add_custom_response

Add Custom Response Plugin

This plugin intercepts 5xx server error responses and replaces them with a 302 redirect to a custom error page. It preserves the original status code in a custom header (Origin-Status) for debugging purposes. Use this plugin when you need to provide a user-friendly error page for server errors, improve user experience during outages, or centralize error handling across multiple backend services. It operates during the response headers processing phase.

How It Works

1. The proxy receives an HTTP response from the upstream server and invokes the plugin's on_http_response_headers callback.
2. The plugin reads the :status pseudo-header and parses it as an integer.
3. If the status code is in the 5xx range (500-599), the plugin calls send_http_response to generate a new 302 (Found) redirect response.
4. The redirect response includes:
   • Location header pointing to a custom error page (https://storage.googleapis.com/www.example.com/server-error.html)
   • Origin-Status header containing the original 5xx status code for logging/debugging
5. The plugin returns Action::Continue, and the proxy sends the redirect response to the client instead of the original 5xx error.

Implementation Notes

• Status parsing: The plugin extracts the :status pseudo-header; C++ uses absl::SimpleAtoi().
• Redirect generation: A 302 redirect is sent directly from the plugin instead of returning the upstream response. C++ requires returning FilterHeadersStatus::ContinueAndEndStream after sending the local response to halt the original response body.
• Diagnostic headers: The original status code is preserved in the custom Origin-Status header.
• Redirection URL: The destination error page is hardcoded as a constant string.

Configuration

No configuration required. The redirect URL is hardcoded as a constant:

• C++: redirect_page = "https://storage.googleapis.com/www.example.com/server-error.html"
• Go: redirectPage = "https://storage.googleapis.com/www.example.com/server-error.html"
• Rust: REDIRECT_PAGE = "https://storage.googleapis.com/www.example.com/server-error.html"

Build

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

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

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

# Go
bazelisk build //samples/add_custom_response: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_custom_response/tests.textpb \
    --plugin /mnt/bazel-bin/samples/add_custom_response/plugin_rust.wasm

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

Expected Behavior

Derived from tests.textpb:

Scenario	Description
NoRedirect	Does nothing for successful responses.
DoRedirect	Intercepts a 5xx response and redirects the client to the custom error page while preserving the original status code.

Available Languages

• Rust
• C++
• Go