4.5 Active Objects
In the task-based frameworks illustrated
throughout most of this chapter, threads are used to propel
conceptually active messages sent among conceptually passive
objects. However, it can be productive to approach some design
problems from the opposite perspective � active objects
sending each other passive messages.
To illustrate, consider an active object that
conforms to the WaterTank description in Chapter
1:
pseudoclass ActiveWaterTank extends Thread { // Pseudocode
// ...
public void run() {
for (;;) {
accept message;
if (message is of form addWater(float amount)) {
if (currentVolume >= capacity) {
if (overflow != null) {
send overflow.addWater(amount);
accept response;
if (response is of form OverflowException)
reply response;
else ...
else ...
else ...
}
else if (message is of form removeWater(float amount)) {
...
}
}
}
}
Pseudocode is used here because there is no
built-in syntax for passing messages from one active object to
another, only for direct invocation among passive objects. However, as discussed in �
4.1.1, similar issues may be encountered even when class=docTextHighlight>using passive objects.
Any of the solutions described there apply equally well here:
adopting message formats of various kinds, transported across
streams, channels, event queues, pipes, sockets, and so on. In
fact, as shown in the WebService example leading off
this chapter, it is easy to add task-based constructions to
designs otherwise based on active objects. Conversely, most
task-based designs discussed in this chapter work equally well
when some objects are active rather than passive.
Further, the use of Runnables as
messages leads to a boring but universal (at least in some
senses) form of active object: a minor variant of a common
worker thread design that also conforms to the initial
abstract characterization of active objects as interpreters in
�
1.2.4:
class ActiveRunnableExecutor extends Thread {
Channel me = ... // used for all incoming messages
public void run() {
try {
for (;;) {
((Runnable)(me.take())).run();
}
}
catch (InterruptedException ie) {} // die
}
}
Of course, such classes are not very useful
unless they also include internal methods that manufacture
Runnables to execute and/or send to other active
objects. It is possible, but unnatural, to write entire
programs in this fashion.
However, many components in reactive systems
can be usefully construed as active objects that operate under
more constrained rules and message-passing disciplines. This
includes especially those objects that interact with other
computers or devices, often the main externally visible
objects in a program.
In distributed frameworks such as CORBA and
RMI, externally visible active objects are themselves ascribed
interfaces listing the messages that they accept. Internally,
they usually have a more uniform structure than does
ActiveWaterTank. Typically, they contain a main run
loop that repeatedly accepts external requests, dispatches to
internal passive objects providing the corresponding service,
and then constructs reply messages that are sent back to
clients. (The internal passive objects are the
ones explicitly programmed when class=docTextHighlight>using CORBA and RMI.
The active objects, sometimes known as class=docEmphasis>skeletons, are usually generated
automatically by tools.)
It is very possible to take an active,
actor-style approach to the design of other components as
well. One reason for designing entire systems from this point
of view is to take advantage of well-developed theory and
design techniques associated with particular sets of rules
surrounding active entities and their messages. The remainder
of this section gives a brief overview of the most well-known
and influential such framework, CSP.
4.5.1 CSP
C.A.R. Hoare's theory of Communicating
Sequential Processes (CSP) provides both a formal approach to
concurrency and an associated set of design techniques. As discussed in the Further Readings in �
4.5.2, there are a number of closely related approaches,
but CSP has had the largest impact on class=docTextHighlight>concurrent design and class=docTextHighlight>programming.
CSP has served as the basis of class=docTextHighlight>programming languages (including
occam), was influential in the design of others
(including Ada), and can be supported in the class=docTextHighlight>Java programming language
through the use of library classes.
The following account illustrates the JCSP
package developed by Peter Welch and colleagues. The package
is available via links from the online supplement. This
section provides only a brief synopsis. Interested readers
will want to obtain copies of the package, its documentation,
and related texts.
4.5.1.1 Processes and channels
A CSP process
can be construed as a special kind of actor-style object, in
which:
Processes have no method interface and no
externally invocable methods. Because there are no invocable
methods, it is impossible for methods to be invoked by
different threads. Thus there is no need for explicit
locking.
Processes communicate only by reading and
writing data across class=docEmphasis>channels.
Processes have no identity, and so cannot
be explicitly referenced. However, channels serve as analogs
of references (see �
1.2.4), allowing communication with whichever process is
at the other end of a channel.
Processes need not spin forever in a loop
accepting messages (although many do). They may read and
write messages on various channels as desired.
A CSP channel
can be construed as a special kind of Channel, in
which:
All channels are synchronous (see �
3.4.1.4), and so contain no internal buffering.
(However, you can construct class=docEmphasis>processes that perform
buffering.)
Channels support only read ("?") and write
("!") operations carrying data values. The operations behave
in the same way as take and put.
The most fundamental channels are
one-to-one. They may be connected only to a single pair of
processes, a writer and a reader. Multiple-reader and
multiple-writer channels may also be defined.
