This is an example filter for Ambassador Pro that injects an
X-Wikipedia header with the URL for a random Wikipedia article
before passing the request off to the backend service. It uses
https://en.wikipedia.org/wiki/Special:Random to obtain the URL; as an
example of a middleware needing to talk to an external service.
- A Linux or MacOS host (WSL on Windows is okay)
- Docker
- rsync
The Plugin is compiled by building a container with a stable Golang environment and using rsync to copy files to and from the container.
To compile all .go files in the root of this repository just run
```shell
$ make DOCKER_REGISTRY=...
```
It will generate a Docker image named
```shell
DOCKER_REGISTRY/amb-sidecar-custom:VERSION
```
where:
- VERSION is
git describe --tags --always --dirty. - DOCKER_REGISTRY is the
$DOCKER_REGISTRYenvironment variable, orlocalhost:31000if the variable is not set.
To generate a Docker image with a custom name run
```shell
$ make DOCKER_IMAGE={{REGISTRY}}/{{IMAGE_NAME}}:{{IMAGE_TAG}}
```
To build .go files in a non-root directory run
```shell
$ make DOCKER_REGISTRY=... PLUGIN_DIR=...
```
where `PLUGIN_DIR` is the directory for your `.go` files.
To compile for a specific version of Ambassador Pro, set APRO_VERSION:
$ make APRO_VERSION=0.2.2-rc2 DOCKER_REGISTRY=...
When switching Ambassador Pro versions, it may be necessary to edit
the go.mod file.
Push that amb-sidecar-plugin Docker image to a registry that your
cluster has access to. You can do this by running make push DOCKER_REGISTRY=....
Use that image you just pushed instead of
docker.io/datawire/ambassador_pro:amb-sidecar when deploying
Ambassador Pro. For more details on deployment, consult the Filter documentation.
In package main, define a functions named PluginMain with this
signature:
func PluginMain(w http.ResponseWriter, r *http.Request) { … }
The *http.Request is the incoming HTTP request that you have the
opportunity to mutate or intercept, which you do via the
http.ResponseWriter.
You may mutate the headers by calling w.Header().Set(HEADERNAME, VALUE). Finalize this by calling w.WriteHeader(http.StatusOK)
If you call w.WriteHeader() with any value other than 200
(http.StatusOK) instead of modifying the request, the middleware has
taken over the request. You can call w.Write() to write the body of
an error page.
This is all very similar to writing an Envoy ext_authz authorization service.