// Send implements the client.Sender interface. It verifies
// permissions and looks up the appropriate range based on the
// supplied key and sends the RPC according to the specified options.
//
// If the request spans multiple ranges (which is possible for Scan or
// DeleteRange requests), Send sends requests to the individual ranges
// sequentially and combines the results transparently.
//
// This may temporarily adjust the request headers, so the proto.Call
// must not be used concurrently until Send has returned.
func (ds *DistSender) Send(_ context.Context, call proto.Call) {
args := call.Args
finalReply := call.Reply
// Verify permissions.
if err := ds.verifyPermissions(call.Args); err != nil {
call.Reply.Header().SetGoError(err)
return
}
// In the event that timestamp isn't set and read consistency isn't
// required, set the timestamp using the local clock.
if args.Header().ReadConsistency == proto.INCONSISTENT && args.Header().Timestamp.Equal(proto.ZeroTimestamp) {
// Make sure that after the call, args hasn't changed.
defer func(timestamp proto.Timestamp) {
args.Header().Timestamp = timestamp
}(args.Header().Timestamp)
args.Header().Timestamp = ds.clock.Now()
}
// If this is a bounded request, we will change its bound as we receive
// replies. This undoes that when we return.
boundedArgs, argsBounded := args.(proto.Bounded)
if argsBounded {
defer func(bound int64) {
boundedArgs.SetBound(bound)
}(boundedArgs.GetBound())
}
defer func(key proto.Key) {
args.Header().Key = key
}(args.Header().Key)
// Retry logic for lookup of range by key and RPCs to range replicas.
curReply := finalReply
for {
call.Reply = curReply
curReply.Header().Reset()
var desc, descNext *proto.RangeDescriptor
var err error
for r := retry.Start(ds.rpcRetryOptions); r.Next(); {
// Get range descriptor (or, when spanning range, descriptors).
// sendAttempt below may clear them on certain errors, so we
// refresh (likely from the cache) on every retry.
desc, descNext, err = ds.getDescriptors(call)
// getDescriptors may fail retryably if the first range isn't
// available via Gossip.
if err != nil {
if rErr, ok := err.(util.Retryable); ok && rErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
continue
}
break
}
err = func() error {
// Truncate the request to our current range, making sure not to
// touch it unless we have to (it is illegal to send EndKey on
// commands which do not operate on ranges).
if descNext != nil {
defer func(endKey proto.Key) {
args.Header().EndKey = endKey
}(args.Header().EndKey)
args.Header().EndKey = desc.EndKey
}
leader := ds.leaderCache.Lookup(proto.RaftID(desc.RaftID))
// Try to send the call.
replicas := newReplicaSlice(ds.gossip, desc)
// Rearrange the replicas so that those replicas with long common
// prefix of attributes end up first. If there's no prefix, this is a
// no-op.
order := ds.optimizeReplicaOrder(replicas)
// If this request needs to go to a leader and we know who that is, move
// it to the front.
if !(proto.IsRead(args) && args.Header().ReadConsistency == proto.INCONSISTENT) &&
leader.StoreID > 0 {
if i := replicas.FindReplica(leader.StoreID); i >= 0 {
replicas.MoveToFront(i)
order = rpc.OrderStable
}
}
return ds.sendRPC(desc.RaftID, replicas, order, args, curReply)
}()
if err != nil {
// For an RPC error to occur, we must've been unable to contact any
// replicas. In this case, likely all nodes are down (or not getting back
// to us within a reasonable amount of time).
// We may simply not be trying to talk to the up-to-date replicas, so
// clearing the descriptor here should be a good idea.
// TODO(tschottdorf): If a replica group goes dead, this will cause clients
// to put high read pressure on the first range, so there should be some
// rate limiting here.
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
} else {
err = curReply.Header().GoError()
}
if err != nil {
if log.V(1) {
log.Warningf("failed to invoke %s: %s", call.Method(), err)
}
// If retryable, allow retry. For range not found or range
// key mismatch errors, we don't backoff on the retry,
// but reset the backoff loop so we can retry immediately.
switch tErr := err.(type) {
case *proto.RangeNotFoundError, *proto.RangeKeyMismatchError:
// Range descriptor might be out of date - evict it.
