## sharedobj/src/SharedObj.i3

-*- Mode: Modula-3 -*-
*
* (bm@cs.columbia.edu) or Steven Feiner (feiner@cs.columbia.edu)
* at the Computer Science Dept., Columbia University,
* 1214 Amsterdam Ave. Mailstop 0401, New York, NY, 10027.
*
* Copyright (C) 1995, 1996 by The Trustees of Columbia University in the
* City of New York.  Blair MacIntyre, Computer Science Department.
* See file COPYRIGHT-COLUMBIA for details.
*
* Author          : Blair MacIntyre
* Created On      : Thu Mar 30 17:21:53 1995
* Update Count    : 108
*
* $Source: /usr/cvs/cm3/m3-comm/sharedobj/src/SharedObj.i3,v$
* $Date: 2001-12-02 13:41:16$
* $Author: wagner$
* $Revision: 1.2$
*
* $Log: SharedObj.i3,v$
* Revision 1.2  2001-12-02 13:41:16  wagner
* add copyright notes, fix overrides for cm3, and make everything compile(except tests)
*
* modified: sharedobj/src/LocalObjectSpace.i3
* modified: sharedobj/src/LocalObjectSpace.m3
* modified: sharedobj/src/Message.i3
* modified: sharedobj/src/Message.m3
* modified: sharedobj/src/ObjCopy.i3
* modified: sharedobj/src/ObjCopy.m3
* modified: sharedobj/src/ObjectInfo.i3
* modified: sharedobj/src/ObjectInfo.m3
* modified: sharedobj/src/ObjectSpace.i3
* modified: sharedobj/src/ObjectSpace_FindObjCallBack_v1.i3
* modified: sharedobj/src/ObjectSpace_FindObjCallBack_v1.m3
* modified: sharedobj/src/ObjectSpace_FindObjCallBack_v2.i3
* modified: sharedobj/src/ObjectSpace_FindObjCallBack_v2.m3
* modified: sharedobj/src/ObjectSpace_T_v1.i3
* modified: sharedobj/src/ObjectSpace_T_v1.m3
* modified: sharedobj/src/ObjectSpace_T_v2.i3
* modified: sharedobj/src/ObjectSpace_T_v2.m3
* modified: sharedobj/src/SharedObj.i3
* modified: sharedobj/src/SharedObjError.i3
* modified: sharedobj/src/SharedObjF.i3
* modified: sharedobj/src/SharedObjF.m3
* modified: sharedobj/src/SharedObjRT.i3
* modified: sharedobj/src/SharedObjRT.m3
* modified: sharedobj/src/SharedObjRTF.i3
* modified: sharedobj/src/SharedObjRep.i3
* modified: sharedobj/src/SharedObjRep.m3
* modified: sharedobj/src/SharedObjStubLib.i3
* modified: sharedobj/src/SharedObjStubLib.m3
* modified: sharedobj/src/SpaceConn.i3
* modified: sharedobj/src/SpaceConn.m3
* modified: sharedobj/src/WeakerRef.i3
* modified: sharedobj/src/WeakerRef.m3
* modified: sharedobj/src/m3makefile
* modified: sharedobj/src/m3overrides
* modified: sharedobj/tests/netobjtest/src/m3makefile
* modified: sharedobj/tests/obsequence/src/m3makefile
* modified: sharedobj/tests/tracker/src/m3makefile
*
* Revision 1.1.1.1  2001/12/02 13:14:14  wagner
* Blair MacIntyre's sharedobj package
*
* Revision 1.4  1998/05/11 02:34:15  bm
*
* Revision 1.3  1997/03/12 21:50:40  bm
* Bug fix.
*
* Revision 1.2  1996/11/22 19:01:32  bm
*
*
* HISTORY

The shared object package is designed to complement Network objects. A network object allows an object to be shared by multiple processes, possibly on different machines, by creating surrogate objects in all processes except the one in which the object actually exists. From the point of view of the programmer and the executing program, the surrogate object behaves exactly like the original object. However, all method calls to the surrogates are synchronously sent to the original object where they are executed, with return values or exceptions propogated back to the calling process.

For many applications, this is sufficient and has many desirable properties: there are no sychronization issues, calls are synchronous so exceptions propogate across processes, global garbage collection is performed, etc. However, for some applicates, the paradigm is not appropriate. The sychronous nature method calls restricts the frequency and of checking the object for changes. More seriously, interested parties are restricted to polling an object if they are interested in changes. Finally, all operations on an object, no matter how trivial, require a remote procedure call, which takes a significant amount of time.

