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Serve a cache hit with no database read or write (closes #309)
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A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5
This commit is contained in:
clawbot
2026-10-21 15:39:29 +00:00
committed by sneak
parent a19cca2f34
commit c0dd1d140f
24 changed files with 1106 additions and 1438 deletions
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internal/imgcache/cache.go
+67 -218
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@@ -12,7 +12,6 @@ import (
"sync/atomic"
"time"
lru "github.com/hashicorp/golang-lru/v2"
"sneak.berlin/go/pixa/internal/httpfetcher"
"sneak.berlin/go/pixa/internal/imagerequest"
)
@@ -23,9 +22,11 @@ var (
ErrNegativeCache = errors.New("negative cache hit")
)
// metaCacheSize is how many variants' content types metaCache holds. A
// variant not among them is served as before, reading its .meta file.
const metaCacheSize = 10000
// maxPendingAccessTimes is how many access times of variants served from
// the cache, and how many of cached sources a miss used, may each wait to be
// written, at about 80 bytes each. An access time handed over while that
// many of its kind wait is dropped.
const maxPendingAccessTimes = 10000
// cleanExpiredBatchSize is the most expired negative cache rows one
// statement of CleanExpired deletes. Each batch is a statement of its
@@ -82,19 +83,23 @@ type Cache struct {
evictionCancel context.CancelFunc
// The pending counts: hits, misses, upstream fetches and transforms
// whose write to the database missed its request's deadline, kept
// here until they are written by the goroutine that
// StartPendingCountWrites starts. As for eviction, the channels are
// created in NewCache: pendingCountsAdded wakes that goroutine, and
// pendingCountsDone is closed when it returns. pendingCountsCancel,
// set by StartPendingCountWrites, cancels its context, which tells it
// to finish.
// that requests counted, kept here until they are written by the
// goroutine that StartPendingCountWrites starts, so that no request
// waits for the database to count it. pendingVariantAccessTimes holds
// the variants served from the cache, and pendingSourceAccessTimes the
// cached sources a miss used, whose access time is still to be written
// by that goroutine. As for eviction, the channels are created in
// NewCache, so that requests can hand over access times before that
// goroutine starts; pendingCountsDone is closed when it returns.
// pendingCountsCancel, set by StartPendingCountWrites, cancels its
// context, which tells it to finish.
pendingHits atomic.Int64
pendingMisses atomic.Int64
pendingUpstreamFetches atomic.Int64
pendingUpstreamFetchBytes atomic.Int64
pendingTransforms atomic.Int64
pendingCountsAdded chan struct{}
pendingVariantAccessTimes chan imagerequest.VariantKey
pendingSourceAccessTimes chan imagerequest.SourceKey
pendingCountsDone chan struct{}
pendingCountsCancel context.CancelFunc
@@ -102,11 +107,6 @@ type Cache struct {
// readHitAndMissCounts to read them
pendingCountsWriteMutex sync.Mutex
// metaCache holds the content types of the variants most recently
// stored or served, so a hit does not read the variant's .meta file.
// It never stands in for the variant file, which is always opened.
metaCache *lru.Cache[imagerequest.VariantKey, string]
// reconciliationPageSize is the most rows one read of a content table
// returns in the reconciliation pass and in Stats. newCache sets it to
// defaultReconciliationPageSize; tests set it smaller.
@@ -134,11 +134,6 @@ func newCache(
log = slog.Default()
}
metaCache, err := lru.New[imagerequest.VariantKey, string](metaCacheSize)
if err != nil {
return nil, fmt.Errorf("failed to create variant content type cache: %w", err)
}
c := &Cache{
db: db,
config: config,
@@ -146,10 +141,10 @@ func newCache(
disabled: config.DisableDiskCache,
evictionPressure: make(chan struct{}, 1),
evictionDone: make(chan struct{}),
metaCache: metaCache,
pendingCountsAdded: make(chan struct{}, 1),
pendingCountsDone: make(chan struct{}),
pendingVariantAccessTimes: make(chan imagerequest.VariantKey, maxPendingAccessTimes),
pendingSourceAccessTimes: make(chan imagerequest.SourceKey, maxPendingAccessTimes),
pendingCountsDone: make(chan struct{}),
reconciliationPageSize: defaultReconciliationPageSize,
}
@@ -166,7 +161,7 @@ func newCache(
}
variants, err := NewVariantStorage(
filepath.Join(config.StateDir, "cache", "variants"), log,
filepath.Join(config.StateDir, "cache", "variants"),
)
if err != nil {
return nil, fmt.Errorf("failed to create variant storage: %w", err)
@@ -192,45 +187,11 @@ func newCache(
return c, nil
}
// LookupResult contains the result of a cache lookup.
