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I'm a software developer who is willing to improve his software design skills. I think software should not only work but have a solid and elegant design too to be reusable and extensive to later purposes.

Now I'm in search of some help figuring out good practices for specific problems. Actually, I'm trying to find out how to design a piece of software that would be extensible via plug-ins.

The questions I have are the following:

  • Should plug-ins be able to access each others functionality? This would bring dependencies I guess.
  • What should the main application offer to the plug-ins if I want to let's say develop a multimedia software which would play videos and music but which could later on get functionality added over plug-ins that would let's say be able to read video status (time, bps, ...) and display it. Would this mean that the player itself would have to be part of the main program and offer services to plug-ins to get such information or would there be a way to develop the player as a plug-in also but offer in some way the possibility to other plug-ins to interact with it?

As you see, my questions are mainly for learning purposes as I strive to improve my software development skills.

I hope to find help here and apologize if something is wrong with my question.

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The OSGI is one plug-in based architecture. osgi.org/Main/HomePage If you want to see a working example of the OSGI architecture, take a look at how Eclipse is put together. –  Gilbert Le Blanc Sep 4 '12 at 13:11
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@gekod If you want to get better at design study and practice design principles, starting with SOLID en.wikipedia.org/wiki/SOLID_(object-oriented_design) in my opinion studying design principles are the best primer for beginning your journey into really analyzing code from a design perspective. –  Jimmy Hoffa Sep 5 '12 at 4:11

1 Answer 1

up vote 8 down vote accepted

Pretty much any software that "wants" to be extended over time, by multiple contributors who are not closely linked or coordinating, can benefit from a plug-in architecture of some sort. Common examples are operating systems, CAD and GIS tools, drawing and image manipulation tools, text editors and word processors, IDEs, web browsers, web content management systems, and programming language and frameworks. Plugins are the mainstay of extensible systems.

Plug-in architectures typically use duck typing. The architect defines a common set of methods (e.g. open, close, play, stop, seek, etc.), which each plugin then implements (either entirely or in part). Some methods are mandatory, while others may be optional, or useful only in specific cases.

When the main program initially runs, it checks one or more "plugin areas" (such as known ./plugins directories) for the existence of plugins. Those found are loaded into the program.

Often plugins must exist at the time the main program runs. The Unix kernel and the Apache web server typically operate this way; they must be restarted to "see" and use new plugins. Plugins may be more dynamic however; here the main program periodically re-checks for newly-added or changed plugins (for example by comparing a stored plugins-last-loaded timestamp with the "last modified" timestamp for a plugins directory). The main program would then (re-)load plugins--either all of them, in the simple/naive case, or just the new/changed ones, if it's more sophisticated.

There is often a "registration" requirement, with each plugin not just being code, but also including some metadata that communicates how the plugin integrates into the whole. A music player plugin, for example, might be required to state what kind(s) of files it can play, what processor architecture(s) it can run on, what resources it needs (e.g. how much memory it needs to be allocated), and other attributes required for the main program to decide which plugin to use to play which file.

The mechanisms for plugin registration, loading, and interaction with the main program is quite language- and framework-specific. Because there's a lot of "orchestration" going on, with some functions handled by the main program and some by its plugins (of which there might be quite a few), setting up a program for extensibility requires care and consideration, and an architectural view of the program as "a system" rather than "a single piece of code."

Most large-scale projects will have already chosen a plugin framework, or designed their own. There are also a number of generic plugin frameworks designed to simplify making your program into an extensible system.

(Answer to Question 1) While plug-ins can use each other's functionality, they typically would do so through the pre-defined methods/APIs the architect laid out. The use of such "duck typing" helps avoid super-interdenency, and means that it's not necessarily clear whether a given feature is provided by "core" code or a plugin. Indeed, having adopted a plug-in strategy, many developers implement even "core" features as plugins--just ones that are shipped with the main program. While having spaghetti tangles of plugins isn't ideal, it's not uncommon to see some plug-ins requiring the existence of other plug-ins.

(Answer to Question 2) As an architect, the main thing you offer plugins is an architecture--e.g. a set of methods via which they are setup, registered, and invoked, and a design and set of requirements in which the plugins will operate. The main program, while running, usually exposes many if not all of its internal data structures and methods to plugins. This is obviously a security exposure. A number of sandboxing techniques can be (and increasingly are being) used, but most often, plugins are "trusted" code, operating as if they are part of the main program.

For further reading:

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I wrote the above as though a program only has one kind of plug-in. That makes things simpler, but in reality, many apps support multiple kinds of plug-in: E.g. data readers, data transformers, data formatters. Or drawing objects, drawing effects, file format readers, file format writers. Or music format players, music information displayers, music animators. –  Jonathan Eunice Sep 4 '12 at 20:17
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Your posted links really are worth reading and your explanation is top level. Thank you for taking time to share such valuable information with the learning ones! If only everybody could behave in such a helpful way, people would get much more out of it! It's people like you who contribute to the quality of such sites. –  gekod Sep 5 '12 at 10:12
    
Given the increasing popularity of "interfaces" in Java, Go, Julia, and other languages, I should note that plugins in those languages will likely implement a defined plugin interface, and not be pure (untyped) duck typing. –  Jonathan Eunice Jul 16 at 17:19

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