[chore] Refactor allocation strategies (open-telemetry#2928)

* Refactor consistent-hashing strategy

* Refactor per-node strategy

* Refactor least-weighted strategy

* Minor allocation strategy refactor

* Add some common allocation strategy tests

* Fix collector and target reassignment

* Minor allocator fixes

* Add changelog entry

* Fix an incorrect comment

.chloggen/refactor-strategies.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. collector, target allocator, auto-instrumentation, opamp, github action)
+component: target allocator
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Refactor allocation strategies
+
+# One or more tracking issues related to the change
+issues: [2928]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: The performance of the per-node strategy was massively improved as part of this change.

cmd/otel-allocator/allocation/allocator.go

@@ -0,0 +1,310 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package allocation
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/go-logr/logr"
+	"github.com/prometheus/client_golang/prometheus"
+
+	"github.com/open-telemetry/opentelemetry-operator/cmd/otel-allocator/diff"
+	"github.com/open-telemetry/opentelemetry-operator/cmd/otel-allocator/target"
+)
+
+/*
+	Target Allocator will serve on an HTTP server exposing /jobs/<job_id>/targets
+	The targets are allocated using the least connection method
+	Target Allocator will need information about the collectors in order to set the URLs
+	Keep a Map of what each collector currently holds and update it based on new scrape target updates
+*/
+
+var _ Allocator = &allocator{}
+
+func newAllocator(log logr.Logger, strategy Strategy, opts ...AllocationOption) Allocator {
+	chAllocator := &allocator{
+		strategy:                      strategy,
+		collectors:                    make(map[string]*Collector),
+		targetItems:                   make(map[string]*target.Item),
+		targetItemsPerJobPerCollector: make(map[string]map[string]map[string]bool),
+		log:                           log,
+	}
+	for _, opt := range opts {
+		opt(chAllocator)
+	}
+
+	return chAllocator
+}
+
+type allocator struct {
+	strategy Strategy
+
+	// collectors is a map from a Collector's name to a Collector instance
+	// collectorKey -> collector pointer
+	collectors map[string]*Collector
+
+	// targetItems is a map from a target item's hash to the target items allocated state
+	// targetItem hash -> target item pointer
+	targetItems map[string]*target.Item
+
+	// collectorKey -> job -> target item hash -> true
+	targetItemsPerJobPerCollector map[string]map[string]map[string]bool
+
+	// m protects collectors, targetItems and targetItemsPerJobPerCollector for concurrent use.
+	m sync.RWMutex
+
+	log logr.Logger
+
+	filter Filter
+}
+
+// SetFilter sets the filtering hook to use.
+func (a *allocator) SetFilter(filter Filter) {
+	a.filter = filter
+}
+
+// SetTargets accepts a list of targets that will be used to make
+// load balancing decisions. This method should be called when there are
+// new targets discovered or existing targets are shutdown.
+func (a *allocator) SetTargets(targets map[string]*target.Item) {
+	timer := prometheus.NewTimer(TimeToAssign.WithLabelValues("SetTargets", a.strategy.GetName()))
+	defer timer.ObserveDuration()
+
+	if a.filter != nil {
+		targets = a.filter.Apply(targets)
+	}
+	RecordTargetsKept(targets)
+
+	a.m.Lock()
+	defer a.m.Unlock()
+
+	// Check for target changes
+	targetsDiff := diff.Maps(a.targetItems, targets)
+	// If there are any additions or removals
+	if len(targetsDiff.Additions()) != 0 || len(targetsDiff.Removals()) != 0 {
+		a.handleTargets(targetsDiff)
+	}
+}
+
+// SetCollectors sets the set of collectors with key=collectorName, value=Collector object.