type LookupResult struct {
Hit bool
CacheKey imagerequest.VariantKey
ContentType string
SizeBytes int64
CacheStatus CacheStatus
}
// Lookup checks if a processed variant exists on disk. Hits touch the
// variant's LRU timestamp; a disabled cache always misses.
func (c *Cache) Lookup(
ctx context.Context, req *imagerequest.Request,
) (*LookupResult, error) {
cacheKey := imagerequest.CacheKey(req)
// Check variant storage directly - no DB needed for cache hits
if !c.disabled && c.variants.Exists(cacheKey) {
c.touchVariant(ctx, cacheKey)
return &LookupResult{
Hit: true,
CacheKey: cacheKey,
CacheStatus: CacheHit,
}, nil
}
return &LookupResult{
Hit: false,
CacheKey: cacheKey,
CacheStatus: CacheMiss,
}, nil
}
// GetVariant returns a reader, size, and content type for a cached
// variant. The content type comes from metaCache, or else from the
// variant's .meta file and is then kept in metaCache. A variant with
// no .meta file is served as application/octet-stream, which is not
// kept.
// GetVariant opens a cached variant and returns a reader, its size and its
// content type, read from the variant file's first bytes. It opens that one
// file and makes no database call. A variant not in the cache, or any
// variant of a disabled cache, is ErrNotFound; a variant file whose first
// bytes are not those of an output format is another error.
func (c *Cache) GetVariant(
cacheKey imagerequest.VariantKey,
) (io.ReadCloser, int64, string, error) {
@@ -238,20 +199,7 @@ func (c *Cache) GetVariant(
return nil, 0, "", ErrNotFound
}
contentType, known := c.metaCache.Get(cacheKey)
if !known {
return c.loadVariantWithMeta(cacheKey)
}
reader, size, err := c.variants.LoadWithSize(cacheKey)
if err != nil {
// The file is gone, e.g. deleted outside pixa
c.metaCache.Remove(cacheKey)
return nil, 0, "", err
}
return reader, size, contentType, nil
return c.variants.LoadWithContentType(cacheKey)
}
// StoreSource stores fetched source content: it writes the file named by
@@ -321,13 +269,11 @@ func (c *Cache) StoreVariant(
return nil
}
size, err := c.variants.Store(cacheKey, content, contentType)
size, err := c.variants.Store(cacheKey, content)
if err != nil {
return err
}
c.metaCache.Add(cacheKey, contentType)
_, err = c.db.ExecContext(ctx, `
INSERT INTO variant_content (cache_key, size_bytes, content_type)
VALUES (?, ?, ?)
@@ -348,9 +294,9 @@ func (c *Cache) StoreVariant(
// LookupSource checks if we have cached source content for a request.
// Returns the source key and content type if found, or empty values
// if not; a row whose file is missing is not found. Hits touch the
// source's LRU timestamp; a disabled cache always reports no cached
// source.
