Introduce unchecked Collectors

Fixes #47 . See there for more detailed discussion.

Signed-off-by: beorn7 <beorn@soundcloud.com>

prometheus/collector.go

@@ -29,24 +29,35 @@ type Collector interface {
 	// collected by this Collector to the provided channel and returns once
 	// the last descriptor has been sent. The sent descriptors fulfill the
 	// consistency and uniqueness requirements described in the Desc
-	// documentation. (It is valid if one and the same Collector sends
-	// duplicate descriptors. Those duplicates are simply ignored. However,
-	// two different Collectors must not send duplicate descriptors.) This
-	// method idempotently sends the same descriptors throughout the
-	// lifetime of the Collector. If a Collector encounters an error while
-	// executing this method, it must send an invalid descriptor (created
-	// with NewInvalidDesc) to signal the error to the registry.
+	// documentation.
+	//
+	// It is valid if one and the same Collector sends duplicate
+	// descriptors. Those duplicates are simply ignored. However, two
+	// different Collectors must not send duplicate descriptors.
+	//
+	// Sending no descriptor at all marks the Collector as “unchecked”,
+	// i.e. no checks will be performed at registration time, and the
+	// Collector may yield any Metric it sees fit in its Collect method.
+	//
+	// This method idempotently sends the same descriptors throughout the
+	// lifetime of the Collector.
+	//
+	// If a Collector encounters an error while executing this method, it
+	// must send an invalid descriptor (created with NewInvalidDesc) to
+	// signal the error to the registry.
 	Describe(chan<- *Desc)
 	// Collect is called by the Prometheus registry when collecting
 	// metrics. The implementation sends each collected metric via the
 	// provided channel and returns once the last metric has been sent. The
-	// descriptor of each sent metric is one of those returned by
-	// Describe. Returned metrics that share the same descriptor must differ
-	// in their variable label values. This method may be called
-	// concurrently and must therefore be implemented in a concurrency safe
-	// way. Blocking occurs at the expense of total performance of rendering
-	// all registered metrics. Ideally, Collector implementations support
-	// concurrent readers.
+	// descriptor of each sent metric is one of those returned by Describe
+	// (unless the Collector is unchecked, see above). Returned metrics that
+	// share the same descriptor must differ in their variable label
+	// values.
+	//
+	// This method may be called concurrently and must therefore be
+	// implemented in a concurrency safe way. Blocking occurs at the expense
+	// of total performance of rendering all registered metrics. Ideally,
+	// Collector implementations support concurrent readers.
 	Collect(chan<- Metric)
 }
 

prometheus/doc.go

@@ -121,7 +121,17 @@
 // NewConstSummary (and their respective Must… versions). That will happen in
 // the Collect method. The Describe method has to return separate Desc
 // instances, representative of the “throw-away” metrics to be created later.
-// NewDesc comes in handy to create those Desc instances.
+// NewDesc comes in handy to create those Desc instances. Alternatively, you
+// could return no Desc at all, which will marke the Collector “unchecked”.  No
+// checks are porformed at registration time, but metric consistency will still
+// be ensured at scrape time, i.e. any inconsistencies will lead to scrape
+// errors. Thus, with unchecked Collectors, the responsibility to not collect
+// metrics that lead to inconsistencies in the total scrape result lies with the
+// implementer of the Collector. While this is not a desirable state, it is
+// sometimes necessary. The typical use case is a situatios where the exact
+// metrics to be returned by a Collector cannot be predicted at registration
+// time, but the implementer has sufficient knowledge of the whole system to
+// guarantee metric consistency.
 //
 // The Collector example illustrates the use case. You can also look at the
 // source code of the processCollector (mirroring process metrics), the

prometheus/registry.go

@@ -15,7 +15,6 @@ package prometheus
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"os"
 	"runtime"
@@ -68,7 +67,8 @@ func NewRegistry() *Registry {
 
