Draw a signal from one participant to another like this:

# This is a comment.            
Alice->Bob: Filled arrow
Alice->>Bob: Open arrow
Bob-->Alice: Dotted line
Bob-->>Alice: Dotted Line, open arrow
            

The participants are automatically created when they are used. Use the "-->" syntax to draw a dotted line.

If you want to participants to be shown in a different order than they are used, declare them first using the participant keyword. You can also rename them this way to save typing.

participant Bob
participant Alice
participant "I have a really\nlong name" as L

Alice->Bob: Authentication Request
Bob->Alice: Authentication Response
Bob->L: Log transaction
            

A participant can send a signal to itself. This will result in an arrow that turns back on itself.

You may break the text into multiple lines by using "\n".

Alice->Alice: This is a signal to self.\nIt also demonstrates \nmultiline \ntext.
            

You can group signals together using the alt/else, opt, and loop keywords. All of them can take a text description that will be displayed in the group header. Use the end keyword to signal the end of a group. The groups may be nested to any depth.

Alice->Bob: Authentication Request
alt successful case
Bob->Alice: Authentication Accepted
else some kind of failure
Bob->Alice: Authentication Failure
opt
    loop 1000 times
        Alice->Bob: DNS Attack
    end
end
else Another type of failure
Bob->Alice: Please repeat
end
            

You can add notes to your diagram. Notes can be placed to the left of a participant or to the right of a participant. In addition, you can centre a note over one or more participants.

If a note contains more than one line, it will be not be word-wrapped. Instead, it will be formatted exactly as written.

participant Alice
participant Bob

note left of Alice 
This is displayed 
left of Alice.
end note
note right of Alice: This is displayed right of Alice.
note over Alice: This is displayed over Alice.
note over Alice, Bob: This is displayed over Bob and Alice.
            

Use the + and - with signals to denote object activation. While activated, the participant's lifeline will be highlighted. + will activate the receiver, and - will deactivate the sender.

You can use the destroy keyword to destroy a participant. The participant's lifeline will end at the previous signal.

User->+A: DoWork
A->+B: <<createRequest>>
B->+C: DoWork
C-->B: WorkDone
destroy C
B-->-A: RequestCreated
A->User: Done
            

You can explain the sequences that follow
simply by indenting your explanations 
with a space.

Alice->Bob: Wow!                
            

You can make different things happen at the same time using the parallel keyword.

parallel {
Client->Proxy: Request
Proxy->Server: Forwarded Request
note right of Server: Web proxy in operation
}
            

A rounded box can be useful to show state information. The syntax is identical to the note keyword, except that state is used.

participant Client
participant Server
parallel {
state over Server: LISTEN
state over Client: CLOSED
}
Client->Server: SYN
parallel { 
state over Client: SYN-SENT
state over Server: SYN_RECEIVED
}
Server->Client: ACK
            

Autonumbering will automatically prefix all signals with a number. To use it, include the line autonumber followed by the number to start with. To turn off autonumbering, use "autonumber off"

autonumber 2
UE->GANC: Register Request
GANC->UE: Register Reject
            

Change the way the bottom of your diagram looks using option footer=.

option footer=none
Alice->Alice:
            

option footer=bar
Alice->Alice: