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Kuvel

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Server-discovery Velocity plugin for Minecraft servers running in a Kubernetes cluster.

// repository documentation

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# Kuvel
## Overview
Kuvel, A service discovery plugin for Velocity. It will automatically discover Minecraft servers and
register/unregister them within Velocity.

## Features

* Monitor Minecraft pods in a Kubernetes cluster and automatically register/unregister them within
  Velocity
* Create a LoadBalancer server and distribute players trying to join to the linked servers.
* Synchronize server names across multiple Velocity instances using Redis

## Installing

The Plugin can be downloaded
from [Releases](https://github.com/AzisabaNetwork/Kuvel/releases/latest). Download `Kuvel.jar` and
install it into Velocity plugins directory. The config file requires initial setup as seen below.

```yml
# The kubernetes namespace to use for the server discovery.
namespace: ""
# The prefix to use for the keys of the server labels or annotations.
label-key-prefix: "kuvel.azisaba.net"
# Server name synchronization by Redis is required in load-balanced environments using multiple Velocity instances.
redis:
  group-name: "develop"
  connection:
    hostname: "redis"
    port: 6379
    # username is optional. if you have authentication enabled, you can use it here. Otherwise you can leave it blank or null.
    username: "default"
    # password is optional. if you have authentication enabled, you can use it here. Otherwise you can leave it blank or null.
    password: "password"

# label-selectors are used to filter Pods and ReplicaSets to be registered.
label-selectors:
  - "kuvel.azisaba.net/enable-server-discovery=true"
```

Alternatively you can use environment variables to configure Kuvel. The environment variable will override
 the config.yml and are `KUVEL_NAMESPACE`, `KUVEL_LABEL_KEY_PREFIX`, `KUVEL_REDIS_GROUPNAME`, 
`KUVEL_REDIS_CONNECTION_HOSTNAME`, `KUVEL_REDIS_CONNECTION_PORT`, `KUVEL_REDIS_CONNECTION_USERNAME`, and 
`KUVEL_REDIS_CONNECTION_PASSWORD`.

In order for Kuvel to monitor the server, you must request permission from Kubernetes. For Velocity pods, please allow get/list/watch to Pods
and ReplicaSets.

```yml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: velocity-account
  namespace: default
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: velocity-clusterrolebiding
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: view
subjects:
- kind: ServiceAccount
  name: velocity-account
  namespace: default
```
```yml
# Apply ServiceAccount to the Velocity pod
apiVersion: apps/v1
kind: ...
# ...
spec:
  serviceAccountName: velocity-account
# ...
```

## Enable Service Discovery on the Minecraft Servers

To tell Kuvel that the pod is a Minecraft server, use the Kubernetes label service.  
The label keys and values are specified in the config file. By default, the label key
is `kuvel.azisaba.net/enable-server-discovery` and the value is `true`.

The following labels are also used for some other features.

If server names longer than 63 characters are desired, the `kuvel.azisaba.net/preferred-server-name` annotation can be used instead of the label.
If `kuvel.azisaba.net/disable-name-suffix=true` is set on a Pod, Kuvel will register the server name exactly as `preferred-server-name` (no `-1` suffix). If multiple replicas are detected, Kuvel logs an error and skips the extra pods. This label is intended for single-replica Pods only.
If `kuvel.azisaba.net/disable-load-balancer=true` is set on a ReplicaSet or Deployment metadata, Kuvel will skip creating the virtual load balancer server for that ReplicaSet. The backing Pods are still registered normally.

### Pod

```yml
apiVersion: v1
kind: Pod
metadata:
  name: test-server
  labels:
    kuvel.azisaba.net/enable-server-discovery: "true" # Required for Kuvel to detect Minecraft servers. Depends on your config.
    kuvel.azisaba.net/preferred-server-name: "test-server" # Required for Kuvel to name the server
    # kuvel.azisaba.net/initial-server: "true" # Uncomment this line if you want to make this server the initial server.   
    # kuvel.azisaba.net/disable-name-suffix: "true" # Use the exact preferred name (no -1 suffix); single replica only.
spec:
  containers:
    - name: test-server
      image: itzg/minecraft-server:java8
      ports:
        - containerPort: 25565
```

### Deployment
```yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: test-server-deployment
spec:
  replicas: 2
  selector:
    matchLabels:
      app: test-server-deployment
  template:
    metadata:
      labels:
        app: test-server-deployment
        kuvel.azisaba.net/enable-server-discovery: "true" # Required for Kuvel to detect Minecraft servers. Depends on your config.
        kuvel.azisaba.net/preferred-server-name: "test-server" # Required for Kuvel to name the server
        # kuvel.azisaba.net/initial-server: "true" # Uncomment this line if you want to make this server the initial server.
    spec:
      containers:
        - name: test-server
          image: itzg/minecraft-server:java8
          ports:
            - containerPort: 25565
```

In both cases, the server is registered under the name `test-server`.

However, if there are two or more servers with the same name, a number will be assigned after the server name. For example, if there are two pods with the server name `test-server`, one will be `test-server` and the other will be `test-server-1`.

## Load Balancer

On parallelizable servers such as Lobby servers, it is sometimes desirable to distribute the number of players as evenly as possible. This is where Kuvel's LoadBalancer feature comes in handy.

```yml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: lobby-deployment
  labels:
    kuvel.azisaba.net/enable-server-discovery: "true"
    kuvel.azisaba.net/preferred-server-name: "lobby"
    # kuvel.azisaba.net/initial-server: "true" # Uncomment this line if you want to make this load balancer server the initial server.
    # kuvel.azisaba.net/disable-load-balancer: "true" # Uncomment this line to disable only the load balancer entry.
spec:
  replicas: 3
  selector:
    matchLabels:
      app: lobby-deployment
  template:
    metadata:
      labels:
        app: lobby-deployment
        kuvel.azisaba.net/enable-server-discovery: "true"
        kuvel.azisaba.net/preferred-server-name: "lobby"
        # kuvel.azisaba.net/initial-server: "true" # Uncomment this line if you want to make this server the initial server.
    spec:
      containers:
        - name: lobby
          image: itzg/minecraft-server:java8
          ports:
            - containerPort: 25565
```

By applying a Label to a Deployment, Kuvel's LoadBalancer feature can be activated. Kuvel's LoadBalancer has the following features.

1. Distribute players who try to join the Load Balancer server randomly to the pods under ReplicaSet.
2. Synchronize with Kubernetes ReplicaSet and automatically register/unregister forwarding destinations.

Using this, you can implement a mechanism to randomly connect to `lobby-1`, `lobby-2`, or `lobby-3` when `/server lobby` is invoked.

## Synchronize Server Names in Multi Velocity Environments

In a Kubernetes cluster, pods can be created at almost the same time, and this can cause different 
Velocity servers to have different registration names. This can cause fatal issues in a parallel 
Velocity environment. Kuvel provides name synchronization using Redis to avoid this
issue. Kuvel uses keys whose key name begins with `kuvel:`.

On 1.x, this feature was optional, but from 2.0.0, this setting is enabled by default.

## Commands

Use `/kuvel` to inspect, update, and repair server registration state.

- `/kuvel status` - show registration summary and missing registrations
- `/kuvel list <pods|loadbalancers>` - list UID to server-name mappings
- `/kuvel register <podUid> <serverName>` - register a pod by UID
- `/kuvel unregister <podUid>` - unregister a pod by UID
- `/kuvel setname <podUid> <serverName>` - change a pod's registered server name
- `/kuvel repair` - re-register missing entries and clean broken mappings

## License
[GNU General Public License v3.0](LICENSE)

Kuvel

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