ballast

# Ballast

Timing-balanced test sharding for ExUnit.

`mix test --partitions` deals files out round-robin regardless of how long they
take, so one slow shard holds up CI. Ballast records how long each file takes
and balances the shards so they finish together.

    $ mix ballast.plan --shards 4
    73 files, 0 without history, max_cases 36, plan 69e9b4be5416

    shard  files  ballast  round-robin
        1     41    31.4s        38.9s
        2     22    31.4s        32.1s
        3      3    31.5s        15.6s
        4      7    31.5s        35.4s

    slowest shard: 31.5s (round-robin: 38.9s)

## Setup

With [Igniter](https://hexdocs.pm/igniter):

    $ mix igniter.install ballast

The installer makes the changes below. To set Ballast up by hand instead:

```elixir
# mix.exs
def cli do
  [preferred_envs: ["ballast.test": :test, "ballast.merge": :test, "ballast.plan": :test]]
end

defp deps do
  [{:ballast, "~> 0.1", only: :test, runtime: false}]
end
```

```elixir
# test/test_helper.exs
ExUnit.start(formatters: [ExUnit.CLIFormatter, Ballast.Formatter])
```

The formatter does nothing under a plain `mix test`.

Without the `preferred_envs` entries, Mix runs the tasks in `:dev`, where the
dependency is not loaded, and reports `The task "ballast.test" could not be found`.

## Use

    $ mix ballast.test --shard 3/8          # run shard 3 of 8, write tmp/ballast/shard-3-of-8.json
    $ mix ballast.merge --check             # check that shards 1..8 share a plan and cover every file once
    $ mix ballast.merge                     # write tmp/ballast/timings.json for the next run
    $ mix ballast.plan --shards 8           # show the split without running it

Other `mix test` options, such as `--exclude`, `--warnings-as-errors` and
`--failed`, are passed through unchanged. Test paths narrow which files get
sharded. `--partitions` is rejected, because `--shard` replaces it.

`mix help ballast.test`, `mix help ballast.plan` and `mix help ballast.merge`
list every option.

With no snapshot, Ballast splits exactly like `--partitions`.

## Keeping shards consistent

Every shard computes the plan on its own machine. They only agree if they see
the same test files and the same snapshot.

- Keep `tmp/ballast/timings.json` in the CI cache, not in git. Pick the cache
  entry in one job and have every shard restore exactly that entry. If each
  shard job looks up the newest entry itself, another run can save a newer
  snapshot in between, and two shards of one run plan from different
  snapshots.
- Run `mix ballast.merge --check` on every CI run. Without it, disagreeing
  shards silently skip or repeat files.

## GitHub Actions

```yaml
name: CI

on:
  push:
    branches: [main]
  pull_request:

env:
  MIX_ENV: test

jobs:
  # Looks up the newest snapshot once, so that every shard restores the same one.
  timings:
    runs-on: ubuntu-24.04
    outputs:
      key: ${{ steps.lookup.outputs.cache-matched-key }}
    steps:
      - id: lookup
        uses: actions/cache/restore@v6
        with:
          path: tmp/ballast/timings.json
          key: ballast-timings-${{ github.run_id }}
          restore-keys: ballast-timings-
          lookup-only: true

  test:
    needs: timings
    runs-on: ubuntu-24.04
    strategy:
      fail-fast: false
      matrix:
        shard: [1, 2, 3, 4]
    steps:
      - uses: actions/checkout@v7
      - uses: erlef/setup-beam@v1
        with: { elixir-version: "1.20", otp-version: "27" }
      - run: mix deps.get
      # No snapshot yet: skip the restore and plan like --partitions.
      - if: needs.timings.outputs.key != ''
        uses: actions/cache/restore@v6
        with:
          path: tmp/ballast/timings.json
          key: ${{ needs.timings.outputs.key }}
          fail-on-cache-miss: true
      - run: mix ballast.test --shard ${{ matrix.shard }}/${{ strategy.job-total }}
      - uses: actions/upload-artifact@v7
        with:
          name: ballast-shard-${{ matrix.shard }}
          path: tmp/ballast/shard-*.json

  verify:
    needs: test
    runs-on: ubuntu-24.04
    steps:
      - uses: actions/checkout@v7
      - uses: erlef/setup-beam@v1
        with: { elixir-version: "1.20", otp-version: "27" }
      - run: mix deps.get
      - uses: actions/download-artifact@v8
        with:
          { pattern: ballast-shard-*, path: tmp/ballast, merge-multiple: true }
      - run: mix ballast.merge --check
      - if: github.ref == 'refs/heads/main'
        run: mix ballast.merge
      # Cache entries are immutable, so every run on main saves a new one and
      # restore-keys picks the newest.
      - if: github.ref == 'refs/heads/main'
        uses: actions/cache/save@v6
        with:
          path: tmp/ballast/timings.json
          key: ballast-timings-${{ github.run_id }}
```

`path` must be the same in every cache step, because it is part of the cache
version. `fail-on-cache-miss` fails a shard whose entry was evicted after the
lookup, instead of letting it plan from no history.

To hand the snapshot to the shards as an artifact instead, upload it in the
`timings` job (with `lookup-only` removed) and download it in each shard. On
the first run there is nothing to upload, so allow both steps to come up
empty:

```yaml
      # timings job, after the restore
      - uses: actions/upload-artifact@v7
        with:
          name: ballast-timings
          path: tmp/ballast/timings.json
          if-no-files-found: ignore

      # test job, instead of the restore
      - uses: actions/download-artifact@v8
        continue-on-error: true
        with: { name: ballast-timings, path: tmp/ballast }
```

`continue-on-error` also hides real download failures. A shard that missed
the snapshot plans from no history, and `mix ballast.merge --check` rejects
the run.

## Details

- Sync modules add up; async modules overlap up to `max_cases`. A shard costs
  `sync + max(longest async module, async / max_cases)`.
- New files get the median weight of the known ones until they have a timing
  of their own.
- With `--shard`, `--cover` exports `cover/ballast-N.coverdata` and prints no
  summary. Collect the `cover/` directories and run `mix test.coverage` for the
  combined report and threshold.
- `mix ballast.test --shard 3/8 --failed` reruns the failures of shard 3. The
  plan is computed on the full suite first, so the shard keeps its files.
  (`--partitions --failed` re-partitions the failed files.)
- Partial runs (`--failed`, `--stale`, `--only`, `-n`,
  `--repeat-until-failure`, `--dry-run`, `FILE:LINE`) are never recorded.
- `mix ballast.merge` refuses shards with failures, because a failing test is
  not a representative timing. `--partial` overrides this, for bootstrapping or
  repairing a snapshot.

## Not supported yet

Umbrella roots, `FILE:LINE` together with `--shard`, and splitting a single
file across shards.

## License

Licensed under either of:

- [Apache License, Version 2.0](./LICENSE-APACHE)
- [MIT license](./LICENSE-MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this project shall be dual licensed as above, without any
additional terms or conditions.