596 Chapter 13 Concurrency
A rendezvous can occur in two basic ways in this simple example. First,
the receiver task, Task_Example, can be waiting for another task to send a
message to the Entry_1 entry. When the message is sent, the rendezvous
occurs. This is the situation described earlier. Second, the receiver task can be
busy with one rendezvous, or with some other processing not associated with
a rendezvous, when another task attempts to send a message to the same entry.
In that case, the sender is suspended until the receiver is free to accept that
message in a rendezvous. If several messages arrive while the receiver is busy,
the senders are queued to wait their turn for a rendezvous.
The two rendezvous just described are illustrated with the timeline dia-
grams in Figure 13.4.Tasks need not have entry points. Such tasks are called actor tasks because
they do not wait for a rendezvous in order to do their work. Actor tasks can
rendezvous with other tasks by sending them messages. In contrast to actor
tasks, a task can have accept clauses but not have any code outside those
accept clauses, so it can only react to other tasks. Such a task is called a
server task.
An Ada task that sends a message to another task must know the entry
name in that task. However, the opposite is not true: A task entry need notFigure 13.4
Two ways a rendezvous
with Task_Example
can occur
AcceptAcceptTask_ExampleTask_ExampleTask_ExampleSenderSenderTask_Example