dp_DistributeObject dp_UndistributeObject dp_setf dp_AppendTriggerUnique dp_AppendTrigger dp_getf dp_SetTrigger dp_ReleaseTrigger dp_ClearTrigger dp_GetTriggers - Tcl-DP support for replicated, distributed objects
The distributed object commands described here are part of the Tcl-DP (Tcl Distributed Programming) extension to Tcl/Tk. These Tcl/Tk commands are used to manage objects which are distributed among several Tcl/Tk processes. The Tcl/Tk processes must be linked by Tcl-DP remote procedure call (RPC) connections.
An object in Tcl-DP is a procedure, just like a widget instance of Tk is a procedure. A procedural object must take as its first argument a method selector. In other words, if object is the name of a procedural object:
object method ?args ...?
Objects can have slots. Objects which are to be distributed using Tcl-DP must handle three methods, configure and slot-value, which access the slots of the object, and destroy, which destroys the object.
The configure method must handle three different forms of arguments, which makes it very much like the configure option for Tk widgets:
object configure ?-slot? object configure ?-slot value ...? object configure
When the Tcl/Tk expression "object configure" is evaluated, the return value is a list of triplets of the form {-slot default current}, one triplet for each slot of the object. The slot gives the name of the slot, default gives the default value of the slot when the object is created, and current gives the current value of the slot within the object. Furthermore, when the command "object configure -slot" is evaluated, the return value should be the triplet associated with the given slot. The command "object configure -slot value" will set the value of the slot of the object to the given value.
The slot-value method must take one argument:
object slot-value ?slot?
The slot-value method returns the current value of a slot of an object.
The destroy method takes no arguments:
object destroy
The destroy method frees up all memory associated with the ob- ject, and deletes the command "object" from the interpreter.
Objects are usually defined in C for speed, with a Tcl access in- terface, just like Tk widgets are defined in C but have Tcl ac- cess interfaces. Objects can also be defined using Tcl/Tk scripting only, as Tcl-DP supplies several utility procedures for creating objects with Tcl/Tk scripting only.
Each object to be distributed must also support a creator command or makeCmd. This is the command used to create an object. It should be of the form "makeCmd objName ?-slot1 value1 -slot2 value2 ...?". This command should create a procedural object called objName and initialize the slots the of new object.
For example, suppose a point object named p1 has two slots, x and y, and the default value for each slot is 0. Suppose the current value of x is 1 and the current value of y is 2. If the makeCmd is called point, then to create such an object, evaluate the command "point p1 -x 1 -y 2". Then the command "p1 config" will have the return value of {{-x 0 1} {-y 0 2}}, the command "p1 config -x" will have the return value of -x 0 1, and the command "p1 slot-value x" will have the return value of 1.
Several assumptions were made in the development of the distributed object system. First, the graph of RPC connected processes is assumed to be acyclic. If this assumption is violated, deadlock could result. Second, each slot of an object has an owner process. Although the value of a slot can be set in any process, it is most efficient to do so in the owner process.
dp_setf object slot value
The dp_setf command is used to set the value of a slot in a distributed object. The side effect of this command includes replicating the new value of the slot to all processes where the object is distributed.
dp_getf object slot
The dp_getf command is used to get the current value of a slot in a distributed object. It is the counterpart of the dp_setf command.
dp_DistributeObject object processes makeCmd
Dp_DistributeObject arranges for object to be distributed to the given processes. The argument object should be the name of the object to be distributed, processes should be a list of sockId's (which we can use with dp_RPC) which represent connections to remote processes, and makeCmd should be the creator command for the object, as described above.
dp_UndistributeObject object processes
Dp_UndistributeObject arranges for the slots of object to not be shared anymore with remote processes. The processes (which should be a list of sockId's which represent dp_RPC connections to remote processes) supplied should be the same as or a subset of the processes originally passed into dp_DistributeObject. The object still exists in all processes, but its slots are no longer shared.
dp_SetTrigger when object slot trigger-list
Dp_SetTrigger is used to cause each Tcl command in trigger-list to be evaluated whenever a call to dp_setf causes the slot of object to change value. If when is "before," each command in trigger-list will be evaluated before the slot is updated. If when is "after," each command in trigger-list will be evaluated after the slot is set. The following commands can also be used to manipulate this trigger-list.
dp_GetTriggers when object slot cmd
Dp_GetTriggers returns the trigger list associated with slot of object.
dp_AppendTrigger when object slot cmd
Dp_AppendTrigger is used to add cmd to the trigger list associated with slot of object.
dp_AppendTriggerUnique when object slot cmd
Dp_AppendTriggerUnique is used to add cmd to the trigger list associated with slot of object only if cmd is not already part of the trigger list.
dp_ReleaseTrigger when object slot cmd
Dp_ReleaseTrigger deletes cmd from the trigger list associated with slot of object.
dp_ClearTrigger when object slot
Dp_ClearTrigger clears the trigger list associated with slot of object.
Brian Smith (bsmith@cs.berkeley.edu)
Steve Yen (syen@cs.berkeley.edu)
Stephen Tu (tus@roger-rabbit.cs.berkeley.edu)