# How Do I: Avoid Deadlock Situations in a Web Service?

There are many situations in which deadlocks can be created between communicating web services. However, the following rule will keep you out of trouble most of the time:

Never invoke a synchronous method on the caller to a synchronous method.

When a client invokes a synchronous method on a web service, the client will block waiting for the method to complete. If you attempt to call the client (invoke a callback) while the client is waiting for the original method to complete, then each party will be waiting for the other. This is a deadlock. The same situation could occur if a callback handler attempted to call a synchronous method on it's caller.

If you have a situation where you want to invoke a callback from a method, you can avoid deadlock by making either the method or the callback asynchronous. One simple way to make the method or callback asynchronous is to use message buffering.

To Buffer a Method or Callback

1. In Design View, select the method or callback.
2. In the Property Editor pane, expand the message-buffer property.
3. From the drop-down list next to the enable attribute, select true.

Evidence of a Deadlock

If you create a deadlock in your application, it may not be directly apparent. Typically, one of the two deadlocking requests will time out and a weblogic.transaction.internal.TimedOutException will be thrown. If you see this exception, you probably have a deadlock in your application.

## Related Topics

Building Asynchronous Web Services

Designing Asynchronous Interfaces