// if not; a row whose file is missing is not found. A source found has
// its access time handed to the background writer without waiting (see
// touchSourceContent); a disabled cache always reports no cached source.
func (c *Cache) LookupSource(
ctx context.Context, req *imagerequest.Request,
) (imagerequest.SourceKey, string, error) {
@@ -379,7 +325,7 @@ func (c *Cache) LookupSource(
return "", "", nil
}
c.touchSourceContent(ctx, key)
c.touchSourceContent(key)
return key, contentType, nil
}
@@ -480,92 +426,33 @@ func (c *Cache) Stats(ctx context.Context) (*CacheStats, error) {
// IncrementStats counts a cache hit or miss, and an upstream fetch that read
// fetchBytes bytes, as IncrementUpstreamFetch does. Like the other Increment
// methods, it writes to the database with ctx's deadline but not its
// cancellation, so an ended request is still counted but does not wait for
// the database past its deadline; a count not written by then becomes a
// pending count (see StartPendingCountWrites).
func (c *Cache) IncrementStats(ctx context.Context, hit bool, fetchBytes int64) {
countCtx, cancel := withoutCancelKeepingDeadline(ctx)
defer cancel()
var err error
pendingCount := &c.pendingMisses
// methods, it only adds to the pending counts, which the goroutine
// StartPendingCountWrites starts writes to the database, so it never waits
// for the database or fails.
func (c *Cache) IncrementStats(hit bool, fetchBytes int64) {
if hit {
pendingCount = &c.pendingHits
_, err = c.db.ExecContext(countCtx, `
UPDATE cache_stats
SET hit_count = hit_count + 1,
last_updated_at = CURRENT_TIMESTAMP
WHERE id = 1
`)
c.pendingHits.Add(1)
} else {
_, err = c.db.ExecContext(countCtx, `
UPDATE cache_stats
SET miss_count = miss_count + 1,
last_updated_at = CURRENT_TIMESTAMP
WHERE id = 1
`)
c.pendingMisses.Add(1)
}
switch {
case errors.Is(err, context.DeadlineExceeded):
c.addPendingCount(pendingCount, 1)
case err != nil:
c.log.Warn("failed to count cache hit or miss", "hit", hit, "error", err)
}
c.IncrementUpstreamFetch(ctx, fetchBytes)
c.IncrementUpstreamFetch(fetchBytes)
}
// IncrementUpstreamFetch counts one upstream fetch that read fetchBytes bytes.
// A fetch that read no bytes is not counted.
func (c *Cache) IncrementUpstreamFetch(ctx context.Context, fetchBytes int64) {
func (c *Cache) IncrementUpstreamFetch(fetchBytes int64) {
if fetchBytes <= 0 {
return
}
countCtx, cancel := withoutCancelKeepingDeadline(ctx)
defer cancel()
_, err := c.db.ExecContext(countCtx, `
UPDATE cache_stats
SET upstream_fetch_count = upstream_fetch_count + 1,
upstream_fetch_bytes = upstream_fetch_bytes + ?,
last_updated_at = CURRENT_TIMESTAMP
WHERE id = 1
`, fetchBytes)
switch {
case errors.Is(err, context.DeadlineExceeded):
c.addPendingCount(&c.pendingUpstreamFetches, 1)
c.addPendingCount(&c.pendingUpstreamFetchBytes, fetchBytes)
case err != nil:
c.log.Warn("failed to count upstream fetch",
"fetch_bytes", fetchBytes, "error", err)
}
c.pendingUpstreamFetches.Add(1)
c.pendingUpstreamFetchBytes.Add(fetchBytes)
}
// IncrementTransformCount counts one image transcoded by the image processor.
func (c *Cache) IncrementTransformCount(ctx context.Context) {
countCtx, cancel := withoutCancelKeepingDeadline(ctx)
defer cancel()
_, err := c.db.ExecContext(countCtx, `
UPDATE cache_stats
SET transform_count = transform_count + 1,
last_updated_at = CURRENT_TIMESTAMP
WHERE id = 1
`)
switch {
case errors.Is(err, context.DeadlineExceeded):
c.addPendingCount(&c.pendingTransforms, 1)
case err != nil:
c.log.Warn("failed to count transform", "error", err)
}
func (c *Cache) IncrementTransformCount() {
c.pendingTransforms.Add(1)
}
// readHitAndMissCounts sets the hit and miss counts of stats to those of the
@@ -587,58 +474,33 @@ func (c *Cache) readHitAndMissCounts(ctx context.Context, stats *CacheStats) err
return err
}
// loadVariantWithMeta is GetVariant for a variant metaCache does not
// hold: it reads the content type from the variant's .meta file and
// keeps it in metaCache, unless a StoreVariant has put one there
// meanwhile, as the store's is newer. A read that finds no .meta file,
// as one can when the store's .meta write failed or an eviction removed
// the file after the read opened the variant, serves
// application/octet-stream and keeps nothing, so metaCache only ever
// holds a type read from a .meta file or passed to StoreVariant.
