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package balancer
// In this package we manage a global pool of addresses for gitaly-ruby,
// accessed via the gitaly-ruby:// scheme. The interface consists of the
// AddAddress and RemoveAddress methods. RemoveAddress returns a boolean
// indicating whether the address was removed; this is intended to give
// back-pressure against repeated process restarts.
//
// The gitaly-ruby:// scheme exists because that is the way we can
// interact with the internal client-side loadbalancer of grpc-go. A URL
// for this scheme would be gitaly-ruby://foobar. For gitaly-ruby://
// URL's, the host and port are ignored. So gitaly-ruby://foobar is
// actually a working, valid address.
//
// Strictly speaking this package implements a gRPC 'Resolver'. This
// resolver feeds address list updates to a gRPC 'balancer' which
// interacts with the gRPC client connection machinery. A resolver
// consists of a Builder which returns Resolver instances. Our Builder
// manages the address pool and notifies its Resolver instances of
// changes, which they then propagate into the gRPC library.
//
import (
"time"
"google.golang.org/grpc/resolver"
)
var (
lbBuilder = newBuilder()
)
func init() {
resolver.Register(lbBuilder)
}
const (
// DefaultRemoveDelay is the minimum time between successive address removals.
DefaultRemoveDelay = 1 * time.Minute
)
// AddAddress adds the address of a gitaly-ruby instance to the load
// balancer.
func AddAddress(a string) {
lbBuilder.addAddress <- a
}
// RemoveAddress removes the address of a gitaly-ruby instance from the
// load balancer. Returns false if the pool is too small to remove the
// address.
func RemoveAddress(addr string) bool {
ok := make(chan bool)
lbBuilder.removeAddress <- addressRemoval{ok: ok, addr: addr}
return <-ok
}
type addressRemoval struct {
addr string
ok chan<- bool
}
type addressUpdate struct {
addrs []resolver.Address
next chan struct{}
}
type config struct {
numAddrs int
removeDelay time.Duration
}
type builder struct {
addAddress chan string
removeAddress chan addressRemoval
addressUpdates chan addressUpdate
configUpdate chan config
// for testing only
testingRestart chan struct{}
}
// ConfigureBuilder changes the configuration of the global balancer
// instance. All calls that interact with the balancer will block until
// ConfigureBuilder has been called at least once.
func ConfigureBuilder(numAddrs int, removeDelay time.Duration) {
cfg := config{
numAddrs: numAddrs,
removeDelay: removeDelay,
}
if cfg.removeDelay <= 0 {
cfg.removeDelay = DefaultRemoveDelay
}
if numAddrs <= 0 {
panic("numAddrs must be at least 1")
}
lbBuilder.configUpdate <- cfg
}
func newBuilder() *builder {
b := &builder{
addAddress: make(chan string),
removeAddress: make(chan addressRemoval),
addressUpdates: make(chan addressUpdate),
configUpdate: make(chan config),
testingRestart: make(chan struct{}),
}
go b.monitor()
return b
}
// Scheme is the name of the address scheme that makes gRPC select this resolver.
const Scheme = "gitaly-ruby"
func (*builder) Scheme() string { return Scheme }
// Build ignores its resolver.Target argument. That means it does not
// care what "address" the caller wants to resolve. We always resolve to
// the current list of address for local gitaly-ruby processes.
func (b *builder) Build(_ resolver.Target, cc resolver.ClientConn, _ resolver.BuildOption) (resolver.Resolver, error) {
cc.NewServiceConfig(`{"LoadBalancingPolicy":"round_robin"}`)
return newGitalyResolver(cc, b.addressUpdates), nil
}
// monitor serves address list requests and handles address updates.
func (b *builder) monitor() {
p := newPool()
notify := make(chan struct{})
cfg := <-b.configUpdate
lastRemoval := time.Now()
// This channel is intentionally nil so that our 'select' below won't
// send messages to it. We do this to prevent sending out invalid (empty)
// messages during boot.
var addressUpdates chan addressUpdate
for {
au := addressUpdate{next: notify}
for _, a := range p.activeAddrs() {
au.addrs = append(au.addrs, resolver.Address{Addr: a})
}
if len(au.addrs) > 0 && addressUpdates == nil {
// Start listening for address update requests
addressUpdates = b.addressUpdates
}
select {
case addressUpdates <- au:
// We have served an address update request
case addr := <-b.addAddress:
p.add(addr)
notify = broadcast(notify)
case removal := <-b.removeAddress:
if time.Since(lastRemoval) < cfg.removeDelay || p.activeSize() < cfg.numAddrs-1 {
removal.ok <- false
break
}
if !p.remove(removal.addr) {
removal.ok <- false
break
}
removal.ok <- true
lastRemoval = time.Now()
notify = broadcast(notify)
case cfg = <-b.configUpdate:
// We have received a config update
case <-b.testingRestart:
go b.monitor()
b.configUpdate <- cfg
return
}
}
}
// broadcast returns a fresh channel because we can only close them once
func broadcast(ch chan struct{}) chan struct{} {
close(ch)
return make(chan struct{})
}
// gitalyResolver propagates address list updates to a
// resolver.ClientConn instance
type gitalyResolver struct {
clientConn resolver.ClientConn
started chan struct{}
done chan struct{}
resolveNow chan struct{}
addressUpdates chan addressUpdate
}
func newGitalyResolver(cc resolver.ClientConn, auCh chan addressUpdate) *gitalyResolver {
r := &gitalyResolver{
started: make(chan struct{}),
done: make(chan struct{}),
resolveNow: make(chan struct{}),
addressUpdates: auCh,
clientConn: cc,
}
go r.monitor()
// Don't return until we have sent at least one address update. This is
// meant to avoid panics inside the grpc-go library.
<-r.started
return r
}
func (r *gitalyResolver) ResolveNow(resolver.ResolveNowOption) {
r.resolveNow <- struct{}{}
}
func (r *gitalyResolver) Close() {
close(r.done)
}
func (r *gitalyResolver) monitor() {
notify := r.sendUpdate()
close(r.started)
for {
select {
case <-notify:
notify = r.sendUpdate()
case <-r.resolveNow:
notify = r.sendUpdate()
case <-r.done:
return
}
}
}
func (r *gitalyResolver) sendUpdate() chan struct{} {
au := <-r.addressUpdates
r.clientConn.NewAddress(au.addrs)
return au.next
}
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