Sequence Diagrams Reference

A Lifeline is shown using a symbol that consists of a rectangle forming its “head” followed by a vertical line (which may be dashed) that represents the lifetime of the participant.

Anonymous lifeline has no name - arbitrary representative of class.

Selector could be used to specify some lifeline from collection.

Execution (full name - execution specification, informally called activation) is interaction fragment which represents a period in the participant's lifetime when it is

• executing a unit of behavior or action within the lifeline,
• sending a signal to another participant,
• waiting for a reply message from another participant.

The duration of an execution is represented by two execution occurrences - the start occurrence and the finish occurrence.

Execution is represented as a thin grey or white rectangle on the lifeline.

Execution can be represented by a wider labeled rectangle, where the label usually identifies the action that was executed.

Overlapping executions on the same lifeline are represented by overlapping rectangles.

Message is a named element that defines one specific kind of communication between lifelines of an interaction. The message specifies not only the kind of communication, but also the sender and the receiver. Sender and receiver are normally two occurrence specifications (points at the ends of messages).

A message is shown as a line from the sender message end to the receiver message end. The line must be such that every line fragment is either horizontal or downwards when traversed from send event to receive event. The send and receive events may both be on the same lifeline. The form of the line or arrowhead reflects properties of the message.

Depending on the type of action that was used to generate the message, message could be one of:

• synchronous call
• asynchronous call
• asynchronous signal
• create
• delete
• reply

Synchronous call typically represents operation call - send message and suspend execution while waiting for response. Synchronous Messages are shown with filled arrow head.

Asynchronous call - send message and proceed immediately without waiting for return value. Asynchronous Messages have an open arrow head.

Asynchronous signal message corresponds to asynchronous send signal action.

Create message is sent to lifeline to create itself. Note, that it is weird but common practice in OOAD to send create message to a nonexisting object to create itself. In real life, create message is sent to some runtime environment.

Create message is shown as a dashed line with open arrowhead (same as reply), and pointing to created lifeline's head.

Delete message (called stop in previous versions of UML) is sent to terminate another lifeline. The lifeline usually ends with a cross in the form of an X at the bottom denoting destruction occurrence.

UML 2.3 specification provides neither specific notation for delete message nor a stereotype. Until they provide some notation, we can use custom «destroy» stereotype.

Reply message to an operation call is shown as a dashed line with open arrow head.

Depending on whether message send event and receive events are present, message could be one of:

• complete message
• lost message
• found message
• unknown message (default)

Lost Message is a message where the sending event is known, but there is no receiving event. It is interpreted as if the message never reached its destination. Lost messages are denoted with as a small black circle at the arrow end of the message.

Found Message is a message where the receiving event is known, but there is no (known) sending event. It is interpreted as if the origin of the message is outside the scope of the description. This may for example be noise or other activity that we do not want to describe in detail.

Found messages are denoted with a small black circle at the starting end of the message.

Destruction occurrence is a message occurrence which represents the destruction of the instance described by the lifeline. It may result in the subsequent destruction of other objects that this object owns by composition. No other occurrence may appear below the destruction on a given lifeline.

Complete UML name of the occurrence is destruction occurrence specification. Until UML 2.4 it was called destruction event, and earlier - stop.

The destruction of instance is depicted by a cross in the form of an X at the bottom of a lifeline.

A state invariant is an interaction fragment which represents a runtime constraint on the participants of the interaction. It may be used to specify different kinds of constraints, such as values of attributes or variables, internal or external states, etc.

State invariant is usually shown as a constraint in curly braces on the lifeline.

It could also be shown as a state symbol representing the equivalent of a constraint that checks the state of the object represented by the lifeline. This could be either the internal state of the classifier behavior of the corresponding classifier or some external state based on a "black-box" view of the lifeline.

Interaction operator could be one of:

• alt - alternatives
• opt - option
• loop - iteration
• break - break
• par - parallel
• strict - strict sequencing
• seq - weak sequencing
• critical - critical region
• ignore - ignore
• consider - consider
• assert - assertion
• neg - negative

The interaction operator alt means that the combined fragment represents a choice or alternatives of behavior. At most one of the operands will be chosen. The chosen operand must have an explicit or implicit guard expression that evaluates to true at this point in the interaction.

