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 an 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 "thenContext" object and is expected to return the same object to the next function in the chain - ether directly or via the Promise:

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 that:

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

API

Properties of "thenContext" object:

• event, ctx, cb - event, context and callback the AWS Lambda function is called with
• config: undefined - to share custom configuation / params between ".then" steps / modules
• res: {} - response to pass to the "callback"

Methods of "thenContext" object:

• promisify() - returns itself as the immediately resolved promise
• finilize(thenCtx, arguments) - log, prepare response and invoke "callback" with the response prepared

Tests

npm test

License

ISC © Vadim Konstantinov