groupcache

groupcache

groupcache is a caching and cache-filling library, intended as a replacement for memcached in many cases.

For API docs and examples, see http://godoc.org/github.com/tochemey/groupcache/v2

Note: This repository is made readonly. The new features will be handled in this new repo

Table of Content

• Overview
• Modifications
• Comparison to Memcached
• Loading Process
• Example
• Clustering
  • Discovery Providers
    • Kubernetes
    • mDNS
    • NATS

Overview

A modified version of group cache with support:

• upgrade the protobuf API.
• service discovery
• simple logger interface with a default logger based upon uber zap library
• reorganise the proto definitions into a folder called: protos. See how to generate the pbs here

Modifications from original library

In addition to these modifications,

• Logger interface to help add custom logging framework
• Service Discovery to help discover other group cache automatically. At the moment the following providers are implemented:
  • the kubernetes api integration is fully functional
  • the mDNS and DNS-SD
  • the NATS integration is fully functional
• HTTPPoolOpts is no longer exposed to the caller. It has been renamed to httpPoolOpts. The same applies to HTTPPool. This allows to start groupcache with a service discovery provider.
• Upgrade the protocol buffer API

Comparing Groupcache to memcached

Like memcached, groupcache:

• shards by key to select which peer is responsible for that key

Unlike memcached, groupcache:

• does not require running a separate set of servers, thus massively reducing deployment/configuration pain. groupcache is a client library as well as a server. It connects to its own peers.

• comes with a cache filling mechanism. Whereas memcached just says "Sorry, cache miss", often resulting in a thundering herd of database (or whatever) loads from an unbounded number of clients (which has resulted in several fun outages), groupcache coordinates cache fills such that only one load in one process of an entire replicated set of processes populates the cache, then multiplexes the loaded value to all callers.

• does not support versioned values. If key "foo" is value "bar", key "foo" must always be "bar".

Loading process

In a nutshell, a groupcache lookup of Get("foo") looks like:

(On machine #5 of a set of N machines running the same code)

1. Is the value of "foo" in local memory because it's super hot? If so, use it.

2. Is the value of "foo" in local memory because peer #5 (the current peer) is the owner of it? If so, use it.

3. Amongst all the peers in my set of N, am I the owner of the key "foo"? (e.g. does it consistent hash to 5?) If so, load it. If other callers come in, via the same process or via RPC requests from peers, they block waiting for the load to finish and get the same answer. If not, RPC to the peer that's the owner and get the answer. If the RPC fails, just load it locally (still with local dup suppression).

Example

import (
    "context"
    "fmt"
    "log"
    "time"

    "github.com/tochemey/groupcache/v2"
    "github.com/tochemey/groupcache/v2/discovery"
    "github.com/tochemey/groupcache/v2/discovery/kubernetes"
)

func ExampleUsage() {

    // NOTE: It is important each node running the groupcache has the env vars properly set:
    // GROUP_PORT, NODE_NAME and NODE_IP
    // That the service discovery can properly identify the running instance

    // Create an instance of the discovery service.
    // For instance let us use kubernetes
    provider := kubernetes.New()

    // Create the discovery options
    // For kubernetes we only need the namespace and the application name
    application := "users"
    namespace := "default"
	 
    options := discovery.Config{
        kubernetes.ApplicationName: application,
        kubernetes.Namespace:       namespace,
    }

    // Create an instance of the service discovery
    serviceDiscovery := discovery.NewServiceDiscovery(provider, options)

    // Create an instance of the cluster
    ctx := context.Background()
    node := groupcache.NewNode(ctx, serviceDiscovery)
    
    // Start the cluster node
    err := node.Start(ctx)
    
    // Stop the cluster node
    defer node.Stop(ctx)
	 
    // Create a new group cache with a max cache size of 3MB
    group := groupcache.NewGroup("users", 3000000, groupcache.GetterFunc(
        func(ctx context.Context, id string, dest groupcache.Sink) error {

            // Returns a protobuf struct `User`
            user, err := fetchUserFromMongo(ctx, id)
            if err != nil {
                return err
            }

            // Set the user in the groupcache to expire after 5 minutes
            return dest.SetProto(&user, time.Now().Add(time.Minute*5))
        },
    ))

    user := new(User)

    ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*500)
    defer cancel()

    if err := group.Get(ctx, "12345", groupcache.ProtoSink(user)); err != nil {
        log.Fatal(err)
    }

    fmt.Printf("-- User --\n")
    fmt.Printf("Id: %s\n", user.Id)
    fmt.Printf("Name: %s\n", user.Name)
    fmt.Printf("Age: %d\n", user.Age)
    fmt.Printf("IsSuper: %t\n", user.IsSuper)

    // Remove the key from the groupcache
    if err := group.Remove(ctx, "12345"); err != nil {
        log.Fatal(err)
    }
}

Clustering

The cluster engine depends upon the discovery mechanism to find other nodes in the cluster.

At the moment the following providers are implemented:

• the kubernetes api integration is fully functional
• the mDNS and DNS-SD
• the NATS integration is fully functional

Note: One can add additional discovery providers using the following interface

In addition, one needs to set the following environment variables irrespective of the discovery provider to help identify the host node on which the cluster service is running:

• NODE_NAME: the node name. For instance in kubernetes one can just get it from the metadata.name
• NODE_IP: the node host address. For instance in kubernetes one can just get it from the status.podIP
• GROUP_PORT: the port used by the discovery provider to communicate.

Note: Depending upon the discovery provider implementation, the NODE_NAME and NODE_IP can be the same.

Built-in Discovery Providers

Kubernetes Discovery Provider Setup

To get the kubernetes discovery working as expected, the following pod labels need to be set:

• app.kubernetes.io/part-of: set this label with the actor system name
• app.kubernetes.io/component: set this label with the application name
• app.kubernetes.io/name: set this label with the application name

In addition, each node is required to have the following port open with the following ports name for the cluster engine to work as expected:

• group-port: help the gossip protocol engine. This is actually the kubernetes discovery port

const (
    namespace = "default"
    applicationName = "accounts"
)
// instantiate the k8 discovery provider
disco := kubernetes.NewDiscovery()
// define the discovery options
discoOptions := discovery.Config{
    kubernetes.ApplicationName: applicationName,
    kubernetes.Namespace:       namespace,
}
// define the service discovery
serviceDiscovery := discovery.NewServiceDiscovery(disco, discoOptions)
// start the cluster

You’ll also have to grant the Service Account that your pods run under access to list pods. The following configuration can be used as a starting point. It creates a Role, pod-reader, which grants access to query pod information. It then binds the default Service Account to the Role by creating a RoleBinding. Adjust as necessary:

kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: pod-reader
rules:
  - apiGroups: [""] # "" indicates the core API group
    resources: ["pods"]
    verbs: ["get", "watch", "list"]
---
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: read-pods
subjects:
  # Uses the default service account. Consider creating a new one.
  - kind: ServiceAccount
    name: default
roleRef:
  kind: Role
  name: pod-reader
  apiGroup: rbac.authorization.k8s.io

mDNS Discovery Provider Setup

• Service Name: the service name
• Domain: The mDNS discovery domain
• Port: The mDNS discovery port
• IPv6: States whether to lookup for IPv6 addresses.

NATS Discovery Provider Setup

To use the NATS discovery provider one needs to provide the following:

• NATS Server Address: the NATS Server address
• NATS Subject: the NATS subject to use
• Application Name: the application name

const (
    natsServerAddr = "nats://localhost:4248"
    natsSubject = "groupcache-gossip"
    applicationName = "accounts"
)
// instantiate the NATS discovery provider
disco := nats.NewDiscovery()
// define the discovery options
discoOptions := discovery.Config{
    ApplicationName: applicationName,
    NatsServer:      natsServer,
    NatsSubject:     natsSubject,
}
// define the service discovery
serviceDiscovery := discovery.NewServiceDiscovery(disco, discoOptions)
// start the cluster

Contribution

Contributions are welcome! The project adheres to Semantic Versioning and Conventional Commits. This repo uses Earthly.

To contribute please:

• Fork the repository
• Create a feature branch
• Submit a pull request

Protobuf stub Generation

To generate any proto definition add the proto definition into the protos folder and run the following command:

earthly +protogen