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package protocol
import (
"context"
"sync"
"time"
"github.com/seaweedfs/seaweedfs/weed/glog"
)
// partitionReader maintains a persistent connection to a single topic-partition
// and streams records forward, eliminating repeated offset lookups
// Pre-fetches and buffers records for instant serving
type partitionReader struct {
topicName string
partitionID int32
currentOffset int64
fetchChan chan *partitionFetchRequest
closeChan chan struct{}
// Pre-fetch buffer support
recordBuffer chan *bufferedRecords // Buffered pre-fetched records
bufferMu sync.Mutex // Protects offset access
handler *Handler
connCtx *ConnectionContext
}
// bufferedRecords represents a batch of pre-fetched records
type bufferedRecords struct {
recordBatch []byte
startOffset int64
endOffset int64
highWaterMark int64
}
// partitionFetchRequest represents a request to fetch data from this partition
type partitionFetchRequest struct {
requestedOffset int64
maxBytes int32
maxWaitMs int32 // MaxWaitTime from Kafka fetch request
resultChan chan *partitionFetchResult
isSchematized bool
apiVersion uint16
}
// newPartitionReader creates and starts a new partition reader with pre-fetch buffering
func newPartitionReader(ctx context.Context, handler *Handler, connCtx *ConnectionContext, topicName string, partitionID int32, startOffset int64) *partitionReader {
pr := &partitionReader{
topicName: topicName,
partitionID: partitionID,
currentOffset: startOffset,
fetchChan: make(chan *partitionFetchRequest, 200), // Buffer 200 requests to handle Schema Registry's rapid polling in slow CI environments
closeChan: make(chan struct{}),
recordBuffer: make(chan *bufferedRecords, 5), // Buffer 5 batches of records
handler: handler,
connCtx: connCtx,
}
// Start the pre-fetch goroutine that continuously fetches ahead
go pr.preFetchLoop(ctx)
// Start the request handler goroutine
go pr.handleRequests(ctx)
glog.V(2).Infof("[%s] Created partition reader for %s[%d] starting at offset %d (sequential with ch=200)",
connCtx.ConnectionID, topicName, partitionID, startOffset)
return pr
}
// preFetchLoop is disabled for SMQ backend to prevent subscriber storms
// SMQ reads from disk and creating multiple concurrent subscribers causes
// broker overload and partition shutdowns. Fetch requests are handled
// on-demand in serveFetchRequest instead.
func (pr *partitionReader) preFetchLoop(ctx context.Context) {
defer func() {
glog.V(2).Infof("[%s] Pre-fetch loop exiting for %s[%d]",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID)
close(pr.recordBuffer)
}()
// Wait for shutdown - no continuous pre-fetching to avoid overwhelming the broker
select {
case <-ctx.Done():
return
case <-pr.closeChan:
return
}
}
// handleRequests serves fetch requests SEQUENTIALLY to prevent subscriber storm
// CRITICAL: Sequential processing is essential for SMQ backend because:
// 1. GetStoredRecords may create a new subscriber on each call
// 2. Concurrent calls create multiple subscribers for the same partition
// 3. This overwhelms the broker and causes partition shutdowns
func (pr *partitionReader) handleRequests(ctx context.Context) {
defer func() {
glog.V(2).Infof("[%s] Request handler exiting for %s[%d]",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID)
}()
for {
select {
case <-ctx.Done():
return
case <-pr.closeChan:
return
case req := <-pr.fetchChan:
// Process sequentially to prevent subscriber storm
pr.serveFetchRequest(ctx, req)
}
}
}
// serveFetchRequest fetches data on-demand (no pre-fetching)
func (pr *partitionReader) serveFetchRequest(ctx context.Context, req *partitionFetchRequest) {
startTime := time.Now()
result := &partitionFetchResult{}
defer func() {
result.fetchDuration = time.Since(startTime)
select {
case req.resultChan <- result:
