Cluster vs. Farm

"Farm was generally used in load balancing scenarios and Clusters are generally used in high available(resilant) solutions.  In the web applications, a Farm is also a cluster because putting a switch infront of more than one machine and the DNS record for the web site is nothing but the IP address of the switch.  The switch depending on how  busy a server is routes the traffic to different machines.

Cluster, is usually a two machine node where one machine is active and other is passive (or active/active in true cluster environment).  But the processing is handled by only one machine and the other one is dormant.  When the active machine goes down, the other node takes over.

Farm is mainly used in web applications (stateless apps) where as clusters are used in database, server (windows service) type applications."

.NET Session State

https://msdn.microsoft.com/en-us/library/ms178586(v=vs.100).aspx
UPDATE: newer version of all this at: https://docs.microsoft.com/en-us/iis/application-frameworks/scenario-build-an-aspnet-website-on-iis/planning-step-2-plan-asp-net-settings

and
https://docs.microsoft.com/en-us/iis/application-frameworks/scenario-build-an-aspnet-website-on-iis/configuring-step-2-configure-asp-net-settings

Session-State Modes

.NET Framework 4

Other Versions

ASP.NET session state supports several different storage options for session data. Each option is identified by a value in theSessionStateMode enumeration. The following list describes the available session state modes:

• InProc mode, which stores session state in memory on the Web server. This is the default.
• StateServer mode, which stores session state in a separate process called the ASP.NET state service. This ensures that session state is preserved if the Web application is restarted and also makes session state available to multiple Web servers in a Web farm.
• SQLServer mode stores session state in a SQL Server database. This ensures that session state is preserved if the Web application is restarted and also makes session state available to multiple Web servers in a Web farm.
• Custom mode, which enables you to specify a custom storage provider.
• Off mode, which disables session state.

You can specify which mode you want ASP.NET session state to use by assigning a SessionStateMode enumeration values to the modeattribute of the sessionState element in your application's Web.config file. Modes other than InProc and Off require additional parameters, such as connection-string values as discussed later in this topic. You can view the currently selected session state by accessing the value of the HttpSessionState.Mode property.

In-Process Mode

In-process mode is the default session state mode and is specified using the InProc SessionStateMode enumeration value. In-process mode stores session state values and variables in memory on the local Web server. It is the only mode that supports theSession_OnEnd event. For more information about the Session_OnEnd event, see Session-State Events.

Caution:
If you enable Web-garden mode by setting the webGarden attribute to true in the processModel element of the application's Web.config file, do not use InProc session state mode. If you do, data loss can occur if different requests for the same session are served by different worker processes.

State Server Mode

StateServer mode stores session state in a process, referred to as the ASP.NET state service, that is separate from the ASP.NET worker process or IIS application pool. Using this mode ensures that session state is preserved if the Web application is restarted and also makes session state available to multiple Web servers in a Web farm.

To use StateServer mode, you must first be sure the ASP.NET state service is running on the server used for the session store. The ASP.NET state service is installed as a service when ASP.NET and the .NET Framework are installed. The ASP.Net state service is installed at the following location:

systemroot\Microsoft.NET\Framework\versionNumber\aspnet_state.exe

To configure an ASP.NET application to use StateServer mode, in the application's Web.config file do the following:

• Set the mode attribute of the sessionState element to StateServer.
• Set the stateConnectionString attribute to tcpip=serverName:42424.

  Note:
  To improve the security of your application when using StateServer mode, it is recommended that you protect yourstateConnectionString value by encrypting the sessionState section of your configuration file. For details, see Encrypting Configuration Information Using Protected Configuration.

The following example shows a configuration setting for StateServer mode where session state is stored on a remote computer named SampleStateServer:

<configuration>
  <system.web>
    <sessionState mode="StateServer"
      stateConnectionString="tcpip=SampleStateServer:42424"
      cookieless="false"
      timeout="20"/>
  </system.web>
</configuration>

Note:
Objects stored in session state must be serializable if the mode is set to StateServer. For information on serializable objects, see the SerializableAttribute class.