 // NewPedanticRegistry returns a registry that checks during collection if each
 // collected Metric is consistent with its reported Desc, and if the Desc has
-// actually been registered with the registry.
+// actually been registered with the registry. Unchecked Collectors (those whose
+// Describe methed does not yield any descriptors) are excluded from the check.
 //
 // Usually, a Registry will be happy as long as the union of all collected
 // Metrics is consistent and valid even if some metrics are not consistent with
@@ -98,6 +98,14 @@ type Registerer interface {
 	// returned error is an instance of AlreadyRegisteredError, which
 	// contains the previously registered Collector.
 	//
+	// A Collector whose Describe method does not yield any Desc is treated
+	// as unchecked. Registration will always succeed. No check for
+	// re-registering (see previous paragraph) is performed. Thus, the
+	// caller is responsible for not double-registering the same unchecked
+	// Collector, and for providing a Collector that will not cause
+	// inconsistent metrics on collection. (This would lead to scrape
+	// errors.)
+	//
 	// It is in general not safe to register the same Collector multiple
 	// times concurrently.
 	Register(Collector) error
@@ -108,7 +116,9 @@ type Registerer interface {
 	// Unregister unregisters the Collector that equals the Collector passed
 	// in as an argument.  (Two Collectors are considered equal if their
 	// Describe method yields the same set of descriptors.) The function
-	// returns whether a Collector was unregistered.
+	// returns whether a Collector was unregistered. Note that an unchecked
+	// Collector cannot be unregistered (as its Describe method does not
+	// yield any descriptor).
 	//
 	// Note that even after unregistering, it will not be possible to
 	// register a new Collector that is inconsistent with the unregistered
@@ -243,6 +253,7 @@ type Registry struct {
 	collectorsByID        map[uint64]Collector // ID is a hash of the descIDs.
 	descIDs               map[uint64]struct{}
 	dimHashesByName       map[string]uint64
+	uncheckedCollectors   []Collector
 	pedanticChecksEnabled bool
 }
 
@@ -300,9 +311,10 @@ func (r *Registry) Register(c Collector) error {
 			}
 		}
 	}
-	// Did anything happen at all?
+	// A Collector yielding no Desc at all is considered unchecked.
 	if len(newDescIDs) == 0 {
-		return errors.New("collector has no descriptors")
+		r.uncheckedCollectors = append(r.uncheckedCollectors, c)
+		return nil
 	}
 	if existing, exists := r.collectorsByID[collectorID]; exists {
 		return AlreadyRegisteredError{
@@ -376,7 +388,8 @@ func (r *Registry) MustRegister(cs ...Collector) {
 // Gather implements Gatherer.
 func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	var (
-		metricChan        = make(chan Metric, capMetricChan)
+		checkedMetricChan   = make(chan Metric, capMetricChan)
+		uncheckedMetricChan = make(chan Metric, capMetricChan)
 		metricHashes        = map[uint64]struct{}{}
 		wg                  sync.WaitGroup
 		errs                MultiError          // The collected errors to return in the end.
@@ -384,11 +397,15 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	)
 
 	r.mtx.RLock()
-	goroutineBudget := len(r.collectorsByID)
+	goroutineBudget := len(r.collectorsByID) + len(r.uncheckedCollectors)
 	metricFamiliesByName := make(map[string]*dto.MetricFamily, len(r.dimHashesByName))
-	collectors := make(chan Collector, len(r.collectorsByID))
+	checkedCollectors := make(chan Collector, len(r.collectorsByID))
+	uncheckedCollectors := make(chan Collector, len(r.uncheckedCollectors))
 	for _, collector := range r.collectorsByID {
-		collectors <- collector
+		checkedCollectors <- collector
+	}
+	for _, collector := range r.uncheckedCollectors {
+		uncheckedCollectors <- collector
 	}
 	// In case pedantic checks are enabled, we have to copy the map before
 	// giving up the RLock.
@@ -405,12 +422,14 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	collectWorker := func() {
 		for {
 			select {
-			case collector := <-collectors:
-				collector.Collect(metricChan)
-				wg.Done()
+			case collector := <-checkedCollectors:
+				collector.Collect(checkedMetricChan)
+			case collector := <-uncheckedCollectors:
+				collector.Collect(uncheckedMetricChan)
 			default:
 				return
 			}
+			wg.Done()
 		}
 	}
 
@@ -418,51 +437,94 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	go collectWorker()
 	goroutineBudget--
 
-	// Close the metricChan once all collectors are collected.
+	// Close checkedMetricChan and uncheckedMetricChan once all collectors
+	// are collected.
 	go func() {
 		wg.Wait()
-		close(metricChan)
+		close(checkedMetricChan)
+		close(uncheckedMetricChan)
 	}()
 
