howfastly 2026.815.0

measure your connection speed to the fastly network
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HowFastly

Binary Cache:

HowFastly measures your connection speed to the Fastly network, running on Fastly Compute. Unlike fastly-debug.com, which only reports connectivity diagnostics, HowFastly measures throughput and latency, inspired by speed.cloudflare.com.

To install with cargo:

RUSTFLAGS='--cfg reqwest_unstable' cargo install howfastly

Or run without installing:

nix run \
  --substituters 'https://cache.ysun.co' \
  --trusted-public-keys 'cache.ysun.co-1:WxPYwT5g3kt9XhUhHPpNLZKI9HIOsVVAuqSHpok8Qt4=' \
  github:stepbrobd/howfastly#howfastly

See stepbrobd/inc#howfastly for packaging.

Nix users: multiple public binary cache instances like cache.nixos.org and cache.ysun.co are fronted by Fastly, so speedtest results are a fairly accurate representation of how fast binary cache fetches will run on your connection.

Methodology:

  • 25 unloaded latency pings (median, jitter, min, avg)
  • Download transfers from 100 kB to 100 MB, uploads from 100 kB to 50 MB
  • Per size iteration counts (8/8/6/4/2, larger transfers have lower relative variance so fewer repeats)
  • 30s time budget per direction (roughly 640 MB transferred worst case)
  • Reports p90 speed with loaded latency per direction (bufferbloat)
  • Server processing time calculated from samples using Server-Timing header
  • No packet loss measurement (this would requires raw UDP probes, which speed.cloudflare.com sends through a WebRTC TURN server native to the Cloudflare network, Fastly Compute cannot emit raw L3 frames, and Fastly does not have freely usable TURN server)

GitHub Action

HowFastly can also be used as a composite action that measures a runner's connection speed to Fastly (note that the workflow downloads a prebuilt binary from this repository's releases and verifies it against the release checksums, regardless, you should verify everything carefully). Pinning a release tag runs that release's binary. Any other ref (either a branch name or a commit SHA), runs the latest release. Supported runners are Linux x64/arm64 and macOS arm64.

jobs:
  speedtest:
    runs-on: ubuntu-latest
    steps:
      - id: howfastly
        uses: stepbrobd/howfastly@master
        with:
          # release tag to install
          # empty uses the pinned tag or the latest release
          version: ""
          # extra CLI flags appended to the default run
          args: -m 10m --ipv4
          # shell command run instead of the default invocation
          # mutually exclusive with args
          # howfastly is on PATH
          # write JSON to $HOWFASTLY_RESULTS to keep outputs and the summary
          # command: howfastly --format json > "$HOWFASTLY_RESULTS"
      - env:
          DOWNLOAD: ${{ steps.howfastly.outputs.download-mbps }}
        run: echo "download p90 ${DOWNLOAD} Mbps"

Outputs:

  • download-mbps, upload-mbps: 90th-percentile throughput
  • latency-ms: Median unloaded latency
  • pop: Fastly POP that served the test
  • json: Full results as compact JSON
  • results-path: Pretty-printed results file, point actions/upload-artifact at it to keep an artifact

Each run also writes a human-readable summary to the workflow run page.

Sponsorship disclaimer

Not an official Fastly product or project. This project is generously supported by Fastly through the Fast Forward program. The views and content of this project are solely those of the author and do not imply endorsement by Fastly.

License

Apache-2.0