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Using Perpetuum in your Application

Perpetuum Mobile

Perpetuum is a code generator and data model for Petri nets used to describe the event loops of asynchronous systems. Here is how to use this software.

The following explains how you can use Perpetuum in your Applications.

Identify Actions and Activities

The first thing you will want to do, is list the actions that you need to have in your problem domain. Actions are quick, snappy functions that return a state of success after initiating a possibly larger process, such as a forked process that creates an RSA key, or the configuration of your web browser, or sending off an LDAP query.

As a general design rule, Actions should never block/wait on anything.

Actions will be bound to Petri-net transitions, where they implement Fast `n' Final Firing, meaning, they take no (virtual) time, or in computational terms, they run atomically, or in database terms, they are isolated operations. In Perpetuum, they are indeed run like that.

Larger compositions must necessarily run concurrently, which can be properly synchronised with Petri net formalisms. We call such compositions of actions or transitions plus intermediate waiting times as Activiites. Activities are sub-processes that take time to complete. These are not implemented through blocking, but by counting on an event that signals completion or another form of progress. As part of the activity support framework, Perpetuum will help you connect its constituent actions together through such things as request identifiers, or process IDs. In addition, data can be carried between the actions and even the events.

Design your Initial Processes

Even though there is no strict requirement for you to do it immediately, you probably want to try if your identified set of actions and activities match your problem domain, and if they suffice for designing the process structures that you have in mind. So go ahead, and design your process.

We deliberately call this your initial process, because Processes can still be modified during Deployment. This is the great strengt of Perpetuum; it does not hard-code the process logic but instead generate it from a Petri net. Change the design, regenerate your code and you are ready for another brand of action.

Perhaps it is not entirely correct to say that processes are still open for change during deployment; in fact, New processes involve rebuilding the Perpetuum application but there are no concerns about their impact on the set of actions and activities that were coded in support for (a class of) processes.

To facilitate this style of working, we do advise you to have configuration parameters for the process diagrams to use. This can be manipulated easily through the CMaked configuration mechanism, even through a GUI if so desired!

Validate your Process Models

The generation of code from Petri nets constitutes little more than a mapping to arrays that define the structure of the networks, just enough for the schedulers to run. The runtime does not check if there are deadlocks or livelocks, or any other concurrency problems.

You validate your Process Models before you run them.

Petri nets have one great advantage, and that is a plethora of analysis tools. They let you be sure that there are no intrinsic problems in your models, even before you run them. This helps us to ignore such aspects at run time, and thus be smaller, faster, and cuter.

Use graphical tools where you can. Sure, it is simpler to enter textual data, but it is not as pleasant for people who design new diagrams during deployment. Graphical tools exist for many process modelling tasks with Petri nets:

  • View and Edit process models
  • Simulation of process models
  • Validation of process models
  • Animation of process models

Pretty neat for a mathematically accurate model, uh? The things to watch for are that you have the right kind of Petri nets, because there are as many as the mathematicians working on them; but we are quite mainstream in processing only:

  • Simple P/T Petri nets
  • Arcs may define multiplicity in numeric inscriptions
  • Inhibitor arcs

Implement your Activities

Activities are useful for you while designing your processes, but they barely have meaning to Perpetuum. The only parts of interest are:

  • What data is stored between actions
  • What data is used to recognise events

You will end up specifying a data structure, or several, and attaching those to the Petri net token. They form the "colour" of your token.

Please note that tokens are recnogised by their colour; those parts used to find back tokens must be unique. TODO: Or could we trigger multiple, based on pattern? Do we want that? Well, not initially ;-)

Any scoping of the data you rely on is up to you. Things need not be kept solely inside an activity, and activities need not even be hierarchically structured! The turning side of the coin is that you cannot count on data privacy. There is no data security either; the various colours are assumed to collaborate (by not overwriting data outside their allocated room).

Your token data will be initialised to all-zero bytes. More wordy: All tokens are created equal; they are all black. On top of the initialisation, there is support for initialisation and cleanup actions.

Implement your Actions

Your code should always

#include <perpetuum/model.h>

Your actions have the following function signature:

trans_retcode_t (*my_action) (
			transref_t transition,
			time_t *nowp,
			void *opt_evdata);

Note the mention of opt_data; this is a data structure that can pass in data from an event.

These functions should do a snappy bit of work, without blocking or waiting, and return one of the values

#define TRANS_FAILURE (~(trans_retcode_t)0)
#define TRANS_DELAY(t) ((trans_retcode)(t))

Note how there could be misinterpretations for TRANS_DELAY(0) as well as TRANS_DELAY(~0) and since some delays may be derived from external data sources (such as a TTL delay), there is a safer variation,

		(((t)==TRANS_SUCCESS)? \
			1: \
			(((t)>=TRANS_FAILURE)? \
				(TRANS_FAILURE-1): \

You have been warned!

If you need to lookup transitions or places by name, then you could; efficient lookup functions are generated as minimal perfect hash functions TODO:AT_SOME_POINT, but you are of course still advised to look for them while being setup (rather than for individual calls). Functions to help you with that are

placeref_t find_place (const petrinet_colour_t *pnc, const char *name);
transref_t find_trans (const petrinet_colour_t *pnc, const char *name);

Be vigilent however -- why do you need this information? Normally, your concern should be local, and not care for other places and transitions. Any communication should be done over the token colours / instance data.

Implement your Events

Events are the subject of semantic navel staring ;-) though we are taking a practical angle initially.

Usually, a transition that wants to respond to an event will return TRANS_MAXDELAY when it is triggered by the scheduler. It can detect such situations by looking at the opt_evdata parameter, which is always NULL when invoked from the scheduler, and usually non-NULL when called from an event handler.

A typical transition action following this line would look like

trans_retcode_t my_action (PARMDEF_COMMA(pnc)
						transref_t tr,
						time_t *nowp,
						void *opt_evdata) {
	if (opt_evdata == NULL) {
		/* Called from the scheduler, refuse to make the transition */
	} else {
		/* Called from event, process data and try to return success */
		... = ((my_data_t *) opt->evdata)->...;

The event callback should be directed to one particular Petri net instance. the reference in pnc indicates which.

When the event is the result of a response to a request that was sent before in an action on a transition, then it may be prudent to also have a timeout transition starting from the same place as the transition holding this action. This way, the wait for a response can be kept limited and any token that indicates work should be going on can always be consumed after due delay.

Connect Actions and Events with your Process Model

Perpetuum is used by two kinds of developers; one is the initial developer of an application, the other is a deploying party. The former controls actions and events, as well as an initial process model; the latter controls only process models. In both cases, there is a need to connect events and actions to the transitions in the process model.

There is an automatic default, which is to use the name of a transition up to an optional slash as an identity. Slash and beyond are considered remarks or informal distinctions between different places where an event or action is used. They are different transitions in the models, but trigger the same action or event (and tools sometimes force different names, which we override with the slash subnaming scheme).

So, by using proper names in your process model you will automatically connect to actions and events in the backend.

TODO: Do we want to override this binding in any way?

Build your Application

Application building is normally done only by application developers, but Perpetuum may also ask for it during deployment, when new process models are used. This means that a gentle mechanism is required. For make that workable, we are relying on CMake and its graphical configuration interface.

TODO: Configure (in CMake), then build, test, package, run