namespace.go

// Copyright (c) 2016 Uber Technologies, Inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package storage

import (
	"errors"
	"fmt"
	iofs "io/fs"
	"math"
	"runtime"
	"sync"
	"time"

	"github.com/m3db/m3/src/dbnode/namespace"
	"github.com/m3db/m3/src/dbnode/persist"
	"github.com/m3db/m3/src/dbnode/persist/fs"
	"github.com/m3db/m3/src/dbnode/sharding"
	"github.com/m3db/m3/src/dbnode/storage/block"
	"github.com/m3db/m3/src/dbnode/storage/bootstrap"
	"github.com/m3db/m3/src/dbnode/storage/bootstrap/result"
	"github.com/m3db/m3/src/dbnode/storage/index"
	"github.com/m3db/m3/src/dbnode/storage/index/convert"
	"github.com/m3db/m3/src/dbnode/storage/repair"
	"github.com/m3db/m3/src/dbnode/storage/series"
	"github.com/m3db/m3/src/dbnode/tracepoint"
	"github.com/m3db/m3/src/dbnode/ts"
	"github.com/m3db/m3/src/dbnode/ts/writes"
	"github.com/m3db/m3/src/m3ninx/doc"
	idxpersist "github.com/m3db/m3/src/m3ninx/persist"
	"github.com/m3db/m3/src/x/clock"
	"github.com/m3db/m3/src/x/context"
	xerrors "github.com/m3db/m3/src/x/errors"
	"github.com/m3db/m3/src/x/ident"
	"github.com/m3db/m3/src/x/instrument"
	xopentracing "github.com/m3db/m3/src/x/opentracing"
	xresource "github.com/m3db/m3/src/x/resource"
	xsync "github.com/m3db/m3/src/x/sync"
	xtime "github.com/m3db/m3/src/x/time"

	opentracinglog "github.com/opentracing/opentracing-go/log"
	"github.com/uber-go/tally"
	"go.uber.org/zap"
)

var (
	errNamespaceAlreadyClosed    = errors.New("namespace already closed")
	errNamespaceIndexingDisabled = errors.New("namespace indexing is disabled")
	errNamespaceReadOnly         = errors.New("cannot write to a read only namespace")
)

type commitLogWriter interface {
	Write(
		ctx context.Context,
		series ts.Series,
		datapoint ts.Datapoint,
		unit xtime.Unit,
		annotation ts.Annotation,
	) error
}

type commitLogWriterFn func(
	ctx context.Context,
	series ts.Series,
	datapoint ts.Datapoint,
	unit xtime.Unit,
	annotation ts.Annotation,
) error

func (fn commitLogWriterFn) Write(
	ctx context.Context,
	series ts.Series,
	datapoint ts.Datapoint,
	unit xtime.Unit,
	annotation ts.Annotation,
) error {
	return fn(ctx, series, datapoint, unit, annotation)
}

var commitLogWriteNoOp = commitLogWriter(commitLogWriterFn(func(
	ctx context.Context,
	series ts.Series,
	datapoint ts.Datapoint,
	unit xtime.Unit,
	annotation ts.Annotation,
) error {
	return nil
}))

type createEmptyWarmIndexIfNotExistsFn func(blockStart xtime.UnixNano) error

type dbNamespace struct {
	sync.RWMutex

	closed             bool
	readOnly           bool
	shutdownCh         chan struct{}
	id                 ident.ID
	shardSet           sharding.ShardSet
	blockRetriever     block.DatabaseBlockRetriever
	namespaceReaderMgr databaseNamespaceReaderManager
	opts               Options
	metadata           namespace.Metadata
	nopts              namespace.Options
	seriesOpts         series.Options
	nowFn              clock.NowFn
	snapshotFilesFn    snapshotFilesFn
	log                *zap.Logger
	bootstrapState     BootstrapState
	repairsAny         bool

	// schemaDescr caches the latest schema for the namespace.
	// schemaDescr is updated whenever schema registry is updated.
	schemaListener xresource.SimpleCloser
	schemaDescr    namespace.SchemaDescr

	// Contains an entry to all shards for fast shard lookup, an
	// entry will be nil when this shard does not belong to current database
	shards []databaseShard

	increasingIndex increasingIndex
	commitLogWriter commitLogWriter
	reverseIndex    NamespaceIndex

	createEmptyWarmIndexIfNotExistsFn createEmptyWarmIndexIfNotExistsFn

	tickWorkers            xsync.WorkerPool
	tickWorkersConcurrency int
	statsLastTick          databaseNamespaceStatsLastTick

	metrics               databaseNamespaceMetrics
	tickOptions           TickOptions
	tickSeqNo             int64 // The sequence number of the current tick.
	shouldTrackTopMetrics bool
}

type databaseNamespaceStatsLastTick struct {
	sync.RWMutex
	activeSeries int64
	activeBlocks int64
	index        databaseNamespaceIndexStatsLastTick
}

type databaseNamespaceIndexStatsLastTick struct {
	numDocs     int64
	numBlocks   int64
	numSegments int64
}

type databaseNamespaceMetrics struct {
	bootstrap           instrument.MethodMetrics
	flushWarmData       instrument.MethodMetrics
	flushColdData       instrument.MethodMetrics
	flushIndex          instrument.MethodMetrics
	snapshot            instrument.MethodMetrics
	write               instrument.MethodMetrics
	writeTagged         instrument.MethodMetrics
	read                instrument.MethodMetrics
	fetchBlocks         instrument.MethodMetrics
	fetchBlocksMetadata instrument.MethodMetrics
	queryIDs            instrument.MethodMetrics
	aggregateQuery      instrument.MethodMetrics

	unfulfilled             tally.Counter
	bootstrapStart          tally.Counter
	bootstrapEnd            tally.Counter
	snapshotSeriesPersist   tally.Counter
	writesWithoutAnnotation tally.Counter

	shards databaseNamespaceShardMetrics
	tick   databaseNamespaceTickMetrics
	status databaseNamespaceStatusMetrics

	repairDifferingPercent tally.Gauge
	repairComparedBlocks   tally.Counter
	repairDifferingBlocks  tally.Counter
	repairMismatchBlocks   tally.Counter
	repairMissingBlocks    tally.Counter
	repairExtraBlocks      tally.Counter
}

type databaseNamespaceShardMetrics struct {
	add         tally.Counter
	close       tally.Counter
	closeErrors tally.Counter
}

type databaseNamespaceTickMetrics struct {
	activeSeries           tally.Gauge
	metricCardinality      tally.Scope // holds multiple gauges.
	expiredSeries          tally.Counter
	activeBlocks           tally.Gauge
	wiredBlocks            tally.Gauge
	unwiredBlocks          tally.Gauge
	pendingMergeBlocks     tally.Gauge
	madeUnwiredBlocks      tally.Counter
	madeExpiredBlocks      tally.Counter
	mergedOutOfOrderBlocks tally.Counter
	errors                 tally.Counter
	index                  databaseNamespaceIndexTickMetrics
	evictedBuckets         tally.Counter
    // How many nanoseconds for running a metric tracking tick.
	tickDuratingNano tally.Gauge
}

type databaseNamespaceIndexTickMetrics struct {
	numBlocks        tally.Gauge
	numDocs          tally.Gauge
	numSegments      tally.Gauge
	numBlocksSealed  tally.Counter
	numBlocksEvicted tally.Counter
}

// databaseNamespaceStatusMetrics are metrics emitted at a fixed interval
// so that summing the value of gauges across hosts when graphed summarizing
// values at the same fixed intervals can show meaningful results (vs variably
// emitted values that can be aggregated across hosts to see a snapshot).
type databaseNamespaceStatusMetrics struct {
	activeSeries tally.Gauge
	activeBlocks tally.Gauge
	index        databaseNamespaceIndexStatusMetrics
}

type databaseNamespaceIndexStatusMetrics struct {
	numDocs     tally.Gauge
	numBlocks   tally.Gauge
	numSegments tally.Gauge
}

