lb.md

The Service Load Balancer

The hydra-http-plugin uses an internal load balancer that is responsible for the selection of the "optimum" computing node for a target service in the micro-service cluster. It is used for internal HTTP calls and requests proxy.

The Load Balancer(lb) selection strategy can be extended for custom requirements. Built-in supported strategies are:

• race: The load balancer select N number of available nodes(ordered by their presence report) and perform a health check, the faster node on respond is selected. The configuration options are described below:

hydra.use(new HydraHttpPlugin({
    lb: {
        strategy: {
            name: 'race',           // strategy name
            timeout: 3000,          // call timeout
            nodes: 3,               // number of nodes to call
            healthPath: '_health'   // health check endpoint, for example: http://127.0.0.1:3000/_health
        }
    }
}));

• last-presence: (default) The load balancer select the node who last reported presence status. This method is intended to be used when the service is already balanced or clustered, Nginx Load Balancer for example.
  Usage and configuration:

hydra.use(new HydraHttpPlugin({
    lb: {
        strategy: {
            name: 'last-presence'  // strategy name
        }
    }
}));

Implementing custom strategies

More strategies to be implemented in coming versions. However, passing a custom strategy is also pretty simple:

hydra.use(new HydraHttpPlugin({
    lb: {
        strategy: {
            name: 'last-presence',          
            handler: async(config, presences, hydra, service) => {
                let service = presences[0]; // presences[0] contains the presence details of the last node who reported available

                return `http://${service.ip}:${service.port}`;
            }
        }
    }
}));

Usage

// getting the lb config
let lbConfig = hydra.http.lb.config;

// get the URL base for a custom service name
let urlBase = await hydra.http.lb.translate('service-name');

Capturing lb events for post processing and monitoring

hydra.on('http-plugin-lb', e => {
    
});