How does service discovery and client-side load balancing work in a Go microservice system?

When service B needs to call service A, it must discover A's current addresses (since pods restart and scale). Go gRPC has built-in pluggable load balancing and name resolution for integrating with Consul, etcd, or Kubernetes DNS.

// Option 1: Kubernetes DNS + round-robin (simplest)
// k8s headless service: userservice.default.svc.cluster.local
// → resolves to ALL pod IPs, not just one VIP
conn, err := grpc.NewClient(
    "dns:///userservice.default.svc.cluster.local:9090",
    grpc.WithDefaultServiceConfig(`{"loadBalancingPolicy":"round_robin"}`),
    grpc.WithTransportCredentials(insecure.NewCredentials()),
)

// Option 2: consul service discovery
import resolverv2 "github.com/mbobrovskyi/grpc-consul-resolver"

resolverv2.RegisterDefault(
    resolverv2.NewResolver(
        "consul://localhost:8500/user-service",
        resolverv2.WithHealthCheck(true),
    ),
)

// Option 3: manual custom resolver for testing / dev
type staticResolver struct{}

func (staticResolver) Build(target resolver.Target,
    cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
    addrs := []resolver.Address{
        {Addr: "localhost:9090"},
        {Addr: "localhost:9091"},
    }
    cc.UpdateState(resolver.State{Addresses: addrs})
    return &staticRes{cc: cc, addrs: addrs}, nil
}

// Load balancing policies in gRPC:
// round_robin:  cycle through all addresses
// pick_first:   always use first healthy address (default)
// grpclb:       server-side load balancing (deprecated)
// rls:          routing lookup service

Service mesh alternative: tools like Istio, Linkerd, and Cilium implement load balancing, circuit breaking, retries, and mTLS in a sidecar proxy — removing these concerns from application code entirely. The Go service makes plain gRPC calls; the mesh handles distribution transparently.

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