Modularity/Getting Started/Building modular things

And an attempt to get the vocabulary consistent

Author: Stephen Tweedie

Building modular things TOC

1. The package
2. The module
3. The stack
4. The image or artifact
5. How do these parts fit together?

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Summary

We have some basic terminology confusion around modules. Is a container image the same thing as a module? Is a software collection a single module, or a group of modules? We can often get away with being vague, but for technical planning we need to be able to distinguish between all these concepts.

I propose we use these terms:

• Package. Essentially, the same thing as an rpm. In the future it might be non-rpm content but should fit the same role.
• Module. A set of packages tested and released together as a distinct unit, complete with the metadata needed to manage it as a unit. May depend on other modules.
• Stack. A complete tree of modules. A stack can be thought of as a top-level module, with the understanding that we’re implicitly including all of that module’s dependencies in the stack.
• Artifact or image. An actual set of bits built out of modules, in a format intended to be distributed or deployed in some way.

Generally, these serve distinct purposes. A module is a building block; a stack contains all the software for a complete solution; an artifact is a concrete object containing a stack (or stacks) for distribution to users.

We will also distinguish between:

• A Build of a package: a process which involves compiling source code and creating a packaged output; and
• A Compose of a module: a process which assembles pre-compiled packages into an organised module, but which includes no compilation step itself.