Moving over to the Patterns Blog

As you may have guessed from this rather stagnant blog, I haven't been working on my model editor for a while now. I have had to focus on getting the rent paid and so my clients are getting all my time these days. (Believe me, if I had a rich uncle, I would do nothing but work on this, but no luck there I'm afraid!)

Fortunately, though, progress is being made by my colleague Andrew Mangogna on some useful components that I hope to integrate or merge with eventually. We've been coordinating closely and I'm very excited about his TclRAL (Tcl Relational Action Language) project. It started out as an in-memory relational extension to Tcl. Now there is an object action language on top of that and a nice model compiler/architecture in the works.

But I think I am going to focus my future posts in two areas right now. I will start posting in the Patterns Blog (should be a link to it over on the right somewhere). And I may rename that Blog to include more than just patterns. Also, as you may have noticed, I am micro-blogging on Twitter. That way, if I am too lazy to do a proper blog post, at least you can see what I am working on.

So for now, this blog is going in the deep freeze until I can allocate some serious time to build that action language metamodel. (Or find a partner that wants to fund my activities!)

Oh, yes, and one more thing. I have been spending time creating some exciting new Executable UML courses that are compatible with the tools of multiple vendors. I am hoping to debut the first as a UC Extension course this fall.

Okay, so for now, catch me on the other blog and twitter. Stay in touch!

- Leon

Away but I'm back!

Still alive! I've been working on a model review for one of my clients, but I should have some time this week to push the action meta forward and catch up on the discussion groups.

- Leon

Progress on the AL metamodel

The notes posted here have been a great help in getting the model started. I've made a lot of progress in the last few days. For now I am focusing on specification (non-runtime) information. Three specification subsystems are emerging - Action Blocks, Actions and Flow. Action Blocks is very small - it just captures the relationship among Operations, States and Services with Action Blocks and Parameters. Actions specializes what I've been calling "processes" into subclassifications. The action subclasses are then related to metamodel components like Class, Attribute, Event etc. For example, the Transmitter Action must specify an Event. By the way, I'm renaming "Transmitter" to Broadcaster to match the UML action semantics vocabulary. I keep referring to the UML terminology as much as possible. Finally, the Flow subsystem captures the graph nature of interconnected Actions. It consists of classes like Flow Type, Node Type, Pin, Process, Store and so forth.

Naturally, I will post my models once I have a reviewable baseline.

Linking and Unlinking

Processes (continued)

You may have noticed that relationship linking was not mentioned in the last batch of notes. It's not like I forgot them - alright I admit it - I forgot them. But I'm kind of glad I did as a bit of thought was required to sort out this seemingly trivial activity. And even then, I'm not sure I've got the best plan - but it helps to get things down on virtual paper. Remember that nothing in this blog is set in stone - just explorations to fuel the modeling activity. So feel free to register your comments if you think I'm going astray. Going astray is one of my specialties.

Linker Processes

A linker process creates or modifies links in a relationship. The link activity described in Mellor/Balcer's Executable UML (p. 120) is a primitive accessor that will be implemented as a core behavior in the metamodel. The primitive link activity takes two object references and a relationship name. (Though a perspective must also be specified to create a link on a binary reflexive association). That, notwithstanding, there is still work to be done before we can supply two distinct object references to our primitive linker.

Assume that we route two object flows into a linker process. During run-time, any number of objects may be streaming along a given flow. Some object flows may come up empty and some may contain too many object references. How do we match up multiple object references from each flow? One approach would be to distinguish "single" object flows and stores from "many" object flows and stores. This is what is done in many executable UML action languages. Wouldn't it be cool though, if we could shift the burden of the one-many selection/store distinction away from the modeler to the underlying execution model? The downside is that we must imbue the action execution engine with the smarts to multitudinous disparity in a predictable way. In that spirit, here we go. These are the link activities processes that must be supported:

• association linker
• association unlinker
• generalized object creator (normal object creator redefined)
  
• generalized object eraser (normal object creator redefined)
  
• reclassifier
  

We start using the / symbol to separate a role or informal name from a defined element. So if we see something like 1 / object_flow, we know that the name or phrase left of the /, "1" in this case, is just an informal name whereas the element on the right has a definition somewhere in the action language notes. In other words, there is an object_flow named "1".