func newDatabaseNamespaceMetrics(
	scope tally.Scope,
	opts instrument.TimerOptions,
) databaseNamespaceMetrics {
	shardsScope := scope.SubScope("dbnamespace").SubScope("shards")
	tickScope := scope.SubScope("tick")
	indexTickScope := tickScope.SubScope("index")
	statusScope := scope.SubScope("status")
	indexStatusScope := statusScope.SubScope("index")
	bootstrapScope := scope.SubScope("bootstrap")
	snapshotScope := scope.SubScope("snapshot")
	repairScope := scope.SubScope("repair")
	return databaseNamespaceMetrics{
		bootstrap:           instrument.NewMethodMetrics(scope, "bootstrap", opts),
		flushWarmData:       instrument.NewMethodMetrics(scope, "flushWarmData", opts),
		flushColdData:       instrument.NewMethodMetrics(scope, "flushColdData", opts),
		flushIndex:          instrument.NewMethodMetrics(scope, "flushIndex", opts),
		snapshot:            instrument.NewMethodMetrics(scope, "snapshot", opts),
		write:               instrument.NewMethodMetrics(scope, "write", opts),
		writeTagged:         instrument.NewMethodMetrics(scope, "write-tagged", opts),
		read:                instrument.NewMethodMetrics(scope, "read", opts),
		fetchBlocks:         instrument.NewMethodMetrics(scope, "fetchBlocks", opts),
		fetchBlocksMetadata: instrument.NewMethodMetrics(scope, "fetchBlocksMetadata", opts),
		queryIDs:            instrument.NewMethodMetrics(scope, "queryIDs", opts),
		aggregateQuery:      instrument.NewMethodMetrics(scope, "aggregateQuery", opts),

		unfulfilled:             bootstrapScope.Counter("unfulfilled"),
		bootstrapStart:          bootstrapScope.Counter("start"),
		bootstrapEnd:            bootstrapScope.Counter("end"),
		snapshotSeriesPersist:   snapshotScope.Counter("series-persist"),
		writesWithoutAnnotation: scope.Counter("writes-without-annotation"),

		shards: databaseNamespaceShardMetrics{
			add:         shardsScope.Counter("add"),
			close:       shardsScope.Counter("close"),
			closeErrors: shardsScope.Counter("close-errors"),
		},
		tick: databaseNamespaceTickMetrics{
			activeSeries:           tickScope.Gauge("active-series"),
			metricCardinality:      tickScope.SubScope("top_metric_"), // "top_metric__xyz" for metric "xyz"
			expiredSeries:          tickScope.Counter("expired-series"),
			activeBlocks:           tickScope.Gauge("active-blocks"),
			wiredBlocks:            tickScope.Gauge("wired-blocks"),
			unwiredBlocks:          tickScope.Gauge("unwired-blocks"),
			pendingMergeBlocks:     tickScope.Gauge("pending-merge-blocks"),
			madeUnwiredBlocks:      tickScope.Counter("made-unwired-blocks"),
			madeExpiredBlocks:      tickScope.Counter("made-expired-blocks"),
			mergedOutOfOrderBlocks: tickScope.Counter("merged-out-of-order-blocks"),
			errors:                 tickScope.Counter("errors"),
			index: databaseNamespaceIndexTickMetrics{
				numDocs:          indexTickScope.Gauge("num-docs"),
				numBlocks:        indexTickScope.Gauge("num-blocks"),
				numSegments:      indexTickScope.Gauge("num-segments"),
				numBlocksSealed:  indexTickScope.Counter("num-blocks-sealed"),
				numBlocksEvicted: indexTickScope.Counter("num-blocks-evicted"),
			},
			evictedBuckets: tickScope.Counter("evicted-buckets"),
			tickDuratingNano: tickScope.Gauge("metric-tracking-duration-nano"),
		},
		status: databaseNamespaceStatusMetrics{
			activeSeries: statusScope.Gauge("active-series"),
			activeBlocks: statusScope.Gauge("active-blocks"),
			index: databaseNamespaceIndexStatusMetrics{
				numDocs:     indexStatusScope.Gauge("num-docs"),
				numBlocks:   indexStatusScope.Gauge("num-blocks"),
				numSegments: indexStatusScope.Gauge("num-segments"),
			},
		},
		repairDifferingPercent: repairScope.Gauge("differing-percent"),
		repairComparedBlocks:   repairScope.Counter("compared-blocks"),
		repairDifferingBlocks:  repairScope.Counter("differing-blocks"),
		repairMismatchBlocks:   repairScope.Counter("mismatch-blocks"),
		repairMissingBlocks:    repairScope.Counter("missing-blocks"),
		repairExtraBlocks:      repairScope.Counter("extra-blocks"),
	}
}

func newDatabaseNamespace(
	metadata namespace.Metadata,
	namespaceRuntimeOptsMgr namespace.RuntimeOptionsManager,
	shardSet sharding.ShardSet,
	blockRetriever block.DatabaseBlockRetriever,
	increasingIndex increasingIndex,
	commitLogWriter commitLogWriter,
	opts Options,
) (databaseNamespace, error) {
	var (
		nopts = metadata.Options()
		id    = metadata.ID()
	)
	if !nopts.WritesToCommitLog() {
		commitLogWriter = commitLogWriteNoOp
	}

	iops := opts.InstrumentOptions()
	logger := iops.Logger().With(zap.String("namespace", id.String()))
	iops = iops.
		SetLogger(logger).
		SetMetricsScope(iops.MetricsScope().Tagged(map[string]string{
			"namespace": id.String(),
		}))
	opts = opts.SetInstrumentOptions(iops)

	scope := iops.MetricsScope().SubScope("database")

	tickWorkersConcurrency := int(math.Max(1, float64(runtime.GOMAXPROCS(0))/8))
	tickWorkers := xsync.NewWorkerPool(tickWorkersConcurrency)
	tickWorkers.Init()

	seriesOpts := NewSeriesOptionsFromOptions(opts, nopts.RetentionOptions()).
		SetStats(series.NewStats(scope)).
		SetColdWritesEnabled(nopts.ColdWritesEnabled())
	if err := seriesOpts.Validate(); err != nil {
		return nil, fmt.Errorf(
			"unable to create namespace %v, invalid series options: %v",
			metadata.ID().String(), err)
	}

	var (
		index NamespaceIndex
		err   error
	)
	if metadata.Options().IndexOptions().Enabled() {
		index, err = newNamespaceIndex(metadata, namespaceRuntimeOptsMgr,
			shardSet, opts)
		if err != nil {
			return nil, err
		}
	}

	n := &dbNamespace{
		id:                     id,
		shutdownCh:             make(chan struct{}),
		shardSet:               shardSet,
		blockRetriever:         blockRetriever,
		namespaceReaderMgr:     newNamespaceReaderManager(metadata, scope, opts),
		opts:                   opts,
		metadata:               metadata,
		nopts:                  nopts,
		seriesOpts:             seriesOpts,
		nowFn:                  opts.ClockOptions().NowFn(),
		snapshotFilesFn:        fs.SnapshotFiles,
		log:                    logger,
		increasingIndex:        increasingIndex,
		commitLogWriter:        commitLogWriter,
		reverseIndex:           index,
		tickWorkers:            tickWorkers,
		tickWorkersConcurrency: tickWorkersConcurrency,
		metrics:                newDatabaseNamespaceMetrics(scope, iops.TimerOptions()),
		tickOptions:            opts.TickOptions(),
		tickSeqNo:              0,
		shouldTrackTopMetrics:  opts.TickOptions().TopMetricsToTrack > 0 && opts.TickOptions().MaxMapLenForTracking > 0 && opts.TickOptions().TopMetricsTrackingTicks > 0,
	}

	n.createEmptyWarmIndexIfNotExistsFn = n.createEmptyWarmIndexIfNotExists

	sl, err := opts.SchemaRegistry().RegisterListener(id, n)
	// Fail to create namespace is schema listener can not be registered successfully.
	// If proto is disabled, err will always be nil.
	if err != nil {
		return nil, fmt.Errorf(
			"unable to register schema listener for namespace %v, error: %v",
			metadata.ID().String(), err)
	}
	n.schemaListener = sl
	n.assignShardSet(shardSet, assignShardSetOptions{
		needsBootstrap:    nopts.BootstrapEnabled(),
		initialAssignment: true,
	})
	go n.reportStatusLoop(opts.InstrumentOptions().ReportInterval())

	return n, nil
}

// SetSchemaHistory implements namespace.SchemaListener.
func (n *dbNamespace) SetSchemaHistory(value namespace.SchemaHistory) {
	n.Lock()
	defer n.Unlock()

	schema, ok := value.GetLatest()
	if !ok {
		n.log.Error("can not update namespace schema to empty", zap.Stringer("namespace", n.ID()))
		return
	}

	metadata, err := namespace.NewMetadata(n.ID(), n.nopts.SetSchemaHistory(value))
	if err != nil {
		n.log.Error("can not update namespace metadata with empty schema history", zap.Stringer("namespace", n.ID()), zap.Error(err))
		return
	}

	n.schemaDescr = schema
	n.metadata = metadata
}

func (n *dbNamespace) reportStatusLoop(reportInterval time.Duration) {
	ticker := time.NewTicker(reportInterval)
	defer ticker.Stop()
	for {
		select {
		case <-n.shutdownCh:
			return
		case <-ticker.C:
			n.statsLastTick.RLock()
			n.metrics.status.activeSeries.Update(float64(n.statsLastTick.activeSeries))
			n.metrics.status.activeBlocks.Update(float64(n.statsLastTick.activeBlocks))
			n.metrics.status.index.numDocs.Update(float64(n.statsLastTick.index.numDocs))
			n.metrics.status.index.numBlocks.Update(float64(n.statsLastTick.index.numBlocks))
			n.metrics.status.index.numSegments.Update(float64(n.statsLastTick.index.numSegments))
			n.statsLastTick.RUnlock()
		}
	}
}

