EGo Splash

This is a demo of how to build a Golang filter for Envoy, based on the Envoy Filter Example project, by using Go’s CGo feature.

It is still a little rough on the edges, though probably good enough to help understand most of the challenges and benefits of this approach.

Design Principles

  • Provide a Go-native experience for Go developers
  • Support zero-copy data access where possible
  • Unopinionated interface: reflect the envoy C++ interface classes as close as possible


  • Parsing and passing filter configuration from C to Go with zero-copy and leveraging Envoy’s protobuf validation mechanism
  • Communication between C and Go with zero-copy and strongly parallel interface design
  • Allow the use of goroutines with minimal discipline required (Pin/Post/Unpin)
  • Creating new filters without having to touch C code (except when “lifting up” new envoy interfaces)
  • Support interactions with SDS via Envoy’s built-in client


This project is still in its infancy, and there are a couple of technical challenges still waiting to be solved.

Dependency Cycles

In order to call Go functions from C, the go libraries need to be built first (as this produces the required C headers). However, the Go libraries also have a dependency on C libraries (to facilitate calls to envoy functions).

This can, in most cases, be mitigated by careful separation of the functions needed for the Go->C interactions from the functions implementing the envoy L7 filter interfaces.

When there are functions that are needed on both sides, this often requires creativity. A good example for this is the onPost handler, which needs to be passed as a callback on the Go->C side, but has an implementation that needs to make a C->Go call.

Since this obviously isn’t a limitation of linkers or C, the situation could probably mitigated by extending the Bazel tooling (for example, by separating the generation of Go C headers and the Go library).

CGo limitations

Currently, there can only be one CGo library, and its main() function needs to reside in the same package containing the CGo code. This means that egofilters needs to be referenced from the ego package and can’t contain CGo code. This is probably the most significant issue because it is in direct opposition to the desired architecture.

Replication of Envoy classes as Go interfaces

This is a work intensive process, and the only reason we chose to take on this challenge is that it typically is a one-off effort that will become less notable over time.

Keeping up with Envoy’s changing C++ interface also requires regular effort, and in fact, this project currently doesn’t account for different Envoy versions, although it probably should.




To set up bazel on MacOS, install Homebrew and then

brew install bazelbuild/tap/bazelisk
brew install coreutils wget cmake libtool go bazel automake ninja clang-format autoconf aspell

(Don’t worry if you get a warning for a symlink that couldn’t be updated because bazelisk is sitting on it — this is by design)


git submodule update --init  # clone the linked envoy tag into /envoy
bazel build //:envoy         # build envoy with the new libraries added

Note: This version requires Envoy v1.14

Trying It Out

This demo includes two example filters:

  • getheader is a very basic filter that showcases using go-routines and the Go runtime library for HTTP requests. Its sole purpose is to retrieve response header hdr from a GET request for URL src, and inject it as request header key into the request to the upstream service.

  • security is a more elaborate piece outlining a framework to accomodate many different custom authentication methods, to show how things might pan out at scale. This is showcased via a simple pseudo-HMAC filter with the actual checksum calculations delegated to a separate REST service to once more demonstrate how to align Go-routines and envoy worker threads as well as to motivate the use of SDS based secrets.

To play with it, go to the repository root, and do

bazel run ego-demo

This spins up a simple echo service as upstream for all requests, an HMAC authentication provider sidecar, and the Envoy proxy configured to showcase the Go-based filters provided with this demo.

curl -iX GET ',world!'

Note the X-Getheader-Result in the response which was injected into the request by the getheader filter

curl -iX GET '' -H 'Authorization: client_id:123'

This should produce a 401 Unauthorized response

curl -iX GET '' -H 'Authorization: client_id:17'

The expectation would be a 200 OK response.

curl -iX GET '' -H 'Authorization: client_id:15'

This should also result in a 200 OK response, and a x-custom-auth-signature-hmac-sha256: 200_nnn response header (which clearly isn’t the SHA256 HMAC but just the status code with the length of the response body appended).


The Go code is integrated with bazel via rules_go. The bazel rules areautomatically generated by gazelle, and interfaces mocks are generated via mockery.

Running Tests

The EGo interface layer still deserves a bit more rigorous testing, only some mind-numbing permutation testing for the most critical pieces has already been implemented so far, please budget a couple of minutes for this to run.

bazel test //ego/...

Since the HMAC filter is a gutted version of a more elaborate thing, coverage is relatively good (although not very pretty to read).

bazel test //egofilters/...

Note for MacOS users

Please see Envoy issue #10478 if some of the C++ tests fail with this message:

dyld: Symbol not found: _program_invocation_name

This is most likely means XCode needs to be installed.

Updating the Go Rules

From the repository root, do

bazel run gazelle

Adding new Go Dependencies

From the repository root, do

bazel run //:gazelle -- update-repos -from_file=go.mod

Bazel and Go Interface Mocks

Make sure you have installed mockery. Then, from the repository root, do

tools/  # update generated protobuf bindings

cd egofilters/mock
go generate

cd ../../ego/test/go/mock
go generate

cd ../../../..
bazel run gazelle    # just in case

Bazel integration of this is work in progress…