# GraphQL over WebSocket Protocol extension

The GraphQL over WebSocket Protocol used by `fastify-gql` follows apollo's protocol defined (here)[https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md]. In order to achieve gateway-to-service communication and handle `connection_init` per client, an extension is used on the protocol for server-to-server communication.

## Message structure extension

This is the extended structure of each message:
```
export interface OperationMessage {
  payload?: any;
  id?: string;
  type: string;

  extensions?: Array<OperationExtension>;
}

export interface OperationExtension {
  type: string;
  payload?: any;
}
```

## Server -> Server
### `connectionInit` extension

Gateway uses this extension to share the `connection_init` payload with a service when the connection is already established between gateway and services.

```
export interface ConnectionInitExtension extends OperationExtension {
  type: string;
  payload?: Object;
}
```

* `type: String` : 'connectionInit'
* `payload: Object` : optional parameters that the client specifies in connectionParams