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Perpetuum: Generating Fast Code from Petri-nets

Perpetuum Mobile

Perpatuum is a code generator for event loops that uses Petri-nets as their input. In a Petri-net, transitions fire in zero time or, more practically, in an atomic action. This is used to intitiate and detect events. Petri-nets help to reroute processes in a verifiable manner.

This work was initiated for the Key Manager component of the IdentityHub in the InternetWide Architecture. This Key Manager is confronted with controlling LDAP, PKCS #11, Kerberos, DNS/DANE/DNSSEC, and internal facilities for authorisation. In general, it is safe to say that

  • The relation between the tasks should be tightly managed;
  • These relations are fairly complicated;
  • These relations may vary over time and across deployments.

To avoid ending up with a monolithic Key Manager that is only good for one purpose, we dediced to create Perpetuum as a coordinator of the individual, simple tasks. The coordination is based on descriptions in Petri-nets, which means that:

  • Graphical tools exist to overview and modify the system process design;
  • Tools exist to prove various correctness requirements of a new design;
  • Graphical tools exist for simulation and animation of the new design.

Given that the remainder of the problem consists of relatively simple actions, such as "send a message over LDAP", this takes away quite a lot of the burden when adapting the Key Manager to new situations, be they driven by technical advances or operations in different contexts.

Programming Model

The Petri Net engine is principally invoked with events to process. It is designed to handle these events one by one; the Petri Net is part of the work-distributing part of an event loop. Actions that are started from the Petri Net are therefore assumed to be snappy actions that will return with virtually no delay.

Snappy actions are not so surprising for an event-driven system. There should be no waiting for other threads or processes, but instead an action is started in the background, and once ready its results will be picked up by another event. So, instead of creating a new private key we will issue a command to have one. And indeed, the work will be sent off over Remote PKCS #11, for which we devised an asynchronous ASN.1 model, meaning a separate data representation for a remote procedure call and its completion.

Transitions fire up such snappy actions, but do not necessarily assume that they will succeed. In fact, the adagio is to try a transition and see if it worked. The return value can be one of TRANS_SUCCESS, TRANS_FAILURE, TRANS_MAXDELAY or TRANS_DELAY(numsec) where the latter means to delay the work for numsec seconds, perhaps because a resource is locked or because a timer should expire before we may continue. Only when the returned value is TRANS_SUCCESS is it possible to have a lasting result or ongoing activity. The other return values indicate that the attempt to fire up the action has failed, and any half-done work has been rolled back. [TODO: INTRODUCE IDEMPOTENCE]

While developing snappy chunks, you need not worry about the Petri net in which your code is running. You should of course be aware that some pairing between starting and ending a background task is useful, but even the connection between your code and the Petri Net are done by the same person who designs the Petri net.

Language Support: C and Erlang (for now)

The current implementation has support for C and Erlang. It has the potential of covering much more, but the compiler probably needs to learn about structure, which was hardly available during our first iteration. The compiler is reliable, but also quite linearly maps out the work to be done in one long list of actions.

The model for C is kept very general; the terminology above refers to event loops but does not limit you to one particular brand. The representation in memory is straightforward, albeit quite compact and built with the memory constrainsts of the smallest embedded system in mind.

The model for Erlang is based on its variable-sized integer type, which can hold the state of a complete Petri Net in just one of these integers. In addition, it needs guiding structures which are static for any but the wildest-sized applications. These static structures are shared between all processes, allowing for a very high number of Petri Net instances running in parallel. This is not exceptional; in fact, Erlang developers generally aim for such structures. Read more about this design in the document on Bitfield Vectors.

It is possible to use a variable-sized integer type in C as well. Most crypto-libraries offer such facilities. There can also be more elaborate choices about process and communication models, but given the wide-ranging choices offered by C, certainly when compared to Erlang, we've left such specialisation for C coding style as an exercise to the Perpetuum user. We'll focus on the process algebraic aspects.

Petri Net Design

To design a Petri net, you need not be concerned with the implementation of the actions that are started from your net, or how events enter the Petri net. All you need to do is make the connection to the action or event by their name. Do read how these may be related; it will often be the case that an action starts a background process and reports back on its success or failure through an event.

