<!--
  - Authors:
  -   Seif Haridi (seif@sics.se)
  -   Nils Franzen (nilsf@sics.se)
  -
  - Copyright:
  -   Seif Haridi, Nils Franzen, 1998
  -
  - Last change:
  -   $Date: 2000/02/08 07:46:49 $ by $Author: nilsf $
  -   $Revision: 1.13 $
  -
  - This file is part of Mozart, an implementation
  - of Oz 3
  -    http://www.mozart-oz.org
  -
  - See the file "LICENSE" or
  -    http://www.mozart-oz.org/LICENSE.html
  - for information on usage and redistribution
  - of this file, and for a DISCLAIMER OF ALL
  - WARRANTIES.
  -
  -->

<CHAPTER ID="chapter.objectscc">
<TITLE>Objects and Concurrency</TITLE>

<P>As we have seen, objects in Oz are stateful data
structures. Threads are the active computation entities. Threads can
communicate either by message passing using ports, or through common
shared objects. Communication through shared objects requires the
ability to serialize concurrent operations on objects so that the
object state is kept coherent after each such an operation. In Oz, we
separate the issue of acquiring exclusive access of an object from the
object system. This gives us the ability to perform coarse-grain
atomic operation on a set of objects, a very important requirement, e.g. in
distributed database systems. The basic mechanism in Oz to get
exclusive access is through locks.


<SECTION id=section.objectscc.lock><TITLE/Locks/
<P>The purpose of a lock is to mediate exclusive access to a shared
resource between threads. Such a mechanism is typically made safer and
more robust by restricting this exclusive access to a critical
region. On entry into the region, the lock is secured and the thread
is granted exclusive access rights to the resource, and when execution
leaves the region, whether normally or through an exception, the lock
is released. A concurrent attempt to obtain the same lock will block
until the thread currently holding it has released it.


<SUBSECTION><TITLE/Simple Locks/
<P>In the case of a simple lock, a nested attempt by the same thread
to reacquire the same lock during the dynamic scope of a critical
section guarded by the lock will block. We say <EM>reentrancy</EM> is
not supported. Simple locks can be modeled in Oz as follows, where
<<Code>> is a nullary procedure encapsulating the computation to be
performed in the critical section.  The lock is represented as a
procedure: when applied to some code, it tries to get the lock
by waiting until <<Old>> gets bound to <<unit>>. Notice that the lock is
released upon normal as well as abnormal exit.

<<<
proc {NewSimpleLock ?Lock}
   Cell = {NewCell unit}
in
   proc {Lock Code}
      Old New in 
      try
         {Exchange Cell Old New}
         {Wait Old} {Code} 
      finally New=unit end
   end
end
>>>


<SUBSECTION><TITLE/Atomic Exchange on Object Attributes/
<P>Another implementation is using an object as shown below to
implement a lock. Notice the use of the construct:

<<<Old = lck &lt;- New>>>

<P>Similar to the Exchange operation on cells, this is an atomic
exchange on an object attribute.

<<<
class SimpleLock
   attr lck:unit
   meth init skip end
   meth 'lock'(Code)
     Old New in
     try 
        Old = lck &lt;- New
        {Wait Old} {Code}
     finally New= unit end
   end
end
>>>


<SECTION id=section.objectscc.reentrant><TITLE/Thread-Reentrant Locks/
<P>In Oz, the computational unit is the thread. Therefore an
appropriate locking mechanism should grant exclusive access rights to
threads. As a consequence the non-reentrant simple lock mechanism
presented above is inadequate. A thread-reentrant lock allows the same
thread to reenter the lock, i.e. to enter a dynamically nested
critical region guarded by the same lock. Such a lock can be acquired
by at most one thread at a time.  Concurrent threads that attempt to
get the same lock are queued. When the lock is released, it is granted
to the thread standing first in line etc. Thread-reentrant locks can
be modeled in Oz as follows:

