I'm using JBM 1.4SP3 (or may be SP4), to send messages to a clustered MDB on multiple servers. Now these MDBs are defined as MDB singletons (1 maximum MDB instance per server). The reason for this is the MDB uses a native library that is not thread safe so it can not allow more than thread to enter at any given time, however, it's totally OK to have multiple of these MDBs running on different machines simultaneously. What I found is that JBM will send messages to these MDBs in a round robin fashion but it doesn't seem to detect if a MDB is busy. My observations show that if a singleton MDB is busy and JBM chooses that MDB to consume it's messages, the message will wait on the queue even if other MDBs on other matches are idle. Obviously the desired behavior is for JBM to know that a singleton MDB is not available (busy) and send it off to another MDB that is available. That does not appear to be happending. I tried setting prefetch size to 1 but that did not help. Any ideas on how to get this to work properly? Conceptually what I'm trying to do is simple as mud but I can't get it to work properly. View the original post : http://www.jboss.org/index.html?module=bb&op=viewtopic&p=4247333#... Reply to the post : http://www.jboss.org/index.html?module=bb&op=posting&mode=reply&a...