4.5.1.2 Composition
Much of the elegance of CSP stems from its
simple and analytically tractable composition rules. The "S"
in CSP stands for Sequential,
so basic processes perform serial computations on internal
data (for example adding numbers, conditional tests,
assignment, looping). Higher-level processes are built by
composition; for a channel c, variable x,
and processes P and Q:
c?x -> P |
Reading from c enables
P |
c!x -> P |
Writing to c enables P
|
P ; Q |
P followed by Q |
P || Q |
P and Q in parallel
|
P [] Q |
P or Q (but not both)
|
The choice operator P []
Q requires that P and Q both be
communication-enabled processes (of form d?y -> R
or d!y -> R). The choice of which process runs
depends on which communication is ready: Nothing happens until
one or both communications are ready. If one is (or becomes)
ready, that branch is taken. If both are (or become) ready,
either choice may be taken (nondeterministically).
4.5.1.3 JCSP
The JCSP package supports CSP-based design in
a straightforward way. It consists of an execution framework
that efficiently supports CSP constructs represented via
interfaces, classes, and methods, including:
Interfaces ChannelInput
(supporting read), ChannelOutput
(supporting write) and Channel (supporting
both) operate on Object arguments, but special
versions for int arguments are also provided. The
principal implementation class is One2OneChannel
that supports use only by a single reader and a single
writer. But various multiway channels are also provided.
Interface CSProcess describes
processes supporting only method run.
Implementation classes Parallel and
Sequence (and others) have constructors that accept
arrays of other CSProcess objects and create
composites.
The choice operator [] is
supported via the Alternative class. Its
constructor accepts arrays with elements of type
Guard. Alternative supports a
select method that returns an index denoting which
of them can (and then must) be chosen. A fairSelect
method works in the same way but provides additional
fairness guarantees � over the course of multiple selects,
it will choose fairly among all ready alternatives rather
than always selecting one of them. The only usages of
Alternative demonstrated below use guard type
AltingChannelInput, which is implemented by
One2OneChannel.
Additional utilities include
CSProcess implementations such as Timer
(which does delayed writes and can also be used for
time-outs in Alternative), Generate (which
generates number sequences), Skip (which does
nothing at all � one of the CSP primitives), and classes
that permit interaction and display via AWT.
4.5.1.4 Dining philosophers
As a classic demonstration, consider the
famous Dining Philosophers problem. A table holds five forks
(arranged as pictured) and a bowl of spaghetti. It seats five
philosophers, each of whom eat for a while, then think for a
while, then eat, and so on. Each philosopher requires two
forks � the ones on the left and right � to eat (no one knows
why; it is just part of the story) but releases them when
thinking.
The main problem to be solved here is that,
without some kind of coordination, the philosophers could
starve when they pick up their left forks and then block
forever trying to pick up their right forks which are being
held by other philosophers.
There are many paths to a solution (and yet
more paths to non-solution). We'll demonstrate one described
by Hoare that adds a requirement (enforced by a Butler) that
at any given time, at most four philosophers are allowed to be
seated. This requirement suffices to ensure that at all times
at least one philosopher can eat � if there are only four
philosophers, at least one of them can get both forks. This
solution does not by itself ensure that all five philosophers
eventually eat. But this guarantee can be obtained via use of
Alternative.fairSelect in the Butler class
to ensure fair processing of seating messages.
We'll use a simple, pure CSP
style where all channels are one-to-one and messages have no
content (using
null for messages).
This puts a stronger focus on the synchronization and process
construction issues. The system is composed of a
College with five Philosophers, five
Forks, and one Butler (standing in the bowl
of spaghetti!), connected class=docTextHighlight>using
One2OneChannels.
Since everything must be either a process or
a channel, forks must be processes. A Fork
continuously loops waiting for a message from one of its users
(either its left-hand or right-hand philosopher). When it gets
a message from one indicating a fork pick-up, it waits for
another indicating a fork put-down. (While it
might be more tasteful to indicate pick-ups versus put-downs
via different kinds of messages or message contents, this
protocol using null
messages suffices.)
In JCSP, this can be written as:
class Fork implements CSProcess {
private final AltingChannelInput[] fromPhil;
Fork(AltingChannelInput l, AltingChannelInput r) {
fromPhil = new AltingChannelInput[] { l, r };
}
public void run() {
Alternative alt = new Alternative(fromPhil);
for (;;) {
int i = alt.select(); // await message from either
fromPhil[i].read(); // pick up
fromPhil[i].read(); // put down
}
}
}
The Butler process makes sure that
at most N-1 (i.e., four here) philosophers are seated at any
given time. It does this by enabling both enter and
exit messages if there are enough seats, but only
exit messages otherwise. Because Alternative
operates on arrays of alternatives, this requires a bit of
manipulation to set up. (Some other utilities in JCSP could be
used to simplify this.) The exit channels are placed
before the enter channels in the chans array
so that the proper channel will be read no matter which
Alternative is used. The fairSelect is
employed here to ensure that the same four philosophers are
not always chosen if a fifth is also trying to enter.