To use StateServer mode in a Web farm, you must have the same encryption keys specified in the machineKey element of your Web configuration for all applications that are part of the Web farm. For information on how to create machine keys, see article 313091, "How to create keys by using Visual Basic .NET for use in Forms authentication," in the Microsoft Knowledge Base at http://support.microsoft.com.

SQL Server Mode

SQLServer mode stores session state in a SQL Server database. Using this mode ensures that session state is preserved if the Web application is restarted and also makes session state available to multiple Web servers in a Web farm.

Note:
Objects stored in session state must be serializable if the mode is SQL Server. For information on serializable objects, see theSerializableAttribute class.

To use SQLServer mode, you must first be sure the ASP.NET session state database is installed on SQL Server. You can install the ASP.NET session state database using the Aspnet_regsql.exe tool, as described later in this topic.

To configure an ASP.NET application to use SQLServer mode, do the following in the application's Web.config file:

• Set the mode attribute of the sessionState element to SQLServer.
• Set the sqlConnectionString attribute to a connection string for your SQL Server database.

  Note:
  to improve the security of your application when using SQLServer mode, it is recommended that you protect yoursqlConnectionString value by encrypting the sessionState section of your configuration file. For details, see Encrypting Configuration Information Using Protected Configuration.

The following example shows a configuration setting for SQLServer mode where session state is stored on a SQL Server named "SampleSqlServer":

<configuration>
  <system.web>
    <sessionState mode="SQLServer"
      sqlConnectionString="Integrated Security=SSPI;data 
        source=SampleSqlServer;" />
  </system.web>
</configuration>

Note:

If you specify a trusted connection to your SQL Server in the configuration file using the sessionState element'ssqlConnectionString attribute, the SessionStateModule will connect to SQL Server using SQL Server integrated security. The connection will be made using the ASP.NET process identity or the user credentials supplied for the identity configuration element, if they exist. You can specify that the IIS impersonated identity be used instead by specifying <identity impersonate="true" /> and setting the useHostingIdentity attribute of the sessionState configuration element to false. For more information on the ASP.NET process identity, see Configuring ASP.NET Process Identity and ASP.NET Impersonation.

To configure SQLServer mode for a Web farm, in the configuration file for each Web server, set the sessionState element'ssqlConnectionString attribute to point to the same SQL Server database. The path for the ASP.NET application in the IIS metabase must be identical on all Web servers that share session state in the SQL Server database. For information on steps to resolve the issue when application paths differ between servers, see article 325056, "PRB: Session State Is Lost in Web Farm If You Use SqlServer or StateServer Session Mode," in the Microsoft Knowledge Base at http://support.microsoft.com.

Installing the Session State Database Using the Aspnet_regsql.exe Tool

To install the session state database on SQL Server, run the Aspnet_regsql.exe tool located in thesystemroot\Microsoft.NET\Framework\versionNumber folder on your Web server. Supply the following information with the command:

• Thename of the SQL Server instance, using the -S option.
• The logon credentials for an account that has permission to create a database on SQL Server. Use the -E option to use the currently logged-on user, or use the -U option to specify a user ID along with the -P option to specify a password.
• The -ssadd command-line option to add the session state database.
  By default, you cannot use the Aspnet_regsql.exe tool to install the session state database on SQL Server Express Edition. In order to run the Aspnet_regsql.exe tool to install a SQL Server Express Edition database, you must first enable the Agent XPsSQL Server option using Transact-SQL commands like the following:
  EXECUTE sp_configure 'show advanced options', 1
  RECONFIGURE WITH OVERRIDE
  GO
  
  EXECUTE sp_configure 'Agent XPs', 1
  RECONFIGURE WITH OVERRIDE
  GO
  
  EXECUTE sp_configure 'show advanced options', 0
  RECONFIGURE WITH OVERRIDE
  GO
  

  You must run these Transact-SQL commands for any instance of SQL Server Express Edition where the Agent XPs option is disabled.