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
// On addressing errors, don't backoff; retry immediately.
r.Reset()
if log.V(1) {
log.Warning(err)
}
continue
case *proto.NotLeaderError:
newLeader := tErr.GetLeader()
// Verify that leader is a known replica according to the
// descriptor. If not, we've got a stale replica; evict cache.
// Next, cache the new leader.
if newLeader != nil {
if i, _ := desc.FindReplica(newLeader.StoreID); i == -1 {
if log.V(1) {
log.Infof("error indicates unknown leader %s, expunging descriptor %s", newLeader, desc)
}
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
}
} else {
newLeader = &proto.Replica{}
}
ds.updateLeaderCache(proto.RaftID(desc.RaftID), *newLeader)
if log.V(1) {
log.Warning(err)
}
r.Reset()
continue
case util.Retryable:
if tErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
continue
}
}
}
break
}
// Immediately return if querying a range failed non-retryably.
// For multi-range requests, we return the failing range's reply.
if err != nil {
call.Reply.Header().SetGoError(err)
return
}
if finalReply != curReply {
// This was the second or later call in a multi-range request.
// Combine the new response with the existing one.
if cFinalReply, ok := finalReply.(proto.Combinable); ok {
cFinalReply.Combine(curReply)
} else {
// This should never apply in practice, as we'll only end up here
// for range-spanning requests.
call.Reply.Header().SetGoError(util.Errorf("multi-range request with non-combinable response type"))
return
}
}
// If this request has a bound, such as MaxResults in
// ScanRequest, check whether enough rows have been retrieved.
if argsBounded {
if prevBound := boundedArgs.GetBound(); prevBound > 0 {
if cReply, ok := curReply.(proto.Countable); ok {
if nextBound := prevBound - cReply.Count(); nextBound > 0 {
// Update bound for the next round.
// We've deferred restoring the original bound earlier.
boundedArgs.SetBound(nextBound)
} else {
// Set flag to break the loop.
descNext = nil
}
}
}
}
// If this was the last range accessed by this call, exit loop.
if descNext == nil {
break
}
// In next iteration, query next range.
// It's important that we use the EndKey of the current descriptor
// as opposed to the StartKey of the next one: if the former is stale,
// it's possible that the next range has since merged the subsequent
// one, and unless both descriptors are stale, the next descriptor's
// StartKey would move us to the beginning of the current range,
// resulting in a duplicate scan.
args.Header().Key = desc.EndKey
// This is a multi-range request, make a new reply object for
// subsequent iterations of the loop.
curReply = args.CreateReply()
}
call.Reply = finalReply
}
// sendAttempt is invoked by Send and handles retry logic and cache eviction
// for a call sent to a single range. It returns a retry status, which is Break
// on success and either Break, Continue or Reset depending on error condition.
// This method is expected to be invoked from within a backoff / retry loop to
// retry the send repeatedly (e.g. to continue processing after a critical node
// becomes available after downtime or the range descriptor is refreshed via
// lookup).
func (ds *DistSender) sendAttempt(desc *proto.RangeDescriptor, call proto.Call) (retry.Status, error) {
leader := ds.leaderCache.Lookup(proto.RaftID(desc.RaftID))
// Try to send the call.
replicas := newReplicaSlice(ds.gossip, desc)
// Rearrange the replicas so that those replicas with long common
// prefix of attributes end up first. If there's no prefix, this is a
// no-op.
order := ds.optimizeReplicaOrder(replicas)
args := call.Args
reply := call.Reply
// If this request needs to go to a leader and we know who that is, move
// it to the front.
if !(proto.IsRead(args) && args.Header().ReadConsistency == proto.INCONSISTENT) &&
leader.StoreID > 0 {
if i := replicas.FindReplica(leader.StoreID); i >= 0 {
replicas.MoveToFront(i)
order = rpc.OrderStable
}
}
err := ds.sendRPC(desc.RaftID, replicas, order, args, reply)
if err != nil {
// For an RPC error to occur, we must've been unable to contact any
// replicas. In this case, likely all nodes are down (or not getting back
// to us within a reasonable amount of time).
// We may simply not be trying to talk to the up-to-date replicas, so
// clearing the descriptor here should be a good idea.
// TODO(tschottdorf): If a replica group goes dead, this will cause clients
// to put high read pressure on the first range, so there should be some
// rate limiting here.