func (n *dbNamespace) Options() namespace.Options {
	return n.nopts
}

func (n *dbNamespace) StorageOptions() Options {
	return n.opts
}

func (n *dbNamespace) ID() ident.ID {
	return n.id
}

func (n *dbNamespace) Metadata() namespace.Metadata {
	// NB(r): metadata is updated in SetSchemaHistory so requires an RLock.
	n.RLock()
	result := n.metadata
	n.RUnlock()
	return result
}

func (n *dbNamespace) Schema() namespace.SchemaDescr {
	n.RLock()
	schema := n.schemaDescr
	n.RUnlock()
	return schema
}

func (n *dbNamespace) NumSeries() int64 {
	var count int64
	for _, shard := range n.OwnedShards() {
		count += shard.NumSeries()
	}
	return count
}

func (n *dbNamespace) Shards() []Shard {
	n.RLock()
	shards := n.shardSet.AllIDs()
	databaseShards := make([]Shard, len(shards))
	for i, shard := range shards {
		databaseShards[i] = n.shards[shard]
	}
	n.RUnlock()
	return databaseShards
}

func (n *dbNamespace) AssignShardSet(shardSet sharding.ShardSet) {
	n.assignShardSet(shardSet, assignShardSetOptions{
		needsBootstrap:    n.nopts.BootstrapEnabled(),
		initialAssignment: false,
	})
}

type assignShardSetOptions struct {
	needsBootstrap    bool
	initialAssignment bool
}

func (n *dbNamespace) assignShardSet(
	shardSet sharding.ShardSet,
	opts assignShardSetOptions,
) {
	var (
		incoming        = make(map[uint32]struct{}, len(shardSet.All()))
		createdShardIds []uint32
		existing        []databaseShard
		closing         []databaseShard
	)
	for _, shard := range shardSet.AllIDs() {
		incoming[shard] = struct{}{}
	}

	n.Lock()
	metadata := n.metadata
	existing = n.shards
	for _, shard := range existing {
		if shard == nil {
			continue
		}
		if _, ok := incoming[shard.ID()]; !ok {
			closing = append(closing, shard)
		}
	}
	n.shardSet = shardSet
	n.shards = make([]databaseShard, n.shardSet.Max()+1)
	for _, shard := range n.shardSet.AllIDs() {
		// We create shards if its an initial assignment or if its not an initial assignment
		// and the shard doesn't already exist.
		if !opts.initialAssignment && int(shard) < len(existing) && existing[shard] != nil {
			n.shards[shard] = existing[shard]
			continue
		}

		// Otherwise it's the initial assignment or there isn't an existing
		// shard created for this shard ID.
		n.shards[shard] = newDatabaseShard(metadata, shard, n.blockRetriever,
			n.namespaceReaderMgr, n.increasingIndex, n.reverseIndex,
			opts.needsBootstrap, n.opts, n.seriesOpts)
		createdShardIds = append(createdShardIds, shard)
		// NB(bodu): We only record shard add metrics for shards created in non
		// initial assignments.
		if !opts.initialAssignment {
			n.metrics.shards.add.Inc(1)
		}
	}

	if len(createdShardIds) > 0 {
		n.log.Info("created new shards",
			zap.Stringer("namespace", n.ID()),
			zap.Uint32s("shards", createdShardIds),
			zap.Bool("initialAssignment", opts.initialAssignment),
			zap.Bool("needsBootstrap", opts.needsBootstrap))
	}

	if idx := n.reverseIndex; idx != nil {
		idx.AssignShardSet(shardSet)
	}
	if br := n.blockRetriever; br != nil {
		br.AssignShardSet(shardSet)
	}
	if mgr := n.namespaceReaderMgr; mgr != nil {
		mgr.assignShardSet(shardSet)
	}

	n.Unlock()
	n.closeShards(closing, false)
}

func (n *dbNamespace) closeShards(shards []databaseShard, blockUntilClosed bool) {
	var wg sync.WaitGroup
	// NB(r): There is a shard close deadline that controls how fast each
	// shard closes set in the options.  To make sure this is the single
	// point of control for determining how impactful closing shards may
	// be to performance, we let this be the single gate and simply spin
	// up a goroutine per shard that we need to close and rely on the self
	// throttling of each shard as determined by the close shard deadline to
	// gate the impact.
	closeFn := func(shard databaseShard) {
		defer wg.Done()
		if err := shard.Close(); err != nil {
			n.log.
				With(zap.Uint32("shard", shard.ID())).
				Error("error occurred closing shard", zap.Error(err))
			n.metrics.shards.closeErrors.Inc(1)
		} else {
			n.metrics.shards.close.Inc(1)
		}
	}

	wg.Add(len(shards))
	for _, shard := range shards {
		dbShard := shard
		if dbShard == nil {
			continue
		}
		go closeFn(dbShard)
	}

	if blockUntilClosed {
		wg.Wait()
	}
}

func (n *dbNamespace) Tick(c context.Cancellable, startTime xtime.UnixNano) error {
	// Allow the reader cache to tick.
	n.namespaceReaderMgr.tick()

	// Fetch the owned shards.
	shards := n.OwnedShards()
	if len(shards) == 0 {
		return nil
	}

	n.RLock()
	nsCtx := n.nsContextWithRLock()
	n.RUnlock()

	// Tick through the shards at a capped level of concurrency.
	var (
		r           tickResult
		multiErr    xerrors.MultiError
		l           sync.Mutex
		wg          sync.WaitGroup
		tickOptions = TickOptions{TopMetricsToTrack: 0}
	)
	n.tickSeqNo++
	tickStartTimeNano := xtime.Now()
	if n.shouldTrackTopMetrics && (n.tickSeqNo%int64(n.tickOptions.TopMetricsTrackingTicks) == 0) {
		tickOptions = n.tickOptions
		r.trackTopMetrics()
		n.log.Info("Will track top metrics for this tick.",
			zap.Int("TopMetricsTrackingTicks", n.tickOptions.TopMetricsTrackingTicks),
			zap.Int64("tickSeqNo", n.tickSeqNo),
		)
	}
	for _, shard := range shards {
		shard := shard
		wg.Add(1)
		n.tickWorkers.Go(func() {
			defer wg.Done()

			if c.IsCancelled() {
				return
			}
			shardResult, err := shard.Tick(c, startTime, nsCtx, tickOptions)

			l.Lock()
			r.merge(shardResult, tickOptions.TopMetricsToTrack)
			multiErr = multiErr.Add(err)
			l.Unlock()
		})
	}

	wg.Wait()

	// Tick namespaceIndex if it exists.
	var (
		indexTickResults namespaceIndexTickResult
		err              error
	)
	if idx := n.reverseIndex; idx != nil {
		indexTickResults, err = idx.Tick(c, startTime)
		if err != nil {
			multiErr = multiErr.Add(err)
		}
	}

	// NB: we early terminate here to ensure we are not reporting metrics
	// based on in-accurate/partial tick results.
	if err := multiErr.FinalError(); err != nil || c.IsCancelled() {
		n.log.Debug("tick cancelled", zap.Error(err))
		return err
	}

	n.statsLastTick.Lock()
	n.statsLastTick.activeSeries = int64(r.activeSeries)
	n.statsLastTick.activeBlocks = int64(r.activeBlocks)
	n.statsLastTick.index = databaseNamespaceIndexStatsLastTick{
		numDocs:     indexTickResults.NumTotalDocs,
		numBlocks:   indexTickResults.NumBlocks,
		numSegments: indexTickResults.NumSegments,
	}
	n.statsLastTick.Unlock()

