The following flow service sequence is an alternative to a Try-Catch block.
Figure 1. Try-Catch Sequence in WebMethods
Sequence
The outer sequence exits on success; the first inner sequence exits on failure and the second inner sequence exits on done.
A sequence block consists of any number of operations that needs to be executed in the placed order. There are three options available for instructing a sequence to exit - FAILURE (default), SUCCESS and DONE. When a sequence exits, the next closest sibling statement or sequence will be executed unless the enclosing sequence instructs to exit on success.
Try Block
In the above case, if the risky operation pub.flow:debugLog (first) runs successfully, the SEQUENCE (exit on failure) will exit until the last statement is run, and exit normally. Because the enclosing SEQUENCE (exit on success) receives the successful indication from the first inner sequence, the enclosing sequence exits, and the second inner sequence will not execute.
Catch Block
If the risky operation pub.flow:debugLog (first) fails, the first inner sequence will exit and because it is not a successful exit, the enclosing sequence will continue to execute the next statement or sequence, and then the second sequence SEQUENCE (exit on done) executes.
Other thoughts
The Try-Catch sequence is useful in wrapping many risky operations such as the out-of-box http client, jms client. However, this pattern prevents any type of exception propagating to the caller operation. Sometimes a risky operation needs to be retried with WebMethods built-in retry feature, hence make sure the ISRuntimeException is rethrown to allow this exception to propagate to the top-level service.