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
} else {
err = reply.Header().GoError()
}
if err != nil {
if log.V(1) {
log.Warningf("failed to invoke %s: %s", call.Method(), err)
}
// If retryable, allow retry. For range not found or range
// key mismatch errors, we don't backoff on the retry,
// but reset the backoff loop so we can retry immediately.
switch tErr := err.(type) {
case *proto.RangeNotFoundError, *proto.RangeKeyMismatchError:
// Range descriptor might be out of date - evict it.
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
// On addressing errors, don't backoff; retry immediately.
return retry.Reset, err
case *proto.NotLeaderError:
newLeader := tErr.GetLeader()
// Verify that leader is a known replica according to the
// descriptor. If not, we've got a stale replica; evict cache.
// Next, cache the new leader.
if newLeader != nil {
if i, _ := desc.FindReplica(newLeader.StoreID); i == -1 {
if log.V(1) {
log.Infof("error indicates unknown leader %s, expunging descriptor %s", newLeader, desc)
}
ds.rangeCache.EvictCachedRangeDescriptor(args.Header().Key, desc)
}
} else {
newLeader = &proto.Replica{}
}
ds.updateLeaderCache(proto.RaftID(desc.RaftID), *newLeader)
return retry.Reset, err
case util.Retryable:
if tErr.CanRetry() {
return retry.Continue, err
}
}
return retry.Break, err
}
return retry.Break, nil
}
// sendChunk is in charge of sending an "admissible" piece of batch, i.e. one
// which doesn't need to be subdivided further before going to a range (so no
// mixing of forward and reverse scans, etc).
func (ds *DistSender) sendChunk(ctx context.Context, ba proto.BatchRequest) (*proto.BatchResponse, error) {
// TODO(tschottdorf): prepare for removing Key and EndKey from BatchRequest,
// making sure that anything that relies on them goes bust.
ba.Key, ba.EndKey = nil, nil
isReverse := ba.IsReverse()
trace := tracer.FromCtx(ctx)
// The minimal key range encompassing all requests contained within.
// Local addressing has already been resolved.
// TODO(tschottdorf): consider rudimentary validation of the batch here
// (for example, non-range requests with EndKey, or empty key ranges).
from, to := keys.Range(ba)
var br *proto.BatchResponse
// Send the request to one range per iteration.
for {
options := lookupOptions{
useReverseScan: isReverse,
}
var curReply *proto.BatchResponse
var desc *proto.RangeDescriptor
var needAnother bool
var err error
for r := retry.Start(ds.rpcRetryOptions); r.Next(); {
// Get range descriptor (or, when spanning range, descriptors). Our
// error handling below may clear them on certain errors, so we
// refresh (likely from the cache) on every retry.
descDone := trace.Epoch("meta descriptor lookup")
var evictDesc func()
desc, needAnother, evictDesc, err = ds.getDescriptors(from, to, options)
descDone()
// getDescriptors may fail retryably if the first range isn't
// available via Gossip.
if err != nil {
if rErr, ok := err.(retry.Retryable); ok && rErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
continue
}
break
}
// If there's no transaction and op spans ranges, possibly
// re-run as part of a transaction for consistency. The
// case where we don't need to re-run is if the read
// consistency is not required.
if needAnother && ba.Txn == nil && ba.IsRange() &&
ba.ReadConsistency != proto.INCONSISTENT {
return nil, &proto.OpRequiresTxnError{}
}
// It's possible that the returned descriptor misses parts of the
// keys it's supposed to scan after it's truncated to match the
// descriptor. Example revscan [a,g), first desc lookup for "g"
// returns descriptor [c,d) -> [d,g) is never scanned.
// We evict and retry in such a case.
if (isReverse && !desc.ContainsKeyRange(desc.StartKey, to)) || (!isReverse && !desc.ContainsKeyRange(from, desc.EndKey)) {
evictDesc()
continue
}
curReply, err = func() (*proto.BatchResponse, error) {
// Truncate the request to our current key range.
untruncate, numActive, trErr := truncate(&ba, desc, from, to)
if numActive == 0 {
untruncate()
// This shouldn't happen in the wild, but some tests
// exercise it.
return nil, util.Errorf("truncation resulted in empty batch on [%s,%s): %s",
from, to, ba)
}
defer untruncate()
if trErr != nil {
return nil, trErr
}
// TODO(tschottdorf): make key range on batch redundant. The
// requests within dictate it anyways.
ba.Key, ba.EndKey = keys.Range(ba)
reply, err := ds.sendAttempt(trace, ba, desc)
ba.Key, ba.EndKey = nil, nil
if err != nil {
if log.V(0 /* TODO(tschottdorf): 1 */) {
log.Warningf("failed to invoke %s: %s", ba, err)
}
}
return reply, err
}()
// If sending succeeded, break this loop.
if err == nil {
break
}
// Error handling below.