-	// Drain metricChan in case of premature return.
+	// Drain checkedMetricChan and uncheckedMetricChan in case of premature return.
 	defer func() {
-		for range metricChan {
+		if checkedMetricChan != nil {
+			for range checkedMetricChan {
+			}
+		}
+		if uncheckedMetricChan != nil {
+			for range uncheckedMetricChan {
+			}
 		}
 	}()
 
-collectLoop:
+	// Copy the channel references so we can nil them out later to remove
+	// them from the select statements below.
+	cmc := checkedMetricChan
+	umc := uncheckedMetricChan
+
 	for {
 		select {
-		case metric, ok := <-metricChan:
+		case metric, ok := <-cmc:
 			if !ok {
-				// metricChan is closed, we are done.
-				break collectLoop
+				cmc = nil
+				break
 			}
 			errs.Append(processMetric(
 				metric, metricFamiliesByName,
 				metricHashes,
 				registeredDescIDs,
 			))
+		case metric, ok := <-umc:
+			if !ok {
+				umc = nil
+				break
+			}
+			errs.Append(processMetric(
+				metric, metricFamiliesByName,
+				metricHashes,
+				nil,
+			))
 		default:
-			if goroutineBudget <= 0 || len(collectors) == 0 {
+			if goroutineBudget <= 0 || len(checkedCollectors)+len(uncheckedCollectors) == 0 {
 				// All collectors are already being worked on or
 				// we have already as many goroutines started as
-				// there are collectors. Just process metrics
-				// from now on.
-				for metric := range metricChan {
+				// there are collectors. Do the same as above,
+				// just without the default.
+				select {
+				case metric, ok := <-cmc:
+					if !ok {
+						cmc = nil
+						break
+					}
 					errs.Append(processMetric(
 						metric, metricFamiliesByName,
 						metricHashes,
 						registeredDescIDs,
 					))
+				case metric, ok := <-umc:
+					if !ok {
+						umc = nil
+						break
 					}
-				break collectLoop
+					errs.Append(processMetric(
+						metric, metricFamiliesByName,
+						metricHashes,
+						nil,
+					))
+				}
+				break
 			}
 			// Start more workers.
 			go collectWorker()
 			goroutineBudget--
 			runtime.Gosched()
 		}
+		// Once both checkedMetricChan and uncheckdMetricChan are closed
+		// and drained, the contraption above will nil out cmc and umc,
+		// and then we can leave the collect loop here.
+		if cmc == nil && umc == nil {
+			break
+		}
 	}
 	return normalizeMetricFamilies(metricFamiliesByName), errs.MaybeUnwrap()
 }

prometheus/registry_test.go

@@ -34,7 +34,22 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+// uncheckedCollector wraps a Collector but its Describe method yields no Desc.
+type uncheckedCollector struct {
+	c prometheus.Collector
+}
+
+func (u uncheckedCollector) Describe(_ chan<- *prometheus.Desc) {}
+func (u uncheckedCollector) Collect(c chan<- prometheus.Metric) {
+	u.c.Collect(c)
+}
+
 func testHandler(t testing.TB) {
+	// TODO(beorn7): This test is a bit too "end-to-end". It tests quite a
+	// few moving parts that are not strongly coupled. They could/should be
+	// tested separately. However, the changes planned for v0.10 will
+	// require a major rework of this test anyway, at which time I will
+	// structure it in a better way.
 
 	metricVec := prometheus.NewCounterVec(
 		prometheus.CounterOpts{
@@ -496,6 +511,18 @@ collected metric's label constname is not utf8: "\xff"
 				externalMetricFamilyWithInvalidLabelValue,
 			},
 		},
+		{ // 17
+			headers: map[string]string{
+				"Accept": "application/json",
+			},
+			out: output{
+				headers: map[string]string{
+					"Content-Type": `text/plain; version=0.0.4; charset=utf-8`,
+				},
+				body: expectedMetricFamilyAsText,
+			},
+			collector: uncheckedCollector{metricVec},
+		},
 	}
 	for i, scenario := range scenarios {
 		registry := prometheus.NewPedanticRegistry()
@@ -510,7 +537,7 @@ collected metric's label constname is not utf8: "\xff"
 		}
 
 		if scenario.collector != nil {
-			registry.Register(scenario.collector)
+			registry.MustRegister(scenario.collector)
 		}
 		writer := httptest.NewRecorder()
 		handler := prometheus.InstrumentHandler("prometheus", promhttp.HandlerFor(gatherer, promhttp.HandlerOpts{}))