Decouple websocket server and message processing

[Mayuri-Mense]

we are trying to implement the ocpp message processing by referring the AWS blog post - https://aws.amazon.com/blogs/iot/building-an-ocpp-compliant-electric-vehicle-charge-point-operator-solution-using-aws-iot-core/

In the post the websocket is running inside the fargate container which forwards the message to IOT topic, subscribed lambda gets invoked when message is received.

Please help if you can point how to segregate or provide the wrappers to do so.

[xBlaz3kx]

Could you clarify what you mean by processing? I assume you want to create a distributed (scalable) CPMS application. Just copy the AWS concept and create a dedicated OCPP service, which will handle the communication with the charge points, and then just sent the messages to dedicated services, which will handle your application logic.

This is what we've done in our company - we've used RabbitMQ instead of IoT core. Just be cautious because you need to handle the incoming messages (sent by the application to the charge point), so routing will be a bit more complex.

There's a similar answer in #59.

[V4G4X]

Hi @xBlaz3kx,
Can you elaborate more on how it is done at your company? And is it anything like what I've shown below?

By "decoupled processing" we mean a particular server(Fargate) to only run the WebSockets server, and maintain connections with the clients. We want this server to offload the processing (i.e. taking the request object and returning the response object) to a Lambda through an MQTT broker (IoT Core).

This is what we're aiming for:

                Pushes                                                                   
┌────────┐     on topic         ┌─────────────┐     Triggers      ┌────────────────────┐ 
│Fargate ├─────────────────────►│IoT Core     ├─────────────────► │Lambda              │ 
│(Server)│◄─────────────────────┤(MQTT Broker)│◄──────────────────┤(Compute            │ 
└────────┘      Listens         └─────────────┘                   │ that processes     │ 
                 on topic                           Listens       │   request-response)│ 
                                                                  └────────────────────┘ 

And this is how I pictured this library/repo being used:

  ┌────────┐                                                                                                                
  │Fargate │                                                                                                                
  │(Server)│                                                                                                                
  └────────┘                                                                                                                
      ▲                                                                                                                     
      │                                                                                                                     
      │                                                                                                                     
Currently we can only use this repo here,                                                                                   
Because we can't see how to decouple the websocket server, and the part that processes the response.                        

And I want to know whether this is possible in this repo:

                 Pushes                                                                     
 ┌────────┐     on topic         ┌─────────────┐     Triggers      ┌────────────────────┐   
 │Fargate ├─────────────────────►│IoT Core     ├─────────────────► │Lambda              │   
 │(Server)│◄─────────────────────┤(MQTT Broker)│◄──────────────────┤(Compute            │   
 └────────┘      Listens         └─────────────┘                   │ that processes     │   
     ▲            on topic                           Listens       │   request-response)│   
     │                                                             └────────────────────┘   
     │                                                                        ▲             
     │                                                                        │             
     │                                                                        │             
Currently we can only use                                               Whereas we want to  
this repo here                                                             use it here      

[xBlaz3kx]

Hey @V4G4X,

I now better understand what you're trying to do - you just want to forward raw websocket messages to a MQTT Broker, process the OCPP request messages in another service, and respond with an OCPP response message (basically mapping websockets to MQTT).

I think thats a good approach, but it might cause a lot of headache if something goes wrong during the forwarding phase. What we did was essentially proxy the OCPP messages through RabbitMQ to application services and vice versa, as RabbitMQ allows the same request-response principle.

As far as I could understand from the code, this is not possible without substantial modification of both the Websocket and OCPPJ server implementations, as the OCPPJ requires the Websocket server as a dependency, and you'd like to split the responsibility between the two.

[V4G4X]

Thanks for the quick reply @xBlaz3kx

I'm having trouble understanding how your approach is different from our approach of "mapping web sockets to MQTT".
Both sound very similar to me.

Could you help me understand this a little better?

[lorenzodonini]

@V4G4X the lib is designed to directly read your WS, hence the suggested architecture is typically:

I'm not familiar with the specific solution offered by AWS but after quickly looking at it I'm left wondering whether this lib is best suited for your solution: you would lose all the state management (such as client state and message queuing) that the library offers. Also you'd have to bootstrap the entire lib on every lambda invocation, which introduces quite a bit of overhead for processing a single message.

