/* There is an application with a microservice architecture. A microservice can be abstracted using the Backend interface. To access a single instance of a microservice, the BackendImpl type, which is already implemented, can be used. There are several dozens of instances of each microservice, each accessible by its address addr. However, individual instances of the microservice are unreliable: they can crash, be unavailable, or overloaded. Therefore, you need to implement a type Balancer that also implements the Backend interface and performs client-side load balancing among the instances of the microservice, choosing the **least loaded** instance each time. */

To implement a Balancer type that implements the Backend interface and distributes load among microservice instances by selecting the least loaded one, you can do the following:

1. Store a list of BackendImpl instances with their addresses and current load.
2. On each call to the Backend interface method, select the instance with the minimum load.
3. Delegate the call to the selected instance.

Example implementation in Go:

type Backend interface {
    DoRequest(req Request) (Response, error)
    Load() int // returns current load
}

type BackendImpl struct {
    addr string
    // ...
}

func (b *BackendImpl) DoRequest(req Request) (Response, error) {
    // request implementation
}

func (b *BackendImpl) Load() int {
    // returns current load, e.g., number of active requests
}

type Balancer struct {
    backends []Backend
    mu       sync.Mutex
}

func (bal *Balancer) DoRequest(req Request) (Response, error) {
    bal.mu.Lock()
    defer bal.mu.Unlock()

    // Select Backend with minimum load
    var target Backend
    minLoad := int(^uint(0) >> 1) // max int
    for _, b := range bal.backends {
        load := b.Load()
        if load < minLoad {
            minLoad = load
            target = b
        }
    }

    if target == nil {
        return Response{}, errors.New("no available backends")
    }

    return target.DoRequest(req)
}

Thus, the Balancer acts as a proxy, distributing requests to the least loaded microservice instance, which increases system resilience and performance.