	n.metrics.tick.activeSeries.Update(float64(r.activeSeries))
	n.metrics.tick.expiredSeries.Inc(int64(r.expiredSeries))
	n.metrics.tick.activeBlocks.Update(float64(r.activeBlocks))
	n.metrics.tick.wiredBlocks.Update(float64(r.wiredBlocks))
	n.metrics.tick.unwiredBlocks.Update(float64(r.unwiredBlocks))
	n.metrics.tick.pendingMergeBlocks.Update(float64(r.pendingMergeBlocks))
	n.metrics.tick.madeExpiredBlocks.Inc(int64(r.madeExpiredBlocks))
	n.metrics.tick.madeUnwiredBlocks.Inc(int64(r.madeUnwiredBlocks))
	n.metrics.tick.mergedOutOfOrderBlocks.Inc(int64(r.mergedOutOfOrderBlocks))
	n.metrics.tick.evictedBuckets.Inc(int64(r.evictedBuckets))
	n.metrics.tick.index.numDocs.Update(float64(indexTickResults.NumTotalDocs))
	n.metrics.tick.index.numBlocks.Update(float64(indexTickResults.NumBlocks))
	n.metrics.tick.index.numSegments.Update(float64(indexTickResults.NumSegments))
	n.metrics.tick.index.numBlocksEvicted.Inc(indexTickResults.NumBlocksEvicted)
	n.metrics.tick.index.numBlocksSealed.Inc(indexTickResults.NumBlocksSealed)
	n.metrics.tick.errors.Inc(int64(r.errors))
	for _, metric := range r.metricToCardinality {
		if metric.cardinality >= tickOptions.MinCardinalityToTrack {
			// If the gauge does not exist, it will be created on the fly.
			n.metrics.tick.metricCardinality.Gauge(string(metric.name)).Update(float64(metric.cardinality))
		}
	}
	tickDurationNano := xtime.Since(tickStartTimeNano).Nanoseconds()
	n.metrics.tick.tickDuratingNano.Update(float64(tickDurationNano))

	return nil
}

func (n *dbNamespace) Write(
	ctx context.Context,
	id ident.ID,
	timestamp xtime.UnixNano,
	value float64,
	unit xtime.Unit,
	annotation []byte,
) (SeriesWrite, error) {
	callStart := n.nowFn()

	if n.ReadOnly() {
		n.metrics.write.ReportError(n.nowFn().Sub(callStart))
		return SeriesWrite{}, errNamespaceReadOnly
	}

	shard, nsCtx, err := n.shardFor(id)
	if err != nil {
		n.metrics.write.ReportError(n.nowFn().Sub(callStart))
		return SeriesWrite{}, err
	}

	opts := series.WriteOptions{
		TruncateType: n.opts.TruncateType(),
		SchemaDesc:   nsCtx.Schema,
	}
	seriesWrite, err := shard.Write(ctx, id, timestamp,
		value, unit, annotation, opts)
	if err == nil && len(annotation) == 0 {
		n.metrics.writesWithoutAnnotation.Inc(1)
	}
	n.metrics.write.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return seriesWrite, err
}

func (n *dbNamespace) WriteTagged(
	ctx context.Context,
	id ident.ID,
	tagResolver convert.TagMetadataResolver,
	timestamp xtime.UnixNano,
	value float64,
	unit xtime.Unit,
	annotation []byte,
) (SeriesWrite, error) {
	callStart := n.nowFn()

	if n.ReadOnly() {
		n.metrics.writeTagged.ReportError(n.nowFn().Sub(callStart))
		return SeriesWrite{}, errNamespaceReadOnly
	}

	if n.reverseIndex == nil {
		n.metrics.writeTagged.ReportError(n.nowFn().Sub(callStart))
		return SeriesWrite{}, errNamespaceIndexingDisabled
	}

	shard, nsCtx, err := n.shardFor(id)
	if err != nil {
		n.metrics.writeTagged.ReportError(n.nowFn().Sub(callStart))
		return SeriesWrite{}, err
	}

	opts := series.WriteOptions{
		TruncateType: n.opts.TruncateType(),
		SchemaDesc:   nsCtx.Schema,
	}
	seriesWrite, err := shard.WriteTagged(ctx, id, tagResolver, timestamp,
		value, unit, annotation, opts)
	if err == nil && len(annotation) == 0 {
		n.metrics.writesWithoutAnnotation.Inc(1)
	}
	n.metrics.writeTagged.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return seriesWrite, err
}

func (n *dbNamespace) WritePendingIndexInserts(
	pending []writes.PendingIndexInsert,
) error {
	if n.reverseIndex == nil {
		return errNamespaceIndexingDisabled
	}
	return n.reverseIndex.WritePending(pending)
}

func (n *dbNamespace) SeriesRefResolver(
	shardID uint32,
	id ident.ID,
	tags ident.TagIterator,
) (bootstrap.SeriesRefResolver, bool, error) {
	n.RLock()
	shard, owned, err := n.shardAtWithRLock(shardID)
	n.RUnlock()
	if err != nil {
		return nil, owned, err
	}
	resolver, err := shard.SeriesRefResolver(id, tags)
	return resolver, true, err
}

func (n *dbNamespace) QueryIDs(
	ctx context.Context,
	query index.Query,
	opts index.QueryOptions,
) (index.QueryResult, error) {
	ctx, sp, sampled := ctx.StartSampledTraceSpan(tracepoint.NSQueryIDs)
	if sampled {
		sp.LogFields(
			opentracinglog.String("query", query.String()),
			opentracinglog.String("namespace", n.ID().String()),
			opentracinglog.Int("seriesLimit", opts.SeriesLimit),
			opentracinglog.Int("docsLimit", opts.DocsLimit),
			xopentracing.Time("start", opts.StartInclusive.ToTime()),
			xopentracing.Time("end", opts.EndExclusive.ToTime()),
		)
	}
	defer sp.Finish()

	callStart := n.nowFn()
	if n.reverseIndex == nil {
		n.metrics.queryIDs.ReportError(n.nowFn().Sub(callStart))
		err := errNamespaceIndexingDisabled
		sp.LogFields(opentracinglog.Error(err))
		return index.QueryResult{}, err
	}

	if !n.reverseIndex.Bootstrapped() {
		// Similar to reading shard data, return not bootstrapped
		n.metrics.queryIDs.ReportError(n.nowFn().Sub(callStart))
		err := errIndexNotBootstrappedToRead
		sp.LogFields(opentracinglog.Error(err))
		return index.QueryResult{},
			xerrors.NewRetryableError(err)
	}

	res, err := n.reverseIndex.Query(ctx, query, opts)
	if err != nil {
		sp.LogFields(opentracinglog.Error(err))
	}
	n.metrics.queryIDs.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return res, err
}

func (n *dbNamespace) AggregateQuery(
	ctx context.Context,
	query index.Query,
	opts index.AggregationOptions,
) (index.AggregateQueryResult, error) {
	callStart := n.nowFn()
	if n.reverseIndex == nil {
		n.metrics.aggregateQuery.ReportError(n.nowFn().Sub(callStart))
		return index.AggregateQueryResult{}, errNamespaceIndexingDisabled
	}

	if !n.reverseIndex.Bootstrapped() {
		// Similar to reading shard data, return not bootstrapped
		n.metrics.aggregateQuery.ReportError(n.nowFn().Sub(callStart))
		return index.AggregateQueryResult{},
			xerrors.NewRetryableError(errIndexNotBootstrappedToRead)
	}

	res, err := n.reverseIndex.AggregateQuery(ctx, query, opts)
	n.metrics.aggregateQuery.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return res, err
}

func (n *dbNamespace) PrepareBootstrap(ctx context.Context) ([]databaseShard, error) {
	ctx, span, sampled := ctx.StartSampledTraceSpan(tracepoint.NSPrepareBootstrap)
	defer span.Finish()

	if sampled {
		span.LogFields(opentracinglog.String("namespace", n.id.String()))
	}

	var (
		wg           sync.WaitGroup
		multiErrLock sync.Mutex
		multiErr     xerrors.MultiError
		shards       = n.OwnedShards()
	)
	for _, shard := range shards {
		shard := shard
		wg.Add(1)
		go func() {
			defer wg.Done()

			err := shard.PrepareBootstrap(ctx)
			if err != nil {
				multiErrLock.Lock()
				multiErr = multiErr.Add(err)
				multiErrLock.Unlock()
			}
		}()
	}

	wg.Wait()

	if err := multiErr.FinalError(); err != nil {
		return nil, err
	}

	return shards, nil
}

func (n *dbNamespace) ReadEncoded(
	ctx context.Context,
	id ident.ID,
	start, end xtime.UnixNano,
) (series.BlockReaderIter, error) {
	callStart := n.nowFn()
	shard, nsCtx, err := n.readableShardFor(id)
	if err != nil {
		n.metrics.read.ReportError(n.nowFn().Sub(callStart))
		return nil, err
	}
	res, err := shard.ReadEncoded(ctx, id, start, end, nsCtx)
	n.metrics.read.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return res, err
}

func (n *dbNamespace) FetchBlocks(
	ctx context.Context,
	shardID uint32,
	id ident.ID,
	starts []xtime.UnixNano,
) ([]block.FetchBlockResult, error) {
	callStart := n.nowFn()
	shard, nsCtx, err := n.ReadableShardAt(shardID)
	if err != nil {
		n.metrics.fetchBlocks.ReportError(n.nowFn().Sub(callStart))
		return nil, err
	}

	res, err := shard.FetchBlocks(ctx, id, starts, nsCtx)
	n.metrics.fetchBlocks.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return res, err
}

