add_request_header

Add Request Header Plugin

This plugin adds and sets HTTP headers on client requests as they pass through the proxy. It demonstrates the distinction between adding a header (which appends to any existing value) and setting/replacing a header (which overwrites any existing value). Use this plugin when you need to inject metadata, routing hints, or default values into upstream requests. It operates during the request headers processing phase.

How It Works

1. The proxy receives an HTTP request from a client and invokes the plugin's on_http_request_headers callback.
2. The plugin adds a Message: hello header. If the request already contains a Message header, the new value is appended (comma-separated), resulting in Message: <original>, hello.
3. The plugin sets the Welcome header to warm. If the request already contains a Welcome header, the existing value is replaced entirely.
4. The plugin returns Action::Continue, allowing the (now modified) request to proceed to the upstream server.

Implementation Notes

• Appending vs replacing headers: This plugin demonstrates the distinction between "adding" a header (appending with a comma to existing values if present) and "setting" or "replacing" a header (completely overwriting it).

Configuration

No configuration required.

Build

Build the plugin for any supported language from the plugins/ directory:

# Rust
bazelisk build //samples/add_request_header:plugin_rust.wasm

# C++
bazelisk build //samples/add_request_header:plugin_cpp.wasm

# Go
bazelisk build //samples/add_request_header:plugin_go.wasm

Test

Run the unit tests defined in tests.textpb:

# Using Docker (recommended)
docker run -it -v $(pwd):/mnt \
    us-docker.pkg.dev/service-extensions-samples/plugins/wasm-tester:main \
    --proto /mnt/samples/add_request_header/tests.textpb \
    --plugin /mnt/bazel-bin/samples/add_request_header/plugin_rust.wasm

# Using Bazel (all languages)
bazelisk test --test_output=all //samples/add_request_header:tests

Expected Behavior

Derived from tests.textpb:

Scenario	Description
AddsRequestHeader	Injects the new headers when they do not previously exist on the request.
UpdatesRequestHeader	Appends to the existing Message header and overwrites the existing Welcome header.
RequestAndResponse	Verifies that the modifications occur only on the request phase.
CaseInsensitiveUpdates	Properly updates existing headers regardless of their original case.

Available Languages

• Rust
• C++
• Go