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2 The Language: Rationale and Fundamentals 27
Table 2.1 Split construct–pattern mapping
Split construct Pattern
AND-split Parallel split
XOR-split Exclusive choice
Deferred choice
OR-split Multichoice
Thread-split Thread split

where after the pour tea task, the execution thread diverges into multiple concurrent
threads causing the add sugarcube task to execute several times.
Each of these situations can be mapped to a specific pattern that precisely
describes the desired split behavior as shown in Table 2.1. In the case of the XOR-
split, there are two possible ways in which it can be implemented and consequently
there are two patterns that describe these alternatives.
The operation of the XOR-split can be realized in two distinct ways depending on
whether the process itself contains sufficient information to make the required rout-
ing decision when the split construct is reached or whether additional external input
is required to make the decision. If we consider the example shown in Fig. 2.2b, two
possible courses of action are possible when the XOR-split construct is reached and
these are recognized by two specific patterns:
Exclusive Choice, where the process contains sufficient information to determine
where to route the thread of control when the decision construct is reached and
does so automatically. In the example above, this would immediately result in
either the add milk or add lemon task being initiated when the XOR-split was
reached.
Deferred Choice, where the decision is not made automatically from within the
context of the process but is deferred to an entity in the operational environment.
In the context of the example above, the process would halt at the XOR-split
pending this decision, which would be signaled by either the add milk or the add
lemon task being initiated.
The branching patterns provide various mechanisms for diverging the thread of con-
trol within a process. The following group of control-flow patterns provide various
mechanisms for synchronizing threads of control within a process.

Synchronization Patterns
Synchronization patterns are the logical counterpart to the branching patterns. They
identify various types of synchronization and merging behaviors that are generally
associated with join constructs in a process. Four distinct types of join are commonly
utilized:
The AND-join, which synchronizes the thread of control in multiple branches
and merges them into a single execution thread in an outgoing branch.