To address these problems, and others, the shared object package was created. The model is the opposite of the network object package and is intended to complement rather that replace it. Instead of the object being stored at one location and remote method calls being used to access the object, shared objects are fully replicated in all interested processes, with any updates to the object being applied to all copies.

\section{Differences between Shared and Network Objects}

The easiest way to explain the shared object package is to compare it to the network object package. In order to support full replication, there are some important behavioural differences between network and shared objects. In the discussion below, update methods refer to methods which change the internal state of an object. The local copy is the copy that resides in the process in which the method call is made.

\begin{list} \item Methods which update the shared object are applied to all copies of the shared object. It is important to realize that the actual method is called at all sites. The shared object package guarantees that all updates will be applied to all copies of an object in the same order, but makes to guarantees about the specific order of application of two method calls performed at approximately the same time. \item In order to provide the above guarantee, update method calls are not performed immediately on the local copy of an object. The caller will block until it is time for the update to be performed, then will be unblocked, perform the update, and return. \item Restrictions are placed on the kinds of parameters that can be used with update methods, due to the requirement that these parameters be distributed to all copies so the update method can be applied everywhere. Specifically, network objects, shared objects, data streams (subtypes of Rd.T or Wr.T) and any object that is associated with a particular process should not be used. (The restriction on using network objects and shared objects as arguments to update methods may be lifted someday if absolutely necessary). \item Network object method calls are performed synchronously. Thus, values can be returned and exceptions propogated back to the original site. Shared object update method calls are performed synchronously on the local copy, but asynchronously on all other copies. Thus, return values and exceptions (except the SharedObj.Fatal exception, as described below) are ignored in all copies except the local one. \item Non-update method calls are performed immediately on the local copy. Thus, read access to shared objects is significantly faster that to network objects. \item Since network objects exist at only one location, programmers are free to create subtypes of network objects which still appear as the original network object when sent out over the network. This works because the network object stub generator only generates the surrogate objects that stand in for the object on remote machines: the real object is not changes. Shared objects, on the other hand, must not be subtyped, as the shared object stub generator generates an object which encapsulates the original object, overriding all methods to create the desired behaviour. If subtypes are created and distributed, incorrect behaviour may result. \item Network objects provide no way for anyone with a copy of the network object to be notified when it changes. It is straightforward to add this to a specific network objects explicitely in exactly the same way as one would do it to any other objects. All shared objects, on the other hand, have change notification built in via Callback objects. When a shared object is generated with the stub generator, a corresponding callback object is generated, as described below. \end{list}.

\section{The Shared Object Package}

The primary public interface for using shared objects is described here.

INTERFACE SharedObj;

IMPORT Atom, AtomList, Thread, EventNumber, EmbProxiedObj, Wr,
Pickle2 AS Pickle, Rd;

CONST Brand = "Shared Objects v.1";

TYPE
T <: Public;
Public = EmbProxiedObj.T OBJECT END;

PROCEDURE Init(s: T): T;

SharedObj.T is the root type of all shared objects. All shared object types are required to have their init method call the shared object Init procedure to properly initialize a shared object. Furthermore, all shared object are EmbProxiedObj.Ts, allowing them to be embedded in an interpretted language such as Obliq.

TYPE Code = AtomList.T;

EXCEPTION Error(Code);           (* Error condition. *)
EXCEPTION Fatal(Code);           (* Fatal error condition. *)

Shared objects raise two kinds of exceptions, Error(code) and Fatal(code), where code is an AtomList.T describing the exception. Error exceptions are algorithmic/logic errors that will occur in all copies of an object when a update method is applied. Since it is raised in all copies, they will remain sychronized, and the original caller will receive the exception. Fatal() exceptions, on the other hand, are meant for situations that may not occur on all copies of the object, such as out of memory or disk space, unavailable services, etc. If this exception is raised during an update method of an object, it becomes invalid and must be recreated. This would typcially be done by retrieving another copy from some process that has it.

It is very important that these exceptions be used correctly. Using Error when Fatal should be used will result in copies of the objects being out of sync. Using Fatal when Error would suffice will result in unnecessarily invalidating a copy of the object. Obviously, it is safer to err on the side of raising Fatal too often rather than too little.

VAR                              (*CONST*)

Common exceptions within the shared object runtime system are defined here.

TYPE
Callback = EmbProxiedObj.T OBJECT END;

A Callback object will be subtyped by the shared object stub generator to create a specific Callback object for each specific shared data object. See the example below for the details of specific call callback objects.

TYPE SequenceNumber = EventNumber.T;

The sequence number is the same as an event number. Each update to an object is assigned a sequence number. Users of the package do not currently need to know about sequence numbers.
\subsection{Controlling Shared Object Behaviour}

In order to provide control over shared objects, the following routines are provided.

