node-execution-context

A straightforward library that provides a persistent process-level context wrapper using node "async_hooks" feature.
This library will try to use by default AsyncLocalStorage implementation based if current node version supports it, otherwise it will fallback to raw async_hooks implementation for lower versions which mimics this behaviour.

Installation

``
npm i node-execution-context
`

Getting started

Let's start with creating the context initialisation point of our app, we will take a simple express app for this example

`js
// main.js

const express = require('express');
const Context = require('node-execution-context');
const UserController = require('./controllers/user');
const app = express();
const port = 3000;

const ContextMiddleware = (req, res, next) => {
Context.run(next, { reference: Math.random() });
};

app.use('/', ContextMiddleware);
app.post('/user', UserController.create);

app.listen(port);

`

This will expose any point of your code form this point that handles that request.

`js
// ./controller/user/index.js

const Context = require('node-execution-context');
const mongo = require('../service/mongo');
const logger = require('../service/logger');

export class UserController {
async create (req) {
const { user } = req.body;

// This will return the reference number set by out ContextMiddleware (generated by Math.random())
const { reference } = Context.get();

logger.info('Created user for reference: ', reference);

return await mongo.create('user', user);
}
}
`

API

$3

Runs given callback that will be exposed to the given context.
The context will be exposed to all callbacks and promise chains triggered from the given fn.

$3

Gets the current asynchronous execution context.

> The get result is returned by reference meaning if you wish any immutability applied, it will have to be manually applied.

> This API may throw CONTEXT_DOES_NOT_EXIST error if accessed without initializing the context properly.

$3

Sets the current asynchronous execution context to given value.

> This API may throw CONTEXT_DOES_NOT_EXIST error if accessed without initializing the context properly.

$3

Creates a given context for the current asynchronous execution.
It is recommended to use the run method. This method should be used in special cases in which the run method cannot be used directly.

> Note that if this method will be called not within a AsyncResource context, it will effect current execution context and should be used with caution.

#### Example

`js
const Context = require('node-execution-context');

// context-events.js
const context = { id: Math.random() };

emitter.on('contextual-event', () => {
Context.create(context);
});

// service.js
emitter.on('contextual-event', () => {
Context.get(); // Returns { id: random }
});

// main.js
emitter.emit('contextual-event');

Context.get(); // Returns { id: random }
`

$3

Configures execution context settings.

> Relevant only for node versions lower than v12.17.0.

$3

Returns an monitoring report over the current execution map resources

> Relevant only for node versions lower than v12.17.0.

> Before calling monitor, you should configure execution context to monitor it's nodes. by default the data kept is as possible.

#### Example

`js
const Context = require('node-execution-context');

// Startup
Context.configure({ monitor: true });

// Later on
const usage = Context.monitor();
console.log(usage); // Prints execution context usage report.
`

$3

`js
const Context = require('node-execution-context');

Context.create({
value: true
});

Promise.resolve().then(() => {
console.log(Context.get()); // outputs: {"value": true}

Context.set({
value: false
});

return new Promise((resolve) => {
setTimeout(() => {
console.log(Context.get()); // outputs: {"value": false}

Context.set({
butter: 'fly'
});

process.nextTick(() => {
console.log(Context.get()); // outputs: {"butter": 'fly'}
resolve();
});

}, 1000);

console.log(Context.get()); // outputs: {"value": false}
});
});
`

The following errors can be thrown while accessing to the context API :

| Code | When |
|-|-
| CONTEXT_DOES_NOT_EXIST | When attempting to get / set a context, that has not yet been created.
| MONITOR_MISS_CONFIGURATION | When try to monitor without calling configure` with monitoring option.