Refactor Target as interface for testability.

Future tests around the ``TargetPool`` and ``TargetManager`` and
friends will be a lot easier when the concrete behaviors of
``Target`` can be extracted out.  Plus, each ``Target``, I suspect,
will have its own resolution and query strategy.

retrieval/interface_test.go

@@ -18,6 +18,9 @@ import (
 )
 
 func TestInterfaces(t *testing.T) {
-	var _ scheduler = &healthScheduler{}
-	var _ healthReporter = Target{}
+	var (
+		_ Target         = &target{}
+		_ healthReporter = target{}
+		_ scheduler      = &healthScheduler{}
+	)
 }

retrieval/scheduler_test.go

@@ -14,6 +14,7 @@
 package retrieval
 
 import (
+	"github.com/matttproud/prometheus/utility/test"
 	"testing"
 	"time"
 )
@@ -44,7 +45,7 @@ func (t *fakeTimeProvider) Now() (time time.Time) {
 	return
 }
 
-func TestHealthScheduler(t *testing.T) {
+func testHealthScheduler(t test.Tester) {
 	now := time.Now()
 	var scenarios = []struct {
 		futureHealthState []TargetState
@@ -109,3 +110,13 @@ func TestHealthScheduler(t *testing.T) {
 		}
 	}
 }
+
+func TestHealthScheduler(t *testing.T) {
+	testHealthScheduler(t)
+}
+
+func BenchmarkHealthScheduler(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		testHealthScheduler(b)
+	}
+}

retrieval/target.go

@@ -23,35 +23,87 @@ import (
 	"time"
 )
 
+// The state of the given Target.
 type TargetState int
 
 const (
+	// The Target has not been seen; we know nothing about it, except that it is
+	// on our docket for examination.
 	UNKNOWN TargetState = iota
+	// The Target has been found and successfully queried.
 	ALIVE
+	// The Target was either historically found or not found and then determined
+	// to be unhealthy by either not responding or disappearing.
 	UNREACHABLE
 )
 
+// A healthReporter is a type that can provide insight into its health state.
+//
+// It mainly exists for testability reasons to decouple the scheduler behaviors
+// from fully-fledged Target and other types.
 type healthReporter interface {
+	// Report the last-known health state for this target.
 	State() TargetState
 }
 
-type Target struct {
+// A Target represents an endpoint that should be interrogated for metrics.
+//
+// The protocol described by this type will likely change in future iterations,
+// as it offers no good support for aggregated targets and fan out.  Thusly,
+// it is likely that the current Target and target uses will be
+// wrapped with some resolver type.
+//
+// For the future, the Target protocol will abstract away the exact means that
+// metrics are retrieved and deserialized from the given instance to which it
+// refers.
+type Target interface {
+	// Retrieve values from this target.
+	//
+	// earliest refers to the soonest available opportunity to reschedule the
+	// target for a future retrieval.  It is up to the underlying scheduler type,
+	// alluded to in the scheduledFor function, to use this as it wants to.  The
+	// current use case is to create a common batching time for scraping multiple
+	// Targets in the future through the TargetPool.
+	Scrape(earliest time.Time, results chan Result) error
+	// Fulfill the healthReporter interface.
+	State() TargetState
+	// Report the soonest time at which this Target may be scheduled for
+	// retrieval.  This value needn't convey that the operation occurs at this
+	// time, but it should occur no sooner than it.
+	//
+	// Right now, this is used as the sorting key in TargetPool.
+	scheduledFor() time.Time
+	// The address to which the Target corresponds.  Out of all of the available
+	// points in this interface, this one is the best candidate to change given
+	// the ways to express the endpoint.
+	Address() string
+	// How frequently queries occur.
+	Interval() time.Duration
+}
+
+// target is a Target that refers to a singular HTTP or HTTPS endpoint.
+type target struct {
+	// scheduler provides the scheduling strategy that is used to formulate what
+	// is returned in Target.scheduledFor.
 	scheduler scheduler
 	state     TargetState
 
-	Address    string
-	Deadline   time.Duration
+	address string
+	// What is the deadline for the HTTP or HTTPS against this endpoint.
+	Deadline time.Duration
+	// Any base labels that are added to this target and its metrics.
 	BaseLabels model.LabelSet
 
 	// XXX: Move this to a field with the target manager initialization instead of here.
-	Interval time.Duration
+	interval time.Duration
 }
 
-func NewTarget(address string, interval, deadline time.Duration, baseLabels model.LabelSet) *Target {
-	target := &Target{
-		Address:    address,
+// Furnish a reasonably configured target for querying.
+func NewTarget(address string, interval, deadline time.Duration, baseLabels model.LabelSet) Target {
+	target := &target{
+		address:    address,
 		Deadline:   deadline,
-		Interval:   interval,
+		interval:   interval,
 		BaseLabels: baseLabels,
 	}
 
@@ -69,7 +121,7 @@ type Result struct {
 	Target  Target
 }
 
-func (t *Target) Scrape(earliest time.Time, results chan Result) (err error) {
+func (t *target) Scrape(earliest time.Time, results chan Result) (err error) {
 	result := Result{}
 
 	defer func() {
@@ -92,7 +144,7 @@ func (t *Target) Scrape(earliest time.Time, results chan Result) (err error) {
 
 	request := func() {
 		ti := time.Now()
-		resp, err := http.Get(t.Address)
+		resp, err := http.Get(t.Address())
 		if err != nil {
 			return
 		}
@@ -110,7 +162,7 @@ func (t *Target) Scrape(earliest time.Time, results chan Result) (err error) {
 			return
 		}
 