Association Linker

Links one object to another (or the same object) on an association.

Flows In: { status } 1 / object_flow < 2 / object flow >
Flows Out: < association class / object_flow >
Specification: rnum

Let's explore each variation.

Binary non-reflexive
( Dog Owner - 1 - is owned by -- owns - 1..* - Dog )

The modeler connects two object flows to the linker. The active perspective will be automatically connected to flow 1 and the other to flow 2. For example, one object pool might be labeled lazy_dogs and the other new_owner. Each generates a seperate object flow into the linker.

Binary reflexive
( Aircraft - 0..1 - takes off after -- takes off before - 0..1 - Aircraft )

The modeler specifies which direction to link by routing the active perspective to flow 1 and the passive perspective to flow 2. This cannot be determined automatically, so it is up the modeler to set these as intended. For example, assuming that "takes off before" is the active perspective (arbitrarily in this case), the flow for aircraft_entering_queue would go to flow 1 and the last_in_queue flow would be 2.

Unary (symmetric reflexive) association
( Territory - 1..* -- borders )

Since the perspective is symmetric it doesn't matter which flow is 1 or 2. If, for example, we link Territory Brazil to Venezuela on "borders" it's means exactly the same as if we did the reverse.

Cyclic link
To link an object to itself on a binary reflexive or unary association only one input flow, object flow 1, is required. It is the same result as flowing two different reference to the same object on the 1 and 2 inputs.

Perspective Multiplicity and Flow Cardinality

Consider the 1, 2 object flow inputs into the association linker. Either object flow could be empty, contain a single object reference or stream multiple object references during run-time. Lets refer to the empty, single and multiple quantities of object references as the cardinality of the flow. How should the linker connect the objects from each stream given each possible mix of cardinality?

The empty case is easily squared away. If at least one input is empty then no links can be made since we always link objects referred to in flow 1 with those in flow 2. The only exception is the cyclic link case mentioned earlier. In this case input 2 is not used and no link is made if input 1 is empty.

This leaves us with these flow cardinality combinations for inputs 1,2: (1, 1), (1, 2..*), (2..*, 1), and (2..*, 2..*). Before we can determine how to link the objects referenced in the paired flows, we need to know the multiplicity of the corresponding perspective of each flow.

First, a quick aside regarding association perspectives. In * UML, the "binary" in binary association does not refer to the class on each side. That is because, in the case of a reflexive binary association, there is only one class, e.g. Plane takes off after Plane. Instead "binary" refers to the two perspectives on the association, e.g. takes off before / takes off after. In the metamodel we needed a way to distinguish between each perspective. We could have used a generic label such as the A and B perspective but that's not very descriptive. Most of the time (in English, at least) we can distinguish the active and passive voice. For the perspectives "owns" and "is owned by", for example, we recognize "owns" as the active side. This rule doesn't always work. The perspectives "is before / is after" are both active. But this is a less frequent situation and we are no worse off than the A/B labeling solution.

Now if we follow the convention of running the object flow corresponding to the active perspective into flow input 1 on the linker with the passiver perspective object flow going into input 2, it is easier to describe how the objects will be linked. In the table below we cross all flow combinations with all perspective multiplicities. Conditionality (0..1, 0..*) is removed from consideration so we are just looking at pure multiplicity in the leftmost column. So S means single (0..1 or 1) and M means many (* or 1..*).

			Flow Cardinality (1, 2)
			1, 1	1, 2..*	2..*, 1	2..*, 2..*
Mult (a,p)	S, S		one->one	one->one	one->one	{ one->one }
	S, M		one->one	one->all	first->one	first->all
	M, M		one->one	one->all	all->one	all X all

The plan is to link as many objects together as possible given the supply of object references. The normal (intended use) cases are highlighted in green. Possibly useful in, uh maroon? and shaded for those of doubtful utility. Let's look at the normal cases first.

The association multiplicity and flow cardinality matches in each of the normal cases.

Multiplicity (S, S) - Flow (1, 1)
A pair of object references arrives, one on each flow. The object referenced in flow 1 is linked to the object referenced in flow 2.

Multiplicity (S, M) - Flow (1, 2..*)
The single multiplicity corresponds to the single object flow and the many multiplicity corresponds to the multiple object flow. The single object referenced in flow 1 is linked to all objects referenced in flow 2.

