Class Alternative
- java.lang.Object
-
- org.jcsp.lang.Alternative
-
public class Alternative extends Object
This enables a process to wait passively for and choose between a number ofGuard
events.Shortcut to the Constructor and Method Summaries.
Description
TheAlternative
class enables aCSProcess
to wait passively for and choose between a number ofGuard
events. This is known asALT
ing.Note: for those familiar with the occam multiprocessing language, this gives the semantics of the
ALT
andPRI
ALT
constructs, extended with a built-in implementation of the classicalFAIR
ALT
.The
Alternative
constructor takes an array of guards. Processes that need to Alt over more than one set of guards will need a separateAlternative
instance for each set.Eight types of
Guard
are provided injcsp.lang
:-
AltingChannelInput
: object channel input -- ready if unread data is pending in the channel. -
AltingChannelInputInt
: integer channel input -- ready if unread data is pending in the channel. -
AltingChannelOutput
: object channel output -- ready if a reading process can take the offered data (symmetric
channels only). -
AltingChannelOutputInt
: integer channel output -- ready if a reading process can take the offered data (symmetric
channels only). -
AltingChannelAccept
: CALL accept -- ready if an unaccepted call is pending. -
AltingBarrier
: barrier synchronisation -- ready if all enrolled processes are offering to synchronise. -
CSTimer
: timeout -- ready if the timeout has expired (timeout values are absolute time values, not delays) -
Skip
: skip -- always ready.
By invoking one of the following methods, a process may passively wait for one or more of the guards associated with an
Alternative
object to become ready. The methods differ in the way they choose which guard to select in the case when two or more guards are ready:-
waits for one or more of the guards to become ready. If more than one become ready, it makes an arbitrary choice between them (and corresponds to the occamselect
ALT
). -
also waits for one or more of the guards to become ready. However, if more than one becomes ready, it chooses the first one listed (and corresponds to the occampriSelect
PRI
ALT
). Note: the use ofpriSelect
between channel inputs and a skip guard (at lowest priority) gives us a polling operation on the readiness of those channels. -
also waits for one or more of the guards to become ready. If more than one become ready, it prioritises its choice so that the guard it chose the last time it was invoked has lowest priority this time. This corresponds to a common occam idiom used for real-time applications. IffairSelect
fairSelect
is used in a loop, a ready guard has the guarantee that no other guard will be serviced twice before it will be serviced. This enables an upper bound on service times to be calculated and ensures that no ready guard can be indefinitely starved.
Finally, each guard may be pre-conditioned with a run-time test to decide if it should be considered in the current choice. This allows considerable flexibilty -- for example, we can decide whether timeouts shoud be set, channels refused or polling enabled depending on the run-time state of the Alting process.
Examples
A Fair Multiplexor
This example demonstrates a process that fairly multiplexes traffic from its array of input channels to its single output channel. No input channel will be starved, regardless of the eagerness of its competitors.import org.jcsp.lang.*; public class FairPlex implements CSProcess { private final AltingChannelInput[] in; private final ChannelOutput out; public FairPlex (final AltingChannelInput[] in, final ChannelOutput out) { this.in = in; this.out = out; } public void run () { final Alternative alt = new Alternative (in); while (true) { final int index = alt.fairSelect (); out.write (in[index].read ()); } } }
Note that ifpriSelect
were used above, higher-indexed channels would be starved if lower-indexed channels were continually demanding service. Ifselect
were used, no starvation analysis is possible. Theselect
mechanism should only be used when starvation is not an issue.A Fair Multiplexor with a Timeout and Poisoning
This example demonstrates a process that fairly multiplexes traffic from its input channels to its single output channel, but which timeouts after a user-settable time. Whilst running, no input channel will be starved, regardless of the eagerness of its competitors. The process also illustrates the poisoning of channels, following the timeout.import org.jcsp.lang.*; public class FairPlexTime implements CSProcess { private final AltingChannelInput[] in; private final ChannelOutput out; private final long timeout; public FairPlexTime (final AltingChannelInput[] in, final ChannelOutput out, final long timeout) { this.in = in; this.out = out; this.timeout = timeout; } public void run () { final Guard[] guards = new Guard[in.length + 1]; System.arraycopy (in, 0, guards, 0, in.length); final CSTimer tim = new CSTimer (); final int timerIndex = in.length; guards[timerIndex] = tim; final Alternative alt = new Alternative (guards); boolean running = true; tim.setAlarm (tim.read () + timeout); while (running) { final int index = alt.fairSelect (); if (index == timerIndex) { running = false; } else { out.write (in[index].read ()); } } System.out.println ("\n\r\tFairPlexTime: timed out ... poisoning all channels ..."); for (int i = 0; i < in.length; i++) { in[i].poison (42); // assume: channel immunity < 42 } out.poison (42); // assume: channel immunity < 42 } }
