Strategy in Go

Strategy is a behavioral design pattern that turns a set of behaviors into objects and makes them interchangeable inside original context object.

The original object, called context, holds a reference to a strategy object and delegates it executing the behavior. In order to change the way the context performs its work, other objects may replace the currently linked strategy object with another one.

Learn more about Strategy

Conceptual Example

Suppose you are building an In-Memory-Cache. Since it’s in memory, it has a limited size. Whenever it reaches its maximum size, some entries have to be evicted to free-up space. This can happen via several algorithms. Some of the popular algorithms are:

Least Recently Used (LRU): remove an entry that has been used least recently.
First In, First Out (FIFO): remove an entry that was created first.
Least Frequently Used (LFU): remove an entry that was least frequently used.

The problem is how to decouple our cache class from these algorithms so that we can change the algorithm at run time. Also, the cache class should not change when a new algorithm is being added.

This is were Strategy pattern comes into the picture. It suggests creating a family of the algorithm with each algorithm having its own class. Each of these classes follows the same interface, and this makes the algorithm interchangeable within the family. Let’s say the common interface name is evictionAlgo.

Now our main cache class will embed the evictionAlgo interface. Instead of implementing all types of eviction algorithms in itself, our cache class will delegate all it to the evictionAlgo interface. Since evictionAlgo is an interface, we can run time change the algorithm to either be LRU, FIFO, LFU without any change in cache class.

evictionAlgo.go: Strategy interface

package main

type evictionAlgo interface {
	evict(c *cache)
}

fifo.go: Concrete strategy

package main

import "fmt"

type fifo struct {
}

func (l *fifo) evict(c *cache) {
	fmt.Println("Evicting by fifo strtegy")
}

lru.go: Concrete strategy

package main

import "fmt"

type lru struct {
}

func (l *lru) evict(c *cache) {
	fmt.Println("Evicting by lru strtegy")
}

lfu.go: Concrete strategy

package main

import "fmt"

type lfu struct {
}

func (l *lfu) evict(c *cache) {
	fmt.Println("Evicting by lfu strtegy")
}

cache.go: Context

package main

type cache struct {
	storage      map[string]string
	evictionAlgo evictionAlgo
	capacity     int
	maxCapacity  int
}

func initCache(e evictionAlgo) *cache {
	storage := make(map[string]string)
	return &cache{
		storage:      storage,
		evictionAlgo: e,
		capacity:     0,
		maxCapacity:  2,
	}
}

func (c *cache) setEvictionAlgo(e evictionAlgo) {
	c.evictionAlgo = e
}

func (c *cache) add(key, value string) {
	if c.capacity == c.maxCapacity {
		c.evict()
	}
	c.capacity++
	c.storage[key] = value
}

func (c *cache) get(key string) {
	delete(c.storage, key)
}

func (c *cache) evict() {
	c.evictionAlgo.evict(c)
	c.capacity--
}

main.go: Client code

package main

func main() {
	lfu := &lfu{}
	cache := initCache(lfu)

	cache.add("a", "1")
	cache.add("b", "2")

	cache.add("c", "3")

	lru := &lru{}
	cache.setEvictionAlgo(lru)

	cache.add("d", "4")

	fifo := &fifo{}
	cache.setEvictionAlgo(fifo)

	cache.add("e", "5")

}

output.txt: Execution result

Evicting by lfu strtegy
Evicting by lru strtegy
Evicting by fifo strtegy

Based on: Golang By Example