Maestro: Kubernetes Game Room Scheduler

Goal:

Have an unified system that automatically scales game rooms regardless of the protocol (TCP, UDP). This system is related to a matchmaker but does not handle the specificities of a match such as how many players fit in a room. It only deals with high level room occupation, i.e. is the room occupied or available. The rooms communicate directly with the matchmaker in order to register and unregister themselves from the matchmaking.

Let us define a Game Room Unity (GRU) as a Kubernetes service (type nodePort) associated with a single pod. This restriction is made because in AWS we cannot load balance UDP. We're using containerized applications and Kubernetes in order to simplify the management and scaling of the game rooms.

Architecture:

Maestro is a game room scheduler that is composed by a controller, a watcher, a worker, an API and a CLI. In the future we may have an UI for displaying metrics such as:

% of rooms usage
rate of room occupation increase/decrease
rooms cpu and memory usage
etc.

maestro-controller:

The controller is responsible for managing the Game Room Unities (GRUs). It creates and gracefully terminates GRUs according to auto scaling policies defined by the user. It makes use of the Kubernetes cluster's API endpoints in order to have updated information about the GRUs managed by Maestro. It is also responsible for persisting relevant information in the database and managing rooms statuses.

maestro-watcher:

The watcher ensures that at any time the Game Room Unities (GRUs) state is as expected. If the scaling policies say that one should have 10 GRUs of a given type, the watcher will ask the controller to create or terminate GRUs as needed. The desired state is kept in a database that is consulted by the watcher (via controller) each time it runs. It has a lock so Maestro can be scaled horizontally. Each scheduler (i.e. maestro scalable entity) has its own watcher.

maestro-worker:

The worker ensures that all valid schedulers (i.e. schedulers that exist in the database) have running watchers.

maestro-api:

The API is the connection of Maestro to the external world and with the game room itself. It is responsible for:

Managing GRUs status and healthcheck (status are: creating, ready, occupied, terminating and terminated);
Saving the scheduler config in a database that will be consulted by the watcher;
Managing the pool of GRUs with each GRU host ip and port;

maestro-cli:

The CLI is a wrapper for the maestro-api endpoints.

maestro-client:

A client lib for Unity and cocos2dx responsible for calling maestro HTTP routes defined in the room protocol. It also must catch sigterm/sigkill and handle the room graceful shutdown.

Configuring Maestro Schedulers

Maestro receives a config file (yaml or json) for each scheduler.

The config file must have the following information:

Docker image
Autoscaling policies
Manifest yaml template
1. Default configuration (ENV VARS)
2. Ports and protocols (UDP, TCP)
3. Resources requests (cpu and memory)

Example yaml config:

name: pong-free-for-all     # this will be the name of the kubernetes namespace (it must be unique)
game: pong                  # several configs can refer to the same game
image: pong/pong:v123
affinity: node-affinity     # optional field: if set, rooms will be allocated preferentially to nodes with label "node-affinity": "true"
toleration: node-toleration # optional field: if set, rooms will also be allocated in nodes with this taint
occupiedTimeout: match-time # how much time a match has. If room stays with occupied status for longer than occupiedTimeout seconds, the room is deleted
ports:
  - containerPort: 5050     # port exposed in the container
    protocol: UDP           # supported protocols are TCP and UDP
    name: gamebinary        # name identifying the port (must be unique for a config)
  - containerPort: 8888
    protocol: TCP
    name: websocket
limits:                     # these will be the resources limits applied to the pods created in kubernetes
  memory: "128Mi"           # they are used to decide how many rooms can run in each node
  cpu: "1"                  # more info: https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-ram-container/
shutdownTimeout: 180        # duration in seconds the pod needs to terminate gracefully
autoscaling:
  min: 100                  # minimum amount of GRUs
  up:
    delta: 10               # how many GRUs will be created every time the scaling policy is triggered
    trigger:
      usage: 70             # minimum usage (percentage) that can trigger the scaling policy
      time: 600             # duration in seconds to wait before scaling policy takes place
      threshold: 80        # percentage of the points that are above 'usage' needed to trigger scale up
    cooldown: 300           # duration in seconds to wait before consecutive scaling
  down:
    delta: 2                # how many GRUs will be terminated every time the scaling policy is triggered
    trigger:
      usage: 50             # maximum usage (percentage) the can trigger the scaling policy
      time: 900            
      threshold: 80        # percentage of the points that are above 'usage' needed to trigger scale down
    cooldown: 300          
env:                        # environment variable to be passed on to the container
  - name: EXAMPLE_ENV_VAR
    value: examplevalue
  - name: ANOTHER_ENV_VAR
    value: anothervalue
cmd:                        # if the image can run with different arguments you can specify a cmd
  - "./room-binary"
  - "-serverType"
  - "6a8e136b-2dc1-417e-bbe8-0f0a2d2df431"

A JSON file equivalent to the yaml above can also be used.

