Build a Custom Evaluator

A tutorial on how to build a custom Evaluator and configure Open Match to use it.


The tutorial aims to explain how to :

  • Build your Custom Evaluator
  • Deploy, Run your custom Evaluator and configure Open Match core to use it


Default Evaluator tutorial

It is highly recommended that you run the default Evaluator tutorial as it introduces the core concepts that this tutorial builds upon. After completion, please remember to cleanup the default Evaluator tutorial before proceeding.

Set up Image Registry

Please setup an Image registry(such as Docker Hub or GC Container Registry) to store the Docker Images used in this tutorial. Once you have set this up, here are the instructions to set up a shell variable that points to your registry:


If using GKE, you can populate the image registry using the command below:$(gcloud config list --format 'value(core.project)')

Get the tutorial template

Make a local copy of the tutorials Folder. Checkout release-x.y branch(e.g. release-1.3, release-1.4) of the Open Match Git repository. Use tutorials/custom_evaluator of release-x.y branch as a working copy for all the instructions in this tutorial.

For convenience, set the following variable:


Create the tutorial namespace

Run this command to create a namespace custom-eval-tutorial in which all the components for this tutorial will be deployed.

kubectl create namespace custom-eval-tutorial


It is highly recommended that you read the Evaluator Guide to familiarize yourself with the lifecycle of a Match proposal through the synchronization and evaluation phases. Also, keep the API Reference handy to look up Open Match specific terminology used in this document. The tutorial also customizes the Open Match installation with the configuration of the custom Evaluator. The Customization Guide is a good reference for those changes.

A complete solution for this tutorial is in the folder tutorials/custom_evaluator/solution.

Custom Evaluator

Scenario Overview

For this tutorial, we will use the matchmaking scenario where a Player selects more than one game-mode to be considered for finding a Match. This will result in concurrent MatchFunction executions to generate overlapping results and hence require evaluation. We will build a custom Evaluator and configure Open Match to resolve conflicts using this Evaluator. We will also add the property on the Ticket indicating the time at which the Ticket entered matchmaking queue.

Thus, the only change to the Matchmaker components from the default Evaluator tutorial is that we will not compute the score or the DefaultEvaluationCriteria in the Match Function. The core of the tutorial thus will focus on authoring the Evaluator and customizing Open Match with it.

Building the Evaluator

Please revisit the Evaluator overview section in the default Evaluator tutorial as it specifically calls out the scenario in which a custom Evaluator is needed.

This tutorial provides an Evaluator scaffold ($TUTORIALROOT/evaluator) as a starting point. The basis Evaluator server implementation in $TUTORIALROOT/evaluator/evaluate/server.go provides the following functionality:

  • Creates a gRPC service that listens on the configured port
  • Implements the Evaluate() interface that Open Match prescribes
  • Accepts a stream of proposals in its implementation for this interface and then calls into a simplified helper method.
  • Streams back results from the helper method to Open Match.

Below is the evaluate() helper method in $TUTORIALROOT/evaluator/evaluate/evaluator.go where the core evaluation logic should be implemented. At runtime, Open Match calls the Evaluate() method and streams the proposals for evaluation. This triggers the evaluate() method with a list of proposals. The evaluate() method should de-collide the proposals and return a list of results that Open Match will then return back to the Director.

func evaluate(proposals []*pb.Match) ([]string, error) {
  // Core evaluation logic
  return []string{}, nil

Please add your evaluation logic to this method. Here are some strategies for evaluation you may use when you detect Matches with overlapping Tickets:

  1. Aggregate the wait time of the Tickets on the Match. The Match with higher aggregate wait time is selected and the rest are discarded.
  2. Match with the Ticket with the longest wait time gets selected and others discarded.

These are just suggestions. You can also experiment with your own evaluation criteria for this scenario.

As a reference, you may check the implementation of the tutorial solution Evaluator that uses strategy 2.

Build and Push

Now that we have a custom Evaluator, please run the below commands in the $TUTORIALROOT to build and push the Evaluator to your configured image registry.

docker build -t $REGISTRY/custom-eval-tutorial-evaluator evaluator/
docker push $REGISTRY/custom-eval-tutorial-evaluator

Deploy and Customize

Run the below command in the $TUTORIALROOT path to deploy the custom Evaluator to the custom-eval-tutorial namespace:

sed "s|REGISTRY_PLACEHOLDER|$REGISTRY|g" evaluator/evaluator.yaml | kubectl apply -f -

We need to update the Open Match ConfigMap with details of the custom Evaluator. The Customization Guide provides some details on customizing your Open Match installation. The updated ConfigMap for the custom Evaluator is present in the customization.yaml. Run the below command in the $TUTORIALROOT path to customize Open Match installation:

kubectl apply -f customization.yaml --namespace open-match

Build, Deploy, Run Matchmaker

Build and Push Container images

Now that you have customized these components, please run the below commands from $TUTORIALROOT to build new images and push them to your configured image registry.

docker build -t $REGISTRY/custom-eval-tutorial-frontend frontend/
docker push $REGISTRY/custom-eval-tutorial-frontend
docker build -t $REGISTRY/custom-eval-tutorial-director director/
docker push $REGISTRY/custom-eval-tutorial-director
docker build -t $REGISTRY/custom-eval-tutorial-matchfunction matchfunction/
docker push $REGISTRY/custom-eval-tutorial-matchfunction

Deploy and Run

Run the below command in the $TUTORIALROOT path to deploy the MatchFunction, Game Frontend. the Director to the custom-eval-tutorial namespace:

sed "s|REGISTRY_PLACEHOLDER|$REGISTRY|g" matchmaker.yaml | kubectl apply -f -


All the components in this tutorial simply log their progress to stdout. To see the progress, run the below commands:

kubectl logs -n custom-eval-tutorial pod/custom-eval-tutorial-frontend
kubectl logs -n custom-eval-tutorial pod/custom-eval-tutorial-director
kubectl logs -n custom-eval-tutorial pod/custom-eval-tutorial-matchfunction

To check the logs from the custom Evaluator, run the following commands:

kubectl logs -n custom-eval-tutorial pod/custom-eval-tutorial-evaluator


Run the below command to remove all the components of this tutorial:

kubectl delete namespace custom-eval-tutorial

This will delete all the components deployed in this tutorial. Open Match core in open-match namespace can then be reused for other exercises but you will need to re-customize it.

Last modified January 18, 2024