Multiplicity (M, M) - Flow (2..*, 2..*)
Each object references supplied in one flow is linked to each object reference from the other flow to create a cross product. So active(1, 2), passive (3, 4, 5) would yield links 1-3, 1-4, 1-5, 2-3, 2-4, 2-5

In the next case, some object references may be ignored.

Multiplicity (S, M) - Flow (2..*, 2..*)
Each object referenced in flow 1 is linked to the next object in flow 2. This pairing continues until there are no more objects in flow 1. Any remaining objects referenced in flow 2 are dropped.

In the remaining cases, some object references will be ignored.

Multiplicity (S, M) or (M, M) - Flow (1, 1)
Since only two object references are delivered, there is only one linking choice. The two referenced objects are linked together.

Multiplicity (S, S) - Flow (1, 2..*) or (2..*, 1)
The single object referenced on either flow can only be linked once. Consequently, only one link can be made with all remaining object references dropped.

Multiplicity (S, M) - Flow (2..*, 1) or (2..*, 2..*)
Since only one object referenced in flow 1 can be linked, the first object referenced in flow 1 is linked to the first or only object referenced in flow 2. All other object references are dropped.

Multiplicity (M, M) - Flow (1, 2..*) or (2..*, 1)
The object referenced from the flow with cardinality 1 is linked to all objects referenced in the flow with cardinality 2..*.

Association Unlinker

An association unlinker breaks existing links on an association relationship. If there is an association class present, each corresponding association object will be removed automatically.

Flows In: { status } 1 / object_flow < 2 / object flow >
Flows Out: None
Specification: rnum

Unlinking can be defined with or without targets specified. Use of object flows 1 and 2 vary depending on the type of association.

Binary non-reflexive
( Dog Owner - 1 - is owned by -- owns - 1..* - Dog )

Target specified

We can unlink one or more objects in one class from one or more specific objects in another class. Each set of object references is input in a separate flow. Object references form the active perspective will automatically be assigned to flow 1. For example, one object pool might be labeled lazy_dogs and the other new_owner. Each link that exists between a member of flow 1 to a member of flow 2 will be erased.

Target not specified

If we use only one object flow, all links to the referenced objects in that flow will be erased. Let's say we flow from an object pool named dead_dogs into the unlinker. Each link to an object referenced in dead_dogs will be broken.

A quick aside about unconditional constraints. We can see from the multplicity on the example association that a Dog Owner must own at least one Dog. In each of our unlink examples one or more dog owners could have been left dogless. To maintain data integrity, the modeler will need to specify some activity to either delete the owner objects or assign new dogs. Now, we may find a way to make this burden easier as we flesh out the action language. But that's going to require some serious analysis. So, for now, assume that no magic maintains unconditional integrity unless otherwise specified.

Binary reflexive
( Router - 0..* - sends mail to -- gets mail from - 0..* - Router )

Target specified

The modeler specifies which direction to unlink by routing the active perspective to flow 1 and the passive perspective to flow 2. This cannot be determined automatically, so it is up the modeler to set these as intended. Let's say that we have a Router object C that both sends and receives mail from a Router object D. The active perspective should be "sends mail to". To unlink in the send direction, from C to D, the object flow with the reference to D ("sends data to" - D) must be 1 (active) with C on flow 2 (passive). If unlinking in both directions was desired, the "not specified" method is probably a better choice.

Target not specified

Only one input flow is supplied in this case. For example, to unlink any object sending or receiving from C, output a reference to C on one object flow into the unlinker. All links involving C on the specified association will be erased.

Unary (symmetric reflexive) association
( Territory - 1..* -- borders )

Target specified

Two object flows are required and assignment to 1 and 2 is arbitrary. Each object referenced in one flow is unlinked with any connected object referenced in the other flow.

Target not specified

Only one object flow is required. All links are broken to each object referenced in the flow.

Cyclic unlink (target specified is itself)
To unlink an object from itself on a binary reflexive or unary association only object flow 1, is required. It is the same result as flowing two different reference to the same object on the 1 and 2 inputs.

