Types of Messages

Synchronous Message

It’s a message where the caller object has to wait for the receiving object to finish execution.

It always has an associated return values or acknowledgement of being successfully completed. So, object will still in activation until it receives a respond.

Simple Message

A message that transfer control from the sender to the recipient object.

It’s one way message. An example for that is the return from synchronous message. The return message transfers the control to the sender object.


Simple Message

Asynchronous Message

The caller object doesn’t have to wait for the receiving object to finish execution. The arrow in this case is shown as an open arrowhead.

Asynchronous messages are common in concurrent, real time systems, in which several objects operation in parallel

We won’t show the asynchronous message here, but you may find it later.

Objects in Sequence Diagram

Objects Created And Destroyed

We can end the lifetime of an object, after finishing it’s job, by putting the letter ‘X’ as show below, so, the object won’t exist anymore.

These objects are created on the fly to control sequence of interaction, and deleted once it’s finished. So, their life-line start from the point of creation to the point of destruction.

Objects Created And Won’t Be Destroyed

Remember? when we said that when an arrow points to an object box, this means a new instance of this object will be created.

Those are the objects that are created, but they won’t be destroyed. Not all of the objects that are created should be deleted.

There isn’t such kind of objects in this diagram, but imagine that the RegisterationForm object has been created but it’s not destroyed.

Objects Neither Created Nor Destroyed

Those are the objects where their life-time extends from the top to the bottom of the diagram.


Objects neither created nor destroyed

Conditional & Looping Interaction


If there is different actions to take place based on different conditions, you can surround these action with what’s called “alt” frame. So, you will show the interactions in case of both conditions.


If there are messages that need to happen multiple times, you can surround this with what’s called a “loop” frame.

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