func (n *dbNamespace) FetchBlocksMetadataV2(
	ctx context.Context,
	shardID uint32,
	start, end xtime.UnixNano,
	limit int64,
	pageToken PageToken,
	opts block.FetchBlocksMetadataOptions,
) (block.FetchBlocksMetadataResults, PageToken, error) {
	callStart := n.nowFn()
	shard, _, err := n.ReadableShardAt(shardID)
	if err != nil {
		n.metrics.fetchBlocksMetadata.ReportError(n.nowFn().Sub(callStart))
		return nil, nil, err
	}

	res, nextPageToken, err := shard.FetchBlocksMetadataV2(ctx, start, end, limit,
		pageToken, opts)
	n.metrics.fetchBlocksMetadata.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return res, nextPageToken, err
}

func (n *dbNamespace) Bootstrap(
	ctx context.Context,
	bootstrapResult bootstrap.NamespaceResult,
) error {
	ctx, span, sampled := ctx.StartSampledTraceSpan(tracepoint.NSBootstrap)
	defer span.Finish()

	if sampled {
		span.LogFields(opentracinglog.String("namespace", n.id.String()))
	}

	callStart := n.nowFn()

	n.Lock()
	if n.bootstrapState == Bootstrapping {
		n.Unlock()
		n.metrics.bootstrap.ReportError(n.nowFn().Sub(callStart))
		return errNamespaceIsBootstrapping
	}
	n.bootstrapState = Bootstrapping
	nsCtx := n.nsContextWithRLock()
	n.Unlock()

	n.metrics.bootstrapStart.Inc(1)

	success := false
	defer func() {
		n.Lock()
		if success {
			n.bootstrapState = Bootstrapped
		} else {
			n.bootstrapState = BootstrapNotStarted
		}
		n.Unlock()
		n.metrics.bootstrapEnd.Inc(1)
	}()

	if !n.nopts.BootstrapEnabled() {
		success = true
		n.metrics.bootstrap.ReportSuccess(n.nowFn().Sub(callStart))
		return nil
	}

	// Bootstrap shards using at least half the CPUs available
	workers := xsync.NewWorkerPool(int(math.Ceil(float64(runtime.GOMAXPROCS(0)) / 2)))
	workers.Init()

	var (
		bootstrappedShards = bootstrapResult.Shards
		multiErr           = xerrors.NewMultiError()
		mutex              sync.Mutex
		wg                 sync.WaitGroup
	)
	n.log.Info("bootstrap marking all shards as bootstrapped",
		zap.Stringer("namespace", n.id),
		zap.Int("numShards", len(bootstrappedShards)))
	for _, shard := range n.OwnedShards() {
		// Make sure it was bootstrapped during this bootstrap run.
		shardID := shard.ID()
		bootstrapped := false
		for _, elem := range bootstrappedShards {
			if elem == shardID {
				bootstrapped = true
				break
			}
		}
		if !bootstrapped {
			// NB(r): Not bootstrapped in this bootstrap run.
			n.log.Debug("skipping already bootstrapped shard",
				zap.Uint32("shard", shardID),
				zap.Stringer("namespace", n.id))
			continue
		}

		if shard.IsBootstrapped() {
			// No concurrent bootstraps, this is an invariant since
			// we only select bootstrapping the shard for a run if it's
			// not already bootstrapped.
			err := instrument.InvariantErrorf(
				"bootstrapper already bootstrapped shard: %d", shardID)
			mutex.Lock()
			multiErr = multiErr.Add(err)
			mutex.Unlock()
			continue
		}

		// Check if there are unfulfilled ranges
		if ranges, ok := bootstrapResult.DataResult.Unfulfilled().Get(shardID); ok && !ranges.IsEmpty() {
			continue
		} else if n.reverseIndex != nil {
			if ranges, ok := bootstrapResult.IndexResult.Unfulfilled().Get(shardID); ok && !ranges.IsEmpty() {
				continue
			}
		}

		wg.Add(1)
		shard := shard
		workers.Go(func() {
			err := shard.Bootstrap(ctx, nsCtx)

			mutex.Lock()
			multiErr = multiErr.Add(err)
			mutex.Unlock()

			wg.Done()
		})
	}
	wg.Wait()

	if n.reverseIndex != nil {
		indexResults := bootstrapResult.IndexResult.IndexResults()
		n.log.Info("bootstrap index with bootstrapped index segments",
			zap.Int("numIndexBlocks", len(indexResults)))
		err := n.reverseIndex.Bootstrap(indexResults)
		multiErr = multiErr.Add(err)
	}

	markAnyUnfulfilled := func(
		bootstrapType string,
		unfulfilled result.ShardTimeRanges,
	) error {
		shardsUnfulfilled := int64(unfulfilled.Len())
		n.metrics.unfulfilled.Inc(shardsUnfulfilled)
		if shardsUnfulfilled == 0 {
			return nil
		}

		errStr := unfulfilled.SummaryString()
		errFmt := "bootstrap completed with unfulfilled ranges"
		n.log.Error(errFmt,
			zap.Error(errors.New(errStr)),
			zap.String("namespace", n.id.String()),
			zap.String("bootstrapType", bootstrapType))
		return fmt.Errorf("%s: %s", errFmt, errStr)
	}

	r := bootstrapResult
	if err := markAnyUnfulfilled("data", r.DataResult.Unfulfilled()); err != nil {
		multiErr = multiErr.Add(err)
	}
	if n.reverseIndex != nil {
		if err := markAnyUnfulfilled("index", r.IndexResult.Unfulfilled()); err != nil {
			multiErr = multiErr.Add(err)
		}
	}

	err := multiErr.FinalError()
	n.metrics.bootstrap.ReportSuccessOrError(err, n.nowFn().Sub(callStart))

	// NB(r): "success" is read by the defer above and depending on if true/false
	// will set the namespace status as bootstrapped or not.
	success = err == nil
	return err
}

func (n *dbNamespace) WarmFlush(
	blockStart xtime.UnixNano,
	flushPersist persist.FlushPreparer,
) error {
	// NB(rartoul): This value can be used for emitting metrics, but should not be used
	// for business logic.
	callStart := n.nowFn()

	n.RLock()
	if n.bootstrapState != Bootstrapped {
		n.RUnlock()
		n.metrics.flushWarmData.ReportError(n.nowFn().Sub(callStart))
		return errNamespaceNotBootstrapped
	}
	nsCtx := n.nsContextWithRLock()
	n.RUnlock()

	if n.ReadOnly() || !n.nopts.FlushEnabled() {
		n.metrics.flushWarmData.ReportSuccess(n.nowFn().Sub(callStart))
		return nil
	}

	// check if blockStart is aligned with the namespace's retention options
	bs := n.nopts.RetentionOptions().BlockSize()
	if t := blockStart.Truncate(bs); !blockStart.Equal(t) {
		return fmt.Errorf("failed to flush at time %v, not aligned to blockSize", blockStart.String())
	}

	multiErr := xerrors.NewMultiError()
	shards := n.OwnedShards()
	for _, shard := range shards {
		if !shard.IsBootstrapped() {
			n.log.
				With(zap.Uint32("shard", shard.ID())).
				Debug("skipping warm flush due to shard not bootstrapped yet")
			continue
		}

		flushState, err := shard.FlushState(blockStart)
		if err != nil {
			return err
		}
		// skip flushing if the shard has already flushed data for the `blockStart`
		if flushState.WarmStatus.DataFlushed == fileOpSuccess {
			continue
		}

		// NB(xichen): we still want to proceed if a shard fails to flush its data.
		// Probably want to emit a counter here, but for now just log it.
		if err := shard.WarmFlush(blockStart, flushPersist, nsCtx); err != nil {
			detailedErr := fmt.Errorf("shard %d failed to flush data: %v",
				shard.ID(), err)
			multiErr = multiErr.Add(detailedErr)
		}
	}

	res := multiErr.FinalError()
	n.metrics.flushWarmData.ReportSuccessOrError(res, n.nowFn().Sub(callStart))
	return res
}