Generalized Object Creator

Our definition of creator from the previous post must be extended to accommodate the peculiarities of generalized/specialized objects. In a * UML generalization, a single object is represented by two instances, one in the superclass and one in a subclass. The creator must create both of these instances. The name of the subclass must be specified.

If an object participates in more than one generalization, all participating instances must be inserted by into the metamodel by the creator. Consider the following arrangement of generalizations:

A <R1 [ B | C ], C <R2 [ D | E ], F <R3 [ E | B ]

Here we have three generalizations R1, R2 and R3 with superclasses A, C and F respectively. To create an object in this example model, a non-specialized (leaf) subclass must be specified. If we specify E, for example, instances will be created in classes E, F, C and A. For subclasses, our object creator both creates and links. Any initialization activity coordinated amongst the instances must be modeled explicitly. Only the instance creation/linking is automatic.

Multidirectional generalization requires the specification of one subclass per direction:

G <R4 [ H | I ], G <R5 [ J | K ]

The set of possible creation specifications are all combinations of non-specialized subclasses linked to a common superclass. So in the model example above we have: (H, K), (H, J), (I, J), (I, K).

One attribute flow per created super or subclass instance may be provided. (This means that the definition of an attribute flow in the previous post must be amended to include a class_name in the flow content. And since I can just do that now, I will. Done.)

Specification: 1{ non-specialized_subclass_name }
Flows In: { status } { attribute }
Flows Out: < object >

The output flow returns a reference to each specified subclass. References to the newly created superclass instances can be found using with a selector process traversing the generalization(s).

Generalized Object Eraser

We will now extend our eraser process so that it can handle generalizations properly. An object in a generalization, super or subclass, is represented by two instances. If an object participates in multiple generalizations there will be one more instance per additional generalization. Every one of these related instances will be eliminated by the object eraser.

Referring back to the generalized object creator example models, let's say that we specify a reference to an instance of class F. Following instances must be erased: F, either E or B, A and if E then also C

The modeler must be careful to not specify deletion until all cleanup and coordination for every instance involved has been accomplished.

Flows In: { status } object

Reclassifier

A reclassifer migrates a subclass instance in a generalization to another subclass at the same level. This means that the original subclass instance must be erased and a new instance created in the destination subclass. Both subclasses must be at the same level in the same generalization. Based on our earlier model examples in the creator and eraser, you can probably guess that the activity doesn't necessarily stop there!

Let's say that we want to migrate C to B. We must delete and unlink the D or E subclass under C (but keep the D/E superclass F intact. Then we delete the C instance, create a B instance and link the B instance to both the A and F superclasses. Any supplied attribute data can be used to initialized the values in B (B only since that is the only created instance).

Once again, it is the modelers responsibility to ensure that any coordination and cleanup activity in the subclasses to be deleted is completed before triggering the reclassification.

Specification: from_subclass_name, to_subclass_name
Flows In: { status } { attribute }
Flows Out: < object >

The output flow returns a reference to the newly created subclass instance.

Conclusion

Okay, that's all for now. I'm going to need some spicy indian food and bad television before getting back to the metamodel.

- Leon

Action Language Notes

Here are some rough notes on the proposed anatomy of the * UML action language. All semantic elements can be represented as text, a la SMALL, or as graphical symbols a la Scrall. For visualization purposes, graphical symbols are less restrictive than text statements. Eventually we will establish a clear connection between these elements and the corresponding UML 2.0 action semantics. For now though, we need to build something that works well, with a minimum of components.

A parser will yield the elements described here so, at this level, we don't care about notation or syntax. We do want to distill the essence of an action block specification so that we can build a machine that can execute any action block. Rather than build a full blown action language all at once, it is probably best to define the core elements and implement a primitive machine. Then, with some experimentation, we can extend or rethink the language to evolve something truely useful.

Ss the goal of these notes is sort out the concepts required to support the specification of action components as well as the run-time execution of those components in an action language interpreter. Whereas SMALL and SCRALL focus on syntax primarily, we need to sort out the underlying mechanics and integration with the metamodel. The next step will be to start modeling those concepts. Incremental versions of the action language metamodel will be posted at sourceforge for anyone interested.

Here are some questions we hope to answer:

• What configurations of data are exchanged? How many do we need?
• Do we have just one flow that can transfer any kind of data?
• Or do we create a separate flow type for each configuration of data?
• What kind of processes do we have? What are the inputs and outputs?
• ?

