npm package: @fibery/correlation-id
Description: Correlation id helper based on async local storage
Author: Vizydrop Team
Latest version: 5.0.1

# Correlation Id

## How to use

Create correlation id instance

```javascript
import {createCorrelationId} from '@vizydrop/correlation-id';
const correlationId = createCorrelationId();
```

Integrate with `@fibery/vizydrop-logger`

```javascript
const {createLogger} = require(`@fibery/vizydrop-logger`);

const logger = createLogger({
    correlationId: {
        enabled: true,
        getCorrelationId: () => correlationId.correlator.getId(),
        emptyValue: `nocorrelation`,
    },
});
```

Register middleware. Support `koa` and `express`

```javascript
// koa
app.use(correlationId.koaMiddleware);
// express
app.use(correlationId.expressMiddleware);
```

Enhance request so each request will contain correlation id http header.

```javascript
const request = require(`request`);
const correlatedRequest = correlationId.enhanceRequest(request);
correlatedRequest.get(`http://anotherservice:10020/data`);
```

Enhance http proxy so each proxied request will contain correlation id http header.

```javascript
const httpProxy = require(`http-proxy`);
const proxy = httpProxy.createProxyServer({
    target: `http://anotherservice:10020/`,
});
correlationId.enhanceHttpProxy(proxy);
```

Run jobs. Jobs usually do not go through `express/koa` middleware so correlation id should be generated manually.

```javascript
function jobTask() {}

function runJob() {
    correlationId.correlator.withId(correlator.generateDefaultId(), () => {
        jobTask();
    });
}
```

## Settings

Custom settings can be passed as an object to `createCorrelationId` function.

-   `generateDefaultId` - function that should return new correlation id. By default [`crypto.randomUUID`](https://nodejs.org/api/crypto.html#cryptorandomuuidoptions) is used.

## Correlation ID API

-   `expressMiddleware` - express middleware that runs next middlewares in scope of correlation id async hook
-   `koaMiddleware` - koa middleware that runs next middlewares in scope of correlation id async hook
-   `enhanceHttpRequest` - takes request and return new request instance that adds correlation id header by default
-   `enhanceHttpProxy` - register additional listener that adds correlation id header by default
-   `correlator.getId()` - returns current correlation id
-   `correlator.withId(id, fn)` - run function and all subsequent function with specified correlation id
-   `correlator.bind()` - binds provided function to current execution context. Actually now it's a simple alias to AsyncResource.bind https://nodejs.org/api/async_context.html#integrating-asyncresource-with-eventemitter
-   `correlator.generateId()` - generates new correlation id

## Known issues after 5.0.0

Since `bindEmitter` method was removed and we don't bind `req` in express and koa middlewares there might be some edge cases where correlation-id gets lost.
According to the https://github.com/nodejs/node/issues/33723 it's not recommended to bind whole emitter as execution in different context might be intentional so packages should maintain correct async context on their side. And it was actually fixed in many packages like express.js, body-parser, [on-finished](https://github.com/jshttp/on-finished), fastify and etc.
In most cases where it's still a problem this can be worked around with `correlator.bind` (see https://nodejs.org/api/async_context.html#integrating-asyncresource-with-eventemitter). Following example shows how to workaround this for dead koa-morgan package:

```
<!-- WAS -->
app.use(morgan(
    `:method :status :url (:res[content-length] bytes) :response-time ms`,
    {
        stream: {
            write: (text) => logger.info(text.trim()), // no correlation id here, context is lost
        },
        immediate: false,
    },
));

<!-- Should become -->
app.use(async (ctx, next) => {
    await morgan(
        `:method :status :url (:res[content-length] bytes) :response-time ms`,
        {
            stream: {
                write: correlator.bind((text) => logger.info(text.trim())), // correlation-id is here as we bound the callback to current async context
            },
            immediate: false,
        },
    )(ctx, next);
});
```

## Known issues 4.3.0.

-   does not work well with `bluebird.promisifyAll`. Alternative solution is to explicitly promisify using native promise

```javascript
const redis = require(`redis`);
const util = require(`util`);

const client = redis.createClient();

client.setAsync = util.promisify(client.set).bind(client);
client.getAsync = util.promisify(client.get).bind(client);
```

-   does not work well with `mongoose` callbacks. Alternative solution is to use promisified functions.

```javascript
const mongoose = require(`mongooose`);
mongoose.Promise = global.Promise;

EntityModel.find({name: `name`}).then((value) => {
    // do something
});
```