Failover and workload balancing

When using the ICM, failover and workload balancing work the same as in standard Domino® clusters. Domino® computes the server availability index based on all open sessions, whether they are from Notes® clients, HTTP clients, or other Domino® services.

To limit the workload of a server and manage failover, you use the same settings, such as SERVER_RESTRICTED and SERVER_AVAILABILITY_THRESHOLD, as in a standard cluster. For database availability, you also use the same settings, such as marking a database Out of service or Pending delete.

Note: Unlike in a standard Domino® cluster, the ICM can direct a client to a server that is in the MAXUSERS state, if no other server is available. The NOTES.INI setting, Server_MaxUsers, applies only to NRPC; it does not apply to HTTP.

The ICM maintains the following information so that it can find a replica when a client asks for one:

  • Information about which databases are available in the cluster and where they are stored. The ICM obtains this information from the Cluster Database Directory.
  • Information about the availability of each server. The ICM obtains this information each time it probes the servers in the cluster.
  • Information about which Web servers are configured for HTTP and which are configured for HTTPS. The ICM obtains this information from the Server documents of each server in the cluster.

To determine which replica of a database to open, the ICM does the following:

  • Determines where replicas reside and whether they are marked out of service or pending delete.
  • Checks the server availability index of each server that contains a replica.
  • Checks the availability of the server by pinging the HTTP port or the HTTPS port, depending on the client request.
  • Eliminates any servers that are not reachable or are RESTRICTED.
  • Eliminates any servers that are BUSY or in the MAXUSERS state.
  • Selects a server from those remaining. If there are no servers remaining, the ICM chooses a server that is BUSY or in the MAXUSERS state, if one is available. If there are multiple servers remaining, the ICM chooses the server with the lightest current workload.

After choosing the server to access, the ICM looks at the Server document to determine which port to use to access the server.

When a server fails

When an HTTP client is connected to a server that fails, the client receives a message saying that the server is not responding. To fail over to a different replica, if one is available, the user must contact the ICM again. The user can do this in the following ways:

  • Click the Back button in the browser one or more times to connect to a page through the ICM
  • Use a bookmark
  • Type the URL

The user may or may not have to re-authenticate with the new server. This is determined by the following factors:

  • If the user already authenticated with the new server during this session, no authentication is necessary
  • If the HTTP client and the server both support SSL3, re-authentication occurs automatically