// If retryable, allow retry. For range not found or range
// key mismatch errors, we don't backoff on the retry,
// but reset the backoff loop so we can retry immediately.
switch tErr := err.(type) {
case *rpc.SendError:
// For an RPC error to occur, we must've been unable to contact
// any replicas. In this case, likely all nodes are down (or
// not getting back to us within a reasonable amount of time).
// We may simply not be trying to talk to the up-to-date
// replicas, so clearing the descriptor here should be a good
// idea.
// TODO(tschottdorf): If a replica group goes dead, this
// will cause clients to put high read pressure on the first
// range, so there should be some rate limiting here.
evictDesc()
if tErr.CanRetry() {
continue
}
case *proto.RangeNotFoundError, *proto.RangeKeyMismatchError:
trace.Event(fmt.Sprintf("reply error: %T", err))
// Range descriptor might be out of date - evict it.
evictDesc()
// On addressing errors, don't backoff; retry immediately.
r.Reset()
if log.V(1) {
log.Warning(err)
}
// For the remainder of this call, we'll assume that intents
// are fair game. This replaces more complex logic based on
// the type of request.
options.considerIntents = true
continue
case *proto.NotLeaderError:
trace.Event(fmt.Sprintf("reply error: %T", err))
newLeader := tErr.GetLeader()
// Verify that leader is a known replica according to the
// descriptor. If not, we've got a stale replica; evict cache.
// Next, cache the new leader.
if newLeader != nil {
if i, _ := desc.FindReplica(newLeader.StoreID); i == -1 {
if log.V(1) {
log.Infof("error indicates unknown leader %s, expunging descriptor %s", newLeader, desc)
}
evictDesc()
}
} else {
newLeader = &proto.Replica{}
}
ds.updateLeaderCache(proto.RangeID(desc.RangeID), *newLeader)
if log.V(1) {
log.Warning(err)
}
r.Reset()
continue
case retry.Retryable:
if tErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
trace.Event(fmt.Sprintf("reply error: %T", err))
continue
}
}
break
}
// Immediately return if querying a range failed non-retryably.
if err != nil {
return nil, err
}
first := br == nil
if first {
// First response from a Range.
br = curReply
} else {
// This was the second or later call in a cross-Range request.
// Combine the new response with the existing one.
if err := br.Combine(curReply); err != nil {
panic(err)
// TODO(tschottdorf): return nil, err
}
}
// If this request has a bound (such as MaxResults in
// ScanRequest) and we are going to query at least one more range,
// check whether enough rows have been retrieved.
// TODO(tschottdorf): need tests for executing a multi-range batch
// with various bounded requests which saturate at different times.
if needAnother {
// Start with the assumption that all requests are saturated.
// Below, we look at each and decide whether that's true.
// Everything that is indeed saturated is "masked out" from the
// batch request; only if that's all requests does needAnother
// remain false.
needAnother = false
if first {
// Clone ba.Requests. This is because we're multi-range, and
// some requests may be bounded, which could lead to them being
// masked out once they're saturated. We don't want to risk
// removing requests that way in the "master copy" since that
// could lead to omitting requests in certain retry scenarios.
ba.Requests = append([]proto.RequestUnion(nil), ba.Requests...)