Note that ifpriSelect
were used above, higher-indexed guards would be starved if lower-indexed guards were continually demanding service -- and the timeout would never be noticed. Ifselect
were used, no starvation analysis is possible.Sometimes we need to use
priSelect
to impose a specific (as opposed to fair) choice that overcomes the external scheduling of events. For example, if we were concerned that the timeout above should be responded to immediately and unconcerned about the fair servicing of its channels, we should put itsCSTimer
as the first element of itsGuard
array and usepriSelect
.To demonstrate
FairPlexTime
, consider:import org.jcsp.lang.*; import org.jcsp.plugNplay.*; class FairPlexTimeTest { public static void main (String[] args) { final One2OneChannel[] a = Channel.one2OneArray (5, 0); // poisonable channels (zero immunity) final One2OneChannel b = Channel.one2One (0); // poisonable channels (zero immunity) final long timeout = 5000; // 5 seconds new Parallel ( new CSProcess[] { new Generate (a[0].out (), 0), new Generate (a[1].out (), 1), new Generate (a[2].out (), 2), new Generate (a[3].out (), 3), new Generate (a[4].out (), 4), new FairPlexTime (Channel.getInputArray (a), b.out (), timeout), new Printer (b.in (), "FairPlexTimeTest ==> ", "\n") } ).run (); } }
whereGenerate
sends its given Integer down its output channel as often as it can. This results in continuous demands on FairPlexTime by all its clients and demonstrates its fair servicing of those demands.The
Generate
andPrinter
are programmed to deal with being poisoned. Here is the run() method for Generate:public void run() { try { while (true) { out.write (N); } } catch (PoisonException p) { // the 'out' channel must have been posioned ... nothing left to do! } }
In general, there will be things to do – especially if there is more than one channel. For example, here is the catch block at the end of the run() method forDelta
(which has a single input channel-end, in, and an array of output channel-ends, out):} catch (PoisonException p) { // don't know which channel was posioned ... so, poison them all! int strength = p.getStrength (); // use same strength of poison in.poison (strength); for (int i = 0; i < out.length; i++) { out[i].poison (strength); } }
A Simple Traffic Flow Regulator
TheRegulate
process controls the rate of flow of traffic from its input to output channels. It produces a constant rate of output flow, regardless of the rate of its input. At the end of each timeslice defined by the required output rate, it outputs the last object input during that timeslice. If nothing has come in during a timeslice, the previous output will be repeated (note: this will be anull
if nothing has ever arrived). If the input flow is greater than the required output flow, data will be discarded.The interval (in msecs) defining the output flow rate is given by a constructor argument. This can be changed at any time by sending a new interval (as a
Long
) down thereset
channel.Note: this example shows how simple it is to program time-regulated functionality like that performed by
java.awt.Component.repaint
.package org.jcsp.plugNplay; import org.jcsp.lang.*; public class Regulate implements CSProcess { private final AltingChannelInput in, reset; private final ChannelOutput out; private final long initialInterval; public Regulate (final AltingChannelInput in, final AltingChannelInput reset, final ChannelOutput out, final long initialInterval) { this.in = in; this.reset = reset; this.out = out; this.initialInterval = initialInterval; } public void run () { final CSTimer tim = new CSTimer (); final Guard[] guards = {reset, tim, in}; // prioritised order final int RESET = 0; // index into guards final int TIM = 1; // index into guards final int IN = 2; // index into guards final Alternative alt = new Alternative (guards); Object x = null; // holding object long interval = initialInterval; long timeout = tim.read () + interval; tim.setAlarm (timeout); while (true) { switch (alt.priSelect ()) { case RESET: interval = ((Long) reset.read ()).longValue (); timeout = tim.read (); // fall through case TIM: out.write (x); timeout += interval; tim.setAlarm (timeout); break; case IN: x = in.read (); break; } } } }
To demonstrate
Regulate
, consider:class RegulateTest { public static void main (String[] args) { final One2OneChannel a = Channel.one2One (); final One2OneChannel b = Channel.one2One (); final One2OneChannel c = Channel.one2One (); final One2OneChannel reset = Channel.one2one (new OverWriteOldestBuffer (1)); new Parallel ( new CSProcess[] { new Numbers (a.out ()), // generate numbers new FixedDelay (250, a.in (), b.out ()), // let them through every quarter second new Regulate (b.in (), reset.in (), c.out (), 1000), // initially sample every second new CSProcess () { public void run () { Long[] sample = {new Long (1000), new Long (250), new Long (100)}; int[] count = {10, 40, 100}; while (true) { for (int cycle = 0; cycle < sample.length; cycle++) { reset.write (sample[cycle]); System.out.println ("\nSampling every " + sample[cycle] + " ms ...\n"); for (int i = 0; i < count[cycle]; i++) { Integer n = (Integer) c.read (); System.out.println ("\t==> " + n); } } } } } } ).run (); } }
