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+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 9"""
+ date="2025-01-27T14:46:43Z"
+ content="""
+Circling back to this, I think the fork in the road is whether this is
+about git-annex providing this and that feature to support external special
+remotes that compute, or whether git-annex gets a compute special
+remote of its own with some simpler/better extension interface
+than the external special remote protocol.
+
+Of course, git-annex having its own compute special remote would not
+preclude other external special remotes that compute. And for that matter,
+a single external special remote could implement an extension interface.
+
+---
+
+Thinking about how a generic compute special remote in git-annex could
+work, multiple instances of it could be initremoted:
+
+       git-annex initremote convertfiles type=compute program=csv-to-xslx
+       git-annex initremote cutvideo type=compute program=ffmpeg-cut
+
+Here the "program" parameter would cause a program like 
+`git-annex-compute-ffmpeg-cut` to be run to get files from that instance
+of the compute special remote. The interface could be as simple as it
+being run with the key that it is requested to compute, and outputting
+the paths to the all keys it was able to compute. (So allowing for 
+"request one key, receive many".) Perhaps also with some way to indicate
+progess of the computation.
+
+It would make sense to store the details of computations in git-annex
+metadata. And a compute program can use git-annex commands to get files
+it depends on. Eg, `git-annex-compute-ffmpeg-cut` could run:
+
+       # look up the configured metadata
+       starttime=$(git-annex metadata --get compute-ffmpeg-starttime --key=$requested)
+       endtime=$(git-annex metadata --get compute-ffmpeg-endtime --key=$requested)
+       source=$(git-annex metadata --get compute-ffmpeg-source --key=$requested)
+
+       # get the source video file
+       git-annex get --key=$source
+       git-annex examinekey --format='${objectpath}' $source
+
+It might be worth formalizing that a given computed key can depend on other
+keys, and have git-annex always get/compute those keys first.
+
+When asked to store a key in the compute special remote, it would verify
+that the key can be generated by it. Using the same interface as used to
+get a key.
+
+This all leaves a chicken and egg problem, how does the user add a computed
+file if they don't know the key yet?
+
+The user could manually run the commands that generate the computed file,
+then `git-annex add` it, and set the metadata. Then `git-annex copy --to`
+the compute remote would verify if the file can be generated, and add it if
+so. This seems awkward, but also nice to be able to do manually.
+
+Or, something like VURL keys could be used, with an interface something
+like this:
+
+       git-annex addcomputed foo --to ffmpeg-cut
+               --input compute-ffmpeg-source=input.mov 
+               --set compute-ffmpeg-starttime=15:00
+               --set compute-ffmpeg-endtime=30:00
+
+All that would do is generate some arbitrary VURL key or similar,
+provisionally set the provided metadata (how?), and try to store the key
+in the compute special remote. If it succeeds, stage an annex pointer
+and commit the metadata. Since it's a VURL key, storing the key in the
+compute special remote would also record the hash of the generated file
+at that point.
+"""]]