Why are git submodules incompatible with svn externals?

Question

There are lots of webpages out there suggesting hackish ways to make svn externals look like git submodules. I have read some accounts of what the difference is, but this doesn't seem very fundamental:

Git submodules link to a particular commit in another project's repository, while svn:externals always fetch the latest revision.

Why does this difference make them so fundamentally incompatible? Isn't there a reasonable default we can assume, such as that most svn:externals point to tags that never move?

Answer 1

The fundamental difference is the composition rule.

In a true component-based approach, you define a configuration, that is:
The list of labels (of SHA1 commits for Git) you need for your project to "work" (i.e. "develop", "compile", "deploy", ...).

Each commit referenced in a configuration help you to get the exact versions of a all tree. There is no exception. Each files of that tree is at the exact version specified by the configuration you have defined.
There can be only one label/SHA1 per module. From one common parent repo, you cannot define a module within a module.
(But a module, which is just a reference to an external Git repo, can have its own submodules definition: the parent repo will only refer the first-level submodule, which in turn will reference whatever submodules it had committed within itself)

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No so in SVN external: you can define directory externals as well as file external, with or without an explicit revision in it.
You can compose various external properties. For instance:

$ svn propget svn:externals calc
third-party/sounds             http://svn.example.com/repos/sounds
third-party/skins -r148        http://svn.example.com/skinproj
third-party/skins/toolkit -r21 http://svn.example.com/skin-maker

The result is not a configuration (one reference for 'calc'), but a composition of selection rules which define the exact "patchwork" you need in the directory 'calc'

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In short, you can not "compute" one SHA1 for a 'calc' submodule which would be the exact equivalent of a bunch of svn:external properties on a 'calc' SVN directory.