}
for i, union := range ba.Requests {
args := union.GetValue()
if _, ok := args.(*proto.NoopRequest); ok {
// NoopRequests are skipped.
continue
}
boundedArg, ok := args.(proto.Bounded)
if !ok {
// Non-bounded request. We will have to query all ranges.
needAnother = true
continue
}
prevBound := boundedArg.GetBound()
cReply, ok := curReply.Responses[i].GetValue().(proto.Countable)
if !ok || prevBound <= 0 {
// Request bounded, but without max results. Again, will
// need to query everything we can. The case in which the reply
// isn't countable occurs when the request wasn't active for
// that range (since it didn't apply to it), so the response
// is a NoopResponse.
needAnother = true
continue
}
nextBound := prevBound - cReply.Count()
if nextBound <= 0 {
// We've hit max results for this piece of the batch. Mask
// it out (we've copied the requests slice above, so this
// is kosher).
ba.Requests[i].Reset() // necessary (no one-of?)
if !ba.Requests[i].SetValue(&proto.NoopRequest{}) {
panic("RequestUnion excludes NoopRequest")
}
continue
}
// The request isn't saturated yet.
needAnother = true
boundedArg.SetBound(nextBound)
}
}
// If this was the last range accessed by this call, exit loop.
if !needAnother {
return br, nil
}
if isReverse {
// In next iteration, query previous range.
// We use the StartKey of the current descriptor as opposed to the
// EndKey of the previous one since that doesn't have bugs when
// stale descriptors come into play.
to = prev(ba, desc.StartKey)
} else {
// In next iteration, query next range.
// It's important that we use the EndKey of the current descriptor
// as opposed to the StartKey of the next one: if the former is stale,
// it's possible that the next range has since merged the subsequent
// one, and unless both descriptors are stale, the next descriptor's
// StartKey would move us to the beginning of the current range,
// resulting in a duplicate scan.
from = next(ba, desc.EndKey)
}
trace.Event("querying next range")
}
}
// Send implements the client.Sender interface. It verifies
// permissions and looks up the appropriate range based on the
// supplied key and sends the RPC according to the specified options.
//
// If the request spans multiple ranges (which is possible for Scan or
// DeleteRange requests), Send sends requests to the individual ranges
// sequentially and combines the results transparently.
//
// This may temporarily adjust the request headers, so the proto.Call
// must not be used concurrently until Send has returned.
func (ds *DistSender) Send(ctx context.Context, call proto.Call) {
args := call.Args
// Verify permissions.
if err := ds.verifyPermissions(call.Args); err != nil {
call.Reply.Header().SetGoError(err)
return
}
trace := tracer.FromCtx(ctx)
// In the event that timestamp isn't set and read consistency isn't
// required, set the timestamp using the local clock.
if args.Header().ReadConsistency == proto.INCONSISTENT && args.Header().Timestamp.Equal(proto.ZeroTimestamp) {
// Make sure that after the call, args hasn't changed.
defer func(timestamp proto.Timestamp) {
args.Header().Timestamp = timestamp
}(args.Header().Timestamp)
args.Header().Timestamp = ds.clock.Now()
}
// If this is a bounded request, we will change its bound as we receive
// replies. This undoes that when we return.
boundedArgs, argsBounded := args.(proto.Bounded)
if argsBounded {
defer func(bound int64) {
boundedArgs.SetBound(bound)
}(boundedArgs.GetBound())
}
_, isReverseScan := call.Args.(*proto.ReverseScanRequest)
// Restore to the original range if the scan/reverse_scan crosses range boundaries.
if isReverseScan {
defer func(key proto.Key) {
args.Header().EndKey = key
}(args.Header().EndKey)
} else {
defer func(key proto.Key) {
args.Header().Key = key
}(args.Header().Key)
}
first := true
// Retry logic for lookup of range by key and RPCs to range replicas.
for {
var curReply proto.Response
var desc, descNext *proto.RangeDescriptor
var err error
for r := retry.Start(ds.rpcRetryOptions); r.Next(); {
// Get range descriptor (or, when spanning range, descriptors). Our
// error handling below may clear them on certain errors, so we
// refresh (likely from the cache) on every retry.
descDone := trace.Epoch("meta descriptor lookup")
// It is safe to pass call here (with its embedded reply) because
// the reply is only used to check that it implements
// proto.Combinable if the request spans multiple ranges.
desc, descNext, err = ds.getDescriptors(call)
descDone()
// getDescriptors may fail retryably if the first range isn't
// available via Gossip.
if err != nil {
if rErr, ok := err.(retry.Retryable); ok && rErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
continue
}
break
}
// At this point reply.Header().Error may be non-nil!
curReply, err = ds.sendAttempt(trace, args, desc)
descKey := args.Header().Key
if isReverseScan {
descKey = args.Header().EndKey
}
if err != nil {
trace.Event(fmt.Sprintf("send error: %T", err))
// For an RPC error to occur, we must've been unable to contact any
// replicas. In this case, likely all nodes are down (or not getting back
// to us within a reasonable amount of time).
