Express.js unique var per request outside routing

Question

In my express application I have a module called helpers thats is required in almost all my routes and modules. This module has a logger method that logs to fluentd (but that's unimportant). While building the data to log I'd like to add a unique identifier of the request, so that all the logs written for the same request have the same unique ID. Using a global var in the app entry point app.use doesn't work because this var would be overwritten every time a new request hits, so the global uuid will change would obviously change in case of high load or long running tasks. The res.locals is not available outside routing, so I can't use it for this matter. Is there a way to create a var that would be unique per request and available in every module or maybe a way to access the res.locals data outside routing? Thank you

EDIT

Maybe an example will help understand better the question. Suppose I have a module called helpers.js like this:

let helpers = {};
helpers.log = (logData, logName) => {
    fluentLogger.emit('', {
        name: logName,
        //uuid: the needed uuid,
        message: logData
    });
}
module.exports = helpers;

Now obviously I can do this in my app.js entry point:

app.use(function (req, res, next) {
    res.locals.uuid = uuid.v4();
    next();
});

and then in every loaded middleware module that requires helpers(adding a new param to the helpers.log method):

const helpers = require('helpers');
router.post('/', (req, res, next) => {
    helpers.log('my log message', 'myLogName', res.locals.uuid);
    next();
});

and this will normally work. But suppose a big or middle size project where there are hundreds of custom modules and models (not middlewares) and a module may require other modules that require other modules that require finally the helpers module. In this case I should pass the res.locals.uuid as a parameter to every method of every method so that I have it available in the logger method. Not a very good idea. Suppose I have a new module called dbmodel.js that is required in a middleware function:

const helpers = require('helpers');
let dbmodel = {};
dbmodel.getSomeData = (someParam) => {
    //some logic
    helpers.log('my log message', 'myLogName');
}
module.exports = dbmodel;

The dbmodel has no idea about the res.locals data if I don't pass it from the middleware, so the helpers.log method will also have no idea about this.

In PHP one would normally write a GLOBAL var in the application's entry point so a hypothetical logger function would have access to this global on every method request from whichever class of the application.

Hope this explanation will help :) Thank you

EDIT 2

The solution for this kind of problems is CLS. Thanks to @robertklep for the hint. A good slideshare explaining exactly the same problem (logger with unique ID) and explaining the CLS solutions can be found here: https://www.slideshare.net/isharabash/cls-and-asynclistener

Express.js unique var per request outside routing

2 Answers