[lorenzodonini]

That being said, all three layers provided in the lib have a clean separation and can potentially be swapped out with custom code. Here is a working example that simply replaces the websocket layer with a dummy (not tested with the full AWS solution though):

package main

import (
    "context"
    "crypto/tls"
    "fmt"
    "net"
    "net/http"
    "time"

    "github.com/aws/aws-lambda-go/lambda"

    "github.com/lorenzodonini/ocpp-go/ocpp1.6/types"

    "github.com/lorenzodonini/ocpp-go/ocpp1.6/core"

    ocpp16 "github.com/lorenzodonini/ocpp-go/ocpp1.6"

    "github.com/lorenzodonini/ocpp-go/ws"
)

// --------- Mock structs ---------

type CloudDummyChannel struct {
    ws.Channel
    ClientID string
}

func (ws *CloudDummyChannel) ID() string {
    return ws.ClientID
}

func (ws *CloudDummyChannel) RemoteAddr() net.Addr {
    return nil
}

func (ws *CloudDummyChannel) TLSConnectionState() *tls.ConnectionState {
    return nil
}

type CloudWebsocketServer struct {
    ws.WsServer
    MessageHandler            func(ws ws.Channel, data []byte) error
    NewClientHandler          func(ws ws.Channel)
    DisconnectedClientHandler func(ws ws.Channel)
    errC                      chan error
    DataC                     chan []byte
}

func (ws *CloudWebsocketServer) Start(port int, listenPath string) {
    // Dummy
}

func (ws *CloudWebsocketServer) Stop() {
    // Dummy
}

func (ws *CloudWebsocketServer) Write(_ string, data []byte) error {
    // Forward the data to the custom channel, so it can be picked up externally
    ws.DataC <- data
    return nil
}

func (ws *CloudWebsocketServer) SetMessageHandler(handler func(ws ws.Channel, data []byte) error) {
    ws.MessageHandler = handler
}

func (ws *CloudWebsocketServer) SetNewClientHandler(handler func(ws ws.Channel)) {
    ws.NewClientHandler = handler
}

func (ws *CloudWebsocketServer) SetDisconnectedClientHandler(handler func(ws ws.Channel)) {
    ws.DisconnectedClientHandler = handler
}

func (ws *CloudWebsocketServer) AddSupportedSubprotocol(_ string) {
    // Dummy
}

func (ws *CloudWebsocketServer) SetCheckClientHandler(handler func(id string, r *http.Request) bool) {
    // Dummy
}

func (ws *CloudWebsocketServer) Errors() <-chan error {
    if ws.errC == nil {
        ws.errC = make(chan error, 1)
    }
    return ws.errC
}

// --------- Custom logic for AWS ---------

func NewCloudWebsocketServer(dataC chan []byte) *CloudWebsocketServer {
    return &CloudWebsocketServer{
        DataC: dataC,
    }
}

type LambdaMessage struct {
    ClientID string
    Message  []byte
}

type CoreHandler struct {
    core.CentralSystemHandler
}

func (c *CoreHandler) OnBootNotification(chargePointId string, request *core.BootNotificationRequest) (confirmation *core.BootNotificationConfirmation, err error) {
    fmt.Println(request)
    // Custom handler logic goes here
    return core.NewBootNotificationConfirmation(types.NewDateTime(time.Now()), 60, core.RegistrationStatusAccepted), nil
}

func HandleRequest(ctx context.Context, msg *LambdaMessage) (out []byte, err error) {
    dataC := make(chan []byte)
    wsDummy := NewCloudWebsocketServer(dataC)
    // Start OCPP server
    ocppServer := ocpp16.NewCentralSystem(nil, wsDummy)
    coreHandler := &CoreHandler{}
    ocppServer.SetCoreHandler(coreHandler)
    ocppServer.Start(0, "")
    // Simulate new client connection
    wsChannel := &CloudDummyChannel{
        ClientID: msg.ClientID,
    }
    wsDummy.NewClientHandler(wsChannel)
    // Pass message to channel
    err = wsDummy.MessageHandler(wsChannel, msg.Message)
    if err != nil {
        return
    }
    // Wait for response
    out = <-dataC
    fmt.Println(string(out))
    // Close OCPP server cleanly to prevent error logs
    ocppServer.Stop()
    // Return response
    return
}

func main() {
    lambda.Start(HandleRequest)
    // Uncomment to test locally
    //_, _ = HandleRequest(context.Background(), &LambdaMessage{
    //  ClientID: "test",
    //  Message:  []byte(`[2,"test","BootNotification",{"chargePointVendor":"test","chargePointModel":"test"}]`),
    //})
}

To efficiently process a single message without starting the entire OCPP server on every request, the lib would require some re-design (I'm not even sure if this would be worth it).

[rogeralsing]

Here is how we usually do it using Proto.Actor and actors + virtual actors: (https://github.com/asynkron/protoactor-go)

We have a WebSocket gateway, which basically just keeps the WebSocket connections. Each WebSocket gets a dedicated local actor. The local actor then calls into the central system cluster, calling the charge-point actor with the ID from the WebSocket.

This means that the central system cluster can be redeployed, bug fixed etc. w/o dropping the WebSocket connections to the chargers.

I would love to give this a try using this lib, assuming I can decouple the WebSocket part from the actual ocpp message processing. I know there have been requests here before about cluster support, this would solve that. While also allowing for other components, such as load balancer actors to communicate with the charge-point actors.