UML in practice – my top fives

I recently came across an interesting publication regarding the adoption and practices of Unified Modeling Language (UML) in the real world. Written by Marian Petre and presented in ICSE’13, titled “UML in practice”. The article stated that,

UML has been described by some as “the lingua franca of software engineering”. Evidence from industry does not necessarily support such endorsements. How exactly is UML being used in industry — if it is? This paper presents a corpus of interviews with 50 professional software engineers in 50 companies and identifies 5 patterns of UML use.

For those that are impartial to reading blog posts, obviously ;), here is great summary of the aforementioned paper from a blog that I follow. http://neverworkintheory.org/2013/06/13/uml-in-practice-2.html.

There are two noteworthy takeaways.

1. After 20 years, UML is not widely adopted due to its complexity and overhead, difficulties in synchronising code and design, and lack of context (UML often deals with architecture and not the whole system)
2. When UML are used, only parts of the diagram notation are adopted; class, sequence, activity, state and use case diagrams. These diagrams have the lowest barrier of entry and provides a very useful and flexible tool for communicating particular view of a system.

For me personally, I too fall under the “selective usage” category. I also believe that the informal nature of UML, particularly when juxtaposed with other formal methods, showcases that UML is great for illustrating overarching architectural breakdown but horrible for specifying detailed implementations.

My experience with UML is a very positive one. Here are my top five tips for some UML hackery:

When I use UML, I…

• follow a particular design methodology, one such as Concurrent Object Modeling and architectural design mEThod (COMET). Doing so will lead to selectively subsetting the notations and diagrams, at the same time applying structure and guidelines for illustrating design and context for the system in question.
• focus on the diagrams that can be easily understood by stakeholders. No point building a state machine that captures non-determinisms and lambda transitions when no stakeholders cares about it. Do, however, provide useful class breakdowns or entity relations if people finds them useful.
• formalise the models depending on the system in question. For example, adding Object Constraint Language (OCL) into your requirement models makes it formal. It can be audited and proven. Formalising UML is suitable for AS 61508 classified systems, compliant to certain Safety Integrity Level (SIL) levels.
• estimation the size of project via use case estimations. This is a great baseline for sanity checks.
• use a good Computer-aided software engineering (CASE) tool. See the following CASE tool comparisons as an example; https://macyves.wordpress.com/2013/01/17/uml-case-tools-for-the-mac/.

Spend some time and get to know this visual language. You will find a clever and flexible way to communicate your ideas and intent to a wider audience and with improved precision and articulation.

Building on varnish-agent2

The varnish-agent2 existing code base is pretty solid,  rather beautiful I might add. These simple words, keep it simple, has been realised by the following rule of thumb.

– Close to 0 configuration
– “Just works”
– Maintainable
– Generic
– Stateless

For those that are keen to get started on the varnish-agent2 code base, I hope that this document will be of some use. I have used a tiny subset of the Concurrent Object Modeling and architectural design mEThod (COMET), particularly the Requirements Modeling, Analysis Modeling and snippets of the actual Design Model.

Lastly, this document mostly serves as a mental note for myself 🙂

Requirements Modeling

The requirement for the vagent2 is to provide an extendible restful interface for varnish cache. In addition, vagent2 act as the point of integration for varnish cache with other systems – e.g. an administrative or monitoring system.

The use cases are simple right now, and is depicted in the use case diagram below.

vagent2 use cases

Analysis Modeling

vagent2 is designed to support the full spectrum of HTTP method types. A user of the vagent2 will issue these HTTP requests and receive JSON data as response where applicable. Furthermore, the vagent2 is built with modules in mind to address a potentially expanding feature set. Lastly, each modules should be able to communicate and reused by another module.

IPC lies at the heart of the varnish-agent2 code base and message passing is the norm here for implementing an event driven model. Each module follows vagent2’s plugin paradigm, and comes equipped with the module’s own private data set. The plugin therefore follows the IPC set of callback methods, such as ipc_start and ipc_run. These methods are assigned to the appropriate functions within each module.

For the module to be exposed as a restful interface, a simple httpd_register method will hook in the module’s reply method of choice, and expose it appropriately.

For any module, the basic dependencies is depicted below.

basic module dependencies

Static Modeling

varnish-agent2 ships with a few core modules, such as the logger, shmlog access, httpd and ipc modules. vstatus and vadmin provides access to the shmlog via Varnish API. Note that as of writing this blog, varnish 3.0.3 was used.

These aforementioned modules provide the building blocks for managing varnish cache. For an overview of the static models, see the class diagram below.

Static overview of vagent2

Dynamic Modeling

Initialisation
The process of initialising a module is rather straightforward. First, add a new init method for the new module in plugins.h, ensure that you have called the init method in main.c, and of course, allocated some memory for the new plugin in main.c too.

This new module must provide an implementation of the new init method. See diagram below depicting vban’s initialisation process.

vban initialisation process

Once initialised, by hooking onto struct agent_plugin* structure, the new module will correspond to the IPC life cycle.

plugin->start  = ipc_start;
plugin->ipc->priv = your_private_plugin_data;
plugin->ipc->cb = your_plugin_callback;

plugin->start is called when your plugin starts. Note that you need to assign a start method if you want the IPC to execute your callback.

plug->ipc->priv refers to a persisted data for your plugin. This can be anything, and as a rule of thumb, this is a good place to hold references to other modules.

plug->ipc-cb refers to the callback method of when ipc_run is issued by another module.

A sample execution path

To tie it all together, the collaboration diagram below illustrate the execution path of issuing a ban to the vagent2. Note that ipc is used to reach the vadmin module.

Issue a ban