We must also keep in mind that varying degrees of parallelism are possible depending on the target architecture. So we must be careful not to define structures that perform only in a sequential context.

Key

Let's use the following symbols:

[ a | b ] - a or b
{ c } - zero, one or many c's
1{ d } - at least one d
< e > - zero or one e - optional
( d, f ) - d and f are grouped together in the same flow
g - exactly one - g is required

Action Block

An action block is a bundle of action language associated with a state, class operation or service. Action language consists of nodes (processes and stores) connected by flows.

Flows

A flow transfers data into, out of, or between nodes. Each flow has a unique numbered identifier and may be optionally named. Duplicate names in the same action block are allowed.

Every flow specifies at least one source and destination. The types and quantities of sources and destinations depends on the flow type.

Specification: Unique_id < Descriptive_name >

Object Flow

An object flow transfers zero or many object references. This flow can be generated by either a selector or a creator process.

Specification: None
Source: [ object_pool | selector | creator ]
Destination: [ object_pool | transmitter | eraser | tester ]
Run-time content: { object_reference }
Uses:

Populate object pool from selector or creator
Supply references to an eraser for deletion
Transfer object pool to other object processors (to be defined later)
Provide input to a tester to evaluate cardinality

Attribute Flow

An attribute flow specifies the transfer of one or more attribute name:value pairs. All attributes in a flow must belong to the same class. Each value must have a data type that matches that of its attribute.

Specification: class_name, 1{ attribute_name }
Source: [ object_pool | transform ]
Destination: [ object_pool | transform | tester | creator ]
Run-time content: { < object_reference > 1{ attribute_name:value } }
Uses:

Read attribute values using object references in an object pool
Write attribute values of a specific object
Write attribute values the same for all objects referenced in an object pool
Supply data to a transform or tester
Supply attribute initialization values to a creator (Cannot carry an object reference)

When writing to specific objects during run-time, the following data is expected:

{ object_ref 1{ attr_name:value }

Each flowing object reference is accompanied by one or more name:value pairs. To obtain the object reference, the origin of the flow must trace back to an object pool store or creator process.

An attribute flow into an object pool containing only name:value pairs and no object reference will write all objects referenced in the pool to the same values.

{ attr_name:value }

We could use this example to set

The following data streams when reading attribute values from an object pool:

{ object_ref 1{ (attr_name:value) }

Here we see that each data item includes an object reference with at least one accompanying attribute name, value pair. The syntax above does not suggest any particular implementation. You don't have to send an attribute name with each value. But there must be some mechansim to ensure that the attribute of each value in an attribute flow can be determined during runtime.

Value Flow

A value flow transfers a single value not associated with any attribute. Each value conforms to a defined user or core data type. Consequently, a value may be arbitarily complex.

Specification: None
Source: [ value_store | transform | service ]
Destination: [ value_store | transform | tester | service | selector ]
Run-time content: value
Uses:

Transfer a temporary value in and out of a value store
Output a value from a class operation, transform or service
Transmit a value to a transmitter for inclusion in a signal
Transmit a comparison value to a selector criteria expression

Status Flow

A process executes when all of its required input data is available. This means that multiple processes could potentially execute in parallel. But we might want to ensure that certain processes wait, even though data is available, until that data is refined further. Or an upstream decision might preclude invocation of a certain process.

A status flow communicates a boolean condition to a downstream process to effect or negate execution. If a status flow is enabled during run-time, the condition is enabled. A process may not execute until all input control flows have been enabled and all input data is available.

Before considering the run-time content of a status flow, we need to understand how flows are managed in general. Every flow, regardless of type, starts off in an "inactive" state. This means that the flow has not yet been populated with any data. Once a flow is populated by whatever process or store provides its input, the flow enters the "activated" state. This means that data is now available. The content of an "active" flow may be "empty". (An object selection that returns no references, for example, would populate an object flow with zero object references). Once all data in a flow has been consumed, the flow enters a final "consumed" state.

When a status flow leaves its "inactive" state, it must be set to either true or false. Any process receiving the false input will not execute. True means that the process may execute. The name of the flow reflects its condition when set. Let's say the name of the flow is "Max Pressure Exceeded" and it feeds into a process named "Close valve". The process may execute only if the value of the flow is True.