PROCEDURE Wait(obj: T; c: Thread.Condition; m: Thread.Mutex := NIL);

It is important that a thread not needlessly block while inside a SharedObj method call. If a thread wishes to wait on a condition variable, it should call Wait instead of Thread.Wait. If no Mutex m is supplied, the implicit mutex is used. The calling thread must have m locked if it is supplied.


If a client needs to execute a sequence of actions atomically on a shared object obj, the client should first call AcquireGlobalLock(obj) to acquire a lock on obj. Once the lock is acquired, it is guaranteed that all updates to obj will occur atomically until the lock is released.

PROCEDURE ReleaseGlobalLock (obj: T) RAISES {Error, Thread.Alerted};

When atomicity of updates is no longer required, ReleaseGlobalLock(obj) will release the lock on obj and allow other clients to apply updates to obj.

PROCEDURE Own (obj: T; willingness: Timeliness := 0)

As desribed above, update methods normally take a significant amount of time to process compared to non-update methods. In fact, the time is comparable to the amount of time taken to process a remote object call on a network object.

However, if one client will be doing most, or all, of the updating of an object, it is not necessary for their update method calls to incur this overhead. The Own(obj,willingness) calls declares the local process to be the owner of shared object obj, and the willingness of the local process to provide direct updates to other processes (see the discussion of Timeliness below). Declaring ownership has a number of side effects: \begin{list} \item The ordering of update events will be performed in this process, significantly reducing the time require for update method calls to complete in process. The time is now comparable to a non-update method call. Unfortunately, the time required for other processes to perform update method calls will be increased somewhat. \item Acquiring and releasing the global lock is significantly faster (analogous to the speed improvement for update method calls). The time required for other processes to acquire and release the global lock will be increased somewhat. \item Network traffic may be reduced for updates performed by this object. \item The time required for updates made by this process to reach certain other copies may be reduced (see the discussion of Timeliness below). \end{list}

PROCEDURE Disown (obj: T) RAISES {Error, Thread.Alerted};

When a process that has declared ownership of an object obj no longer needs, or wishes, to be the owner, Disown(obj) reliquishes ownership.

TYPE Timeliness = [-8 .. 7];

PROCEDURE SetTimeliness (obj: T; value: Timeliness)

SetTimeliness(obj, val) is used to specify to the runtime how timely we want updates to obj to arrive at our local copy. The default timeliness is 0. Larger values imply a greater desire for timely updates.

Timeliness values are currently used when some process declares itself the owner of obj. When declaring ownership, a willingness value is provided. Any copy of the object with a timeliness greater than this willingness value will receive updates directly from the object. As a result, declaring a timeliness of FIRST(SharedObj.Timeliness) will prevent the process from ever receiving updates directly. Similarly, declaring a willingness of LAST(SharedObj.Timeliness) will prevent a process from ever sending udpates directly. Any other combination is obviously possible.

\paragraph{Specials.}

Specials provide for customized pickling of specified data types on every call of read or write in this process. See the Pickle package for the details on Pickle.Specials. The Shared Object system uses the Pickle package to transmit data between hosts, by registering a Pickle.Special for each subtype of SharedObj.T. A SharedObj.Special may be registered for a specific subtype of SharedObj.T and is used by the type's corresponding Pickle.Special routines to read and write the user defined data for that object. By default, all of the user defined data fields of a SharedObj.T are writen and read. As with Pickle.Specials:

\begin{itemize} \item the methods must leave the Rd.T or Wr.T positioned after the last byte read or written;

\item the read method must consume the number of bytes written by the write method;

\item the read method must produce a value equivalent to the one that was given to the write method. \end{itemize}

If these rules are violated, the result could be either a checked runtime error or an invalid result from reading a pickle.

There are many ways to program a special. For example, the write method could modify the value and then call the root special. Or the write method could create a related value and call writer.write or Special.write. Or it could write some data fields individually and call writer.write for selected sub-values of its value. Or it could use mixtures of these techniques.

The SharedObj.Special methods cannot be called themselves. Each SharedObj type will have its own subtype of Special generated, whose default methods will read and write the user defined fields of the object. Each type will also have it's own RegisterSpecial(sp) procedure defined, to set the special to be used for that type. 