+// This method is called when Collectors are added or removed.
+func (a *allocator) SetCollectors(collectors map[string]*Collector) {
+	timer := prometheus.NewTimer(TimeToAssign.WithLabelValues("SetCollectors", a.strategy.GetName()))
+	defer timer.ObserveDuration()
+
+	CollectorsAllocatable.WithLabelValues(a.strategy.GetName()).Set(float64(len(collectors)))
+	if len(collectors) == 0 {
+		a.log.Info("No collector instances present")
+	}
+
+	a.m.Lock()
+	defer a.m.Unlock()
+
+	// Check for collector changes
+	collectorsDiff := diff.Maps(a.collectors, collectors)
+	if len(collectorsDiff.Additions()) != 0 || len(collectorsDiff.Removals()) != 0 {
+		a.handleCollectors(collectorsDiff)
+	}
+}
+
+func (a *allocator) GetTargetsForCollectorAndJob(collector string, job string) []*target.Item {
+	a.m.RLock()
+	defer a.m.RUnlock()
+	if _, ok := a.targetItemsPerJobPerCollector[collector]; !ok {
+		return []*target.Item{}
+	}
+	if _, ok := a.targetItemsPerJobPerCollector[collector][job]; !ok {
+		return []*target.Item{}
+	}
+	targetItemsCopy := make([]*target.Item, len(a.targetItemsPerJobPerCollector[collector][job]))
+	index := 0
+	for targetHash := range a.targetItemsPerJobPerCollector[collector][job] {
+		targetItemsCopy[index] = a.targetItems[targetHash]
+		index++
+	}
+	return targetItemsCopy
+}
+
+// TargetItems returns a shallow copy of the targetItems map.
+func (a *allocator) TargetItems() map[string]*target.Item {
+	a.m.RLock()
+	defer a.m.RUnlock()
+	targetItemsCopy := make(map[string]*target.Item)
+	for k, v := range a.targetItems {
+		targetItemsCopy[k] = v
+	}
+	return targetItemsCopy
+}
+
+// Collectors returns a shallow copy of the collectors map.
+func (a *allocator) Collectors() map[string]*Collector {
+	a.m.RLock()
+	defer a.m.RUnlock()
+	collectorsCopy := make(map[string]*Collector)
+	for k, v := range a.collectors {
+		collectorsCopy[k] = v
+	}
+	return collectorsCopy
+}
+
+// handleTargets receives the new and removed targets and reconciles the current state.
+// Any removals are removed from the allocator's targetItems and unassigned from the corresponding collector.
+// Any net-new additions are assigned to the collector on the same node as the target.
+func (a *allocator) handleTargets(diff diff.Changes[*target.Item]) {
+	// Check for removals
+	for k, item := range a.targetItems {
+		// if the current item is in the removals list
+		if _, ok := diff.Removals()[k]; ok {
+			a.removeTargetItem(item)
+		}
+	}
+
+	// Check for additions
+	assignmentErrors := []error{}
+	for k, item := range diff.Additions() {
+		// Do nothing if the item is already there
+		if _, ok := a.targetItems[k]; ok {
+			continue
+		} else {
+			// TODO: track target -> collector relationship in a separate map
+			item.CollectorName = ""
+			// Add item to item pool and assign a collector
+			err := a.addTargetToTargetItems(item)
+			if err != nil {
+				assignmentErrors = append(assignmentErrors, err)
+			}
+		}
+	}
+
+	// Check for unassigned targets
+	unassignedTargets := len(assignmentErrors)
+	if unassignedTargets > 0 {
+		err := errors.Join(assignmentErrors...)
+		a.log.Info("Could not assign targets for some jobs", "targets", unassignedTargets, "error", err)
+		TargetsUnassigned.Set(float64(unassignedTargets))
+	}
+}
+
+func (a *allocator) addTargetToTargetItems(tg *target.Item) error {
+	a.targetItems[tg.Hash()] = tg
+	if len(a.collectors) == 0 {
+		return nil
+	}
+
+	colOwner, err := a.strategy.GetCollectorForTarget(a.collectors, tg)
+	if err != nil {
+		return err
+	}
+
+	// Check if this is a reassignment, if so, unassign first
+	// note: The ordering here is important, we want to determine the new assignment before unassigning, because
+	// the strategy might make use of previous assignment information