Status might be enabled repeatedly for a series of object references. The example taken from figure 6.16.1 of the SMALL paper shows this situation. Crates are sorted into large and small sizes. A flow of object references in SMALL, each with an attached guard condition, flows to two separate attribute write processes.

Each status flow in this example flows object references as well as status.

Specification: None
Source: tester
Destination: any process, including another tester
Run-time content: <>

There are some cases where, instead of a status flow, a value flow with a boolean data type could be used to similar effect. But it would not be possible to preclude execution of a downstream process with this method.

Nodes

A node is either a Store or a Process. Some configuration of data can flow into a node, out of a node or in both directions.

Stores

A store is a node where data is organized for access by one or more flows. It is more accurate to say that a store makes data available than to say that anything is actually being stored. They are freshly initiated when an action block is invoked and destroyed when the action block completes. So the scope of a store, regardless of type, is the current action block.

Object Pool

An object pool stores a set of object references. The object references may be created by the output object flow of either a selector or an object creator. Every object pool has exactly one object flow input. During run-time, an object pool may receive zero, one or many object references. So it is possible to end up with an empty object pool after the object flow is activated.

Any number of attribute flows may enter an object pool. Each such flow specifies a write to the attributes of any objects referenced in the pool. Also, any number of attribute flows may stem from an object pool. Each such flow specifies the reading of attribute values from referenced objects.

Flows In: { attribute } { object }
Flows Out: { attribute } { object }

Each action block in an instance based state has automatic access to an object pool initialized to contain a reference to the current instance. This special object pool is always named "self". Not sure whether we need an object flow to fill this store or whether we make a special rule and say that the self store has no input.

Value Store

A value store holds a single value defined by a data type. Consequently the content of a value store can be arbitrarily complex. A value store is populated by at least one value flow. Any write to a value store replaces any prior value. A value flow that exits a value store reads the current value.

A parameter value in a state action block or a class operation has no input value flow. The value is automatically made available when the block is initialized.

Flows In: { value }
Flows Out: { value }

Processes

A process is a specialized activity that has exercises some influence on metamodel content. The inputs, outputs and activity depends on the process type.

Selector

A selector takes as input a class, a quantity [ 1 | * ], an optional criteria expression and an optional path expression. It locates the specified object or objects in the designated class and outputs an object flow populated with matching object references. If no matching objects are found, zero references are produced in the object flow.

Flows In: { status } { value }
Flows Out: object
Specification: ( class_name ) ( quantity ) < criteria > < path >

Creator

A creator makes a new instance of an object. A reference to the newly created object may be provided by an output object flow. An input attribute flow can be used to initialize any of the object's attributes.

Specification: class_name
Flows In: { status } < attribute >
Flows Out: < object >

Eraser

An eraser deletes any objects referenced in an incoming object flow.

Flows In: { status } object

Transmitter

A transmitter accepts an object flow and value flow for each parameter. A signal is generated to each object referenced in the flow.

Specification: event_specification { value_flow_name, parameter_name }
Flows In: { status } { object } { value }

Tester

A tester accepts status, value and/or attribute flows and produces one or more status flows.

Specification: Internal comparison expressions (NOT an action block)
Flows In: { status } { value } { attribute }
Flows Out: 1{ status }

Transform

A transform is a process that, as its name implies, transforms one or more input values to produce a one or more output values. Calculations may be performed, data manipulated and coverted, values tested and assignments made. A transform cannot directly access the class or state models. This means that objects (including "self") may not be selected, attributes read or written or signals generated. Local value stores may be allocated and accessed. A transform cannot produce a status flow - you need a tester process for that.

Specification: Internal transformation expressions (NOT an action_block)
Flows In: { status } { attribute } { value }
Flows Out: { attribute } { value }

Example:

Transform Compute_volume

Inputs( Attribute_flow:Dimensions )
Output( Attribute_flow:Volume )

Volume.Volume = Dimensions.Length * Dimensions.Width * Dimensions.Depth
Volume.Object = Dimensions.Object

In the example above, a transform named Compute_volume is defined. It's single input is declared as an attribute flow. The local name Dimensions is applied. An output attribute flow is declared and named Volume. The L, W, D components of the input flow were defined outside the transform, but can be referenced internally. The single component V of the output flow is dynamically declared internally. It can then be referenced downstream. The object reference received in the Dimensions attribute flow is passed along to the Volume attribute flow.