The interaction operator opt means that the combined fragment represents a choice of behavior where either the (sole) operand happens or nothing happens. An option is semantically equivalent to an alternative combined fragment where there is one operand with non-empty content and the second operand is empty.

If loop has no bounds specified, it means potentially infinite loop with zero as lower bound and infinite upper bound.

If only min-int is specified, it means that upper bound is equal to the lower bound, and loop will be executed exactly the specified number of times.

If both bounds are specified, loop will iterate minimum the min-int number of times and at most the max-int number of times.

Besides iteration bounds loop could also have an interaction constraint - a Boolean expression in square brackets. To add to the other confusions, UML 2.3 also calls both of them guards.

UML tries to shuffle the simplest form of for loop and while loop which causes weird UML 2.3 loop semantics on p.488: "after the minimum number of iterations have executed and the Boolean expression is false the loop will terminate". This is clarified - with opposite meaning - on the next page as "the loop will only continue if that specification evaluates to true during execution regardless of the minimum number of iterations specified in the loop."

The interaction operator break represents a breaking or exceptional scenario that is performed instead of the remainder of the enclosing interaction fragment.

Note, UML allows only one level - directly enclosing interaction fragment - to be abandoned. This could become really annoying if double loop or loop with other combined fragments should be broken.

The interaction operator par defines potentially parallel execution of behaviors of the operands of the combined fragment. Different operands can be interleaved in any way as long as the ordering imposed by each operand is preserved.

Parallel combined fragment has a notational shorthand for the common situations where the order of events on one lifeline is insignificant. In a coregion area of a lifeline restricted by horizontal square brackets all directly contained fragments are considered as separate operands of a parallel combined fragment.

The interaction operator strict requires a strict sequencing (order) of the operands on the first level within the combined fragment.

Weak sequencing seq is defined by the set of traces with these properties:

• The ordering of occurrence specifications within each of the operands are maintained in the result.
• Occurrence specifications on different lifelines from different operands may come in any order.
• Occurrence specifications on the same lifeline from different operands are ordered such that an occurrence specification of the first operand comes before that of the second operand.

Thus weak sequencing seq reduces to a parallel merge when the operands are on disjunct sets of participants. Weak sequencing reduces to strict sequencing when the operands work on only one participant.

The interaction operator critical defines that the combined fragment represents a critical region. A critical region is a region with traces that cannot be interleaved by other occurrence specifications (on the lifelines covered by the region). This means that the region is treated atomically by the enclosing fragment and can't be interleaved, e.g. by parallel operator.

Interaction operator ignore means that there are some messages that are not shown within this combined fragment. These message types can be considered insignificant and are implicitly ignored if they appear in a corresponding execution.

The list of ignored messages follows the operand enclosed in a pair of curly braces "{" and "}". Ignore operation is typically combined with other operations such as "assert ignore {m, s}."

The interaction operator consider defines which messages should be considered within this combined fragment, meaning that any other message will be ignored.

The list of considered messages follows the operand enclosed in a pair of curly braces "{" and "}". Consider operation is typically combined with other operations such as "assert consider {m, s}."

The interaction operator assert means that the combined fragment represents the assertion that the sequences of the assert operand are the only valid continuations (must be satisfied by a correct design of the system). All other continuations result in an invalid trace.

The interaction operator neg describes combined fragment of traces that are defined to be negative (invalid). Negative traces are the traces which occur when the system has failed. All interaction fragments that are different from the negative are considered positive, meaning that they describe traces that are valid and should be possible.

Interaction use is interaction fragment which allows to use (or call) another interaction. Large and complex sequence diagrams could be simplified with interaction uses. It is also common reusing some interaction between several other interactions.

The interaction use is shown as a combined fragment with operator ref.

The syntax of the interaction use of the ref operator is:

interaction-use ::= [ attribute-name '=' ] [ collaboration-use   '.' ]   interaction-name [ io-arguments ]   [ ':' return-value ]
io-arguments ::= '(' io-argument   [ ',' io-argument ]*   ')'
io-argument ::= in-argument   |   'out'   out-argument