// We may simply not be trying to talk to the up-to-date replicas, so
// clearing the descriptor here should be a good idea.
// TODO(tschottdorf): If a replica group goes dead, this will cause clients
// to put high read pressure on the first range, so there should be some
// rate limiting here.
ds.rangeCache.EvictCachedRangeDescriptor(descKey, desc, isReverseScan)
} else {
err = curReply.Header().GoError()
}
if err == nil {
break
}
if log.V(1) {
log.Warningf("failed to invoke %s: %s", call.Method(), err)
}
// If retryable, allow retry. For range not found or range
// key mismatch errors, we don't backoff on the retry,
// but reset the backoff loop so we can retry immediately.
switch tErr := err.(type) {
case *proto.RangeNotFoundError, *proto.RangeKeyMismatchError:
trace.Event(fmt.Sprintf("reply error: %T", err))
// Range descriptor might be out of date - evict it.
ds.rangeCache.EvictCachedRangeDescriptor(descKey, desc, isReverseScan)
// On addressing errors, don't backoff; retry immediately.
r.Reset()
if log.V(1) {
log.Warning(err)
}
continue
case *proto.NotLeaderError:
trace.Event(fmt.Sprintf("reply error: %T", err))
newLeader := tErr.GetLeader()
// Verify that leader is a known replica according to the
// descriptor. If not, we've got a stale replica; evict cache.
// Next, cache the new leader.
if newLeader != nil {
if i, _ := desc.FindReplica(newLeader.StoreID); i == -1 {
if log.V(1) {
log.Infof("error indicates unknown leader %s, expunging descriptor %s", newLeader, desc)
}
ds.rangeCache.EvictCachedRangeDescriptor(descKey, desc, isReverseScan)
}
} else {
newLeader = &proto.Replica{}
}
ds.updateLeaderCache(proto.RangeID(desc.RangeID), *newLeader)
if log.V(1) {
log.Warning(err)
}
r.Reset()
continue
case retry.Retryable:
if tErr.CanRetry() {
if log.V(1) {
log.Warning(err)
}
trace.Event(fmt.Sprintf("reply error: %T", err))
continue
}
}
break
}
// Immediately return if querying a range failed non-retryably.
// For multi-range requests, we return the failing range's reply.
if err != nil {
call.Reply.Header().SetGoError(err)
return
}
if first {
// Equivalent of `*call.Reply = curReply`. Generics!
dst := reflect.ValueOf(call.Reply).Elem()
dst.Set(reflect.ValueOf(curReply).Elem())
} else {
// This was the second or later call in a multi-range request.
// Combine the new response with the existing one.
if cReply, ok := call.Reply.(proto.Combinable); ok {
cReply.Combine(curReply)
} else {
// This should never apply in practice, as we'll only end up here
// for range-spanning requests.
call.Reply.Header().SetGoError(util.Errorf("multi-range request with non-combinable response type"))
return
}
}
first = false
// If this request has a bound, such as MaxResults in
// ScanRequest, check whether enough rows have been retrieved.
if argsBounded {
if prevBound := boundedArgs.GetBound(); prevBound > 0 {
if cReply, ok := curReply.(proto.Countable); ok {
if nextBound := prevBound - cReply.Count(); nextBound > 0 {
// Update bound for the next round.
// We've deferred restoring the original bound earlier.
boundedArgs.SetBound(nextBound)
} else {
// Set flag to break the loop.
descNext = nil
}
}
}
}
// If this was the last range accessed by this call, exit loop.
if descNext == nil {
break
}
if isReverseScan {
// In next iteration, query previous range.
// We use the StartKey of the current descriptor as opposed to the
// EndKey of the previous one.
args.Header().EndKey = desc.StartKey
} else {
// In next iteration, query next range.
// It's important that we use the EndKey of the current descriptor
// as opposed to the StartKey of the next one: if the former is stale,
// it's possible that the next range has since merged the subsequent
// one, and unless both descriptors are stale, the next descriptor's
// StartKey would move us to the beginning of the current range,
// resulting in a duplicate scan.
args.Header().Key = desc.EndKey
}
trace.Event("querying next range")
}
}
