Load balancing

Moleculer has several built-in load balancing strategies. If a service is running on multiple node instances, ServiceRegistry uses these strategies to select a single node from the available ones.

Built-in strategies

To configure strategy, set strategy broker options under registry property. It can be either a name (in case of built-in strategies) or a Strategy class which inherited from BaseStrategy (in case of custom strategies).

RoundRobin strategy

This strategy selects a node based on round-robin algorithm.

Usage

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "RoundRobin"
    }
};

Random strategy

This strategy selects a node randomly.

Usage

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Random"
    }
};

CPU usage-based strategy

This strategy selects a node that has the lowest CPU usage. Since the node list can be very long, it gets samples and selects the node with the lowest CPU usage only from a sample instead of the whole node list.

Usage

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "CpuUsage"
    }
};

Strategy options

Name	Type	Default	Description
sampleCount	Number	3	The number of samples. To turn of sampling, set to 0.
lowCpuUsage	Number	10	The low CPU usage percent (%). The node which has lower CPU usage than this value is selected immediately.

Usage with custom options

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "CpuUsage",
        strategyOptions: {
            sampleCount: 3,
            lowCpuUsage: 10
        }
    }
};

Latency-based strategy

This strategy selects a node that has the lowest latency, measured by periodic ping commands. Notice that the strategy only ping one node / host. Since the node list can be very long, it gets samples and selects the host with the lowest latency only from a sample instead of the whole node list.

Usage

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Latency"
    }
};

Strategy options

Name	Type	Default	Description
sampleCount	Number	5	The number of samples. If you have a lot of hosts/nodes, it’s recommended to increase the value. To turn of sampling, set to 0.
lowLatency	Number	10	The low latency (ms). The node which has lower latency than this value is selected immediately.
collectCount	Number	5	The number of measured latency per host to keep in order to calculate the average latency.
pingInterval	Number	10	Ping interval in seconds. If you have a lot of host/nodes, it’s recommended to increase the value.

Usage with custom options

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Latency",
        strategyOptions: {
            sampleCount: 15,
            lowLatency: 20,
            collectCount: 10,
            pingInterval: 15
        }
    }
};

Sharding strategy

Shard invocation strategy is based on consistent-hashing algorithm. It uses a key value from context params or meta to route the request to nodes. It means that requests with same key value will be routed to the same node.

Example of a shard key name in context params

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Shard",
        strategyOptions: {
            shardKey: "name"
        }
    }
};

Example of a shard key user.id in context meta

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Shard",
        strategyOptions: {
            shardKey: "#user.id"
        }
    }
};

If shard key is in context’s meta it must be declared with a # at the beginning. The actual # is ignored.

Strategy options

Name	Type	Default	Description
shardKey	String	null	Shard key
vnodes	Number	10	Number of virtual nodes
ringSize	Number	2^32	Size of the ring
cacheSize	Number	1000	Size of the cache

All available options of Shard strategy

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "Shard",
        strategyOptions: {
            shardKey: "#user.id",
            vnodes: 10,
            ringSize: 1000,
            cacheSize: 1000
        }
    }
};

Overwrite global options

You can overwrite globally defined load balancing strategy in action/event definitions.

Using ‘Shard’ strategy for ‘hello’ action instead of global ‘RoundRobin’

// moleculer.config.js
module.exports = {
    registry: {
        strategy: "RoundRobin"
    }
});

// greeter.service.js
module.exports = {
    name: "greeter",
    actions: {
        hello: {
            params: {
                name: "string"
            },
            strategy: "Shard",
            strategyOptions: {
                shardKey: "name"
            }            
            handler(ctx) {
                return `Hello ${ctx.params.name}`;
            }
        }
    }
};

Custom strategy

Custom strategy can be created. We recommend to copy the source of RandomStrategy and implement the select method.

Create custom strategy

const BaseStrategy = require("moleculer").Strategies.Base;

class MyStrategy extends BaseStrategy {
    select(list, ctx) { /*...*/ }
}

module.exports = MyStrategy;

Use custom strategy

const { ServiceBroker } = require("moleculer");
const MyStrategy = require("./my-strategy");

const Strategies = require("moleculer").Strategies
// Add custom strategy to the registry
Strategies.register("myCustomStrategy", MyStrategy)


// moleculer.config.js
module.exports = {
    registry: {
        // Strategy is already registered. Call it by name
        strategy: "myCustomStrategy"
    }
};

Preferring local services

The ServiceBroker first tries to call the local instances of service (if exists) to reduce network latencies. It means, if the given service is available on the local broker, the configured strategy will be skipped and the broker will call the local service always.
This logic can be turned off in broker options with preferLocal: false property under the registry key.

// moleculer.config.js
module.exports = {
    registry: {
        preferLocal: false
    }
};