You can use a “par” fragment to model multiple synchronous calls being executed in parallel. The fragment consists of a rectangle with the label “par”. The rectangle is divided into multiple boxes, each representing a thread of execution. Below an example extracted from the link.

A thread is a lightweight flow that can execute concurrently with other threads within the same process. An active object is an object that owns a process or thread and can initiate control activity. An active class is a class whose instances are active objects.

The parallel combination fragment is drawn using a frame, and you place the text “par” in the frame’s namebox. You then break up the frame’s content section into horizontal operands separated by a dashed line. Each operand in the frame represents a thread of execution done in parallel.

Activity, Sequence, and State Diagrams are all correct ways of showing thread behavior. 1st: (To vs’s comments) There are two sets of diagrams or modeling elements in UML, static structure, as you put it, and behavioral.

A process is an active program i.e. a program that is under execution. A thread shares information like data segment, code segment, files etc. with its peer threads while it contains its own registers, stack, counter etc.

Below are the steps on how to represent a loop:

1. Create a sequence diagram.
2. Place two objects on the sequence diagram.
3. Draw a message from one object to the other.
4. Draw a note over the message and add a description (for example, “Messages 2 through 4 are repeated.”)

The following nodes and edges are typically drawn in a UML sequence diagram : lifeline, execution specification, message, combined fragment, interaction use, state invariant, continuation, destruction occurrence. Major elements of the sequence diagram are shown on the picture below. Major elements of UML sequence diagram.

This symbol is used when a sender must wait for a response to a message before it continues. The diagram should show both the call and the reply. Represented by a solid line with a lined arrowhead. Asynchronous messages don’t require a response before the sender continues. Only the call should be included in the diagram.

Lucidchart supports .vsd and .vdx file import and is a great Microsoft Visio alternative. Almost all of the images you see in the UML section of this site were generated using Lucidchart. Diagramming is quick and easy with Lucidchart.

To understand what a sequence diagram is, it’s important to know the role of the Unified Modeling Language, better known as UML. UML is a modeling toolkit that guides the creation and notation of many types of diagrams, including behavior diagrams, interaction diagrams, and structure diagrams. A sequence diagram is a type of interaction diagram