The structure of actors

Actors perform their computation in a sequence of steps called firings. In each of those steps:

• 1. the actor may consume tokens from its input ports,
• 2. it may modify its internal state,
• 3. it may produce tokens at its output ports.

Consequently, describing an actor involves describing its interface to the outside, the ports, the structure of its internal state, as well as the steps it can perform, what these steps do (in terms of token production and consumption, and the update of the actor state), and how to pick the step that the actor will perform next. This section discusses some of the constructs in the CAL language that deal with these issues. Actions describe the things that happen during a step that an actor takes. In fact, it is accurate to say that a step consists of executing an action. Recall that when an actor takes a step, it may consume input tokens and produce output tokens.

Therefore, input patterns do the following:

• They define the number of tokens (for each port) that will be consumed when the action is executed (fired).
• They declare the variable symbols by which tokens consumed by an action firing will be referred to within the action.
• They define a firing condition for the action, i.e. a condition that must be met for the action to be able to fire.

The output side of an action is a little simpler, the output expressions simply define the number and values of the output tokens that will be produced on each output port by each firing of the action. It is permissible to omit the explicit naming of the port that an input pattern or output expression applies to if an action provides as many input patterns as there are input ports, or output expressions as there are output ports. In such a case, the patterns or expressions are matched by position against the port declarations.

One way of thinking about an actor is as an operator on streams of data — sequences of tokens enter it on its input ports, and sequences of tokens leave it on its output ports. When discussing the operation of an actor, it is often useful to look at it as an operator on streams. Actors can have parameters. They act as constants during the actor execution, and are given a concrete value when an actor is instantiated as part of an actor network. The main purpose of actor parameters is to allow programmers to specify families of related actors, without having to duplicate a lot of code.

Non determinism

A non-deterministic actor is one that, for the same input sequences, allows more than one run and more than one possible output. Non-determinism can be very powerful when used appropriately, but it can also be a very troublesome source of errors. A particular concern is that non-determinism might be introduced into an actor inadvertently, i.e. the author thinks the actor is deterministic even though it isn't. One of the key design goals of the CAL language was to allow the description of non-deterministic actors, while at the same time permitting tools to identify possible sources of non-determinism, so that they can warn the user about them.

A key consequence of a non-deterministic actor like NDMerge is that during an actual execution, its output may depend on the timing of its input. If both its input queues are empty, and NDMerge is waiting for input, then whatever input the next token arrives at may be the one that is copied next to the output. Consequently, the scheduling of activities in the actor network, or the relative speeds of the actors feeding into an actor like NDMerge may affect the output of the system. This may, occasionally, by desirable, and at other times it may not. In any event, it is a property that one needs to be aware of.

One way to look at non-determinism of the kind that makes an actor dependent on the precise timing of token arrivals is that such an actor only appears to be non-deterministic if we look at it as an operator on streams, because that view abstracts from the temporal properties of the execution, and thus purposefully removes information that is used to determine the sequence in which actions fire. From the perspective of the CAL language, this is not entirely accurate, but even so, it is easy to write non-deterministic actors that would not be deterministic even if we knew everything about the timing of the tokens and the actor implementation—such as the following:

Guarded actions

The guard clause of an action contains a number of expressions that all need to be true in order for the action to be fireable. For the first action to be fireable, the incoming token needs to be greater or equal to zero, in which case it will be sent to output P. Otherwise that action cannot fire. Conversely, for the second action to be fireable, the token needs to be less than zero, in which case it is sent to output N. A run of this actor might look like this: An actor could run into trouble if it ever encounters a zero token, because none of its actions will be able to fire on it.

It's not illegal to write actors that terminate on some input, and in fact it may be important to have a few of those in some systems. But it is a pitfall that one needs to be aware of. Secondly, the guard conditions are also disjoint in addition to being exhaustive.

Finally, note that guard conditions can ”peek” at the incoming tokens without actually consuming them — if the guards happen to be false or the action is not fired for some other reason, and if the token is not consumed by another action, then it remains where it is, and will be available for the next firing. (Or it will remain there forever, as in the case of the zero token in front of SplitDead, which is never removed because the actor is dead.)

The Select actor below is another example of the use of guarded actions. It is similar to the NDMerge actor in the sense that it merges two streams (the ones arriving at its A and B input ports). However, it does so according to the (Boolean) values of the tokens arriving at its S input port.

Actors with state

In all the actors so far, nothing an action firing did would in any way affect subsequent firings of actions of the same actor. Using state variables, action firings can leave information behind for subsequent firings of either the same or a different action of the same actor. The way this actor is written, the selection of the next input token and the actual copying of the token to the output is one atomic step.

Note that Select and IterSelect are almost, but not entirely, equivalent. First of all, IterSelect makes twice as many steps in order to process the same number of tokens. Secondly, it actually reads, and therefore consumes, the S input token, irrespective of whether a matching data token is available on A or B.

Atomic expressions

Probably the most fundamental expressions are constants. Another group of basic expressions are variable references. Syntactically, a variable is any sequence of letters and digits. One important property of expressions is that they are guaranteed not to change variables (we also say they have no side effects)—consequently, within an expression, multiple references to the same variable will always yield the same result.

Simple composite expressions

CAL provides operators of two kinds to build expressions: unary and binary. A unary operator in CAL is always a prefix operator, i.e. it appears before its single operand. A binary operator occurs between its two operands.

Control flow

As in most other programming languages, there are constructs to control the order in which the statements within a program are executed. The part of this loop that directly follows the 'foreach keyword is a generator, much like those in list comprehensions.

Action

• Input patterns: declaring variables
• Guard: specifying enabling conditions
• Output expressions: computing output tokens
• Body: modifying the actor state

OpenDF framework

Open RVC-CAL compiler