Service

A service is a blackbox process provided by either an external service domain or in the caller's domain. No model references such as objects or attributes may be exchanged with the caller. Only status and values may be communicated.

Flows In: { status } { value }
Flows Out: { status } { value }
Specification: action_block

Operation

An operation is an activity defined on a class independent of any state chart. Operations are object-based so that a separate context executes for each object. All selected object references remain available to any invoked operation. This makes sense since the object references in an object pool are valid until the state action completes and the operation must return before that. An operation may produce a single value output. (When we design the exception throw-catch mechansim there will be an additional way for an operation to pass data out).

Flows In: { status } { value } { object } { attribute }
Flows Out: { status } { value } { object } { attribute }
Specification: action_block

Conclusion

Again, these are just rough notes in preparation for the modeling activity. As usual, the process of modeling will ferret out inconsistency, ambiguity, self contradiction and gaping holes in logic. All of which are surely present! Nonetheless, I thought it might be helpful to expose the current thinking. Feel free to comment if you see any trouble spots or feel I'm heading down the wrong road or have any theory to contribute.

- Leon

Still analyzing...

I had hoped to post my premilinary action language metamodel notes by now, but the quicksand keeps shifting! Still resolving inconsistencies and trying to boil down the necessary concepts. Getting close, though.

Small updates

I haven't posted for a few days so I thought I would just make a quick update. I am currently putting together some notes so that I can start building the action language metamodel. Naturally, you can't model what you don't understand and until I get a grip on this thing there's just not much to model. The good news, though, is that I'm making a lot of progress which I will post shortly.

I have been reading, re-reading, re-re-re-reading every little detail in the SMALL language paper and putting together some notes. As I progress I am finding a number of issues left, intentionally or not, as "exercises for the student".

Primitive processes such as "attribute write", "object create", etc. are pretty clear. But then you see something like "Compute Volume" (figure 6.16.1 on page 23) with an input data flow labeled "N" and an output labeled "Volume". Okay...

Clearly, somewhere inside that process, is the simple v = l * w * d expression. But we see that there will be one (l, w, d) for each Crate object - thus the N on the input. There's no N on the output! But the example doesn't work if we output only one v, so N must have been intended. Now we could say that the action language interpreter automatically binds the output to an object reference since it sees that the input flow carries an object reference. But what happens if we have an process that inputs multiple <...>N flows - each from a different class? Do we merge the output somehow? Or what if we really did intend to output a single value unassociated with any object reference? My point is that there is a lot of room for ambiguity here and the SMALL paper does not spell any of this out.

Now that's just one example. This paper is replete with missing pieces - all due respect to the authors! So I will make my best effort to fill them in, present some notes, and see what you all think.

Action Language Metamodel

I've been pondering the action language metamodel the last couple of days. For reseach, I've printed out the in-depth SMALL paper from back in 97 and xeroxed the two page summary from Executable UML as well as my SCRALL notes.

Goals of this action language are:

1. Emphasize parallel processing and data flows.
2. Ensure that the language is as easy to read as the class and state models.
3. Support a full range of data types and domain bridge mechanisms.
   

I'm really impressed with the quality of the SMALL paper. I will probably just implement it straight out. But how?

I certainly need to model action behaviors (link, create, write attribute, etc.) as primitive operations in the class and state metamodel subystems. For now I will just imagine these as domain functions with appropriate signatures. Example:

::Unlink(object_id1, object_id2, rnum)

Clearly we need a lex-yacc generated parser, which means I need to write up a full grammar. I'm thinking that the intermediate representation produced by the parser will populate an action language metamodel - yet to be built.

So when action language is entered, it will be parsed and the action language metamodel will be populated. When a block of action language is triggered at run-time, the action language metamodel will invoke the primitive operations with appropriate parameter values.

The lex-parser and primitive ops are the easy parts, so I will save those for last. I hope to post progress here in the coming days.

It's a nice day for a walk so I'm heading to the local cafe to ponder further.