TODOs:

Define Architecture
- Validate Kubernetes performance with a large amount of services
Formalize room protocol
Release map
Define config template

Release Map:

Milestone 1:

Goals:
- Create scheduler that scales up with the given policy.
- Delete scheduler.
Tasks:
- maestro-controller
  - scheduler
    - create new scheduler with given config
      - generate Kubernetes manifest template from the config
      - create config in database
      - create namespace in kubernetes
      - create initial GRUs
        
        create service
        
        create pod with service nodePort as an argument
      - delete scheduler
        
        remove config from database
        
        gracefully terminate running scheduler GRUs
        
        remove namespace from kubernetes
  - scheduler state
    - get rooms statuses
    - update rooms status
    - get autoscaling policy (from config persisted in db)
  - scaling
    - create GRUs
  - docs
- maestro-watcher
  - validate rooms status vs autoscaling policy
  - scale cluster
    - up
  - docs
- maestro-api
  - scheduler
    - create new scheduler with given config
    - delete scheduler
  - room protocol routes
    - ping
    - room ready
    - match started
    - match ended
    - address polling
  - docs
- maestro-client
Milestone 2:

Goals:
- Create scheduler that scales down with the given policy.
- Update running scheduler.
- Monitor scheduler rooms metrics.
Tasks:
- maestro-controller
  - scheduler
    - update running scheduler config
      - update config in database
      - update GRUs
        
        launch new GRUs with the updates config
        
        gracefully terminate GRUs running with old config
  - scheduler state
    - report room occupation metrics
  - scaling
    - remove GRUs
- maestro-watcher
  - scale cluster
    - down
- maestro-api
  - scheduler
    - update running scheduler config
  - get rooms metrics
- maestro-cli
  - scheduler
    - create new scheduler with given config
    - update running scheduler config
    - delete scheduler
    - scheduler status
  - docs
- maestro-client
  - cocos2dx support
- UI
  - display rooms metrics
  - scheduler
    - create new scheduler with given config
    - update running scheduler config
    - delete scheduler

Doubts

~~Can Kubernetes handle thousands of services?~~
How to manage different versions running at the same time? Will the matchmaker be responsible for it?
How to properly tune autoscaling policies?

Architecture Validation and Tests

Validating Kubernetes performance

Testing with 30 nodes m4.large and 900 GRUs (pod + service) using a simple image for UDP listener: mendhak/udp-listener:latest.

To be checked:

Observations:

Initial setup: Kubernetes 1.5

While running the 900 pods + services
- kube-system used 30 cores (CPU) and 9Gi (memory usage). Each kube-proxy pod consumes about 1 core.
- syncProxyRules took 16.810688ms
Without any test pods
- kube-system used 1 core (CPU) and 7Gi (memory usage). Each kube-proxy pod consumes about 0.02 core.
- syncProxyRules took 966.091651ms (note: it runs by default every 1000ms)

Changing --iptables-min-sync-period to 10s seems to have improved the CPU usage performance, but the cost is that anytime a new service is created it can take up to 10s until they are available.

This Kubernetes PR might be related to the services scaling problem and it is available in Kubernetes 1.6 and greater.

After changing to Kubernetes 1.6 and running the 900 pods + services:

kube-system used 2 cores (CPU) and 2Gi (memory usage). Each kube-proxy pod consumes about 0.04 core.
syncProxyRules took 1.25s

kube-proxy relevant config options (to be tunned):

--iptables-min-sync-period
--udp-timeout (actually this flag is not used with iptables, in this case kube-proxy is using iptables default timeout: 30s)
-proxy-port-range

Load Test

Integration Tests

The integration tests run on local minikube. The script can download kubectl and minikube if the machine doesn't have them.

The tests won't interfere on running clusters because it creates namespaces with uuid v4 as prefix. All created namespaces are deleted after the tests.

Redis and Postgres run on containers defined on a docker-compose.yaml.

Important: If you have kubectl installed, make sure it has the latest Kubernetes version. If it isn't, probably the tests won't work.

Room Protocol:

Game rooms have four different statuses:

Creating

From the time maestro starts creating the GRU in Kubernetes until a room ready is received.
Ready

From the time room ready is called until a match started is received. It means the room is available for matches.
Occupied

From the time match started is called until a match ended is received. It means the room is not available for matches.
Terminating

From the time a sigkill/sigterm signal is received by the room until the GRU is no longer available in Kubernetes.

Maestro's auto scaling policies are based on the number of rooms that are in ready state.

In order to properly set their statuses, game rooms must call the maestro-api HTTP routes described in the Maestro API docs.

Event Forwarders:

Event forwarders are pluggable components that forwards events like: RoomReady, RoomTerminated, RoomTerminating, etc... to other services.

A event forwarder is a go native plugin that should be compiled and put into bin folder, it should contain a method func NewForwarder(config *viper.Viper) (eventforwarder.EventForwarder) that returns a configured instance of a struct that implements "eventforwarder.EventForwarder".

An example is provided in the plugins/grpc folder, compile it with:

go build -o bin/grpc.so -buildmode=plugin plugins/grpc/forwarder.go

Then to turn it on, include a config like that in the active config file:

forwarders:
  grpc:
    matchmaking: 
      address: "10.0.23.57:10000"
    local:
      address: "localhost:10000"

In this example, maestro will look for a plugin "grpc.so" in the bin folder and create 2 forwarders from it, matchmaking and local one, each using a different address. Then, every time a room is changing states, all forwarders will be called with infos about the change.

There's also a route: /scheduler/{schedulerName}/rooms/{roomName}/playerevent that can be called like that, for example:

curl -X POST -d '{"timestamp":12424124234, "event":"playerJoin", "metadata":{"playerId":"sime"}}' localhost:8080/scheduler/some/rooms/r1/playerevent

It will forward the playerEvent "playerJoin" with the provided metadata and roomId to all the configured forwarders.

For the provided plugin, the valid values for event field are: ['playerJoin','playerLeft']

cscatolini/maestro