// idAndBlockStart is the composite key for the genny map used to keep track of
// dirty series that need to be ColdFlushed.
type idAndBlockStart struct {
	id         []byte
	blockStart xtime.UnixNano
}

type coldFlushReusableResources struct {
	// dirtySeries is a map from a composite key of <series ID, block start>
	// to an element in a list in the dirtySeriesToWrite map. This map is used
	// to quickly test whether a series is dirty for a particular block start.
	//
	// The composite key is deliberately made so that this map stays one level
	// deep, making it easier to share between shard loops, minimizing the need
	// for allocations.
	//
	// Having a reference to the element in the dirtySeriesToWrite list enables
	// efficient removal of the series that have been read and subsequent
	// iterating through remaining series to be read.
	dirtySeries *dirtySeriesMap
	// dirtySeriesToWrite is a map from block start to a list of dirty series
	// that have yet to be written to disk.
	dirtySeriesToWrite map[xtime.UnixNano]*idList
	// idElementPool is a pool of list elements to be used when constructing
	// new lists for the dirtySeriesToWrite map.
	idElementPool *idElementPool
	fsReader      fs.DataFileSetReader
}

func newColdFlushReusableResources(opts Options) coldFlushReusableResources {
	fsReader, err := fs.NewReader(opts.BytesPool(), opts.CommitLogOptions().FilesystemOptions())
	if err != nil {
		return coldFlushReusableResources{}
	}

	return coldFlushReusableResources{
		dirtySeries:        newDirtySeriesMap(),
		dirtySeriesToWrite: make(map[xtime.UnixNano]*idList),
		// TODO(juchan): set pool options.
		idElementPool: newIDElementPool(nil),
		fsReader:      fsReader,
	}
}

func (r *coldFlushReusableResources) reset() {
	for _, seriesList := range r.dirtySeriesToWrite {
		if seriesList != nil {
			seriesList.Reset()
		}
		// Don't delete the empty list from the map so that other shards don't
		// need to reinitialize the list for these blocks.
	}

	r.dirtySeries.Reset()
}

func (n *dbNamespace) ColdFlush(flushPersist persist.FlushPreparer) error {
	// NB(rartoul): This value can be used for emitting metrics, but should not be used
	// for business logic.
	callStart := n.nowFn()

	n.RLock()
	if n.bootstrapState != Bootstrapped {
		n.RUnlock()
		n.metrics.flushColdData.ReportError(n.nowFn().Sub(callStart))
		return errNamespaceNotBootstrapped
	}
	nsCtx := n.nsContextWithRLock()
	repairsAny := n.repairsAny
	n.RUnlock()

	// If repair has run we still need cold flush regardless of whether cold writes is
	// enabled since repairs are dependent on the cold flushing logic.
	enabled := n.nopts.ColdWritesEnabled() || repairsAny
	if n.ReadOnly() || !enabled {
		n.metrics.flushColdData.ReportSuccess(n.nowFn().Sub(callStart))
		return nil
	}

	shards := n.OwnedShards()
	resources := newColdFlushReusableResources(n.opts)

	// NB(bodu): The in-mem index will lag behind the TSDB in terms of new series writes. For a period of
	// time between when we rotate out the active cold mutable index segments (happens here) and when
	// we actually cold flush the data to disk we will be making writes to the newly active mutable seg.
	// This means that some series can live doubly in-mem and loaded from disk until the next cold flush
	// where they will be evicted from the in-mem index.
	var (
		onColdFlushDone OnColdFlushDone
		err             error
	)
	if n.reverseIndex != nil {
		onColdFlushDone, err = n.reverseIndex.ColdFlush(shards)
		if err != nil {
			n.metrics.flushColdData.ReportError(n.nowFn().Sub(callStart))
			return err
		}
	}

	cfOpts := NewColdFlushNsOpts(true)
	onColdFlushNs, err := n.opts.OnColdFlush().ColdFlushNamespace(n, cfOpts)
	if err != nil {
		n.metrics.flushColdData.ReportError(n.nowFn().Sub(callStart))
		return err
	}

	// NB(bodu): Deferred shard cold flushes so that we can ensure that cold flush index data is
	// persisted before persisting TSDB data to ensure crash consistency.
	multiErr := xerrors.NewMultiError()
	shardColdFlushes := make([]ShardColdFlush, 0, len(shards))
	for _, shard := range shards {
		if !shard.IsBootstrapped() {
			n.log.
				With(zap.Uint32("shard", shard.ID())).
				Debug("skipping cold flush due to shard not bootstrapped yet")
			continue
		}
		shardColdFlush, err := shard.ColdFlush(flushPersist, resources, nsCtx, onColdFlushNs)
		if err != nil {
			detailedErr := fmt.Errorf("shard %d failed to compact: %v", shard.ID(), err)
			multiErr = multiErr.Add(detailedErr)
			continue
		}
		shardColdFlushes = append(shardColdFlushes, shardColdFlush)
	}

	// We go through this error checking process to allow for partially successful flushes.
	indexColdFlushError := onColdFlushNs.Done()
	if indexColdFlushError == nil && onColdFlushDone != nil {
		// Only evict rotated cold mutable index segments if the index cold flush was successful
		// or we will lose queryability of data that's still in mem.
		indexColdFlushError = onColdFlushDone()
	}
	if indexColdFlushError == nil {
		// NB(bodu): We only want to complete data cold flushes if the index cold flush
		// is successful. If index cold flush is successful, we want to attempt writing
		// of checkpoint files to complete the cold data flush lifecycle for successful shards.
		for _, shardColdFlush := range shardColdFlushes {
			multiErr = multiErr.Add(shardColdFlush.Done())
		}
	}
	multiErr = multiErr.Add(indexColdFlushError)

	res := multiErr.FinalError()
	n.metrics.flushColdData.ReportSuccessOrError(res, n.nowFn().Sub(callStart))
	return res
}

func (n *dbNamespace) FlushIndex(flush persist.IndexFlush) error {
	callStart := n.nowFn()
	n.RLock()
	if n.bootstrapState != Bootstrapped {
		n.RUnlock()
		n.metrics.flushIndex.ReportError(n.nowFn().Sub(callStart))
		return errNamespaceNotBootstrapped
	}
	n.RUnlock()

	if n.ReadOnly() || !n.nopts.FlushEnabled() || !n.nopts.IndexOptions().Enabled() {
		n.metrics.flushIndex.ReportSuccess(n.nowFn().Sub(callStart))
		return nil
	}

	shards := n.OwnedShards()
	err := n.reverseIndex.WarmFlush(flush, shards)
	n.metrics.flushIndex.ReportSuccessOrError(err, n.nowFn().Sub(callStart))
	return err
}

func (n *dbNamespace) Snapshot(
	blockStarts []xtime.UnixNano,
	snapshotTime xtime.UnixNano,
	snapshotPersist persist.SnapshotPreparer,
) error {
	// NB(rartoul): This value can be used for emitting metrics, but should not be used
	// for business logic.
	callStart := n.nowFn()

	var nsCtx namespace.Context
	n.RLock()
	if n.bootstrapState != Bootstrapped {
		n.RUnlock()
		n.metrics.snapshot.ReportError(n.nowFn().Sub(callStart))
		return errNamespaceNotBootstrapped
	}
	nsCtx = n.nsContextWithRLock()
	n.RUnlock()

	if !n.nopts.SnapshotEnabled() {
		// Note that we keep the ability to disable snapshots at the namespace level around for
		// debugging / performance / flexibility reasons, but disabling it can / will cause data
		// loss due to the commitlog cleanup logic assuming that a valid snapshot checkpoint file
		// means that all namespaces were successfully snapshotted.
		n.metrics.snapshot.ReportSuccess(n.nowFn().Sub(callStart))
		return nil
	}

	var (
		seriesPersist int
		multiErr      xerrors.MultiError
	)

	for _, shard := range n.OwnedShards() {
		log := n.log.With(zap.Uint32("shard", shard.ID()))
		if !shard.IsBootstrapped() {
			log.Debug("skipping snapshot due to shard not bootstrapped yet")
			continue
		}
		snapshotBlockStarts := shard.FilterBlocksNeedSnapshot(blockStarts)
		if len(snapshotBlockStarts) == 0 {
			log.Debug("skipping shard snapshot since no blocks need it")
			continue
		}
		for _, blockStart := range snapshotBlockStarts {
			snapshotResult, err := shard.Snapshot(blockStart, snapshotTime, snapshotPersist, nsCtx)
			if err != nil {
				detailedErr := fmt.Errorf("shard %d failed to snapshot %v block: %w", shard.ID(), blockStart, err)
				multiErr = multiErr.Add(detailedErr)
				continue
			}
			seriesPersist += snapshotResult.SeriesPersist
		}
	}

	n.metrics.snapshotSeriesPersist.Inc(int64(seriesPersist))

	res := multiErr.FinalError()
	n.metrics.snapshot.ReportSuccessOrError(res, n.nowFn().Sub(callStart))
	return res
}

func (n *dbNamespace) NeedsFlush(
	alignedInclusiveStart xtime.UnixNano,
	alignedInclusiveEnd xtime.UnixNano,
) (bool, error) {
	// NB(r): Essentially if all are success, we don't need to flush, if any
	// are failed with the minimum num failures less than max retries then
	// we need to flush - otherwise if any in progress we can't flush and if
	// any not started then we need to flush.
	n.RLock()
	defer n.RUnlock()
	return n.needsFlushWithLock(alignedInclusiveStart, alignedInclusiveEnd)
}

func (n *dbNamespace) needsFlushWithLock(
	alignedInclusiveStart xtime.UnixNano,
	alignedInclusiveEnd xtime.UnixNano,
) (bool, error) {
	var (
		blockSize   = n.nopts.RetentionOptions().BlockSize()
		blockStarts = timesInRange(alignedInclusiveStart, alignedInclusiveEnd, blockSize)
	)

	// NB(prateek): we do not check if any other flush is in progress in this method,
	// instead relying on the databaseFlushManager to ensure atomicity of flushes.

