index.js

const Koa = require('koa');
const Router = require('koa-router');
const AWS = require('aws-sdk');

const app = new Koa();
const router = new Router();

const thumbnails = {};

AWS.config.loadFromPath('./aws_lambda.json');

const lambda = new AWS.Lambda({
  region: 'eu-central-1',
  apiVersion: '2015-03-31',
});

async function getThumbnailUrl(sandboxId) {
  const pullParams = {
    FunctionName: 'codesandbox-thumbnails-lambda',
    InvocationType: 'RequestResponse',
    LogType: 'None',
    Payload: JSON.stringify({ sandboxId }),
  };

  const response = await lambda.invoke(pullParams).promise();
  const payload = JSON.parse(response.Payload);

  return payload.Location;
}

router.get('/:sandboxId.png', async ctx => {
  const { sandboxId } = ctx.params;
  if (!(sandboxId in thumbnails)) {
    thumbnails[sandboxId] = await getThumbnailUrl(sandboxId);
  }
  ctx.redirect(thumbnails[sandboxId]);
});

app
  .use(router.routes())
  .use(router.allowedMethods())
  .listen(9000);

console.log('listening on http://localhost:9000');