While developing the latency benchmark for the serverless databases (DynamoDB, FaunaDB, Upstash), I wished there was an API where I will record the latency numbers and get the histogram back. In this tutorial, I will build such an API where you can record your latency values from any application. It will be a REST API with following methods:
  • record: Records numeric values into the histogram.
  • get: Returns the histogram object.

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Motivation

I will show how easy to develop a generic API using AWS Lambda and Serverless Redis. See code.

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1 Create a Redis (Upstash) Database

Create a database as getting started

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2 Serverless Project Setup

If you do not have it already install serverless framework via: npm install -g serverless In any folder run serverless as below: 
>> serverless

Serverless: No project detected. Do you want to create a new one? Yes
Serverless: What do you want to make? AWS Node.js
Serverless: What do you want to call this project? histogram-api

Project successfully created in 'histogram-api' folder.

You can monitor, troubleshoot, and test your new service with a free Serverless account.

Serverless: Would you like to enable this? No
You can run the “serverless” command again if you change your mind later.
See Using AWS SAM, if you prefer AWS SAM over Serverless Framework.
Inside the project folder create a node project with the command: 
npm init
Then install the redis client and histogram library with: 
npm install ioredis

npm install hdr-histogram-js
Update the serverless.yml as below. Copy your Redis URL from console and replace below: 
service: histogram-api
frameworkVersion: "2"

provider:
  name: aws
  runtime: nodejs12.x
  lambdaHashingVersion: 20201221
  environment:
    REDIS_URL: REPLACE_YOUR_URL_HERE

functions:
  record:
    handler: handler.record
    events:
      - httpApi:
          path: /record
          method: post
          cors: true
  get:
    handler: handler.get
    events:
      - httpApi:
          path: /get
          method: get
          cors: true
This example uses ioredis, you can copy the connection string from the Node tab in the console.

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3 Code

Edit handler.js as below. 
const hdr = require("hdr-histogram-js");
const Redis = require("ioredis");
if (typeof client === "undefined") {
  var client = new Redis(fixUrl(process.env.REDIS_URL));
}
const headers = {
  "Access-Control-Allow-Origin": "*",
  "Access-Control-Allow-Credentials": true,
};
const SIZE = 10000;

module.exports.get = async (event) => {
  if (!event.queryStringParameters || !event.queryStringParameters.name) {
    return {
      statusCode: 400,
      headers: headers,
      body: JSON.stringify({
        message: "Invalid parameters. Name is needed.",
      }),
    };
  }
  const name = event.queryStringParameters.name;
  const data = await client.lrange(name, 0, SIZE);
  const histogram = hdr.build();
  data.forEach((item) => {
    histogram.recordValue(item);
  });

  return {
    statusCode: 200,
    body: JSON.stringify({
      histogram: histogram,
    }),
  };
};

module.exports.record = async (event) => {
  let body = JSON.parse(event.body);
  if (!body || !body.name || !body.values) {
    return {
      statusCode: 400,
      headers: headers,
      body: JSON.stringify({
        message: "Invalid parameters. Name and values are needed.",
      }),
    };
  }
  const name = body.name;
  const values = body.values;
  await client.lpush(name, values);
  return {
    statusCode: 200,
    body: JSON.stringify({
      message: "Success",
      name: name,
    }),
  };
};

function fixUrl(url) {
  if (!url) {
    return "";
  }
  if (url.startsWith("redis://") && !url.startsWith("redis://:")) {
    return url.replace("redis://", "redis://:");
  }
  if (url.startsWith("rediss://") && !url.startsWith("rediss://:")) {
    return url.replace("rediss://", "rediss://:");
  }
  return url;
}
We have two serverless functions above. get takes name as parameter and loads a list from Redis. Then builds a histogram using the values in the list. The record function takes name and values as parameters. It adds the values to the Redis List with name name. The get function calculates the histogram over the latest 10000 latency records. Update the SIZE parameter to change this number. The fixUrl is a helper method which corrects the Redis url format.

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4 Deploy and Try the API

Deploy your functions with: 
serverless deploy
The command will deploy two functions and output two endpoints. Try the endpoints with setting parameters as below: Record latency numbers to perf-test-1: 
curl --header "Content-Type: application/json" -d "{\"name\":\"perf-test-1\", \"values\": [90,80,34,97,93,45,49,57,99,12]}" https://v7xx4aa2ib.execute-api.us-east-1.amazonaws.com/record
Get the histogram for perf-test-1: 
curl https://v7xx4aa2ib.execute-api.us-east-1.amazonaws.com/get?name=perf-test-1

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Batching

It can be costly to call a remote function each time for latency calculation. In your application, you should keep an array or queue as a buffer for the latency numbers, then submit them in batches to the API when the array reaches the batch size. Something like below: 
let records = [];
let batchSize = 1000;
function recordLatency(value) {
  records.push(value);
  if (records.length >= batchSize) {
    // the below submits the records to the API then empties the records array.
    submitToAPI(records);
  }
}