What are the advantages and disadvantages of plug-in based architecture?

Question

I want to do the architectural design for a software that can be used integrate various third party software’s (executable) under one platform.

Standard project types will be added to the platform by default. The project type defines the way in which the different software will be executed and their input and output files.

The user can customize the available standard project type and that will be added to the platform as new project type which defines new custom execution flow.

Also it should support easy extension and customization of the features. I read that plug-in based architecture supports both.

What are the advantages and disadvantages of plug-in based architecture? Do we have any better architecture which can be used for this kind of scenario?

Thanks in advance:)

Answer 1

The benefits of a pluggable system are

• extensibility: the application can be dynamically extended to include new features.
• parallel development: since features can be implemented as separate components, they can be developed in parallel by different teams.
• clear development direction: since the plugin framework ideally provides a well-defined interface and documentation for plugin writers, developers have a clear roadmap for development.
• simplicity: a plugin typically has one function, and so developers have a single focus

But some of these strengths are also weaknesses:

• extensibility: does the plugin interface anticipate the ways plugin writers what to extend the app, or does it restrict extension. Designing extensibility to meet all use cases often takes several iterations, or extremely good requirements analysis.
• maintainability: the provider of the plugin framework not only has to make sure the plugin interface satisfies indented use cases, is clear and well documented, but also that it can evolve. Managing versions and backwards compatibility with existing plugins can be very hard. Hard enough that many practical implementations don't bother, and push the onus on plugin writers to update their plugins with each version.
• complexity: although each plugin works when tested alone, interactions between plugins can cause new problems, with bugs appearing only with certain combinations of plugins.
• testing: testing plugins can be difficult if the plugin system does not provide some form of mock plugin runner for testing, which is sometimes not possible, and testing is only available by running the plugin for real, which slows down development.
• artifical separation: a plugin typically has a single focus, but what constitues a single focus is set by the plugin api provider. If a plugin writer finds he needs a plugin that can reasonably do 2 things (as defined by the plugin api) in close tandem, he may end up having to implement two plugins and find ways of providing communication between them that is not presently provided by the api. He's then having to work around or against the plugin framework.

Designing a good plugin environment has many of the same challenges as designing a good library. If you are producing both the environment and the plugins yourself, then it's not so bad since you can update all the plugins as the environment evolves, but if the plugin api is open to all, then it requires careful planning and execution to get the design right to avoid too many plugin rewrites as the environment evolves.

"Second-system syndrome" described by Fred Brooks advocates that the second system developed is often excessively generic, aiming for ultimate flexibility, sometimes producing a "platform within a platform"/"inner platform effect". A pluggable design is often seen as a way out when requirements are non-existent or underspecified. To compensate, the software is made as flexible as possible to try to handle "whatever comes along".

Appologies if this paints a dreary picture - pluggable systems can be fantastic and offer a lot of strengths, but they come at a high price. Before diving into a pluggable system, it's prudent to draw up requirements for all the plugins that you will need to cover the functionality required. This will then help you decide if the pluggable design is worth the effort, or some simpler approach would serve equally well.

Answer 2

The advantages of a plug-in architecture is obviously the increase in flexibility. This allows other developers to extend your application in ways that did not expect in the first place. Note that there are various plug-in architecture ranging from flexible to extreme flexible. The most flexible one is called a Full Plug-in architecture, which is used in eclipse.

The disadvantage is that to be really flexible you have to develop a solid framework that incorporates loading, unloading and communication between plugins. There will also be a slight performance overhead in communication between plug-ins.

For a discussing on how to create a plug-in architecture take a look at this question.