ckr/url-shortener

'use strict';
// Lambda URL shortener function, called via API Gateway
// Creates an Amazon S3 object with random name and adds metadata for http redirect
const AWS = require('aws-sdk');
const url = require('url');

// configuration to be customized
const S3_Bucket = process.env['S3_BUCKET'];
const S3_Region = process.env['S3_REGION'];
const S3_Prefix = process.env['S3_PREFIX'];

// generate a 7 char shortid
const shortid = () => {
  return 'xxxxxxx'.replace(/x/g, (c) => {
    return (Math.random()*36|0).toString(36);
  });
}

exports.handler = (event, context, cb) => {
  const s3 = new AWS.S3({ region: S3_Region });
  const url_long = event.url_long;
  const cdn_prefix = event.cdn_prefix;
  let retry = 0;    // try at most 3 times to create unique id

  const done = (url_short, error) => {
    cb(null, { url_long: url_long, url_short: url_short, error: error });
  };

  const check_and_create_s3_redirect = (s3_bucket, key_short, url_long) => {
    s3.headObject({ Bucket: s3_bucket, Key: key_short }, (err, data) => {
      if (err) {
        // we should normall have a NotFound error showing that the id is not already in use
        if (err.code === "NotFound") {
          // normal execution path
          s3.putObject({ Bucket: s3_bucket, Key: key_short, Body: "", WebsiteRedirectLocation: url_long, ContentType: "text/plain" }, 
            (err, data) => {
              if (err) { done("", err.message); }
              else {
                const ret_url = "https://" + cdn_prefix + "/" + id_short;
                console.log("Success, short_url = " + ret_url);
                done(ret_url, "");
              }
            });
        } else {
          // treat all other errors as fatal
          done("", "Could not find an suitable name, error: " + err.code);
        }
      } else {
        // we found a duplicate, let's retry a limited number of times
        retry += 1;
        if (retry <= 3) {
          check_and_create_s3_redirect(s3_bucket, key_short, url_long);
        } else {
          // abort after 3 tries
          done("", "Cannot find an unused short id, aborting." );
        }
      }
    });
  }

  // check if url is valid
  const url_check = url.parse(url_long);
  if (!((url_check) && (url_check.host))) { return done("", "Invalid URL format"); }

  console.log("Long URL to shorten: " + url_long);
  const id_short = shortid();
  const key_short = S3_Prefix + "/" + id_short;
  console.log("Short id = " + key_short);
  check_and_create_s3_redirect(S3_Bucket, key_short, url_long);
};