@talema/graphql-azure-servicebus-subscriptions/src/ServiceBusPubSub.ts

A GraphQL subscription library using Azure Service Bus topics as PubSub server

import { PubSubEngine } from "graphql-subscriptions";

import {
  ServiceBusClient,
  ServiceBusMessage,
  ServiceBusReceivedMessage,
  ServiceBusSender,
  ServiceBusReceiver,
  delay,
  isServiceBusError,
  ProcessErrorArgs,
  ServiceBusError,
} from "@azure/service-bus";

import { Subject, Subscription, filter, tap, map } from "rxjs";
import debug from "debug";
import { IEvent, IEventResult } from "./Event";
import { IMessageProcessor, MessageProcessor } from "./MessageProcessor";

export interface ILogger extends Console {}

/**
 * Represents configuration needed to wire up PubSub engine with the ServiceBus topic
 * @property {string} connectionString - The ServiceBus connection string. This would be the Shared Access Policy connection string.
 * @property {string} topicName - This would be the topic where all the events will be published.
 * @property {string} subscriptionName - This would be the ServiceBus topic subscription name.
 */
export interface IServiceBusOptions {
  connectionString: string;
  topicName: string;
  subscriptionName: string;
}

/**
 * An override for the in-memory PubSubEngine which connects to the Azure ServiceBus.
 */

export class ServiceBusPubSub extends PubSubEngine {
  private client: ServiceBusClient;
  private sender: ServiceBusSender;
  private reciever: ServiceBusReceiver;
  private subscriptions = new Map<number, Subscription>();
  private options: IServiceBusOptions;
  private subject: Subject<IEventResult>;
  private debugger = debug("graphql:servicebus");
  private eventNameKey: string = "sub.eventName";
  private subscription: { close(): Promise<void> } | undefined;
  private logger: ILogger;
  private messageProcessor: IMessageProcessor;

  constructor(
    options: IServiceBusOptions,
    logger: ILogger,
    messageProcessor?: IMessageProcessor,
    client?: ServiceBusClient
  ) {
    super();
    this.options = options;
    this.client = client || new ServiceBusClient(this.options.connectionString);
    this.sender = this.client.createSender(this.options.topicName);
    this.reciever = this.client.createReceiver(
      this.options.topicName,
      this.options.subscriptionName
    );
    this.logger = logger;
    this.messageProcessor = messageProcessor || new MessageProcessor(logger);
    this.subject = new Subject<IEventResult>();
  }

  createSubscription(): { close(): Promise<void> } {
    return this.reciever.subscribe({
      processMessage: async (message: ServiceBusReceivedMessage) => {
        await this.messageProcessor.process(this.subject, message);
      },
      processError: async (args: ProcessErrorArgs) => {
        await this.messageProcessor.onError(
          args,
          async (error: ServiceBusError): Promise<void> => {
            if (error.code === "UnauthorizedAccess") {
              await this.subscription?.close();
            }
          }
        );
      },
    });
  }

  async publish(eventName: string, payload: any): Promise<void> {
    try {
      let event: IEvent = {
        body: {
          name: eventName,
          payload: payload,
        },
      };

      event = this.enrichMessage(
        new Map<string, any>([[this.eventNameKey, eventName]]),
        event
      );
      return this.sender.sendMessages(event);
    } catch (error) {
      this.logger.error(error);
    }
  }

  /**
   * Subscribe to a specific event updates. The subscribe method would create a ServiceBusReceiver to listen to all the published events.
   * The method internally would filter out all the received events that are not meant for this subscriber.
   * @property {eventName | string} - published event name
   * @property {onMessage | Function} - client handler for processing received events.
   * @returns {Promise<number>} - returns the created identifier for the created subscription. It would be used to dispose/close any resources while unsubscribing.
   */
  async subscribe(
    eventName: string,
    onMessage: Function,
    options: Object = {}
  ): Promise<number> {
    const id = Date.now() * Math.random();
    this.debugger("sub metadata: ", eventName, onMessage, options);

    if (this.subscriptions.size <= 0) {
      this.subscription = this.createSubscription();
    }

    this.subscriptions.set(
      id,
      this.subject
        .pipe(
          filter(
            (e) =>
              (eventName && e.body.name === eventName) ||
              !eventName ||
              eventName === "*"
          ),
          map((e) => e.body.payload),
          tap((e) => e)
        )
        .subscribe((event) => {
          this.debugger("returned event: ", event);
          onMessage(event);
        })
    );
    return id;
  }

  /**
   * Unsubscribe method would close open connection with the ServiceBus for a specific event handler.
   * @property {subId} - It's a unique identifier for each subscribed client.
   */
  async unsubscribe(subId: number): Promise<boolean> {
    const subscription = this.subscriptions.get(subId) || undefined;
    if (!subscription) return false;

    if (!subscription.closed) {
      subscription.unsubscribe();
      this.subscriptions.delete(subId);
    }

    if (this.subscriptions.size <= 0) await this.subscription?.close();
    return true;
  }

  private enrichMessage(
    attributes: Map<string, any>,
    message: ServiceBusMessage
  ): ServiceBusMessage {
    const enrichedMessage = Object.assign({}, message);

    if (enrichedMessage.applicationProperties == undefined)
      enrichedMessage.applicationProperties = {};

    attributes.forEach((value, key) => {
      if (
        enrichedMessage.applicationProperties !== undefined &&
        enrichedMessage.applicationProperties?.[key] === undefined
      ) {
        enrichedMessage.applicationProperties[key] = value;
      }
    });

    return enrichedMessage;
  }
}