State management in ASP.NET for high availability of data

ASP.NET session is the correct answer for state management. Yes that's it developers have to just use session to maintain state ie.. any data relavant to a user's session. Ways to store session state can mainly be categorized into InProc and OutProc.

InProc

Session state stored as in-memory on the web server.

Websites running in "Inproc" mode is completely dependent on the resources of the server. Limitation for number of users is dependent on the following factors
- Memory of server
- Amount of data stored per user session

As the memory used starts to exceed available memory application pool gets recycled and hence data stored is lost.

Single Web Server

Easy to configure and setup.
To enable session state as "Inproc" update web.config as follows:-

Advantages
- Easy access to data as stored in-memory

Disadvantages
- Data loss due exceeded memory utilization

Web Farm

Another method of increasing user capacity is by enabling sticky sessions on a web farm. This can achieved by using a Network Load Balancing (NLB). NLB has a feature known as "The Request Forwarder" and can be used to configure a cluster (web farm).

Sticky sessions use a HTTP cookie, which is returned for each client request that contains server instance GUID (ie.. first server that handled the request) and session ID allocated for the user. Subsequent requests will pass back server instance GUID and session ID that load balancer identifies and forwards to respective server. For mode detail explanation of this works look at technet article avalaible at http://technet.microsoft.com/en-us/library/bb734910.aspx.

Advantages
- Increased user capacity
- Easy access to data as stored in-memory

Dis-Advantages
- Partial load balance as user's are tied to specific server that served the request
- Still has possibility of data loss due exceeded memory utilization

OutProc

Session state is stored on a state server. Also this can be located on the same machine as web server or can point to another machine on your network. This mode is reliable as restarting web application does not affect state server and hence no data loss.

This mode can be configured either for a single server instance or for multiple servers on a web farm. In case of multiple web servers in a web farm,
- objects stored in session needs to be serializable
- should be installed on the same path in each of the web server in the Web farm.

1)StateServer

Session state is stored on an ASP.NET state service. Session data stored on a remote machine, no data loss due to application pool recycling. But still has the overhead of data loss if machine that has been configured as state management server is restarted.

Advantages
- No data loss due to application pool recycle

Dis-Advantages
- Is still limited based on the memory
- Reduces performance as data is read from a remote machine

2)SQLServer

session state is stored in an SQL Server database. Session data stored in a database because of which data is never lost. Machine restart will still not loose data as data is stored in a database.

Advantages
- No data loss due to application pool recycle - Data stored permanently in a database which has high reliability

Dis-Advantages
- Reduces performance as data read is from a database - Limited to memory allocated to database

3)Custom

Session state is stored on a custom provider. There are various options available for custom provider are AppFabric, Memcached etc.

AppFabric
- distributed in-memory application cache, refer to AppFabric for windows server for more details.
Memcached
- high-performance, distributed memory object caching system, refer to Memcached for more details.
SharedCache
- distributed and replicated memory object caching system, refer to SharedCache for more details.
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