TYPE
Special = OBJECT
METHODS
write(obj: T; writer: Pickle.Writer)

 \section{Callback Objects} For each shared object generated with the shared object stub generator, a corresponding callback object is also generated. The details of this object are best explained via an example. Assume we have the following shared object definition: TYPE Data = Logitech.TrackerPosition; T <: S; S <: Public; Public = SharedObj.T OBJECT METHODS set (READONLY val: Data) RAISES {SharedObj.Error}; get (VAR val: Data) RAISES {SharedObj.Error, Thread.Alerted}; <* SHARED UPDATE METHODS set *> END; The <* SHARED UPDATE METHODS set *> pragma declares that the set() method is an update method. The following callback object will be generated for this shared object: TYPE T <: Public; Public = SharedObj.Callback OBJECT METHODS init(obj: TrackerPosition.T): T; pre_set (READONLY obj: TrackerPosition.T; READONLY val: TrackerPosition.Data): BOOLEAN; pre_anyChange(READONLY obj: TrackerPosition.T); post_set (READONLY obj: TrackerPosition.T; READONLY val: TrackerPosition.Data): BOOLEAN; post_anyChange(READONLY obj: TrackerPosition.T); END; There are three kinds of methods in the callback object. For a callback object cb and a shared object obj, the methods of cb are: \begin{list} \item {\it Initialization}. Calling cb.init(obj) initializes cb to be a callback for obj. When obj changes, one or more methods of cb will be called. \item {\it Specific callbacks}. For each update method in obj, a corresponding pair of methods exist in cb. One, the pre_ method, is called prior to the update and the other, the post_ method, is called just after the update. It is guaranteed the object passed to the pre_ methods represents the state of the object just prior to the update. Similarly, the object passed to the post_ methods represents the state of the object just after the update. The parameters to the callback methods are obj (the object being updated) and the parameters to the corresponding update method. All of the parameters are read only, as it is not permissable to change them in the callback methods. More importantly, it is not permissable to call any update methods on obj from within a method of cb. The return value is used to indicate if the generic callback should be called, as discussed next. \item {\it Generic Callback}. A pair of callbacks, pre_anyChange(obj) and post_anyChange(obj) are called for any changes to the object when the corresponding specific callback returns FALSE. \end{list} To use the callback objects, the programmer should override those methods representing the kinds of change notification desired. The default implementations of the specific callbacks do nothing and return FALSE, causing the generic callback to be involked. The default generic callbacks do nothing. Therefore, if all that is desired is notification that the object has changed, only the generic callbacks need to be overridden. If some combination of specific and generic notification is desired, some specific callbacks and the generic callbacks can be overridden. When overriding a specific callback, the programmer should return TRUE to indicate that the generic callback should not be called for this update. It is permissable, however, to return FALSE and have the generic callback be called if desired. Consider the example where we have a tracker object above and we wish to noticed the left button on the hypothetical tracker changing. Assume that we obj.set(val) is only called if a change actually occurs. Furthermore, assume we can not tell from the val parameter if the button has changed, and that there is a method obj.getLeftButton() that will return the button value. We could set up a callback as follows to notify us of the change. TYPE MyCallback = T OBJECT left: BOOLEAN; OVERRIDES pre_set := MyPreSet; post_set := MyPostSet; post_anyChange := MyPostAnyChange; END; PROCEDURE MyPreSet(cb: MyCallback; READONLY obj: TrackerPosition.T; READONLY val: TrackerPosition.Data): BOOLEAN = BEGIN left := obj.getLeftButton(); END MyPreSet; PROCEDURE MyPostSet(cb: MyCallback; READONLY obj: TrackerPosition.T; READONLY val: TrackerPosition.Data): BOOLEAN = BEGIN IF left # obj.getLeftButton() THEN IO.Put("Left button changed!\n"); RETURN TRUE; END; RETURN FALSE; END MyPostSet; PROCEDURE MyPostAnyChange(cb: MyCallback; READONLY obj: TrackerPosition.T) = BEGIN IO.Put("Something other than the left button changed.\n"); END MyPostAnyChange; Now consider an example where we have a similar tracker object and we still wish to noticed the left button on the hypothetical tracker changing. We could set up a much simpler callback to notify us of the change. TYPE MyCallback = T OBJECT OVERRIDES pre_set := MyPreSet; pre_anyChange := MyPostAnyChange; END; PROCEDURE MyPreSet(cb: MyCallback; READONLY obj: TrackerPosition.T; READONLY val: TrackerPosition.Data): BOOLEAN = BEGIN IF val.left_button # obj.getLeftButton() THEN IO.Put("Left button changed!\n"); RETURN TRUE; END; RETURN FALSE; END MyPreSet; PROCEDURE MyPreAnyChange(cb: MyCallback; READONLY obj: TrackerPosition.T) = BEGIN IO.Put("Something other than the left button changed.\n"); END MyPreAnyChange; Although contrived, these examples show how to use the callbacks.