# Heartbeat Function

This article shows you how to use the heartbeat feature provided by Sermant in your development.

# Function Introduction

The heartbeat function plays a key role in the field of microservice governance. The heartbeat mechanism is used to monitor the status of each service instance, and the heartbeat is periodically reported to inform other services of its own survival status to avoid exceptions caused by instance downtime.

The Sermant reports the metadata of the service instance to Backend through the heartbeat. It also reports information about the loaded service governance plug-ins to inform Backend which service governance plug-ins are carried by the Sermant Agent. The Backend monitoring panel displays service instances that carry Sermant Agents.

Attention: The heartbeat function must be used with the Backend monitoring panel. For details, see the Sermant Backend User Manual.

# Development Example

The development based on heartbeat capability is mainly used to allow plug-in developers to report a small amount of very important information of plug-ins through heartbeat, and can be observed in the monitoring panel. This development example is based on the project created in the documentation for Create your first plugin.

In engineering template\template-plugin module under com.huaweicloud.sermant.template.TemplateDeclarer class add variable heartbeatService framework for the heart service, This parameter is used to set additional heartbeat information：

HeartbeatService heartbeatService = ServiceManager.getService(HeartbeatService.class);

In the before method of Interceptor for themain method, set the heartbeat extra information via the setExtInfo interface:

@Override
public ExecuteContext before(ExecuteContext context) throws Exception {
  heartbeatService.setExtInfo(() -> Collections.singletonMap("status", "started"));
  System.out.println("Good morning!");
  return context;
}

After the development is complete, you can follow the Packaged build process when creating the first plug-in, execute the mvn package under the project root directory
refer to Sermant Backend User Manual Starts backend.
Set the heartbeat switch agent.service.heartbeat.enable to true in file agent/config/config.properties:

# Heartbeat service switch
agent.service.heartbeat.enable=true

Finally, access http://localhost:8900 to view Backend.

As you can see from the figure, there is currently one instance carrying Sermant Agent, and it is running properly, and it carries Sermant Agent and plugins of version 1.0.0

Sermant Agent sends heartbeats to Backend on a regular basis. Backend will renew the heartbeats received from each Sermant Agent, and if they exceed the valid time (can be modified by Instance status parameters). If a heartbeat has not been reported from a node, the node is set to be lost.

# API & Configuration

# API

Get The Heartbeat Service

Get the heartbeat service object, which is used to add additional information to the plug-in heartbeat data. This service is initialized by the framework and can be directly accessed by the plug-in.

HeartbeatService heartbeatService = ServiceManager.getService(HeartbeatService.class);

Add Additional Information About Heartbeat

Developers can add additional information about the heartbeat to the plugin as follows:

heartbeatService.setExtInfo(() -> Collections.singletonMap("key", "value"));

# Configuration

Based on the Project structure created in the first plugin, the heartbeat configuration is set by modifying the config/config.properties file.

Configuration Items	Description	Example Configuration
heartbeat.interval	Heartbeat sending interval time in ms	heartbeat.interval=30000

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