The SMTP and web service part of Message Connector can run as a part of a Windows Network Load Balancing (NLB).

A Message Connector NLB cluster can be used to add scalability, load balancing, and high availability to the SMTP and web service in scenarios, where the sender SMTP or web service client can only be configured to send to a particular server machine defined by a single IP address.

The Windows NLB concept by itself provides failover only for the case when an NLB node in the cluster is completely shut down: NLB directs all TCP-based traffic automatically only to the working NLB nodes. However, NLB does not provide a failover solution for the case when a node is running but the TCP-based services on the node (in our case SMTP and web service listeners) have problems, or if the service is stopped.

Therefore, Message Connector includes a feature called NLB Port Control that monitors the health of its processes. In case of problems, the port control makes the node unavailable.

Simple variant

In this variant, either Windows NLB or a hardware NLB is used to divide load to two or more computers. All computers run in parallel. You can continue to add computers as needed.

Active/active Windows cluster

In the active/active Windows cluster, two independent instances of Message Connector run on a single computer. This variant combines network load balancing with failover.

During normal operation, one of these Message Connectors is active on each computer so that system load is distributed across both computers.

If one machine fails, the passive instance on the second computer is activated and continues to process documents.

The active/active cluster configuration can be used with any input service. However, if you want to use SMTP or web service input with a single IP address, an external load balancer is required.

You can also run KC Plug-In as a clustered service, but this does not support the active/active scenario, because only one instance of KC Plug-In is supported on a computer.