Make that `dvc repro --pull` pulls all missing files.

[peper0]

According to help for dvc repro:

  --pull                Try automatically pulling missing cache for outputs
                        restored from the run-cache.

and that's what it does. Pulls missing files that are outputs restored from the run-cache. But if there are outputs missing from "sources" (i.e. dvc files having only output and no command at all), it won't pull them. These must be pulled separately with a pull command.

Why not change it into "pull whatever is missing and necessary for this repro"? Is there any use case where a user want to download automatically some missing files, but not all of them?

[efiop]

Hi @peper0 !

It is the plan :slightly_smiling_face: For now it is limited to run-cache, but we'll add support for data sources soon. Stay tuned! :slightly_smiling_face:

[dberenbaum]

Comments from https://github.com/iterative/dvc/issues/4871:

Currently the --pull flag have no impact when used together with the --downstream flag. When working in a new repo, one must pull manually all the deps of stage A (if there is too much data handled with dvc to run dvc pull) before running dvc repro --downstream A. It would be great if dvc repro --downstream --pull A would automatically pull A deps.

Linked to #4870 Discord discussion: https://discord.com/channels/485586884165107732/485596304961962003/775412039321387008

Thinking more about it it would be great if --pull would work with -s|--single-item too.

[dberenbaum]

Comment from #4870:

Let's say there is a 3 stages workflow: A -> B -> C:

* A run `a.py` and produce `a.json`,

* B run `b.py` that takes `a.json` as input and produce `b.json`,

* C run `c.py` that takes `b.json` as input and produce `c.json`.

It would be very handy to have a flag in dvc repro to download only what is necessary to rerun the stages that changed. For example, when working on a fresh clone of the repo:

* if I change nothing and run `dvc repro --new-flag` -> dvc download nothing and run nothing.

* if I change c.py and run `dvc repro --new-flag` -> dvc only download b.json and run step C.

* if I change b.py and run `dvc repro --new-flag` -> dvc download a.json and run steps B and C.

This become particularly useful when working with big pipelines that train multiple models. Downloading all the training data for all the model can takes a lot of time and a lot of space on the disk.

[dberenbaum]

Another request in discord: https://discord.com/channels/485586884165107732/485596304961962003/945713992825991188

As noted there, this is really important for remote execution, so I think we need to prioritize this soon. cc @pmrowla @karajan1001

[francesco086]

Hi, I am leaving my comment as a user as suggested by @dberenbaum, I hope it can be helpful.

Let's take the example A -> B -> C mentioned above. After defining the pipeline I want to run each stage in a dedicated container using an orchestrator like Argo (or Kubeflow or Airflow or ...). So, for example, I will have a step in Argo where I want to execute the stage B. Then I would like to be able to do somthing along these lines:

git clone git-repo-containing-the-dvc-files
cd git-repo-containing-the-dvc-files
dvc repro B --new-flag
dvc add -R .
dvc push -R .
git add -A .
git commit -m "executed stage B"
git push

(--new-flag could be something like --pull-direct-dependency-only)

The command dvc repro B --new-flag should ideally pull (download) a.json (output of stage A) only if it changed, and it should run the stage B again only if there was any change.

Note from a design perspective: "There should be one-- an preferably only one --obvious way to do it". I can imagine that a user may want to just pull the data as described without the repro part. I am wondering if it is wise to put everything in one command, or if it rather makes more sense to split what I described in two: pull and repro (already existing). So, something like:

dvc pull B --new-flag && dvc repro B

(--new-flag could be something like --only-changed-direct-dependency) This is up to your judgment.

[pmrowla]

Can't this already be done with dvc pull -d/--with-deps B?

The behavior for repro --pull is definitely confusing and needs to be updated at some point, but I'm not sure it needs to be prioritized since it's already possible with pull + repro. (IMO it may be better to just rename repro --pull to repro --pull-run-cache to make it more obvious what that flag actually does)

[francesco086]

Actually dvc pull -d/--with-deps B would additionally download (pull) the output of B. Moreover, if you do it for stage C it will download a.json too.

[pmrowla]

Ah I see, so what we want would be something along the lines of pull --deps-only.

Although I'm not sure I understand why pulling the output here is a problem. This is also what repro --pull is "intended" to do (since it pulls run-cache, if the result of a known cached run exists DVC will just check out that resulting output instead of reproducing the stage).

[francesco086]

In the perspective of running a stage on a "clean" container, you want to run a stage only if:

1. the output will be different from one already computed - so, in case, don't download needed inputs to save resources.
2. pulling the output to overwrite it is just a waste of resources.

[dberenbaum]

In the initial comment for this issue, there is a good summary: "pull whatever is missing and necessary for this repro."

