NOTE: This
article taken from Code Project which is published by Abhijit Jana, 2 Oct 2010 (Thanks Abhijit for your
wonderful article.)
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.
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
·
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
·
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.