A transaction message is received at a router. A determination is made whether the transaction message includes an affinity. The affinity indicates an administrator-specified preference regarding processing of the transaction message. When the transaction message includes the affinity, a server system to process the transaction message is identified based on the affinity and based on a system affinity of the server system. The system affinity specifies processing characteristics of the server system. The server system includes a set of servers. The transaction message is modified to identify the server system and routed to a message queue.