In Apigee hybrid, the Message Processor (MP) is the workhorse that processes incoming API requests, executes policies, and proxies API requests to target services. To do its work, the MP interacts with the local, runtime Cassandra database and it has read-only access to the host file system, where it loads proxies, resources, target servers, and other required Edge entities at startup.
Internally, each environment is assigned to one and only one message processor, which does the work of processing proxy requests, applying policies and routing traffic to and from target services. Therefore, the host alias determines which message processor receives any given incoming request.
See also About the runtime plane.