envoy-ndjson-to-sse

An Envoy proxy WASM filter that converts NDJSON (Newline Delimited JSON) response bodies into Server-Sent Events (SSE). Use it when a backend returns streaming NDJSON and you want clients to consume it as text/event-stream SSE.

Behavior

• Condition: The filter runs only when the response Content-Type is NDJSON: application/x-ndjson, application/ndjson, or x-ndjson (parameters like ; charset=utf-8 are ignored). Other responses pass through unchanged.
• Response headers: When transforming, sets Content-Type: text/event-stream and removes Content-Length.
• Body: Each non-empty NDJSON line is emitted as one SSE event: data: <trimmed-line>\n\n.
• Streaming: Handles chunked response bodies; incomplete lines are buffered until a newline or end-of-stream.
• Empty lines: Lines that are empty or only whitespace are skipped.

Build

Requires Rust and the wasm32-wasip1 target:

rustup target add wasm32-wasip1
cargo build --target wasm32-wasip1 --release

Output: target/wasm32-wasip1/release/envoy_ndjson_to_sse.wasm

Download pre-built WASM

• Releases: For each release (e.g. v0.1.0), download envoy-ndjson-to-sse-<version>.wasm from the release assets. Create a release by pushing a tag:

  git tag v0.1.0 && git push origin v0.1.0

• CI artifacts: The Actions workflow builds on every push to master. Open the latest successful run → Summary → download the envoy-ndjson-to-sse-wasm artifact (contains envoy-ndjson-to-sse.wasm).

Envoy Configuration

Use the filter in the HTTP connection manager. Example snippet:

http_filters:
  - name: envoy.filters.http.wasm
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.http.wasm.v3.Wasm
      config:
        name: ndjson_to_sse
        root_id: ndjson_to_sse
        vm_config:
          runtime: envoy.wasm.runtimes.v8
          code:
            local:
              filename: /path/to/envoy-ndjson-to-sse.wasm
  - name: envoy.filters.http.router
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.http.router.v3.Router

Point the route to a backend that returns NDJSON (e.g. Content-Type: application/x-ndjson). Clients can then request the same path and receive SSE.

Testing

• Unit tests (NDJSON → SSE conversion, no Envoy):

  cargo test

• Integration with Envoy: See TESTING.md for a full Envoy config example and curl usage.

License

MIT License. See LICENSE.