<!-- DENYS: fixed a race condition bug in previous definition -->
<<<
class ReentrantLock from SimpleLock
   attr Current:unit
   meth 'lock'(Code)
      ThisThread = {Thread.this} in
      if ThisThread == @Current then
	 {Code}
      else
	 proc {Code1}
	    try
	       Current &lt;- ThisThread
	       {Code}
	    finally
	       Current &lt;- unit
	    end
	 end
      in
	 SimpleLock, 'lock'(Code1)
      end
   end
end
>>>

<P>Thread reentrant locks are given syntactic and implementational
support in Oz. They are implemented as subtype of chunks. Oz provides
the following syntax for guarded critical regions:

<<<lock ?{E} then ?{S} end>>>

<P>?{E} is an expression that evaluates to a lock. The construct
blocks until ?{S} is executed. If ?{E} is not a lock, then a type
error is raised.

<LIST>

<ITEM><<{NewLock L}>> creates a new lock <<L>>.   

<ITEM><<{IsLock E>>} returns true iff  <<E >>is a lock.

</LIST>


<SUBSECTION><TITLE/Arrays/
<P>Oz has arrays as chunk subtype. Operations on arrays are defined in
module <<Array>>.

<LIST>

<ITEM><<{NewArray +L +H +I ?A}>> creates an array <<A>>, where <<L>>
is the lower-bound index, <<H>> is the higher-bound index, and <<I>> is
the initial value of the array elements.

<ITEM><<{Array.low +A ?L}>> returns the lower index.

<ITEM><<{Array.high +A ?L}>> returns the higher index.

<ITEM><<{Get +A +I ?R}>> returns <<A[I]>> in <<R>>.

<ITEM><<{Put +A +I X}>> assigns <<X>> to the entry <<A[I]>>.

</LIST>

<P>As a simple illustration of the use of locks consider the program
in <PTR TO="usingLock">. The procedure <<Switch>> transforms negative
elements of an array to positive, and zero elements to the atom
<<zero>>! The procedure <<Zero>> resets all elements to zero.

<P><FIGURE ID="usingLock" CLASS="Figure">
<CAPTION>Using a lock</CAPTION>
<<<
declare A L in
A = {NewArray 1 100 ~5}
L = {NewLock}
proc {Switch A}
   {For {Array.low A} {Array.high A} 1
    proc {$ I} 
       X = {Get A I} in
       if X<0 then {Put A I ~X}
       elseif X == 0  then {Put A I zero} end
       {Delay 100}
    end}
end
proc {Zero A}
   {For {Array.low A} {Array.high A} 1
    proc {$ I} {Put A I 0} {Delay 100} end}
end   
>>>
</FIGURE>

<P>Try the following program.

<<<
local X Y in
   thread {Zero A} X = unit end  
   thread {Switch A} Y = X end 
   {Wait Y} 
   {For 1 10 1 proc {$ I} {Browse {Get A I}} end} 
end
>>>

<P>The elements of the array will be mixed <<0>> and <<zero>>.

<P>Assume that we want to perform the procedures <<Zero>> and
<<Switch>>, each atomically but in an arbitrary order. To do this we
can use locks as in the following example.

<<<
local X Y in
   thread 
      {Delay 100} 
      lock L then {Zero A} end  
      X = unit 
   end 
   thread 
      lock L then {Switch A} end 
      Y = X 
   end 
   {Wait Y} 
   {For 1 10 1 proc {$ I} {Browse {Get A I}} end} 
end
>>>

<P>By Switching the delay statement above between the first and the
second thread, we observe that all the elements of the array either
will get the value <<zero>> or <<0>>. We have no mixed values.

<P CLASS=WARNING>*** Write an example of an atomic transaction on
multiple objects using multiple locks.


