AWS Lambda as chained ".then" steps

A tiny module to write an AWS Lambda handler as execution steps chained by ".then"

Why?

A Lambda handler often runs a number of consequent asynchronous computational steps.

Javascript's Promise API provides a nice way to chain them by ".then" calls.

Moreover, the "middleware" behaviour (like the one in express.js) may easily be incorporated in the "chain", making the code cleaner and simpler.

Usage

As an example, consider a "web-hook" handler that, against a http(s) request, decrypts the data with AWS.KMS service, then queries some DB, then publishes a message on a queue with the AWS.IoT, and finally responds to the original http(s) request whether the data has been published.
Instead of being a "callback hell", the code may be as clean as:

let ThenContext = require('../thenContext');

exports.handler = (event, context, callback) => {
    let thenCtx = new ThenContext(event, context, callback);
    thenCtx.promisify()
        .then(kmsDecrypt)
        .then(queryDBase)
        .then(publishIotMsg)
        .then(thenCtx.finalize)
        .catch(thenCtx.finalize);
};

Every ".then" function is passed with "thenContext" object and is expected to return the same object to the next function in the chain - ether directly or via the Promise resolving to it:

function publishIotMsg(thenCtx) {
    let device = thenCtx.config.device;
    let topic = thenCtx.config.topic;
    let msg = thenCtx.msg;

    return (new Publisher(device))
        .publish(topic, msg)
        .then(() => { return thenCtx });
}

Alternatively, any ".then" function may stop further processing and cause the request to be immediately responded like this:

function kmsDecrypt(thenCtx) {
    // ...
    if (!isAuthorized) return Promise.reject("Unauthorized");
}

API

ThenContext(event, context, callback)

Returns "thenContext" object

"thenContext" object

properties:

• event, ctx, cb - event, context and callback the AWS Lambda function is called with
• config - (undefined by default) custom configuration/params
• res - (empty object by default) response to pass to the "callback"

methods:

• promisify()
  returns itself as the immediately resolved promise

• finilize(thenCtxOrError)
  logs, prepares the response and invokes the "callback" with the response prepared.
  thenCtxOrError defines the arguments the 'callback' will be called with as follows.

• If finilize is called with itself, null/undefined, or empty string, the 'callback' is invoked with "ok" response:

thenCtx.finilize();     // alternatively: thenCtx.finilize(thenCtx);

// providing thenCtx.res.body is not defined, will result in:
callback(null, {
    headers: {'Content-Type': 'application/json'},
    body: '{"result":"Ok"}',
    statusCode: '200'
});

• If thenCtx.res.body is defined, it will be passed to the 'callback':

thenCtx.res.body = '{"device-status": "ONLINE"}';               // mandatory, non-empty
thenCtx.res.headers = {'Content-Type': 'application/json'};     // optional, 'text/plain' by default
thenCtx.res.statusCode = '202';                                 // optional, '200' by default 

thenCtx.finalize();     // alternatively: thenCtx.finilize(thenCtx);

// will result in:
callback(null, {
    headers: {'Content-Type': 'application/json'},
    body: '{"device-status": "ONLINE"}',
    statusCode: '202'
});

• Otherwise, 'callback' is invoked with 'error' response:

thenCtx.finalize(new Error('Unauthorized'));

// no matter if thenCtx.res.body is defined or not, will result in:
callback(null, {
    headers: {'Content-Type': 'text/plain'},
    body: 'Unauthorized',
    statusCode: '400'
});

Tests

npm test

License

ISC © Vadim Konstantinov