Allow splitting reads/writes when load balancing

In addition to specifying one or more `--target` servers to deploy, you
can now also specify one or more `--read-target`. When any read targets
are present, traffic will be routed such that read requests go to read
targets, and write requests go to write targets.

A read request is defined as a `GET` or `HEAD`, and excludes WebSocket
traffic. Everything else is considered to be a write request.

Read requests that immediately follow a write on the same session will
also be treated as writes. In configurations where readers are replicas
of writers, this helps avoid problems where replication lag leads to a
client failing to read an item it just wrote. This "writer affinity" is
controlled by a cookie, and by default the effect lasts for 3 seconds.
It can be adjusted via `--writer-affinity-timeout`, or disabled entirely
by setting the timeout to zero.

Aside from this writer affinity case, writers will not receive any read
requests unless they are also explicitly listed as a `--read-target`

Author: kevinmcconnell

internal/cmd/deploy.go

@@ -28,6 +28,7 @@ func newDeployCommand() *deployCommand {
 	}
 
 	deployCommand.cmd.Flags().StringSliceVar(&deployCommand.args.TargetURLs, "target", []string{}, "Target host(s) to deploy")
+	deployCommand.cmd.Flags().StringSliceVar(&deployCommand.args.ReaderURLs, "read-target", []string{}, "Read-only target host(s) to deploy")
 	deployCommand.cmd.Flags().StringSliceVar(&deployCommand.args.ServiceOptions.Hosts, "host", []string{}, "Host(s) to serve this target on (empty for wildcard)")
 	deployCommand.cmd.Flags().StringSliceVar(&deployCommand.args.ServiceOptions.PathPrefixes, "path-prefix", []string{}, "Deploy the service below the specified path(s)")
 	deployCommand.cmd.Flags().BoolVar(&deployCommand.args.ServiceOptions.StripPrefix, "strip-path-prefix", true, "With --path-prefix, strip prefix from request before forwarding")
@@ -43,6 +44,7 @@ func newDeployCommand() *deployCommand {
 	deployCommand.cmd.Flags().DurationVar(&deployCommand.args.TargetOptions.HealthCheckConfig.Interval, "health-check-interval", server.DefaultHealthCheckInterval, "Interval between health checks")
 	deployCommand.cmd.Flags().DurationVar(&deployCommand.args.TargetOptions.HealthCheckConfig.Timeout, "health-check-timeout", server.DefaultHealthCheckTimeout, "Time each health check must complete in")
 	deployCommand.cmd.Flags().StringVar(&deployCommand.args.TargetOptions.HealthCheckConfig.Path, "health-check-path", server.DefaultHealthCheckPath, "Path to check for health")
+	deployCommand.cmd.Flags().DurationVar(&deployCommand.args.ServiceOptions.WriterAffinityTimeout, "writer-affinity-timeout", server.DefaultWriterAffinityTimeout, "Time after a write before read requests will be routed to readers")
 
 	deployCommand.cmd.Flags().DurationVar(&deployCommand.args.TargetOptions.ResponseTimeout, "target-timeout", server.DefaultTargetTimeout, "Maximum time to wait for the target server to respond when serving requests")
 

internal/cmd/rollout_deploy.go

@@ -23,6 +23,7 @@ func newRolloutDeployCommand() *rolloutDeployCommand {
 	}
 
 	rolloutDeployCommand.cmd.Flags().StringSliceVar(&rolloutDeployCommand.args.TargetURLs, "target", []string{}, "Target host(s) to deploy")
+	rolloutDeployCommand.cmd.Flags().StringSliceVar(&rolloutDeployCommand.args.ReaderURLs, "read-target", []string{}, "Read-only target host(s) to deploy")
 	rolloutDeployCommand.cmd.Flags().DurationVar(&rolloutDeployCommand.args.DeployTimeout, "deploy-timeout", server.DefaultDeployTimeout, "Maximum time to wait for the new target to become healthy")
 	rolloutDeployCommand.cmd.Flags().DurationVar(&rolloutDeployCommand.args.DrainTimeout, "drain-timeout", server.DefaultDrainTimeout, "Maximum time to allow existing connections to drain before removing old target")
 

internal/server/commands.go

@@ -19,6 +19,7 @@ type CommandHandler struct {
 type DeployArgs struct {
 	Service        string
 	TargetURLs     []string
+	ReaderURLs     []string
 	DeployTimeout  time.Duration
 	DrainTimeout   time.Duration
 	ServiceOptions ServiceOptions
@@ -48,6 +49,7 @@ type RemoveArgs struct {
 type RolloutDeployArgs struct {
 	Service       string
 	TargetURLs    []string
+	ReaderURLs    []string
 	DeployTimeout time.Duration
 	DrainTimeout  time.Duration
 }
@@ -113,7 +115,7 @@ func (h *CommandHandler) Close() error {
 }
 
 func (h *CommandHandler) Deploy(args DeployArgs, reply *bool) error {
-	return h.router.DeployService(args.Service, args.TargetURLs, args.ServiceOptions, args.TargetOptions, args.DeployTimeout, args.DrainTimeout)
+	return h.router.DeployService(args.Service, args.TargetURLs, args.ReaderURLs, args.ServiceOptions, args.TargetOptions, args.DeployTimeout, args.DrainTimeout)
 }
 
 func (h *CommandHandler) Pause(args PauseArgs, reply *bool) error {
@@ -139,7 +141,7 @@ func (h *CommandHandler) List(args bool, reply *ListResponse) error {
 }
 
 func (h *CommandHandler) RolloutDeploy(args RolloutDeployArgs, reply *bool) error {
-	return h.router.SetRolloutTargets(args.Service, args.TargetURLs, args.DeployTimeout, args.DrainTimeout)
+	return h.router.SetRolloutTargets(args.Service, args.TargetURLs, args.ReaderURLs, args.DeployTimeout, args.DrainTimeout)
 }
 
 func (h *CommandHandler) RolloutSet(args RolloutSetArgs, reply *bool) error {

internal/server/health_check.go

@@ -16,8 +16,8 @@ const (
 )
 
 var (
-	ErrorHealthCheckRequestTimedOut  = errors.New("Request timed out")
-	ErrorHealthCheckUnexpectedStatus = errors.New("Unexpected status")
+	ErrorHealthCheckRequestTimedOut  = errors.New("request timed out")
+	ErrorHealthCheckUnexpectedStatus = errors.New("unexpected status")
 )
 
 type HealthCheckConsumer interface {