<SECTION id=section.objectscc.lockobject><TITLE/Locking Objects/
<P>To guarantee mutual exclusion on objects one may use the locks
described in the previous subsection. Alternatively, we may declare in
the class that its instance objects can be locked with a default lock
existing in the objects when they are created.  A class with an
implicit lock is declared as follows:

<<<
class ?{C} from ....
   prop locking
  ....
end
>>>

<P>This does not automatically lock the object when one of its methods
is called. Instead we have to use the construct:

<<<lock ?{S} end>>>

<P>inside any method to guarantee exclusive access when ?{S} is executed.
Remember that our locks are thread-reentrant. This implies that:

<LIST>

<ITEM>if we take all objects that we have constructed and enclose each
method body with <<lock ... end>>, and

<ITEM>execute our program with only one thread, then

<ITEM>the program will behave exactly as before

</LIST>

<P>Of course, if we use multiple threads calling methods in multiple
objects, we might deadlock if there is any cyclic dependency. Writing
nontrivial concurrent programs needs careful understanding of the
dependency patterns between threads. In such programs deadlock may
occur whether locks are used or not. It suffices to have a cyclic
communication pattern for deadlock to occur.

<P>The program in <PTR TO="counterClass"> can be refined to work in
concurrent environment by refining it as follows:

<<<
class CCounter from Counter
   prop locking
   meth inc(Value)
      lock Counter,inc(Value) end
   end
   meth init(Value)
      lock Counter,init(Value) end
   end
end
>>>

<P>Let us now study a number of interesting examples where threads not
only perform atomic transactions on objects, but also synchronize
through objects.


<SECTION id=section.objectscc.fifo><TITLE/Concurrent FIFO Channel/
<P>The first example shows a concurrent channel, which is shared among
an arbitrary number of threads. Any producing thread may put
information in the channel asynchronously. A consuming thread has to
wait until information exists in the channel. Waiting threads are
served fairly. <PTR TO="asyncChannel"> shows one possible
realization. This program relies on the use of logical variables to
achieve the desired synchronization. The method <<put/1>> inserts an
element in the channel. A thread executing the method <<get/1>> will
wait until an element is put in the channel. Multiple consuming
threads will reserve their place in the channel, thereby achieving
fairness. Notice that <<{Wait I}>> is done outside an exclusive
region. If waiting was done inside <<lock ... end>> the program would
deadlock. So, as a rule of thumb:

<LIST>

<ITEM>Do not wait inside an exclusive region, if the waking-up action
has to acquire the same lock.

</LIST>

<P><FIGURE ID="asyncChannel" CLASS="Figure">
<CAPTION>An Asynchronous Channel Class</CAPTION>
<<<
class Channel from BaseObject
   prop locking
   attr f r
   meth init
      X in f &lt;- X r &lt;- X
   end
   meth put(I)
      X in lock @r=I|X r&lt;-X end
   end
   meth get(?I)
      X in lock @f=I|X f&lt;-X end {Wait I}
   end
end
>>>


<SECTION id=section.objectscc.monitor><TITLE/Monitors/
<P>The next example shows a traditional way to write
<EM>monitors</EM>. We start by defining a class that defines the
notion of events and the monitor operations <<notify(Event)>> and
<<wait(Event)>> by specializing the class <<Channel>>.

<<<
class Event from Channel
   meth wait
      Channel , get(_)
   end
   meth notify
      Channel , put(unit)
   end
end
>>>

<P>We show here an example of a unit buffer in the traditional monitor
style. The unit buffer behaves in a way very similar to a channel when
it comes to consumers. Each consumer waits until the buffer is
full. In the case of producers only one is allowed to insert an item
in the empty buffer. Other producers have to suspend until the item is
consumed. The program in <PTR TO="ubMonitor"> shows a single buffer
monitor. Here we had to program a signaling mechanism for producers
and consumers. Observe the pattern in <<put/1>> and <<get/1>>
methods. Most execution is done in an exclusive region. If waiting is
necessary it is done outside the exclusive region. This is done by
using an auxiliary variable <<X>>, which gets bound to <<yes>>.  The
<<get/1>> method notifies one producer at a time by setting the
<<empty>> flag and notifying one producer (if any). This is done as an
atomic step. The <<put/1>> method does the reciprocal action.

