Developing custom filters for routing

Sometimes you may want Ambassador Edge Stack to manipulate an incoming request. Some example use cases:

• Inspect an incoming request, and add a custom header that can then be used for routing
• Add custom Authorization headers
• Validate an incoming request fits an OpenAPI specification before passing the request to a target service

Ambassador Edge Stack supports these use cases by allowing you to execute custom logic in Filters. Filters are written in Golang, and managed by Ambassador Edge Stack. If you want to write a filter in a language other than Golang, Ambassador Edge Stack also has an HTTP/gRPC interface for Filters called External.

Prerequisites

Plugin Filters are built as Go plugins and loaded directly into the Ambassador Edge Stack container so they can run in-process with the rest of Ambassador Edge Stack.

To build a Plugin Filter into the Ambassador Edge Stack container you will need

• Linux or MacOS host (Windows Subsystem for Linux is ok)
• Docker
• rsync

The Plugin Filter is built by make which uses Docker to create a stable build environment in a container and rsync to copy files between the container and your host machine.

See the README for more information on how the Plugin works.

Creating and deploying filters

We've created an example filter that you can customize for your particular use case.

1. Start with the example filter: git clone https://github.com/datawire/apro-example-plugin/.

2. Make code changes to param-plugin.go. Note: If you're developing a non-trivial filter, see the rapid development section below for a faster way to develop and test your filter.

3. Run make DOCKER_REGISTRY=..., setting DOCKER_REGISTRY to point to a registry you have access to. This will generate a Docker image named $DOCKER_REGISTRY/amb-sidecar-plugin:VERSION.

4. Push the image to your Docker registry: docker push $DOCKER_REGISTRY/amb-sidecar-plugin:VERSION.

5. Configure Ambassador Edge Stack to use the plugin by creating a Filter and FilterPolicy CRD, as per the filter reference.

6. Update the standard Ambassador Edge Stack manifest to use your Docker image instead of the standard sidecar.

Rapid development of a custom filter

During development, you may want to sidestep the deployment process for a faster development loop. The aes-plugin-runner helps you rapidly develop Ambassador Edge Stack filters locally.

To install the runner, download the latest version:

Note that the plugin runner must match the version of Ambassador Edge Stack that you are running. Place the binary somewhere in your $PATH.

Information about open-source code used in aes-plugin-runner can be found by running aes-plugin-runner --version.

Now, you can quickly test and develop your filter.

1. In your filter directory, type: aes-plugin-runner :8080 ./param-plugin.so.

2. Test the filter by running curl:

Note in the example above the X-Dc header is added. This lets you inspect the changes the filter is making to your HTTP header.

Further reading

For more details about configuring filters and the plugin interface, see the filter reference.