role of Web gardens and web farms in ASP.NET