Creating Our First Service
Making a CDS service with Sero

This Service is pretty simple: it will tell us the current time. More specifically, it will echo the current time back to the Client that makes a request for decision support.
You can find the complete source for this guide in the Sero project at example/cds-hooks-api-guide​

In the src directory, create a folder called current-time then cd into it and create the file get-current-time.js.
Sero includes a Service class and a Card class. Import those at the top of the file:
import { Service, Card } from "";

Every CDS service consists of two things: a configurations option object, and a service handler. The options object contains necessary and optional parameters for creating a CDS Service.
Let's define the following options:
const options = {
id: "get-current-time",
title: "Get the current time",
hook: "patient-view",
description: "This example shows how easy it is to make a CDS hooks service with Sero. This service responds with the current time after being invoked by the patient-view hook",
What do these options mean?
Every service that's made on our CDS Hooks API can be called by a CDS client making requests to <server_url>/cds-services. The id parameter is the unique identifier for a service.
The title and description are a human-readable name and explanation of what the service does. They are required.
Lastly, this service will respond to the patient-view Hook. This is used to tell the a CDS client when to use this decision support (in this case, when a patient record is being view). Hooks also help us define data requirements, which we will cover in a later section.
Luckily, Sero configures this automatically for us.
You can read more about the options available in the Sero reference guide for the CDS Hooks feature:

The Service handler is an async function that runs when a request is made from a Client to a Service. In this case, an HTTP GET request is sent to/cds-services/get-current-time. The id we defined in the options above is what configures this url.
Let's add an empty handler to get-current-time.js
const handler = async (request) => { };
Upon receiving a request, the handler will respond with a new Card displaying the current time. Let's expand our handler:
const handler = async (request) => {
const today = new Date();
const minutes = today.getUTCMinutes();
const time = `${today.getHours()}:${
minutes <= 9 ? "0" : ""
return {
cards: [
new Card({
source: {
label: "Automate Medical, Inc.",
url: "",
summary: `What time is it?`,
detail: `The current time is ${time}`,
indicator: "info",
Our handler now returns an object with a new Card containing information about the current time. Not exactly ground-breaking decision support, but we need to start somewhere!
To round off get-current-time.js, let's add a default export:
export default new Service(options, handler);

Head back to index.js and import the service.
import { Http, CDSHooks, start } from "";
import compareTimeService from "./current-time/current-time.js";
const config = {
cdsHooks: {
services: [compareTimeService],
cors: true,
const http = Http(config);
CDSHooks(config, http);
In order to run the node server locally, package.json needs a new script, as well the type key set to module.
"name": "cds-hooks-api-guide",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
"start": "node src/index.js"
"author": "",
"license": "ISC",
"dependencies": {
"": "^0.0.16"
"type": "module"
The latest version of Sero may be different than the version used in this walkthrough
The server can now be run with npm run start.

For this part of the walkthrough, we'll be using the CDS Hooks sandbox to make requests to our server.
The CDS Hooks Sandbox uses HTTPS. To test our local service, we need to host it behind an HTTPS url. We're going to use ngrok to do this by creating a public URL to our local server.

First, create an account with ngrok. Go through the short walkthrough that guides through basic installation and authentication. Launch ngrok and, with the server running, enter ngrok http The console should now provide a public http and https links for the server.

Head to the CDS Hooks Sandbox. The sandbox simulates three example workflows and let's us test the Service as a Client.
You should see the Patient View workflow on your screen. On the left, we see a simulated EHR event (viewing a patient record). On the right, we see a debug panel showing the request/response lifecycle from the Service.
CDS Hooks Sandbox Patient View
By default, the sandbox shows a default response to a patient-view hook invocation that returns a card with the text Now seeing: Daniel. You can see the Card rendering this information on the left.
Click on the gear in the top right and, in the dropdown, select "Add CDS Services."
Paste the public https link from ngrok into the input area. Append /cds-services to the end of the link, and click "Save." This is known as the "Discovery Endpoint" for our Service.
The CDS Hooks Sandbox will ask Sero for a list of Services in our Discovery Endpoint.
CDS Hooks Sandbox displaying the result of our new decision support Service
Our new Service, get-current-time returns a Card, now displayed on the left, with the current time.
Congratulations! We just built a CDS API with a simple CDS service. In the next section we'll learn more about CDS Hooks, how to deal with incoming requests, and how to work with FHIR Resources.
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On this page
What we'll be building
The code
Service handler
Running the API
Calling the API
Configuring ngrok
Launching the CDS Hooks Sandbox