October '21 Community Gems

A roundup of technical Q&A's from the DVC community. This month: DVC stages, working with outputs, DVC API, and more.

  • Milecia McGregor
  • October 28, 20217 min read
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Is there a command to force reproduce a specific stage of a DVC pipeline?

Good question @wickeat!

You can use dvc repro -f <stage_name>, although this will reproduce the earlier dependency stages in the pipeline up to that point. If you only want to reproduce a single target stage, you can add -s/--single-item to the dvc repro command.

How do you manage a dvc.yaml file for a project that's going to be a big, sparse DAG?

This an awesome use case from @Ian!

Let's say we have this scenario:

  • A new data set is delivered to you every day
  • It needs to be featurized (does not depend on previous days' data)
  • Subsequent stage depends on all days

The recommended approach is to keep all of the previous days and use the foreach syntax, which ensures your DAG still knows about all the previously processed days:

      - 20210101
      - 20210102
      - 20210103
      cmd: python featurize.py ${item}
        - raw/${item}.csv
        - intermediate/${item}.csv
    cmd: python combine.py
      - intermediate
      - combined.csv

That way if you adjusted something in your featurize script, for example, it would automatically reprocess every day's data.

What is the best practice for capturing and saving stdout?

The best practice when using DVC is to pipe each command stdout into a different file with a unique name, like a timestamp, in a directory that becomes the stage output.

If optimizing storage space is a concern, in case the stdout dumps grow a lot, this is what we recommend.

Here's an example of what that might look like if you're using a tool like tee.

  cmd: python src/train.py data/features model.pkl | tee -a 20211021_model.pkl
    - data/features
    - src/train.py
    - train.min_split
    - train.n_est
    - train.seed
    - models/20211026_model.pkl

This will output the stdout from the train stage in the terminal and also save it in a new file with the timestamp as part of the title.

That was a helpful question. Thanks @gregk0!

There is a file in our pipeline that needs to be manually modified and then used as the input to other stages in the pipeline. What would be the best approach for this with DVC?

This is another great use case. Thanks @omarelb!

Let's say that you have a process similar to this.

  • Run the first stage of the pipeline, for example a stage called cleaning
  • Inspect its output, lexicon.txt, and modify it if necessary
  • The modified version of lexicon.txt is then cached and used as input to following stages of the pipeline

You can copy the output and modify and commit it in the copied location so the first stage and its output are separate from the modified file and subsequent stages.

If you want to link the first stage to the rest of the pipeline, you could have your 2nd stage be something like:

  cmd: |
    # To generate lexicon_modified.txt:
    # 1. Run `cp lexicon.txt lexicon_modified.txt`.
    # 2. Check and modify lexicon_modified.txt.
    # 3. Run `dvc commit manual`.
    - lexicon.txt
    - lexicon_modified.txt

To clarify, if you put that manual stage into your dvc.yaml, it should connect the whole pipeline. Each time you run dvc repro and the first stage generates a new lexicon.txt, you will get ERROR: failed to reproduce 'dvc.yaml': output 'lexicon_modified.txt' does not exist because the manual stage doesn't generate the expected output.

You can then manually copy, modify, and commit your new lexicon_modified.txt and run dvc repro again to run the rest of the pipeline.

What is the workflow if I want to remove some files from my dataset registry with DVC?

In this case, assume that the data was added as a folder containing images, which means that there is a single .dvc for the whole folder. You don't need to remove the .dvc file that's tracking the data in that folder.

You can delete the files you want to remove and then re-add the folder using dvc commit. Here's what an example of what that flow might look like.

  • You git clone your data registry.
  • Then dvc pull your data.
  • Delete the files you want to remove.
  • Run dvc commit and git commit to save your changes.

It should be faster to commit, as DVC won't re-add the files to the cache nor will it try to hash them.

Good question @MadsO!

We want to access a private Git repo using dvc.api.read() in a Docker container. How do I pass the credentials to DVC so that we can read DVC files from this repo?

Great question about the API @dashmote!

There are a couple different ways to handle this.

The first option is to use SSH. You'll need to pass GitHub SSH keys into your Docker container and use the git@github.com:username/repo.git URL format when you call the API method.

The other option is to use HTTP. You need to use the https://username:token@github.com/username/repo.git URL format when you call the API method.

You could pass your credentials into your container as environment variables and then do something like:

username = os.environ["GITHUB_USERNAME"]
token = os.environ["GITHUB_TOKEN"]
dvc.api.read(..., repo=f"https://{username}:{token}/...", ...)

Is there a clean way to handle multiple models in the same repo that are trained using the same pipeline?

Let's say your project looks something like this:

├── data
│   ├── customer_1
│   │   ├── input_data.txt
│   │   ├── input_params.yaml
│   │   └── output
│   │       └── model.pkl
│   └── customer_2
│       ├── input_data.txt
│       ├── input_params.yaml
│       └── output
│           └── model.pkl
├── dvc.lock
├── dvc.yaml
└── train_model.py

The simplest way is to copy the dvc.yaml into each model's separate directory, like this:

├── data
│   ├── customer_1
│   │   ├── input_data.txt
│   │   ├── input_params.yaml
│   │   ├── dvc.yaml
│   │   ├── dvc.lock
│   │   └── output
│   │       └── model.pkl
│   └── customer_2
│       ├── input_data.txt
│       ├── input_params.yaml
│       ├── dvc.yaml
│       ├── dvc.lock
│       └── output
│           └── model.pkl
└── train_model.py

Another potential solution is try templating. We'll have a dvc.yaml in the root of the project and add vars to define the model you want to train. Then we'll update the train stage to use the vars like this:

  - model_name: 'customer_2'

    cmd: python train.py
      - data/${model_name}/input_data.txt
      - data/${model_name}/input_params.yaml:
          - batch_size
          - ...

You can learn more about templating in the docs. It essentially lets you add variables to the dvc.yaml to dynamically set values for your stages.

Thanks for the great question @omarelb!

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