UML Sequence Diagram feedback

Hi can anyone tell me their thought开发者_JAVA技巧s on this sequence diagram, if it is correct or whatever needs changing.

Thankyou, your feedback would certainly help a lot.

Full Size Image: http://i.stack.imgur.com/ktPsY.jpg

There seem to be some splits in the sequences, for example there is a gap between steps 2 and 3. Keeping each sequence whole makes the diagram valid and much more readable. Here is an example of what a diagram should look like. Note how the bar for the customer goes on until the very end of the sequence.

The objects have also been chosen pretty confusingly. Aren't Register, Login and Book part of the web server? If they are, you probably should get rid of the Web Server object in this chart.

The return values should be passed back to the object that originated the call like has been correctly done in steps 11 and 12, though there is another gap in the sequence.

Some sequences are incomplete, like step 8. Now the diagram describes the Browse Shows as a call that is made by the user to the Book object, which never provides any return value.

The alternate paths 4 and 5 are incorrectly defined. An else block should be used instead.