	// Check for not started or failed that might need a flush
	for _, shard := range n.shards {
		if shard == nil {
			continue
		}
		for _, blockStart := range blockStarts {
			flushState, err := shard.FlushState(blockStart)
			if err != nil {
				return false, err
			}
			if flushState.WarmStatus.DataFlushed != fileOpSuccess {
				return true, nil
			}
		}
	}

	// All success or failed and reached max retries
	return false, nil
}

func (n *dbNamespace) Truncate() (int64, error) {
	var totalNumSeries int64

	n.RLock()
	shards := n.shardSet.AllIDs()
	for _, shard := range shards {
		totalNumSeries += n.shards[shard].NumSeries()
	}
	n.RUnlock()

	// For now we are simply dropping all the objects (e.g., shards, series, blocks etc) owned by the
	// namespace, which means the memory will be reclaimed the next time GC kicks in and returns the
	// reclaimed memory to the OS. In the future, we might investigate whether it's worth returning
	// the pooled objects to the pools if the pool is low and needs replenishing.
	n.assignShardSet(n.shardSet, assignShardSetOptions{
		needsBootstrap:    false,
		initialAssignment: true,
	})

	// NB(xichen): possibly also clean up disk files and force a GC here to reclaim memory immediately
	return totalNumSeries, nil
}

func (n *dbNamespace) Repair(
	repairer databaseShardRepairer,
	tr xtime.Range,
	opts NamespaceRepairOptions,
) error {
	shouldRun := opts.Force || n.nopts.RepairEnabled()
	if !shouldRun {
		return nil
	}

	n.RLock()
	repairsAny := n.repairsAny
	n.RUnlock()
	if !repairsAny {
		// Only acquire write lock if required.
		n.Lock()
		n.repairsAny = true
		n.Unlock()
	}

	var (
		wg                      sync.WaitGroup
		mutex                   sync.Mutex
		numShardsRepaired       int
		numTotalSeries          int64
		numTotalBlocks          int64
		numSizeDiffSeries       int64
		numSizeDiffBlocks       int64
		numChecksumDiffSeries   int64
		numChecksumDiffBlocks   int64
		peerMetadataComparisons repair.PeerMetadataComparisonResults
		throttlePerShard        time.Duration
	)

	multiErr := xerrors.NewMultiError()
	shards := n.OwnedShards()
	numShards := len(shards)
	if numShards > 0 {
		throttlePerShard = time.Duration(
			int64(repairer.Options().RepairThrottle()) / int64(numShards))
	}

	workers := xsync.NewWorkerPool(repairer.Options().RepairShardConcurrency())
	workers.Init()

	n.RLock()
	nsCtx := n.nsContextWithRLock()
	nsMeta := n.metadata
	n.RUnlock()

	for _, shard := range shards {
		shard := shard

		wg.Add(1)
		workers.Go(func() {
			defer wg.Done()

			ctx := n.opts.ContextPool().Get()
			defer ctx.Close()

			metadataRes, err := shard.Repair(ctx, nsCtx, nsMeta, tr, repairer)

			mutex.Lock()
			if err != nil {
				multiErr = multiErr.Add(err)
			} else {
				numShardsRepaired++
				numTotalSeries += metadataRes.NumSeries
				numTotalBlocks += metadataRes.NumBlocks
				numSizeDiffSeries += metadataRes.SizeDifferences.NumSeries()
				numSizeDiffBlocks += metadataRes.SizeDifferences.NumBlocks()
				numChecksumDiffSeries += metadataRes.ChecksumDifferences.NumSeries()
				numChecksumDiffBlocks += metadataRes.ChecksumDifferences.NumBlocks()
				peerMetadataComparisons = append(peerMetadataComparisons, metadataRes.PeerMetadataComparisonResults...)
			}
			mutex.Unlock()

			if throttlePerShard > 0 {
				time.Sleep(throttlePerShard)
			}
		})
	}

	wg.Wait()

	aggregatePeerComparison := peerMetadataComparisons.Aggregate()
	n.metrics.repairDifferingPercent.Update(aggregatePeerComparison.ComparedDifferingPercent)
	n.metrics.repairComparedBlocks.Inc(aggregatePeerComparison.ComparedBlocks)
	n.metrics.repairDifferingBlocks.Inc(aggregatePeerComparison.ComparedDifferingBlocks)
	n.metrics.repairMismatchBlocks.Inc(aggregatePeerComparison.ComparedMismatchBlocks)
	n.metrics.repairMissingBlocks.Inc(aggregatePeerComparison.ComparedMissingBlocks)
	n.metrics.repairExtraBlocks.Inc(aggregatePeerComparison.ComparedExtraBlocks)

	n.log.Info("repair result",
		zap.String("repairTimeRange", tr.String()),
		zap.Int("numTotalShards", len(shards)),
		zap.Int("numShardsRepaired", numShardsRepaired),
		zap.Int64("numTotalSeries", numTotalSeries),
		zap.Int64("numTotalBlocks", numTotalBlocks),
		zap.Int64("numSizeDiffSeries", numSizeDiffSeries),
		zap.Int64("numSizeDiffBlocks", numSizeDiffBlocks),
		zap.Int64("numChecksumDiffSeries", numChecksumDiffSeries),
		zap.Int64("numChecksumDiffBlocks", numChecksumDiffBlocks),
		zap.Float64("peerComparisonComparedDifferingPercent", aggregatePeerComparison.ComparedDifferingPercent),
		zap.Int64("peerComparisonComparedBlocks", aggregatePeerComparison.ComparedBlocks),
		zap.Int64("peerComparisonComparedDifferingBlocks", aggregatePeerComparison.ComparedDifferingBlocks),
		zap.Int64("peerComparisonComparedMismatchBlocks", aggregatePeerComparison.ComparedMismatchBlocks),
		zap.Int64("peerComparisonComparedMissingBlocks", aggregatePeerComparison.ComparedMissingBlocks),
		zap.Int64("peerComparisonComparedExtraBlocks", aggregatePeerComparison.ComparedExtraBlocks),
	)

	return multiErr.FinalError()
}

func (n *dbNamespace) OwnedShards() []databaseShard {
	n.RLock()
	shards := n.shardSet.AllIDs()
	databaseShards := make([]databaseShard, len(shards))
	for i, shard := range shards {
		databaseShards[i] = n.shards[shard]
	}
	n.RUnlock()
	return databaseShards
}

func (n *dbNamespace) SetIndex(reverseIndex NamespaceIndex) error {
	n.Lock()
	defer n.Unlock()

	if !n.metadata.Options().IndexOptions().Enabled() {
		return errNamespaceIndexingDisabled
	}
	n.reverseIndex = reverseIndex

	return nil
}

func (n *dbNamespace) Index() (NamespaceIndex, error) {
	n.RLock()
	defer n.RUnlock()
	if !n.metadata.Options().IndexOptions().Enabled() {
		return nil, errNamespaceIndexingDisabled
	}
	return n.reverseIndex, nil
}

func (n *dbNamespace) ReadOnly() bool {
	return n.readOnly
}

func (n *dbNamespace) SetReadOnly(value bool) {
	n.readOnly = value
}

func (n *dbNamespace) shardFor(id ident.ID) (databaseShard, namespace.Context, error) {
	n.RLock()
	nsCtx := n.nsContextWithRLock()
	shardID := n.shardSet.Lookup(id)
	shard, _, err := n.shardAtWithRLock(shardID)
	n.RUnlock()
	return shard, nsCtx, err
}

func (n *dbNamespace) readableShardFor(id ident.ID) (databaseShard, namespace.Context, error) {
	n.RLock()
	nsCtx := n.nsContextWithRLock()
	shardID := n.shardSet.Lookup(id)
	shard, err := n.readableShardAtWithRLock(shardID)
	n.RUnlock()
	return shard, nsCtx, err
}

func (n *dbNamespace) ReadableShardAt(shardID uint32) (databaseShard, namespace.Context, error) {
	n.RLock()
	nsCtx := n.nsContextWithRLock()
	shard, err := n.readableShardAtWithRLock(shardID)
	n.RUnlock()
	return shard, nsCtx, err
}

