Core & Common

Why am I getting NATS error. Could not connect to server: Error: connect ECONNREFUSED error message when starting my project?

The NATS server is not part of the Moleculer. You have to install & start it before starting your project. Download it from here: https://nats.io/download/nats-io/nats-server/

[7480] 2019/10/06 14:18:05.801763 [INF] Starting nats-server version 2.0.0
[7480] 2019/10/06 14:18:05.805763 [INF] Git commit [not set]
[7480] 2019/10/06 14:18:05.809763 [INF] Listening for client connections on
[7480] 2019/10/06 14:18:05.809763 [INF] Server id is NCHICRYD3SMATIT6QMO557ZDHQUY5JUYPO25TK4SAQYP7IPCIOGKTIRU
[7480] 2019/10/06 14:18:05.810763 [INF] Server is ready

How can I start services with Moleculer Runner in debug mode?

Use the following command:

$ node --inspect= node_modules/moleculer/bin/moleculer-runner services

How to add V8 flags for Moleculer Runner?

$ node --max-old-space-size=8192 node_modules/moleculer/bin/moleculer-runner services

What happens if I emit an event and the service with the event handler is offline?

Moleculer’s events are fire-and-forget meaning that if the service is offline, the event will be lost. If you want persistent events you should look for the transporters that offer this kind of capabilities.

Why the broker exits without any error when I start my service?

If there is no continuously running process (e.g., transporter connection, API gateway, DB connection) that keeps event loop running then the process will exit. It’s normal behavior and not a bug. If you want to keep your service broker running then you should keep the event loop “busy”. Try to enable the transporter in moleculer.config.js.

API Gateway (moleculer-web)

Why am I getting 413 - request entity too large error message when sending a big POST body?

You should configure the bodyParsers to overwrite the default 100kb POST body limit. More info.

module.exports = {
name: "api",
settings: {
routes: [{
path: "/api",

// Use bodyparser modules
bodyParsers: {
json: { limit: "2MB" },
urlencoded: { extended: true, limit: "2MB" }

Use streaming feature when you want to send big data to a service or receive from a service.

How do I reformat error responses?

You should define an onError hook in API Gateway settings. More info.

// api.service.js
module.exports = {
mixins: [ApiService],
settings: {
// Global error handler
onError(req, res, err) {
res.setHeader("Content-Type", "application/json");
res.writeHead(err.code || 500);
success: false,
message: err.message

Why am I getting 502 - Bad Gateway when api-gateway is behind ALB on AWS?

You need to adjust the keepAliveTimeouts in the HTTP server. You can access the HTTP server instance in created() function of api-gateway. More info here.

module.exports = {
mixins: [ApiService],

created() {
// Ensure all inactive connections are terminated by the ALB, by setting this a few seconds higher than the ALB idle timeout
this.server.keepAliveTimeout = 65000;
// Ensure the headersTimeout is set higher than the keepAliveTimeout due to this nodejs regression bug: https://github.com/nodejs/node/issues/27363
this.server.headersTimeout = 66000;

DB Adapters (moleculer-db)

How can I manage multiple entities/tables per service?

At the moment, Moleculer DB only supports one model per service. This design works well if you are using a NoSQL database, especially Document database, because you can easily nest all child entities. However, for SQL databases things get tricky because you can have multiple and complex relations between the entities/tables. Due to this, its difficult (with the current workforce) to create a solution that will work for everyone. Therefore, for scenarios with multiple entities and relationships you will have to write your own adapter.

moleculer-db violates Domain-Driven Design (DDD)?

moleculer-db is a simple (and optional) service mixin to handle one DB entity/table. By no means it obliges or forces you to change your mindset or your way of implementing/modeling things. If the features provided by the moleculer-db don’t fit your needs then you should write your own service with custom actions.