-		baseLabels := map[string]string{"instance": t.Address}
+		baseLabels := map[string]string{"instance": t.Address()}
 
 		for name, v := range intermediate {
 			asMap, ok := v.(map[string]interface{})
@@ -199,10 +251,18 @@ func (t *Target) Scrape(earliest time.Time, results chan Result) (err error) {
 	return
 }
 
-func (t Target) State() TargetState {
+func (t target) State() TargetState {
 	return t.state
 }
 
-func (t Target) scheduledFor() time.Time {
+func (t target) scheduledFor() time.Time {
 	return t.scheduler.ScheduledFor()
 }
+
+func (t target) Address() string {
+	return t.address
+}
+
+func (t target) Interval() time.Duration {
+	return t.interval
+}

retrieval/targetmanager.go

@@ -24,8 +24,8 @@ import (
 type TargetManager interface {
 	acquire()
 	release()
-	Add(t *Target)
-	Remove(t *Target)
+	Add(t Target)
+	Remove(t Target)
 	AddTargetsFromConfig(config *config.Config)
 }
 
@@ -51,20 +51,20 @@ func (m targetManager) release() {
 	<-m.requestAllowance
 }
 
-func (m targetManager) Add(t *Target) {
-	targetPool, ok := m.pools[t.Interval]
+func (m targetManager) Add(t Target) {
+	targetPool, ok := m.pools[t.Interval()]
 
 	if !ok {
 		targetPool.manager = m
-		log.Printf("Pool %s does not exist; creating and starting...", t.Interval)
-		go targetPool.Run(m.results, t.Interval)
+		log.Printf("Pool %s does not exist; creating and starting...", t.Interval())
+		go targetPool.Run(m.results, t.Interval())
 	}
 
 	heap.Push(&targetPool, t)
-	m.pools[t.Interval] = targetPool
+	m.pools[t.Interval()] = targetPool
 }
 
-func (m targetManager) Remove(t *Target) {
+func (m targetManager) Remove(t Target) {
 	panic("not implemented")
 }
 

retrieval/targetpool.go

@@ -8,7 +8,7 @@ import (
 
 type TargetPool struct {
 	done    chan bool
-	targets []*Target
+	targets []*target
 	manager TargetManager
 }
 
@@ -37,7 +37,7 @@ func (p *TargetPool) Pop() interface{} {
 }
 
 func (p *TargetPool) Push(element interface{}) {
-	p.targets = append(p.targets, element.(*Target))
+	p.targets = append(p.targets, element.(*target))
 }
 
 func (p TargetPool) Swap(i, j int) {
@@ -62,7 +62,7 @@ func (p TargetPool) Stop() {
 	p.done <- true
 }
 
-func (p *TargetPool) runSingle(earliest time.Time, results chan Result, t *Target) {
+func (p *TargetPool) runSingle(earliest time.Time, results chan Result, t *target) {
 	p.manager.acquire()
 	defer p.manager.release()
 
@@ -71,7 +71,7 @@ func (p *TargetPool) runSingle(earliest time.Time, results chan Result, t *Targe
 
 func (p *TargetPool) runIteration(results chan Result) {
 	for i := 0; i < p.Len(); i++ {
-		target := heap.Pop(p).(*Target)
+		target := heap.Pop(p).(*target)
 		if target == nil {
 			break
 		}

retrieval/targetpool_test.go

@@ -15,6 +15,7 @@ package retrieval
 
 import (
 	"container/heap"
+	"github.com/matttproud/prometheus/utility/test"
 	"testing"
 	"time"
 )
@@ -28,7 +29,7 @@ func (s literalScheduler) ScheduledFor() time.Time {
 func (s literalScheduler) Reschedule(earliest time.Time, future TargetState) {
 }
 
-func TestTargetPool(t *testing.T) {
+func testTargetPool(t test.Tester) {
 	type expectation struct {
 		size int
 	}
@@ -113,25 +114,54 @@ func TestTargetPool(t *testing.T) {
 		pool := TargetPool{}
 
 		for _, input := range scenario.inputs {
-			target := Target{
-				Address:   input.address,
+			target := target{
+				address:   input.address,
 				scheduler: literalScheduler(input.scheduledFor),
 			}
 
 			heap.Push(&pool, &target)
 		}
 
+		targets := []Target{}
+
 		if pool.Len() != len(scenario.outputs) {
 			t.Errorf("%s %d. expected TargetPool size to be %d but was %d", scenario.name, i, len(scenario.outputs), pool.Len())
 		} else {
 			for j, output := range scenario.outputs {
-				target := heap.Pop(&pool).(*Target)
+				target := heap.Pop(&pool).(Target)
 
-				if target.Address != output.address {
-					t.Errorf("%s %d.%d. expected Target address to be %s but was %s", scenario.name, i, j, output.address, target.Address)
+				if target.Address() != output.address {
+					t.Errorf("%s %d.%d. expected Target address to be %s but was %s", scenario.name, i, j, output.address, target.Address())
 
 				}
+				targets = append(targets, target)
+			}
+
+			if pool.Len() != 0 {
+				t.Errorf("%s %d. expected pool to be empty, had %d", scenario.name, i, pool.Len())
+			}
+
+			if len(targets) != len(scenario.outputs) {
+				t.Errorf("%s %d. expected to receive %d elements, got %d", scenario.name, i, len(scenario.outputs), len(targets))
+			}
+
+			for _, target := range targets {
+				heap.Push(&pool, target)
+			}
+
+			if pool.Len() != len(scenario.outputs) {
+				t.Errorf("%s %d. expected to repopulated with %d elements, got %d", scenario.name, i, len(scenario.outputs), pool.Len())
 			}
 		}
 	}
 }
+
+func TestTargetPool(t *testing.T) {
+	testTargetPool(t)
+}
+
+func BenchmarkTargetPool(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		testTargetPool(b)
+	}
+}