<P><FIGURE ID="ubMonitor" CLASS="Figure">
<CAPTION>A Unit Buffer Monitor</CAPTION>
<<<
class UnitBufferM
   attr item empty psignal csignal
   prop locking
   meth init
      empty &lt;- true
      psignal &lt;- {New Event init}
      csignal &lt;- {New Event init}
   end
   meth put(I)
      X in
      lock
	 if @empty then
	    item &lt;- I
	    empty &lt;- false
	    X = yes
	    {@csignal notify}
	 else X = no end
      end
      if X == no then
	 {@psignal wait}
	 {self put(I)}
      end
   end
   meth get(I)
      X in
      lock
	 if {Not @empty} then
	    I = @item
	    empty &lt;- true
	    {@psignal notify}
	    X = yes
	 else X = no end
      end
      if X == no then
	 {@csignal wait}
	 {self get(I)}
      end
   end
end
>>>
</FIGURE>

<P>Try the above example by running the following code:

<<<
local
  UB = {New UnitBufferM init} in
  {For 1 15 1
   proc{$ I} thread {UB put(I)} {Delay 500} end end}
  {For 1 15 1
   proc{$ I} thread {UB get({Browse}}{Delay 500} end end}
end
>>>


<SUBSECTION><TITLE/Bounded Buffers Oz Style/
<P>In Oz, it is very rare to write programs in the monitor style shown
above. In general it is very awkward. There is a simpler way to write
a <<UnitBuffer>> class that is not traditional. This is due to the
combination of objects and logic variable, <PTR TO="unitBuffer"> shows 
a simple definition. No locking is needed directly.

<P><FIGURE ID="unitBuffer" CLASS="Figure">
<CAPTION>Unit Buffer</CAPTION>
<<<
class UnitBuffer from BaseObject
   attr prodq buffer
   meth init
      buffer &lt;- {New Channel init}
      prodq &lt;- {New Event init}
      {@prodq notify}
   end
   meth put(I)
      {@prodq wait}
      {@buffer put(I)}
   end
   meth get(?I)
      {@buffer get(I)}
      {@prodq notify}
   end
end
>>>
</FIGURE>

<P>A simple generalization of the above program leads to an arbitrary
size bounded buffer class. This is shown in below. The put and get
methods are the same as before. Only the initialization method is
changed.

<<<
class BoundedBuffer from UnitBuffer
   attr prodq buffer
   meth init(N)
      buffer &lt;- {New Channel init}
      prodq &lt;- {New Event init}
      {For 1 N 1 proc {$ _} {@prodq notify} end}
   end
end      
>>>


<SECTION id=section.objectscc.active><TITLE/Active Objects/
<P>An active object is a thread whose behavior is described by a class
definition. Communication with active objects is through asynchronous
message passing. An active object reacts to received messages by
executing the corresponding methods in its associated class. An active
object executes one method at a time. Therefore locking is not needed
for methods performed by an active object. The interface to an active
object is through Oz ports. Clients of an active object send messages
to the object by sending messages to its associated port. We will show
how to create generically this abstraction. Since active objects
resemble servers receiving messages from clients though a network we
call this abstraction the server abstraction. To create a server <<S>>
from a class Class we execute:

<<<S = {NewServer Class init}>>>

<P>Here <<init>> is the initial object construction method. To get
the basic idea we show first a simplified form of the <<NewServer>>
function. The following function:

<LIST>

<ITEM>creates a port <<Port>>,

<ITEM>creates an object <<Object>>, and finally

<ITEM>creates a thread that serves messages sent to the port, by 
applying the corresponding class methods.

</LIST>

<<<
fun {NewServer Class Init}
   S    % The stream of the port
   Port = {NewPort S}
   Object = {New Class Init}
in
   thread {ForAll S 
           proc{$ M} {Object M} end}
   end
   Port
end
>>>

<P>We would like to add the ability of terminating the thread by
making a protected method <<Close>> accessible to the method in
<<Class>>. This leads us to the following extension of the above
function. We use the exception handling mechanism to jump out of the
receiving loop.

<<<
local
   class Server
     attr close:Close
     meth Close raise closeException end end
   end
in
fun {NewServer Class Init}
   S    % The stream of the port
   Port = {NewPort S}
   Object = {New class $ from Server Class end Init}
in
   thread 
     try  {ForAll S 
           proc{$ M} {Object M} end}
     catch closeException then skip end
   end
   Port
end
>>>
<!--Figure 39: Server Abstraction -->

</CHAPTER>