case <-ctx.Done():
case <-time.After(50 * time.Millisecond):
glog.Warningf("[%s] Timeout sending result for %s[%d]",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID)
}
}()
// Get high water mark
hwm, hwmErr := pr.handler.seaweedMQHandler.GetLatestOffset(pr.topicName, pr.partitionID)
if hwmErr != nil {
glog.Warningf("[%s] Failed to get high water mark for %s[%d]: %v",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID, hwmErr)
result.recordBatch = []byte{}
return
}
result.highWaterMark = hwm
// CRITICAL: If requested offset >= HWM, return immediately with empty result
// This prevents overwhelming the broker with futile read attempts when no data is available
if req.requestedOffset >= hwm {
result.recordBatch = []byte{}
glog.V(3).Infof("[%s] No data available for %s[%d]: offset=%d >= hwm=%d",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID, req.requestedOffset, hwm)
return
}
// Update tracking offset to match requested offset
pr.bufferMu.Lock()
if req.requestedOffset != pr.currentOffset {
glog.V(2).Infof("[%s] Offset seek for %s[%d]: requested=%d current=%d",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID, req.requestedOffset, pr.currentOffset)
pr.currentOffset = req.requestedOffset
}
pr.bufferMu.Unlock()
// Fetch on-demand - no pre-fetching to avoid overwhelming the broker
// Pass the requested offset and maxWaitMs directly to avoid race conditions
recordBatch, newOffset := pr.readRecords(ctx, req.requestedOffset, req.maxBytes, req.maxWaitMs, hwm)
if len(recordBatch) > 0 && newOffset > pr.currentOffset {
result.recordBatch = recordBatch
pr.bufferMu.Lock()
pr.currentOffset = newOffset
pr.bufferMu.Unlock()
glog.V(2).Infof("[%s] On-demand fetch for %s[%d]: offset %d->%d, %d bytes",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID,
req.requestedOffset, newOffset, len(recordBatch))
} else {
result.recordBatch = []byte{}
}
}
// readRecords reads records forward using the multi-batch fetcher
func (pr *partitionReader) readRecords(ctx context.Context, fromOffset int64, maxBytes int32, maxWaitMs int32, highWaterMark int64) ([]byte, int64) {
// Create context with timeout based on Kafka fetch request's MaxWaitTime
// This ensures we wait exactly as long as the client requested
fetchCtx := ctx
if maxWaitMs > 0 {
var cancel context.CancelFunc
fetchCtx, cancel = context.WithTimeout(ctx, time.Duration(maxWaitMs)*time.Millisecond)
defer cancel()
}
// Use multi-batch fetcher for better MaxBytes compliance
multiFetcher := NewMultiBatchFetcher(pr.handler)
fetchResult, err := multiFetcher.FetchMultipleBatches(
fetchCtx,
pr.topicName,
pr.partitionID,
fromOffset,
highWaterMark,
maxBytes,
)
if err == nil && fetchResult.TotalSize > 0 {
glog.V(2).Infof("[%s] Multi-batch fetch for %s[%d]: %d batches, %d bytes, offset %d -> %d",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID,
fetchResult.BatchCount, fetchResult.TotalSize, fromOffset, fetchResult.NextOffset)
return fetchResult.RecordBatches, fetchResult.NextOffset
}
// Fallback to single batch (pass context to respect timeout)
smqRecords, err := pr.handler.seaweedMQHandler.GetStoredRecords(fetchCtx, pr.topicName, pr.partitionID, fromOffset, 10)
if err == nil && len(smqRecords) > 0 {
recordBatch := pr.handler.constructRecordBatchFromSMQ(pr.topicName, fromOffset, smqRecords)
nextOffset := fromOffset + int64(len(smqRecords))
glog.V(2).Infof("[%s] Single-batch fetch for %s[%d]: %d records, %d bytes, offset %d -> %d",
pr.connCtx.ConnectionID, pr.topicName, pr.partitionID,
len(smqRecords), len(recordBatch), fromOffset, nextOffset)
return recordBatch, nextOffset
}
// No records available
return []byte{}, fromOffset
}
// close signals the reader to shut down
func (pr *partitionReader) close() {
close(pr.closeChan)
}
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