class Butler implements CSProcess {
private final AltingChannelInput[] enters;
private final AltingChannelInput[] exits;
Butler(AltingChannelInput[] e, AltingChannelInput[] x) {
enters = e;
exits = x;
}
public void run() {
int seats = enters.length;
int nseated = 0;
// set up arrays for select
AltingChannelInput[] chans = new AltingChannelInput[2*seats];
for (int i = 0; i < seats; ++i) {
chans[i] = exits[i];
chans[seats + i] = enters[i];
}
Alternative either = new Alternative(chans);
Alternative exit = new Alternative(exits);
for (;;) {
// if max number are seated, only allow exits
Alternative alt = (nseated < seats-1)? either : exit;
int i = alt.fairSelect();
chans[i].read();
// if i is in first half of array, it is an exit message
if (i < seats) --nseated; else ++nseated;
}
}
}
The Philosopher processes run
forever in a loop, alternating between thinking and eating.
Before eating, philosophers must first enter their seats, then
get both forks. After eating, they do the opposite. The
eat and think methods are just no-ops here,
but could be fleshed out to (for example) help animate a
demonstration version by reporting status to JCSP channels and
processes that interface into AWT.
class Philosopher implements CSProcess {
private final ChannelOutput leftFork;
private final ChannelOutput rightFork;
private final ChannelOutput enter;
private final ChannelOutput exit;
Philosopher(ChannelOutput l, ChannelOutput r,
ChannelOutput e, ChannelOutput x) {
leftFork = l;
rightFork = r;
enter = e;
exit = x;
}
public void run() {
for (;;) {
think();
enter.write(null); // get seat
leftFork.write(null); // pick up left
rightFork.write(null); // pick up right
eat();
leftFork.write(null); // put down left
rightFork.write(null); // put down right
exit.write(null); // leave seat
}
}
private void eat() {}
private void think() {}
}
Finally, we can create a College
class to represent the parallel composition of the
Forks, Philosophers, and Butler. The channels are constructed class=docTextHighlight>using a JCSP convenience
function create that creates arrays of
channels.
The Parallel constructor accepts an array of
CSProcess, which is first loaded with all of the
participants.
class College implements CSProcess {
final static int N = 5;
private final CSProcess action;
College() {
One2OneChannel[] lefts = One2OneChannel.create(N);
One2OneChannel[] rights = One2OneChannel.create(N);
One2OneChannel[] enters = One2OneChannel.create(N);
One2OneChannel[] exits = One2OneChannel.create(N);
Butler butler = new Butler(enters, exits);
Philosopher[] phils = new Philosopher[N];
for (int i = 0; i < N; ++i)
phils[i] = new Philosopher(lefts[i], rights[i],
enters[i], exits[i]);
Fork[] forks = new Fork[N];
for (int i = 0; i < N; ++i)
forks[i] = new Fork(rights[(i + 1) % N], lefts[i]);
action = new Parallel(
new CSProcess[] {
butler,
new Parallel(phils),
new Parallel(forks)
});
}
public void run() { action.run(); }
public static void main(String[] args) {
new College().run();
}
}
4.5.2 Further Readings
CSP has proven to be a successful approach to
the design and analysis of systems that can be usefully
expressed as bounded sets of identityless, interfaceless
processes communicating via synchronous channels. CSP was
introduced in:
Hoare, C. A. R. Communicating Sequential
Processes, Prentice Hall, 1985.
An updated account appears in:
Roscoe, A. William. The Theory and
Practice of Concurrency, Prentice Hall, 1997.
Several of the texts listed in Chapter
1 (including the book by Burns and Welling in �
1.2.5.4) discuss CSP in the course of describing
constructs in occam and Ada.
Other related formalisms, design
techniques, languages, and frameworks have adopted different
base assumptions that adhere more closely to the
characteristics of other class=docTextHighlight>concurrent systems and/or to
different styles of analysis.
These include Milner's CCS and face=symbol>p-calculus, and Berry's Esterel. See:
Milner, Robin. Communication and
Concurrency, Prentice Hall, 1989.
Berry, Gerard. "The Foundations of Esterel",
in Gordon Plotkin, Colin Stirling, and Mads Tofte (eds.),
Proof, Language and Interaction, MIT Press, 1998.
As package support becomes
available for these and related approaches to class=docTextHighlight>concurrent system design, they
become attractive alternatives to the direct use of
thread-based constructs in the development of systems that are
best viewed conceptually as collections of active
objects.
For example, Triveni is an
approach based in part on Esterel, and is described in:
Colby, Christopher, Lalita Jategaonkar
Jagadeesan, Radha Jagadeesan, Konstantin L�ufer, and Carlos
Puchol. "Objects and Concurrency in Triveni: A
Telecommunication Case Study in class=docTextHighlight>Java", USENIX Conference on
Object-Oriented Technologies and Systems (COOTS),
1998.
Triveni is supported by a class=docTextHighlight>Java programming language
package (see the online supplement). Among its main
differences from CSP is that active objects in Triveni
communicate by issuing events. Triveni also includes
computation and composition rules surrounding the interruption
and suspension of activities upon reception of events, which
adds to expressiveness especially in real-time design
contexts.
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