By default, the Aspnet_regsql.exe tool will create a database named ASPState containing stored procedures that support SQLServermode. Session data itself is stored in the tempdb database by default. You can optionally use the -sstype option to change the storage location of session data. The following table specifies the possible values for the -sstype option:

Option	Description
t	Stores session data in the SQL Server tempdb database. This is the default. If you store session data in the tempdb database, the session data is lost if SQL Server is restarted.
p	Stores session data in the ASPState database instead of in the tempdb database.
c	Stores session data in a custom database. If you specify the c option, you must also include the name of the custom database using the -d option.

For example, the following command creates a database named ASPState on a SQL Server instance named "SampleSqlServer" and specifies that session data is also stored in the ASPState database:

aspnet_regsql.exe -S SampleSqlServer -E -ssadd -sstype p

Note:

If you are running ASP.NET 1.0 or ASP.NET 1.1, you cannot use the Aspnet_regsql.exe tool to configure ASP.NET to store session state in a persistent SQL Server database. However, you can obtain scripts to store session state in a persistent database. For details, see article 311209, "HOW TO: Configure ASP.NET for Persistent SQL Server Session State Management" in the Microsoft Knowledge Base at http://support.microsoft.com. As an alternative, Web servers running ASP.NET 1.0 or ASP.NET 1.1 can direct persistent session state to a SQL Server that has the ASP.NET 2.0 session state schema installed.

In SQLServer mode, you can configure several computers running SQL Server to work as a failover cluster, which is two or more identical computers running SQL Server that store data for a single database. If one computer running SQL Server fails, another server in the cluster can take over and serve requests without session-data loss. To configure SQL Server mode for a failover cluster, you must specify -sstype p when you execute the Aspnet_regsql.exe tool so that session state data is stored in the ASPState database instead of the tempdb database. Storing session state in the tempdb database is not supported for a SQL Server cluster. For more information about setting up SQL Server mode for a failover cluster, see article 323262, "How to use ASP.NET session state SQL Server Mode in a failover cluster" in the Microsoft Knowledge Base at http://support.microsoft.com.

Custom Mode

Custom mode specifies that you want to store session state data using a custom session state store provider. When you configure your ASP.NET application with a Mode of Custom, you must specify the type of the session state store provider using the providerssub-element of the sessionState configuration element. You specify the provider type using an add sub-element and include both atype attribute that specifies the provider's type name and a name attribute that specifies the provider instance name. The name of the provider instance is then supplied to the customProvider attribute of the sessionState element to configure ASP.NET session state to use that provider instance for storing and retrieving session data.

The following example shows elements from a Web.config file that specify that ASP.NET session state use a custom session state store provider:

<configuration>
  <connectionStrings>
    <add name="OdbcSessionServices" 
      connectionString="DSN=SessionState;" />
  </connectionStrings>

  <system.web>
    <sessionState 
      mode="Custom"
      customProvider="OdbcSessionProvider">
      <providers>
        <add name="OdbcSessionProvider"
          type="Samples.AspNet.Session.OdbcSessionStateStore"
          connectionStringName="OdbcSessionServices" 
          writeExceptionsToEventLog="false" />
      </providers>
    </sessionState>
  </system.web>
</configuration>

For more information on custom session state store providers, see Implementing a Session-State Store Provider.

Note:

A custom session state store provider will access any secured resource, such as SQL Server, using the ASP.NET process identity or the user credentials supplied to the identity configuration element, if they exist. You can specify that the IIS impersonated identity be used instead by specifying <identity impersonate="true" /> and setting the useHostingIdentity attribute of thesessionState configuration element to false. For more information on the ASP.NET process identity, see Configuring ASP.NET Process Identity and ASP.NET Impersonation.