The reader may like to consider the danger of deadlock in the above system if thereset
channel were not an overwriting one.Polling
Sometimes, we want to handle incoming channel data if it's there, but get on with something else if all is quiet. This can be done byPRI
ALT
ing the channels we wish to poll against aSKIP
guard:import org.jcsp.lang.*; public class Polling implements CSProcess { private final AltingChannelInput in0; private final AltingChannelInput in1; private final AltingChannelInput in2; private final ChannelOutput out; public Polling (final AltingChannelInput in0, final AltingChannelInput in1, final AltingChannelInput in2, final ChannelOutput out) { this.in0 = in0; this.in1 = in1; this.in2 = in2; this.out = out; } public void run() { final Skip skip = new Skip (); final Guard[] guards = {in0, in1, in2, skip}; final Alternative alt = new Alternative (guards); while (true) { switch (alt.priSelect ()) { case 0: ... process data pending on channel in0 ... break; case 1: ... process data pending on channel in1 ... break; case 2: ... process data pending on channel in2 ... break; case 3: ... nothing available for the above ... ... so get on with something else for a while ... ... then loop around and poll again ... break; } } } }
The above technique lets us poll anyGuard
events, including timeouts. If we just want to poll channels for input events, see thepending
methods of the various ``...2One...'' channels for a more direct and efficient way.Note: polling is an often overused technique. Make sure your design would not be better suited with a blocking ALT and with the `something else' done by a process running in parallel.
To this end, a formal (CSP) model of Java's monitor primitives (theThe `Wot-no-Chickens?' Canteen
This examples demonstrates the use of pre-conditions on theALT
guards. TheCanteen
process buffers a supply of chickens. It can hold a maximum of 20 chickens. Chickens are supplied on thesupply
line in batches of, at most, 4. Chickens are requested by hungry philosophers who share therequest
line to theCanteen
. In response to such requests, one chicken is delivered down thedeliver
line.The
Canteen
refuses further supplies if it has no room for the maximum (4) batch supply. TheCanteen
refuses requests from the philosophers if it has no chickens.import org.jcsp.lang.*; public class Canteen implements CSProcess { private final AltingChannelInput supply; // from the cook private final AltingChannelInput request; // from a philosopher private final ChannelOutput deliver; // to a philosopher public Canteen (final AltingChannelInput supply, final AltingChannelInput request, final ChannelOutput deliver) { this.supply = supply; this.request = request; this.deliver = deliver; } public void run() { final Guard[] guard = {supply, request}; final boolean[] preCondition = new boolean[guard.length]; final int SUPPLY = 0; final int REQUEST = 1; final Alternative alt = new Alternative (guard); final int maxChickens = 20; final int maxSupply = 4; final int limitChickens = maxChickens - maxSupply; final Integer oneChicken = new Integer (1); // ready to go! int nChickens = 0; // invariant : 0 <= nChickens <= maxChickens while (true) { preCondition[SUPPLY] = (nChickens <= limitChickens); preCondition[REQUEST] = (nChickens > 0); switch (alt.priSelect (preCondition)) { case SUPPLY: nChickens += ((Integer) supply.read ()).intValue (); // <= maxSupply break; case REQUEST: Object dummy = request.read (); // we have to still input the signal deliver.write (oneChicken); // preCondition ==> (nChickens > 0) nChickens--; break; } } } }
Contrast the above programming of the canteen as a CSP process rather than a monitor. A monitor cannot refuse a callback when noone has the lock, even though it may not be in a state to process it. In the above, a
supply
method would have to cope with its being called when there is no room to take the supply. Arequest
method would have to be dealt with even though there may be no chickens to deliver. Monitors manage such problems by putting their callers on hold (wait
), but that means that their methods have to rely on each other to get out of any resulting embarassment (usingnotify
). And that means that the logic of those methods has to be tightly coupled, which makes reasoning about them hard. This gets worse the more interdependent methods the monitor has.On the other hand, the above
Canteen
process simply refuses service on itssupply
andrequest
channels if it can't cope, leaving the supplying or requesting processes waiting harmlessly on those channels. The service responses can assume their run-time set pre-conditions and have independent -- and trivial -- logic. When circumstances permit, the blocked processes are serviced in the normal way.Implementation Footnote
ThisAlternative
class and the various channel classes (e.g.One2OneChannel
