Core Messaging Design

[pozylon]

core-messaging is currently responsible for:

• Providing a central director/registry for registration of templates
• Providing module functions to help render text with mjml or mustache

The "message" plugin (which is in plugins) is currently responsible for calling the director to get the correct template, then applies the template to it's input which adds 0 or more work queue items depending on the messaging template

Every core-module currently calls the worker independently and adds message items, example:

sendOrderRejectionToCustomer: async (order, params, { modules }) => {
      const user = await modules.users.findUserById(order.userId);
      const locale = modules.users.userLocale(user);

      await modules.worker.addWork({
        type: 'MESSAGE',
        retries: 0,
        input: {
          ...params,
          locale,
          template: 'ORDER_REJECTION',
          orderId: order._id,
        },
      });

      return order;
    },

Problems with the current design:

• When there is no template registered for a specific message template, there will always be at least 1 MESSAGE queue item for every such event with ERROR state that pollutes the work queue.
• When there is no notifications to be sent in any way, there will always be at least 1 MESSAGE queue item for every message event which pollutes the work queue.
• The core-messaging's core functions are more utils than anything else
• There is no safe way for a module to add a message
• Messaging is spread all over and the call of the context's worker is violating our idea of strict seggregation of modules and services concept
• We cannot track a "read/unread" status of a message even if the messaging plugin would support that

Ideas:

• Provide an event type of the event system when registering a template, getting rid of the explicit calls to addWork(type MESSAGE):

MessagingDirector.registerTemplate(MessageTypes.ORDER_CONFIRMATION, resolveOrderConfirmationTemplate, "ORDER_CONFIRMED");

• Remove the "MESSAGE" plugin and instead extend the core-messaging plugin with a "messages" collection that tracks all messages and their delivered/pending/error & read/unread stati.
• Remove the render utils for mjml and mustache. remove default html email variants. mjml as a dependency currently weighs in at 224 deps, 18.72mb, 5622 files.