func (n *dbNamespace) shardAtWithRLock(shardID uint32) (databaseShard, bool, error) {
	// NB(r): These errors are retryable as they will occur
	// during a topology change and must be retried by the client.
	if int(shardID) >= len(n.shards) {
		return nil, false, xerrors.NewRetryableError(
			fmt.Errorf("not responsible for shard %d", shardID))
	}
	shard := n.shards[shardID]
	if shard == nil {
		return nil, false, xerrors.NewRetryableError(
			fmt.Errorf("not responsible for shard %d", shardID))
	}
	return shard, true, nil
}

func (n *dbNamespace) readableShardAtWithRLock(shardID uint32) (databaseShard, error) {
	shard, _, err := n.shardAtWithRLock(shardID)
	if err != nil {
		return nil, err
	}
	if !shard.IsBootstrapped() {
		return nil, xerrors.NewRetryableError(errShardNotBootstrappedToRead)
	}
	return shard, nil
}

func (n *dbNamespace) Close() error {
	n.Lock()
	if n.closed {
		n.Unlock()
		return errNamespaceAlreadyClosed
	}
	n.closed = true
	shards := n.shards
	n.shards = shards[:0]
	n.shardSet = sharding.NewEmptyShardSet(sharding.DefaultHashFn(1))
	n.Unlock()
	n.namespaceReaderMgr.close()
	n.closeShards(shards, true)
	if retriever := n.blockRetriever; retriever != nil {
		if err := retriever.Close(); err != nil {
			n.log.Error("error when closing blockRetriever",
				zap.Error(err), zap.Stringer("namespace", n.id))
		}
	}
	close(n.shutdownCh)
	if n.reverseIndex != nil {
		return n.reverseIndex.Close()
	}
	if n.schemaListener != nil {
		n.schemaListener.Close()
	}
	return nil
}

func (n *dbNamespace) BootstrapState() BootstrapState {
	n.RLock()
	defer n.RUnlock()
	return n.bootstrapState
}

func (n *dbNamespace) ShardBootstrapState() ShardBootstrapStates {
	n.RLock()
	shardStates := make(ShardBootstrapStates, len(n.shards))
	for _, shard := range n.shards {
		if shard == nil {
			continue
		}
		shardStates[shard.ID()] = shard.BootstrapState()
	}
	n.RUnlock()
	return shardStates
}

func (n *dbNamespace) FlushState(shardID uint32, blockStart xtime.UnixNano) (fileOpState, error) {
	n.RLock()
	defer n.RUnlock()
	shard, _, err := n.shardAtWithRLock(shardID)
	if err != nil {
		return fileOpState{}, err
	}
	flushState, err := shard.FlushState(blockStart)
	if err != nil {
		return fileOpState{}, err
	}
	return flushState, nil
}

func (n *dbNamespace) nsContextWithRLock() namespace.Context {
	return namespace.Context{ID: n.id, Schema: n.schemaDescr}
}

func (n *dbNamespace) AggregateTiles(
	ctx context.Context,
	sourceNs databaseNamespace,
	opts AggregateTilesOptions,
) (int64, error) {
	var (
		callStart = n.nowFn()

		aggregateTilesScope = opts.InsOptions.MetricsScope()
		timerOpts           = n.opts.InstrumentOptions().TimerOptions()
		methodMetrics       = instrument.NewMethodMetrics(
			aggregateTilesScope, "aggregate-tiles", timerOpts)
	)

	processedTileCount, err := n.aggregateTiles(ctx, sourceNs, opts)

	methodMetrics.ReportSuccessOrError(err, n.nowFn().Sub(callStart))

	return processedTileCount, err
}

func (n *dbNamespace) aggregateTiles(
	ctx context.Context,
	sourceNs databaseNamespace,
	opts AggregateTilesOptions,
) (int64, error) {
	var (
		startedAt          = time.Now()
		targetBlockSize    = n.Metadata().Options().RetentionOptions().BlockSize()
		targetBlockStart   = opts.Start.Truncate(targetBlockSize)
		sourceBlockSize    = sourceNs.Options().RetentionOptions().BlockSize()
		lastSourceBlockEnd = opts.End.Truncate(sourceBlockSize)

		scope           = opts.InsOptions.MetricsScope()
		processedShards = scope.Counter("processed-shards")
	)

	if targetBlockStart.Add(targetBlockSize).Before(lastSourceBlockEnd) {
		return 0, fmt.Errorf("tile aggregation must be done within a single target block (start=%s, end=%s, blockSize=%s)",
			opts.Start, opts.End, targetBlockSize.String())
	}

	if n.BootstrapState() != Bootstrapped || sourceNs.BootstrapState() != Bootstrapped {
		return 0, errNamespaceNotBootstrapped
	}

	if len(n.OwnedShards()) == 0 {
		// This is possible during a new node join.
		n.log.Info("no shards owned, skip AggregateTiles")
		return 0, nil
	}

	// Create an empty warm index fileset because the block
	// will not be queryable if it has no warm index.
	if err := n.createEmptyWarmIndexIfNotExistsFn(targetBlockStart); err != nil {
		return 0, err
	}

	// Cold flusher builds the reverse index for target (current) ns.
	cfOpts := NewColdFlushNsOpts(false)
	onColdFlushNs, err := n.opts.OnColdFlush().ColdFlushNamespace(n, cfOpts)
	if err != nil {
		return 0, err
	}

	var (
		processedTileCount int64
		aggregationSuccess bool
	)

	defer func() {
		if aggregationSuccess {
			return
		}
		// Abort building reverse index if aggregation fails.
		if err := onColdFlushNs.Abort(); err != nil {
			n.log.Error("error aborting cold flush",
				zap.Stringer("sourceNs", sourceNs.ID()), zap.Error(err))
		}
	}()

	for _, targetShard := range n.OwnedShards() {
		if !targetShard.IsBootstrapped() {
			n.log.Debug("skipping aggregateTiles due to shard not bootstrapped",
				zap.Uint32("shard", targetShard.ID()))
			continue
		}

		shardProcessedTileCount, err := targetShard.AggregateTiles(
			ctx, sourceNs, n, targetShard.ID(), onColdFlushNs, opts)

		processedTileCount += shardProcessedTileCount
		processedShards.Inc(1)
		if err != nil {
			return 0, fmt.Errorf("shard %d aggregation failed: %v", targetShard.ID(), err)
		}
	}

	// Aggregation success, mark so we don't abort reverse index building (cold flusher).
	aggregationSuccess = true
	if err := onColdFlushNs.Done(); err != nil {
		return 0, err
	}

	n.log.Info("finished large tiles aggregation for namespace",
		zap.Stringer("sourceNs", sourceNs.ID()),
		zap.Stringer("process", opts.Process),
		zap.Bool("memorizeMetricTypes", opts.MemorizeMetricTypes),
		zap.Bool("backfillMetricTypes", opts.BackfillMetricTypes),
		zap.Time("targetBlockStart", targetBlockStart.ToTime()),
		zap.Time("lastSourceBlockEnd", lastSourceBlockEnd.ToTime()),
		zap.Duration("step", opts.Step),
		zap.Int64("processedTiles", processedTileCount),
		zap.Duration("took", time.Now().Sub(startedAt)))

	return processedTileCount, nil
}

func (n *dbNamespace) DocRef(id ident.ID) (doc.Metadata, bool, error) {
	shard, _, err := n.readableShardFor(id)
	if err != nil {
		return doc.Metadata{}, false, err
	}
	return shard.DocRef(id)
}

func (n *dbNamespace) createEmptyWarmIndexIfNotExists(blockStart xtime.UnixNano) error {
	fsOpts := n.opts.CommitLogOptions().FilesystemOptions()

	shardIds := make(map[uint32]struct{}, len(n.OwnedShards()))
	for _, shard := range n.OwnedShards() {
		if shard.IsBootstrapped() {
			shardIds[shard.ID()] = struct{}{}
		}
	}

	fileSetID := fs.FileSetFileIdentifier{
		FileSetContentType: persist.FileSetIndexContentType,
		Namespace:          n.ID(),
		BlockStart:         blockStart,
		VolumeIndex:        0,
	}

	warmIndexOpts := fs.IndexWriterOpenOptions{
		BlockSize:       n.Metadata().Options().IndexOptions().BlockSize(),
		FileSetType:     persist.FileSetFlushType,
		Identifier:      fileSetID,
		Shards:          shardIds,
		IndexVolumeType: idxpersist.DefaultIndexVolumeType,
	}

	warmIndexWriter, err := fs.NewIndexWriter(fsOpts)
	if err != nil {
		return err
	}

	if err = warmIndexWriter.Open(warmIndexOpts); err != nil {
		if xerrors.Is(err, iofs.ErrExist) {
			n.log.Debug("warm index already exists",
				zap.Stringer("namespace", n.id),
				zap.Stringer("blockStart", blockStart),
				zap.Reflect("shardIds", shardIds))
			return nil
		}
		return err
	}

	return warmIndexWriter.Close()
}