Petri nets are a bit like the assembly language of the process algebras. They have little structure, thereby allowing you all the room you need to design a smart process flow -- and to screw up while at it. There are however a lot of tools that can validate your process flow for you. You can not only enter your Petri nets graphically, but you will also find that you can simulate them running, and see tokens move from place to place while doing so. This should be enough to sharpen your intuition about the design process you are in. And as if that isn't enough, there are tools that formally prove all sorts of properties of Petri nets to assure you that it will not run into problems when you deploy them.

Formal Testing Opportunities

Please read the tools document for details on tools for editing, simulating and analysing Petri Nets. A lot of research is inconclusive, but it would be a shame to conclude that none of it can be used until it is complete; some problems in mathematics are simply very hard to solve, if they can be solved at all. It is probably good to use tools even when they constrain patterns more than your gut feeling tells you is possible; the most important thing is that your gut feeling is replaced with thinking about the (hopefully few) cases where it is useful to double-check. You may find that you only need to add a few explicit pathways to your Petri Net to silence a process validation tool.

Think of process algebraic tests as an analogue to type checking. Where type checking can do upfront validation of the data that is allowed in certain variables, it is the task if process validation to check that things happen in the right order. Compare this to using locks everywhere in your code, delved in so deeply that it is difficult to figure out why you had a deadlock. And it may even be difficult to reproduce because, unlike a process validator, your test runs only cover a single trace of time. One day, the cat in the box dies; another day, it does not. Tools can just look at all the days that might occur.

Process logic has its own notation for order. Commonly mentioned languages are LTL and CTL, but there are also diagramming languages similar to state charts. The power of these languages can be, just like with type annotations, that a requirement that underpins the logic of a system can be spelled out and automatically checked after updates to the code, long after the original thinking and possibly by a different person. Once violated, a constraint will ask for attention leading to a rethink of either code or the constraint. This is usefully spent time, in terms of writing reliable code. Again, you want to handle both dead and alive cats in your boxes.

Please read the testing document for details on this package's tools for testing the software produced. Although only applied to a few standard examples, you should find it relatively easy to test the correctness of your Petri Net mapping into code, as made by Perpetuum.

NOTES after discussion with Adriaan de Groot

Both Adriaan and Rick have a background in process algebra. We've had great fun thinking about the practical implications that a system like this could have.

  • Time-consuming processes can justly be run in a place
  • The non-determinism of Petri-nets helps to challenge/cripple formal tools (meaning, avoid that they jump to illegit conclusions) but implementations invariably add choices that may be more concrete (without breaking any formal properties)
  • One way of being concrete in an implementation is: a place can fire when its action is done (but the transition needs to be ready for it too, so waiting may be involved)
  • It is possible to refine one place into a sequence with intermittent transitions, except when the last transition responds to not having a token (through an inhibitor arc) -- in which case these inhibitors must attach to all the intermittent places too
  • This helps to avoid a very common pattern of "trans/init, place/ready, trans/request, place/wait, trans/response, place/done" and instead we can often just write "trans/init, place/ready2response, trans/response" (or so...)
  • Processes run in places need not always exit in the same way; they may also side-step on error or other considerd-useful distinct situations
  • Events reamin valuable on transitions; incoming places define preconditions for them being noticed (?) or for them passing through; we should be able to express both where one can be a simple pattern around the other
  • It is probably also nice to allow lightning-fast actions initiated on transitions; this is indeed suggested by the literature too, where places represent states (of rest); indeed, processes in places can be seen as an abbreviated form of request/wait/response (which is how Petri nets can be used to model activities in UML Activity Diagrams)

With longer-running processes, colour can be endangered; writes may replace values in multiple places at the same time; this might be overcome in a number of ways

  • Use locks, either in the activities or in the surrounding generated code
  • Analyse such that colour aspects (parts of user type space) never overlaps (using k-bound analysis and a few extra places and arcs between conflicting parts?)
  • Use analysis of such extra colour aspects to add the extra places and arcs in the Petri Net that enforce 1-boundedness of the collection of places that access the same piece of information