daemon.go

package shezmu

import (
	"fmt"
	"strings"
	"time"

	"github.com/juju/ratelimit"
)

// Daemon is the interface that contains a set of methods required to be
// implemented in order to be treated as a daemon.
type Daemon interface {
	// Startup implementation should:
	//
	// func (d *DaemonName) Startup() {
	//     // 1. Set up a panic handler
	//     b.HandlePanics(func() {
	//         log.Error("Oh, crap!")
	//     })
	//
	//     // 2. If the daemon is doing some IO it is a good idea to limit the
	//     // rate of its execution
	//     b.LimitRate(10, 1 * time.Second)
	//
	//     // 3. If the daemon is also a consumer we need to subscribe for
	//     // topics that would be consumed by the daemon
	//     b.Subscribe("ProductPriceUpdates", func(p PriceUpdate) {
	// 	       log.Printf("Price for %q is now $%.2f", p.Product, p.Amount)
	//     })
	// }
	Startup()

	// Shutdown implementation should clean up all daemon related stuff:
	// close channels, process the last batch of items, etc.
	Shutdown()

	// String returns the name of a daemon.
	String() string

	// base is a (hack) function that allows the Daemon interface to reference
	// underlying BaseDaemon structure.
	base() *BaseDaemon
}

// BaseDaemon is the parent structure for all daemons.
type BaseDaemon struct {
	self         Daemon
	name         string
	queue        chan<- *task
	logger       Logger
	panicHandler PanicHandler
	shutdown     chan struct{}
	limit        *ratelimit.Bucket
}

// PanicHandler is a function that handles panics. Duh!
type PanicHandler func(error)

// Process creates a task and then adds it to processing queue.
func (d *BaseDaemon) Process(a Actor) {
	if d.limit != nil {
		d.limit.Wait(1)
	}

	d.tryEnqueue(&task{
		daemon:    d.self,
		actor:     a,
		createdAt: time.Now(),
		name:      "Actor",
	})
}

// SystemProcess creates a system task that is restarted in case of failure
// and then adds it to processing queue.
func (d *BaseDaemon) SystemProcess(name string, a Actor) {
	if name == "" {
		name = "SystemProcess"
	}

	d.tryEnqueue(&task{
		daemon:    d.self,
		actor:     a,
		createdAt: time.Now(),
		system:    true,
		name:      name,
	})
}

// LimitRate limits the daemons' processing rate.
func (d *BaseDaemon) LimitRate(times int, per time.Duration) {
	rate := float64(time.Second) / float64(per) * float64(times)
	if rate <= 0 {
		d.Logf("Daemon %s processing rate was limited to %d. Using 1 instead", d.base(), rate)
		rate = 1.0
	}
	d.Logf("Daemon %s processing rate is limited to %.2f ops/s", d.base(), rate)
	d.limit = ratelimit.NewBucketWithRate(rate, 1)
}

// HandlePanics sets up a panic handler function for the daemon.
func (d *BaseDaemon) HandlePanics(f PanicHandler) {
	d.panicHandler = f
}

// ShutdownRequested returns a channel that is closed the moment daemon shutdown
// is requested.
func (d *BaseDaemon) ShutdownRequested() <-chan struct{} {
	return d.shutdown
}

// Continue returns true if daemon should proceed and false if it should stop.
func (d *BaseDaemon) Continue() bool {
	select {
	case <-d.shutdown:
		return false
	default:
		return true
	}
}

// Log logs values using shezmu.Logger.Println function.
func (d *BaseDaemon) Log(v ...interface{}) {
	if d.logger != nil {
		d.logger.Println(append([]interface{}{"[" + d.String() + "]"}, v...)...)
	}
}

// Logf logs values using shezmu.Logger.Printf function.
func (d *BaseDaemon) Logf(format string, v ...interface{}) {
	if d.logger != nil {
		d.logger.Printf("["+d.String()+"] "+format, v...)
	}
}

// Startup is the empty implementation of the daemons' Startup function that
// is inherited and used by default.
func (d *BaseDaemon) Startup() {}

// Shutdown is the empty implementation of the daemons' Shutdown function that
// is inherited and used by default.
func (d *BaseDaemon) Shutdown() {}

// String returns the name of the Deamon unerlying struct.
func (d *BaseDaemon) String() string {
	if d.name == "" {
		d.name = strings.Split(fmt.Sprintf("%T", d.self), ".")[1]
	}

	return d.name
}

// base is a (hack) function that allows the Daemon interface to reference
// underlying BaseDaemon structure.
func (d *BaseDaemon) base() *BaseDaemon {
	return d
}

func (d *BaseDaemon) tryEnqueue(t *task) {
	defer func() {
		if err := recover(); err != nil {
			d.Logf("Failed to enqueue task %q due to process termination", t)
		}
	}()

	d.queue <- t
}

func (d *BaseDaemon) handlePanic(err error) {
	if d.panicHandler != nil {
		d.panicHandler(err)
	}
}