) are mutually dependent monitors -- they see instances of each other and invoke each others' strongly interdependent methods. This logic is inspired by the published algorithms and data structures burnt into the microcode of the transputer some 15 years ago (1984). Getting this logic `right' in the context of Java monitors is something we have done(n + 1)
times, only to find it flawedn
times with an unsuspected race-hazard months (sometimes years) later. Hopefully, we have it right now ... but a proof of correctness is really needed!synchronized
keyword and thewait
,notify
andnotifyAll
methods of theObject
class) has been built. This has been used for the formal verification of the JCSP implementation of channelread
andwrite
, along with the correctness of 2-way channel inputAlternative
s. Details and references are listed under `A CSP Model for Java Threads' on the JCSP web-site. [The proof uses the FDR model checker. Model checkers do not easily allow verification of results containing free variables - such as the correctness of the n-wayAlternative
. An investigation of this using formal transformation of one system of CSP equations into another, rather than model checking is being considered.]The transputer designers always said that getting its microcoded scheduler logic right was one of their hardest tasks. Working directly with the monitor concept means working at a similar level of difficulty for application programs. One of the goals of JCSP is to protect users from ever having to work at that level, providing instead a range of CSP primitives whose ease of use scales well with application complexity -- and in whose implementation those monitor complexities are correctly distilled and hidden.
- Author:
- P.H. Welch and P.D. Austin
- See Also:
Guard
,AltingChannelInput
,AltingChannelInputInt
,AltingChannelAccept
,AltingBarrier
,CSTimer
,Skip
-
-
-
Field Summary
Fields Modifier and Type Field Description protected Object
altMonitor
The monitor synchronising the writers and alting reader
-
Constructor Summary
Constructors Constructor Description Alternative(Guard[] guard)
Construct anAlternative
object operating on theGuard
array of events.
-
Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description int
fairSelect()
Returns the index of one of the ready guards.int
fairSelect(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true.int
priSelect()
Returns the index of one of the ready guards.int
priSelect(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true.int
select()
Returns the index of one of the ready guards.int
select(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true.
-
-
-
Field Detail
-
altMonitor
protected Object altMonitor
The monitor synchronising the writers and alting reader
-
-
Constructor Detail
-
Alternative
public Alternative(Guard[] guard)
Construct anAlternative
object operating on theGuard
array of events. Supported guard events are channel inputs (AltingChannelInput
andAltingChannelInputInt
), CALL channel accepts (AltingChannelAccept
), barriers (AltingBarrier
), timeouts (CSTimer
) and skips (Skip
).- Parameters:
guard
- the event guards over which the select operations will be made.
-
-
Method Detail
-
select
public final int select()
Returns the index of one of the ready guards. The method will block until one of the guards becomes ready. If more than one is ready, an arbitrary choice is made.
-
priSelect
public final int priSelect()
Returns the index of one of the ready guards. The method will block until one of the guards becomes ready. If more than one is ready, the one with the lowest index is selected.
-
fairSelect
public final int fairSelect()
Returns the index of one of the ready guards. The method will block until one of the guards becomes ready. Consequetive invocations will service the guards `fairly' in the case when many guards are always ready. Implementation note: the last guard serviced has the lowest priority next time around.
-
select
public final int select(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true. The method will block until one of these guards becomes ready. If more than one is ready, an arbitrary choice is made.Note: the length of the
preCondition
array must be the same as that of the array of guards with which this object was constructed.- Parameters:
preCondition
- the guards from which to select
-
priSelect
public final int priSelect(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true. The method will block until one of these guards becomes ready. If more than one is ready, the one with the lowest index is selected.Note: the length of the
preCondition
array must be the same as that of the array of guards with which this object was constructed.- Parameters:
preCondition
- the guards from which to select
-
fairSelect
public final int fairSelect(boolean[] preCondition)
Returns the index of one of the ready guards whosepreCondition
index is true. The method will block until one of these guards becomes ready. Consequetive invocations will service the guards `fairly' in the case when many guards are always ready. Implementation note: the last guard serviced has the lowest priority next time around.Note: the length of the
preCondition
array must be the same as that of the array of guards with which this object was constructed.- Parameters:
preCondition
- the guards from which to select
-
-