Description

The MongoDB server can be heavily loaded when using the MongoDB providers for both private and shared session states, if a collection is missing a required index.

For example, when the private and shared session state providers are configured as in the following:

• In the \App_Config\ConnectionStrings.config file:

  <connectionStrings>
  ...  
    <add name="private.sessions" connectionString="mongodb://{user-name}:{password}@{host}/sitecore_private_sessions" />
    <add name="shared.sessions" connectionString="mongodb://{user-name}:{password}@{host}/sitecore_shared_sessions" />
  ...
  </connectionStrings>

• In the Web.config file:

  <sessionState mode="Custom" customProvider="mongo" ... >
    <providers>
    ...
      <add name="mongo"
           type="Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider, Sitecore.SessionProvider.MongoDB"
           sessionType="Private"
           connectionStringName="private.sessions"
           pollingInterval="5"
           compression="true" />
    ...
    </providers>
  </sessionState>

• In the \App_Config\Include\Sitecore.Analytics.Tracking.config file:

  <sharedSessionState defaultProvider="mongo">
    <providers>
    ...
      <add name="mongo"
           type="Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider, Sitecore.SessionProvider.MongoDB"
           sessionType="Shared"
           connectionStringName="shared.sessions"
           pollingInterval="5"
           compression="true" />
    ...
    </providers>
  ...
  </sharedSessionState>

Sitecore XP creates two instances of the Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider class, but only the first one will initialize the sessions collection in MongoDB with the e_1__id.s_1__id.a_1 index. As a result, querying the private session state database uses the index, and querying the shared session state database uses a full collection scan.

Solution

To resolve this issue:

1. Connect to the MongoDB server using a GUI tool, for example Robomongo.
2. Open the shared.sessions database and navigate to the \Collections\sessions\Indexes node.
3. Right-click on the Indexes node and then click the Add Index... command.
4. In the Index Properties dialog, fill in the Name and Keys fields using the following values:
   
   • Name: e_1__id.s_1__id.a_1
   • Keys: { "e" : 1, "_id.s" : 1, "_id.a" : 1 }
5. In the Index Properties dialog, click Save.