README.md

SRV Record Load Balancer library for Go

SRV-LB is a load balancer library designed for use with service discovery solutions that expose an interface through DNS SRV records (e.g. consul or skyDNS)

The library selects a SRV record answer according to specified load balancer algorithm, resolves its A record to an ip, and returns an Address structure:

type Address struct {
	Address string
	Port    uint16
}

To select a DNS server you can use the value from your system's resolv.conf (the default), specify it explicitly when configuring the library, or set it as an ENV variable (e.g. SRVLB_HOST=127.0.0.1:8600 to connect to a local consul agent) at run time.

The library defaults to use a "Round Robin" algorithm, but you can specify another or build your own (see below).

Example:

Default Load Balancer

srvName := "foo.service.fligl.io"
cfg, err := lb.DefaultConfig()
if err != nil {
	panic(err)
}

l := lb.New(cfg, srvName)

address, err := l.Next()
if err != nil {
	panic(err)
}

fmt.Printf("%s", address.String())
// Output: 0.1.2.3:8001

• Uses dns server configured in /etc/resolv.conf
• Uses round robin strategy

or build a generic load balancer

srvName := "foo.service.fligl.io"
cfg, err := lb.DefaultConfig()
if err != nil {
	panic(err)
}

l := lb.NewGeneric(cfg)

address, err := l.Next(srvName)
if err != nil {
	panic(err)
}

fmt.Printf("%s", address.String())
// Output: 0.1.2.3:8001

or configure explicitly

srvName := "foo.service.fligl.io"
cfg := &lb.Config{
	Dns:      dns.NewLookupLib("127.0.0.1:8600"),
	Strategy: random.RandomStrategy,
}
l := lb.New(cfg, srvName)

address, err := l.Next()
if err != nil {
	panic(err)
}

fmt.Printf("%s", address.String())
// Output: 0.1.2.3:8001

Development

tests are run against some fixture dns entries I set up on fligl.io (dig foo.service.fligl.io SRV).

go get -u -t ./...
go test ./...

Build your own load balancing strategy

srv-lb leverages go's init() function to allow you to use your own load balancer strategy without forking the core library. Below is a walkthrough of how to create your own "FancyLB" strategy. For a complete example, see how the "random" strategy is implemented.

Of course, if your strategy would be generally usefull I would lolve a pull request!

The default strategy, RoundRobin, is registered slightly differently to avoid import cycles, so avoid using it as an example

Give your strategy a unique identifier

const FancyStrategy lb.StrategyType = "fancy"

Create a factory (and your implementation of GenericLoadBalancer)

func New(lib dns.Lookup) lb.GenericLoadBalancer {
	return &FancyLB{Dns: lib}
}

Register it with the load balancer

func init() {
	lb.RegisterStrategy(FancyStrategy, New)
}

And then specify it when constructing your load balancer

cfg, _ := lb.DefaultConfig()
cfg.Strategy = fancy.FancyStrategy

l := lb.New(cfg, srvName)

Projects Using SRV-LB

• chinchilla - a rabbitmq to REST bridge