func (c *Cache) loadVariantWithMeta(
cacheKey imagerequest.VariantKey,
) (io.ReadCloser, int64, string, error) {
reader, size, contentType, err := c.variants.LoadWithMeta(cacheKey)
if err != nil {
return nil, 0, "", err
}
if contentType == "" {
return reader, size, fallbackContentType, nil
}
c.metaCache.ContainsOrAdd(cacheKey, contentType)
return reader, size, contentType, nil
}
// touchVariant updates the LRU timestamp of a variant, best-effort:
// a failed touch only makes the entry look colder to eviction.
func (c *Cache) touchVariant(ctx context.Context, cacheKey imagerequest.VariantKey) {
_, err := c.db.ExecContext(ctx, `
UPDATE variant_content SET last_accessed_at = CURRENT_TIMESTAMP
WHERE cache_key = ?
`, string(cacheKey))
if err != nil {
c.log.Debug("failed to touch variant LRU timestamp",
"cache_key", cacheKey, "error", err)
// touchVariant hands the access time of a variant served from the cache to
// the goroutine StartPendingCountWrites starts, which writes it in its next
// batch. It never waits: while maxPendingAccessTimes access times of
// variants wait to be written, this one is dropped, and the variant only
// looks colder to eviction.
func (c *Cache) touchVariant(cacheKey imagerequest.VariantKey) {
select {
case c.pendingVariantAccessTimes <- cacheKey:
default:
}
}
// touchSourceContent updates the LRU timestamp of a cached source,
// best-effort: a failed touch only makes the source look colder.
func (c *Cache) touchSourceContent(ctx context.Context, key imagerequest.SourceKey) {
_, err := c.db.ExecContext(ctx, `
UPDATE source_content SET last_accessed_at = CURRENT_TIMESTAMP
WHERE source_key = ?
`, string(key))
if err != nil {
c.log.Debug("failed to touch source content LRU timestamp",
"source_key", key, "error", err)
// touchSourceContent hands the access time of a cached source a miss used to
// the goroutine StartPendingCountWrites starts, which writes it in its next
// batch. It never waits: while maxPendingAccessTimes access times of sources
// wait to be written, this one is dropped, and the source only looks colder
// to eviction.
func (c *Cache) touchSourceContent(key imagerequest.SourceKey) {
select {
case c.pendingSourceAccessTimes <- key:
default:
}
}
// checkNegativeCache checks if a request is in the negative cache.
// checkNegativeCache checks if a request is in the negative cache. It only
// reads: a row past its expiry is not a hit, and the eviction goroutine
// deletes it with the other expired rows (see CleanExpired).
func (c *Cache) checkNegativeCache(
ctx context.Context, req *imagerequest.Request,
) (bool, error) {
@@ -657,22 +519,9 @@ func (c *Cache) checkNegativeCache(
return false, fmt.Errorf("failed to check negative cache: %w", err)
}
// Check if expired
if time.Now().After(expiresAt) {
// Clean up expired entry
_, err = c.db.ExecContext(ctx, `
DELETE FROM negative_cache
WHERE source_host = ? AND source_path = ? AND source_query = ?
`, req.Source.Host, req.Source.Path, req.Source.Query)
if err != nil {
c.log.Warn("failed to delete expired negative cache entry",
"host", req.Source.Host, "path", req.Source.Path, "error", err)
}
expired := time.Now().After(expiresAt)
return false, nil
}
return true, nil
return !expired, nil
}
// cleanExpiredBatch deletes up to cleanExpiredBatchSize negative cache