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Bazel build rules for machine learning workflows
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Latest commit 52ba5d3 Jun 25, 2019




Computing can be complicated. As programmers, we naturally want to decompose a complex process into simpler steps. but how do those steps fit together? Before long, we always turn to DAGs. When building software, we can express build DAGs in make. In the realm of data pipelines, tools like luigi and airflow similarly create DAGs to describe computations made over data. I thought, "we need to build software to run our data analysis workflows, but we use different tools to describe the DAGs to build the two. Why?". With a little too much time on my hands, I created databazel to unify the two.


databazel is, at its core, a set of bazel rules that runs a machine learning workflow. If you don't know what bazel is, I don't know why you've come this far. This is a pure proof-of-concept of the background discussion above, simply to see if it could be done. As such, it's not very flexible, but it is cool. Currently, it supports three operations: training a model, evaluating a model, and conducting hyperparameter search.

In addition to the bazel rules, there is an example workflow in the mnist directory. databazel itself does not implement all of the steps above - rather, it specifies how they fit together and depend on one another. A hypothetical user would be responsible for writing training and evaluation scripts that conform to databazel's expected interfaces.


To run this example, first run to prep the data. It's lame, I could make data-fetching happen in bazel, but this isn't my day job. You can then run model training by running bazel build --python_top=//:env-databazel-3 //mnist:bzl_training

Why --python_top?

It's what we have to do to make bazel use the python environment set up for our purposes. It's lame. With a different implementation of training we wouldn't need it

Why bazel build?

You may think of model training as being something that you run. But this is a software build tool, so it thinks that it's building something. If you think of it as building a trained model, you'll be able to live with it.

Training a model isn't cool. You know what's cool? Training a bunch of models. Automatically. To do a hyperparameter grid search, do bazel build --python_top=//:env-databazel-3 //mnist:bzl_search

This will kick off training runs for each combination of specified hyperparameters, and then a model evaluation run for each of them, and write out all the results.


Here are some things that people have commented would be cool, but that I haven't done yet or would only make sense if this were being used for real

  • Make use of bazel (remote) cache to re-use artifacts (models, evaluation reports) that have already been generated on previous runs. This actually sorta works.
  • Make use of bazel remote execution to parallelize things like hyperparam search
  • Encode multiple training epochs in bazel. Resume painlessly, in theory.


Here are the components in a little more detail:


  • training_data: Bazel label for location of training data. Can point to a file, or an s3 download link, or whatever the hell else bazel can understand - it'll figure out how to pass it to your training script
  • train_executable: Your script. better make sure it takes the right inputs and writes out its data where it's supposed to
  • model: Name of the file that will be written out. This will be passed to your script, which better write the damn file there.
  • hyperparams: Just what it sounds like. all strings all the time, your code can convert values to the types you want


  • eval_executable: Your evaluation script. Again, this needs to conform to the exact contract that databazel expects
  • test_data: a reference to data like above. The mnist example is a little weird in that it's one big dataset that has both train and test data in it; you can figure out the details within your script
  • model: the bazel label of an instance of model above


This rule does a lot more heavy lifting itself, constructing a series of model and evaluate steps on its own. You do not need to define those on your own to use it. Its arguments are basically the union of all the arguments above.

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