+	if _, ok := a.collectors[tg.CollectorName]; ok && tg.CollectorName != "" {
+		a.unassignTargetItem(tg)
+	}
+
+	tg.CollectorName = colOwner.Name
+	a.addCollectorTargetItemMapping(tg)
+	a.collectors[colOwner.Name].NumTargets++
+	TargetsPerCollector.WithLabelValues(colOwner.String(), a.strategy.GetName()).Set(float64(a.collectors[colOwner.String()].NumTargets))
+
+	return nil
+}
+
+// unassignTargetItem unassigns the target item from its Collector. The target item is still tracked.
+func (a *allocator) unassignTargetItem(item *target.Item) {
+	collectorName := item.CollectorName
+	if collectorName == "" {
+		return
+	}
+	c, ok := a.collectors[collectorName]
+	if !ok {
+		return
+	}
+	c.NumTargets--
+	TargetsPerCollector.WithLabelValues(item.CollectorName, a.strategy.GetName()).Set(float64(c.NumTargets))
+	delete(a.targetItemsPerJobPerCollector[item.CollectorName][item.JobName], item.Hash())
+	if len(a.targetItemsPerJobPerCollector[item.CollectorName][item.JobName]) == 0 {
+		delete(a.targetItemsPerJobPerCollector[item.CollectorName], item.JobName)
+	}
+	item.CollectorName = ""
+}
+
+// removeTargetItem removes the target item from its Collector.
+func (a *allocator) removeTargetItem(item *target.Item) {
+	a.unassignTargetItem(item)
+	delete(a.targetItems, item.Hash())
+}
+
+// removeCollector removes a Collector from the allocator.
+func (a *allocator) removeCollector(collector *Collector) {
+	delete(a.collectors, collector.Name)
+	// Remove the collector from any target item records
+	for _, targetItems := range a.targetItemsPerJobPerCollector[collector.Name] {
+		for targetHash := range targetItems {
+			a.targetItems[targetHash].CollectorName = ""
+		}
+	}
+	delete(a.targetItemsPerJobPerCollector, collector.Name)
+	TargetsPerCollector.WithLabelValues(collector.Name, a.strategy.GetName()).Set(0)
+}
+
+// addCollectorTargetItemMapping keeps track of which collector has which jobs and targets
+// this allows the allocator to respond without any extra allocations to http calls. The caller of this method
+// has to acquire a lock.
+func (a *allocator) addCollectorTargetItemMapping(tg *target.Item) {
+	if a.targetItemsPerJobPerCollector[tg.CollectorName] == nil {
+		a.targetItemsPerJobPerCollector[tg.CollectorName] = make(map[string]map[string]bool)
+	}
+	if a.targetItemsPerJobPerCollector[tg.CollectorName][tg.JobName] == nil {
+		a.targetItemsPerJobPerCollector[tg.CollectorName][tg.JobName] = make(map[string]bool)
+	}
+	a.targetItemsPerJobPerCollector[tg.CollectorName][tg.JobName][tg.Hash()] = true
+}
+
+// handleCollectors receives the new and removed collectors and reconciles the current state.
+// Any removals are removed from the allocator's collectors. New collectors are added to the allocator's collector map.
+// Finally, update all targets' collector assignments.
+func (a *allocator) handleCollectors(diff diff.Changes[*Collector]) {
+	// Clear removed collectors
+	for _, k := range diff.Removals() {
+		a.removeCollector(k)
+	}
+	// Insert the new collectors
+	for _, i := range diff.Additions() {
+		a.collectors[i.Name] = NewCollector(i.Name, i.NodeName)
+	}
+
+	// Set collectors on the strategy
+	a.strategy.SetCollectors(a.collectors)
+
+	// Re-Allocate all targets
+	assignmentErrors := []error{}
+	for _, item := range a.targetItems {
+		err := a.addTargetToTargetItems(item)
+		if err != nil {
+			assignmentErrors = append(assignmentErrors, err)
+			item.CollectorName = ""
+		}
+	}
+	// Check for unassigned targets
+	unassignedTargets := len(assignmentErrors)
+	if unassignedTargets > 0 {
+		err := errors.Join(assignmentErrors...)
+		a.log.Info("Could not assign targets for some jobs", "targets", unassignedTargets, "error", err)
+		TargetsUnassigned.Set(float64(unassignedTargets))
+	}
+}