git-annex/ tips/ multiple remotes accessing the same data store

A remote configures git-annex with a way to access a particular data store. Sometimes, there's more than one way for git-annex to access the same underlying data store. It then makes sense to have multiple remotes.

The most common and easy case of this is when the data store is itself a git-annex repository. For example, using git-annex on your laptop, you might add a ssh remote which works from anywhere. But, when you're located on the same LAN as the server, it might be faster or simpler to use a different ssh url.

    git remote add server someserver.example.com:/data
    git remote add lanserver someserver.lan:/data

git-annex is able to realize automatically that these two remotes both access the same repository (by contacting the repository and looking up its git-annex uuid). You can use git-annex to access which ever remote you want to at a given time.

Special remotes don't store data in a git-annex repository. It's possible to configure two special remotes that access the same underlying data store, at least in theory. Whether it will work depends on how the two special remotes store their data. If they don't use the same filename (or whatever), it might not work.

The case almost guaranteed to work is two special remotes of the same type, and other configuration, but with different urls that point to the same data store. For example, a git-lfs repository can be accessed using http or ssh. So two git-lfs special remotes can be set up, and both access the same data.

    git annex initremote lfs-ssh type=git-lfs encryption=shared url=git@example.com:repo
    git annex initremote --sameas=lfs-ssh lfs-http type=git-lfs url=https://example.com/repo

The `--sameas` parameter tells git-annex that the new special remote uses the same data store as an existing special remote. Note that the encryption= parameter, which is usually mandatory, is omitted. The two necessarily encrypt data in the same way, so it will inherit the encryption configuration. Other configuration is not inherited, so if you need some parameter to initremote to make a special remote behave a certian way, be sure to pass it to both.

Finally, it's sometimes possible to access the same data stored in two special remotes with different types. One combination that works is a directory special remote and a rsync special remote.

    git annex initremote dir type=directory encryption=none directory=/foo
    git annex initremote --sameas=dir rsync type=rsync rsyncurl=localhost:/foo

If a combination does not work, git-annex will be unable to access files in one remote or the other, it could get into a scrambled mess. So it's best to test a a combination carefully before you start using it for real. If you find combinations that work, please edit this page to list them.

## known working combinations

- directory and rsync
- httpalso and directory
- httpalso and rsync
- httpalso and rclone (using git-remote-rclone) (any layout except for frankencase)
- httpalso and any special remote that uses exporttree=yes

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