Protecting duplicate/lost updates against host failures

Method and apparatus for tracking messages and transactions communicated between a number of users and a number of hosts, where the hosts are executing concurrent applications. During a first user session, a first user communicates with a first host. A counter associated with the first user session is incremented when the application creates an output message. The counter is decremented when the output message is released to the first user. A flag associated with the first user session is set and the counter is incremented once the flag is set, if an input message or committing transaction is received from the first user. The flag is cleared and the counter is decremented when the input message or committing transaction is processed by the concurrent application. The counter being decremented indicates there are no dependencies during the first user session with regard to the output message, input message, or committing transaction. If a second user session is initiated with a second host, the second host determines if there are any dependencies with the first user session. A dependency exists if the output message is created but not released, or if the input message or committing transaction is received but not processed. If there are dependencies, the first user session is reestablished to resolve the dependencies, otherwise, the second session is continued.