# The Heartbeat Pattern

Which computers in a global network are offline?

Restaurant chains rely on thousands of computers to conduct business. Think of every credit card machine in every McDonald’s in the world. Any of those computers could stop working without warning. Connectivity outages, power losses and hardware failures are common problems. Knowing when a machine disappears is not easy. Headquarters cannot ping machines through thousands of different private networks. In-store personnel may not notice a lack of connectivity for hours. Failures can even happen when the store is empty with no human around.

How do scientists know black holes exist?

Black holes are unobservable -by definition-. And yet, scientists can know their location. ¹

Tracking transactions

Computers in a global network are like black holes. They cannot be observed. But their effects can be measured.

A computer processing sales is online. The system receiving those transactions can know which computers are online.

Are computers without recent transactions offline?

A lack of transactions does not mean there is a problem. Some markets may have low sales periods. Stores in different timezones have different activity hours. Some retailers may process a very small daily volume.

Heart beats

An average human heart beats approximately 60 times per minute. A lack of heartbeats indicates a serious health problem. That is why ICUs watch them closely.

Heartbeat Pattern

What if every computer in the network sends a small test message every minute?

A system receiving these messages can know which computers are online.
An absence of periodic messages indicates a problem.
Alerts can be sent. And the appropriate personnel can react. Sometimes before there is any business impact.

Advantages

The Heartbeat Pattern solves a critical business problem. With the simplicity of a recurrent test message. It allows headquarters to assess the status of a global network. It can be the foundation to operational intelligence and proactive problem remediation.

This pattern can even be adapted to track other indicators. Complex clients can record internet speed, computer temperature, or last user interaction.

Reference Architecture

The Heartbeat pattern can be implemented with small effort.

Client

Runs recurrent curl commands to POST HTTP requests to an API. The message payload will be a unique computer identifier.

* * * * * curl -i -H "x-api-key: SECRET" -d '{"hostId": "prod-host-1"}' -X POST https://api.retailer.com/v1/hearbeat

Sample crontab file for a computer sending heartbeats

There is no need to write any custom code. Major operating systems support recurrent tasks. And curl runs on any platform.

API

Stores the unique identifiers and the current time in a single table.

Alerts Recurrent Task

Reads all the items in the table.
Considers any computer that hasn’t sent a recent message to be offline.
Triggers the appropriate alerts for each offline computer.