I think it's related to https://github.com/iterative/dvc/issues/5369, and that may be a prerequisite for the best-case scenario. A good solution for #5369 would determine whether there are any modified stages, taking into account what can be checked out from the cache/run-cache or pulled from remote storage.

Ideally, there should be an equivalent to do this but have the pipeline checkout/pull what's needed and run the necessary stages. For each stage, the command should do something like:

1. Check if dependencies have changed.
2. If dependencies are missing, check to see if they are available anywhere locally or remotely. Don't checkout/pull yet.
3. Check the run-cache locally and remotely to determine if the stage needs to be run.
4. If the stage needs to be run, checkout/pull as needed and run the stage.

I don't think dvc pull is sufficient because:

1. I don't want to pull unnecessary data. a. Pulling dependencies from unmodified or upstream stages doesn't help. Dependencies only need to be pulled if other dependencies for that stage are modified. b. Pulling outputs doesn't help. Intermediate outputs that are needed as dependencies of subsequent stages can be pulled later only if that subsequent stage needs to be run.
2. I don't want to have to determine which stages require me to pull. Having dvc pull flags in addition could be helpful for those who don't want to automatically kick off repro, but I don't think they are enough by themselves.

[pmrowla]

I'm still not following

1. I don't want to pull unnecessary data. a. Pulling dependencies from unmodified or upstream stages doesn't help. Dependencies only need to be pulled if other dependencies for that stage are modified.

Determining whether or not the upstream stages are modified requires on having the dependencies available (to see whether or not they have changed)

b. Pulling outputs doesn't help. Intermediate outputs that are needed as dependencies of subsequent stages can be pulled later only if that subsequent stage needs to be run.

In this case, it sounds like you are talking about where I have a pipeline like A -> B -> C, and I want to repro C, so DVC would only check the dependencies for A, since the intermediate outputs for A and B are not actually "required" here, just the deps for A and outputs for C. Is that correct?

DVC doesn't work this way right now, and supporting this would be a bigger change than just repro --pull. Even when you are using run cache, DVC still has to checkout the intermediate outputs, since the "is this changed/unchanged" check is only applied to individual stages, one at a time (meaning DVC needs to pull/checkout the deps/outs for each upstream stage).

[dberenbaum]

Determining whether or not the upstream stages are modified requires on having the dependencies available (to see whether or not they have changed)

I would like to have an option to use the .dvc files and dvc.lock outputs from previous stages to pull missing dependencies. For example, I change and repro A, push the changes, then git clone elsewhere to repro B. Using the updated dvc.lock, DVC should be able to tell that B dependencies have changed, pull the necessary output from stage A, and run stage B.

DVC doesn't work this way right now, and supporting this would be a bigger change than just repro --pull.

You're right, it's a bigger change than a typical command option.

In this case, it sounds like you are talking about where I have a pipeline like A -> B -> C, and I want to repro C, so DVC would only check the dependencies for A, since the intermediate outputs for A and B are not actually "required" here, just the deps for A and outputs for C. Is that correct?

If I want to repro C:

1. Check the dependencies for A. If any are missing but have a corresponding .dvc file, assume that's what should be used in A.
2. If there are changes and no corresponding entry in the run-cache, pull the missing dependencies and run A. Otherwise, skip A and don't pull dependencies (update dvc.lock to reflect any changes retrieved from the run-cache).
3. Check the dependencies for B. If A was skipped, its outputs will be missing. Use its dvc.lock outputs to determine the missing dependencies for B.
4. Repeat this process for the rest of B and C.

[dberenbaum]

@francesco086 You mentioned wanting to have the ability to pull a single stage's dependencies in dvc pull. I think we could have this as a related but separate feature request if we can figure out appropriate syntax. Sorry if I misled by directing you to this issue.

I see two possible approaches:

1. Have flags to pull only dependencies (not outputs), and to limit the depth of dependencies so that they only include those from the previous stage. This might look like dvc pull --deps-only --depth 1, although I think that seems clunky.
2. Have a flag to pull the previous stage's outputs without specifying it by name (since that makes it hard to automate). This might look like dvc pull -n 1 B to get the outputs of A. If B depends on multiple other stages, this becomes more complicated. As long as we document what happens in that scenario (unsupported, collect all such previous stages, or choose one), I think it's fine.

[DXist]

Is it possible to explicitly refer .dvc file as stage dep that provides the required files?

Analogies from Bazel build system:

• http_archive rule provides unpacked archive files. Build targets could explicitly depend on them. When these targets are built only their remote dependencies are fetched.
• bazel fetch command allows to pull all remote dependencies for workspace

[dberenbaum]

After #5369, we should have the ability to determine whether a stage needs to be run without pulling the data locally. At that point, solving this issue seems straightforward (although still more complex than a typical CLI flag):

1. Check the stage status without pulling data locally.
2. If the status isn't clean, pull the dependencies and run the stage.