Setting debug strings dynamically in Loopback 2.0

Question

I use debug strings for debugging Loopback 2.0 application. Loopback documentation says:

The LoopBack framework has a number of built-in debug strings to help with debugging. Specify a string on the command-line via an environment variable as follows:

MacOS and Linux

$ DEBUG=<pattern>[,<pattern>...] node .

Is it possible to change patterns dynamically in runtime? Or is it possible to use environment-specific configuration?

Answer 1

Before I get deeper note that this debug logging facility uses visionmedia's debug module which handles almost all of the logic.

Is it possible to change patterns dynamically in runtime?

Well before any module is loaded, safest and best way I believe is to just manipulate the environmental variables:

if (process.env.NODE_ENV === 'development') {
    process.env.DEBUG = process.env.DEBUG + ',loopback:*';
}

Another way would be to load debug and use it's .enable method:

require('debug').enable('loopabck:*');

But note that it only works if you do this before Loopback is required since it only allows changes before it's instances are created, which is in this case before loopback is loaded. Another thing is that, there might be multiple debug modules installed depending on the dependencies and your package manager(npm@3, npm@2 and yarn behave differently). debug might be in your node_modules directory, or it might be in loopback each module, node_modules directory. So make sure you require all instances of it and enable, if you want to do it this way.

Now if don't want to do it on the startup, well API doesn't allow changes in the runtime. You can view the discussion regarding this here. Though there are some dirty ways to go around it, but these might possibly break in the future so be careful.

Firstly, there's a module called hot-debug which supposedly makes require('debug').enable work on previously created instances also, but when I tried it, it didn't work perfectly and it was buggy, but it's possible it might work fine for you.

If that doesn't work for you another way is to override require('debug').log method. If this is defined, debug will call this method instead of console.log with the formatted the arguments. You can set DEBUG=* and then filter it yourself:

require('debug').log = function (string) {
    if (string.contains('loopback:security')) {
        console.log(string);
    }
};

This way will be slow in production though as all the debug output will be formatted before being filtered even though nothing might be outputted to console.

Another thing to override the require('debug').init method. This is called everytime a new debug instance is created. Since every debug instance uses an enabled property to check if it's enabled we can toggle that.

const debug = require('debug');
const { init } = debug; 
const instances = [];
debug.init = function(debugInstance) {
  init(debugInstance);
  instances.push(debugInstance);
};

// You can call this function later to enable a given namespace like loopback.security.acl
function enableNamespace(namespace) {
  instances.forEach(instance => {
    instance.enabled = instance.namespace === namespace;
  });
}

Though there's a lot of improvement can be done on this, but you get the idea.