Replica sync with many physical servers

[joeblew999]

Pretty nice use of docker and all the things Docker has for Scaling it.

https://github.com/localrivet/dosync/blob/main/internal/replica/README.md

The DB holds the state: https://github.com/localrivet/dosync/blob/main/internal/metrics/database.go

But then I was wondering how global state is known .

[joeblew999]

Just brainstorming one option.... Curious what you think.

The DB gives self autonomy, and use NATS JetStream as your global state coordinator.

// Global state manager using NATS JetStream
type GlobalStateManager struct {
    js nats.JetStreamContext
    nc *nats.Conn
}

func NewGlobalStateManager(natsURL string) (*GlobalStateManager, error) {
    nc, err := nats.Connect(natsURL)
    if err != nil {
        return nil, err
    }
    
    js, err := nc.JetStream()
    if err != nil {
        return nil, err
    }
    
    // Create streams for different state domains
    streams := []string{"replica.state", "replica.events", "replica.metrics"}
    for _, stream := range streams {
        _, err = js.AddStream(&nats.StreamConfig{
            Name:     stream,
            Subjects: []string{stream + ".*"},
            Storage:  nats.FileStorage,
            Replicas: 3, // For high availability
        })
        if err != nil && !strings.Contains(err.Error(), "already exists") {
            return nil, err
        }
    }
    
    return &GlobalStateManager{js: js, nc: nc}, nil
}

Each replica publishes its state changes and subscribes to global state updates:

type ReplicaState struct {
    ReplicaID    string            `json:"replica_id"`
    ServerID     string            `json:"server_id"`
    Timestamp    time.Time         `json:"timestamp"`
    State        map[string]interface{} `json:"state"`
    Version      int64             `json:"version"`
}

func (gsm *GlobalStateManager) PublishStateUpdate(state ReplicaState) error {
    data, err := json.Marshal(state)
    if err != nil {
        return err
    }
    
    subject := fmt.Sprintf("replica.state.%s", state.ServerID)
    _, err = gsm.js.Publish(subject, data)
    return err
}

func (gsm *GlobalStateManager) SubscribeToGlobalState(handler func(ReplicaState)) error {
    _, err := gsm.js.Subscribe("replica.state.*", func(msg *nats.Msg) {
        var state ReplicaState
        if err := json.Unmarshal(msg.Data, &state); err != nil {
            log.Printf("Failed to unmarshal state: %v", err)
            return
        }
        handler(state)
        msg.Ack()
    }, nats.Durable("global-state-consumer"))
    
    return err
}