Activity Diagram

It represents workflows and the sequence of activities. It define what activities user perform during a use case.

Each use case in a use case diagram can be further explored by creating an activity diagram. It's common to create an activity diagram for each use case identified in the use case diagram.

Components:

Activities: Represent tasks or actions performed in the workflow. Each activity is denoted by a rounded rectangle.
Start Node: Indicates the beginning of a process. It is represented by a filled black circle.
End Node: Marks the end of the process flow. It is represented by a black circle with a surrounding border.
Transitions/Arrows: Arrows show the flow or transition from one activity to the next.
Decision Node: Depicted as a diamond shape, it represents branching in the workflow based on conditions. Each outgoing arrow is labeled with a condition.
Merge Node: A diamond shape without conditions where multiple flows join back together.
Fork Node: Represented as a thick horizontal or vertical line, it splits a single flow into multiple parallel flows.
Join Node: A thick line that synchronizes multiple parallel flows into a single flow.
Swimlanes: Vertical or horizontal partitions used to organize activities by the responsible actors or departments. This helps in visually associating actions with particular actors.

Example

+-------------------------+   +---------------------+   +---------------------+
|        Start            |-->|   Search Book       |-->| Book Available?     |
+-------------------------+   +---------------------+   +---------+-----------+
                                                     |           |
                                                     |           v
                                                     |   +---------------------+
                                                     |   | Book Not Available  |
                                                     |   | Notify User         |
                                                     |   +---------------------+
                                                     |           |
                                                     |           v
                                                     |   +---------------------+
                                                     |   |  End                |
                                                     |   +---------------------+
                                                     |
                                                     v
+-------------------------+   +---------------------+   +---------------------+
|  Verify Membership      |-->|  Valid Member?      |-->|  Borrow Book        |
+-------------------------+   +---------+-----------+   +---------------------+
                                        |
                                        |                   +---------------------+
                                        |                   |  Record Borrowing   |
                                        v                   +---------------------+
                              +---------------------+                   |
                              |  Not a Member       |                   |
                              |  Notify User        |                   v
                              +---------------------+   +---------------------+
                                        |               |  Issue Receipt       |
                                        v               +---------------------+
                                  +------------------+             |
                                  |        End       |<-------------+
                                  +------------------+

Example​

Example