// InternalRangeLookup is used to look up RangeDescriptors - a RangeDescriptor
// is a metadata structure which describes the key range and replica locations
// of a distinct range in the cluster.
//
// RangeDescriptors are stored as values in the cockroach cluster's key-value
// store. However, they are always stored using special "Range Metadata keys",
// which are "ordinary" keys with a special prefix prepended. The Range Metadata
// Key for an ordinary key can be generated with the `keys.RangeMetaKey(key)`
// function. The RangeDescriptor for the range which contains a given key can be
// retrieved by generating its Range Metadata Key and dispatching it to
// InternalRangeLookup.
//
// Note that the Range Metadata Key sent to InternalRangeLookup is NOT the key
// at which the desired RangeDescriptor is stored. Instead, this method returns
// the RangeDescriptor stored at the _lowest_ existing key which is _greater_
// than the given key. The returned RangeDescriptor will thus contain the
// ordinary key which was originally used to generate the Range Metadata Key
// sent to InternalRangeLookup.
//
// The "Range Metadata Key" for a range is built by appending the end key of
// the range to the meta[12] prefix because the RocksDB iterator only supports
// a Seek() interface which acts as a Ceil(). Using the start key of the range
// would cause Seek() to find the key after the meta indexing record we're
// looking for, which would result in having to back the iterator up, an option
// which is both less efficient and not available in all cases.
//
// This method has an important optimization: instead of just returning the
// request RangeDescriptor, it also returns a slice of additional range
// descriptors immediately consecutive to the desired RangeDescriptor. This is
// intended to serve as a sort of caching pre-fetch, so that the requesting
// nodes can aggressively cache RangeDescriptors which are likely to be desired
// by their current workload.
func (r *Range) InternalRangeLookup(batch engine.Engine, args *proto.InternalRangeLookupRequest, reply *proto.InternalRangeLookupResponse) []proto.Intent {
if err := keys.ValidateRangeMetaKey(args.Key); err != nil {
reply.SetGoError(err)
return nil
}
rangeCount := int64(args.MaxRanges)
if rangeCount < 1 {
reply.SetGoError(util.Errorf(
"Range lookup specified invalid maximum range count %d: must be > 0", rangeCount))
return nil
}
if args.IgnoreIntents {
rangeCount = 1 // simplify lookup because we may have to retry to read new
}
// We want to search for the metadata key just greater than args.Key. Scan
// for both the requested key and the keys immediately afterwards, up to
// MaxRanges.
startKey, endKey := keys.MetaScanBounds(args.Key)
// Scan inconsistently. Any intents encountered are bundled up, but other-
// wise ignored.
kvs, intents, err := engine.MVCCScan(batch, startKey, endKey, rangeCount,
args.Timestamp, false /* !consistent */, args.Txn)
if err != nil {
// An error here would likely amount to something seriously going
// wrong.
reply.SetGoError(err)
return nil
}
if args.IgnoreIntents && len(intents) > 0 {
// NOTE (subtle): in general, we want to try to clean up dangling
// intents on meta records. However, if we're in the process of
// cleaning up a dangling intent on a meta record by pushing the
// transaction, we don't want to create an infinite loop:
//
// intent! -> push-txn -> range-lookup -> intent! -> etc...
//
// Instead we want:
//
// intent! -> push-txn -> range-lookup -> ignore intent, return old/new ranges
//
// On the range-lookup from a push transaction, we therefore
// want to suppress WriteIntentErrors and return a value
// anyway. But which value? We don't know whether the range
// update succeeded or failed, but if we don't return the
// correct range descriptor we may not be able to find the
// transaction to push. Since we cannot know the correct answer,
// we choose randomly between the pre- and post- transaction
// values. If we guess wrong, the client will try again and get
// the other value (within a few tries).
if rand.Intn(2) == 0 {
key, txn := intents[0].Key, &intents[0].Txn
val, _, err := engine.MVCCGet(batch, key, txn.Timestamp, true, txn)
if err != nil {
reply.SetGoError(err)
return nil
}
kvs = []proto.KeyValue{{Key: key, Value: *val}}
}
}
if len(kvs) == 0 {
// No matching results were returned from the scan. This could
// indicate a very bad system error, but for now we will just
// treat it as a retryable Key Mismatch error.
err := proto.NewRangeKeyMismatchError(args.Key, args.EndKey, r.Desc())
reply.SetGoError(err)
log.Errorf("InternalRangeLookup dispatched to correct range, but no matching RangeDescriptor was found. %s", err)
return nil
}
// Decode all scanned range descriptors, stopping if a range is encountered
// which does not have the same metadata prefix as the queried key.
rds := make([]proto.RangeDescriptor, len(kvs))
for i := range kvs {
// TODO(tschottdorf) Candidate for a ReplicaCorruptionError, once we
// introduce that.
if err = gogoproto.Unmarshal(kvs[i].Value.Bytes, &rds[i]); err != nil {
reply.SetGoError(err)
return nil
}
}
reply.Ranges = rds
return intents
}
// InternalRangeLookup is used to look up RangeDescriptors - a RangeDescriptor
// is a metadata structure which describes the key range and replica locations
// of a distinct range in the cluster.
//
// RangeDescriptors are stored as values in the cockroach cluster's key-value
// store. However, they are always stored using special "Range Metadata keys",
// which are "ordinary" keys with a special prefix appended. The Range Metadata
// Key for an ordinary key can be generated with the `engine.RangeMetaKey(key)`
// function. The RangeDescriptor for the range which contains a given key can be
// retrieved by generating its Range Metadata Key and dispatching it to
// InternalRangeLookup.
//
// Note that the Range Metadata Key sent to InternalRangeLookup is NOT the key
// at which the desired RangeDescriptor is stored. Instead, this method returns
// the RangeDescriptor stored at the _lowest_ existing key which is _greater_
// than the given key. The returned RangeDescriptor will thus contain the
// ordinary key which was originally used to generate the Range Metadata Key
// sent to InternalRangeLookup.
//
// This method has an important optimization: instead of just returning the
// request RangeDescriptor, it also returns a slice of additional range
// descriptors immediately consecutive to the desired RangeDescriptor. This is
// intended to serve as a sort of caching pre-fetch, so that the requesting
// nodes can aggressively cache RangeDescriptors which are likely to be desired
// by their current workload.
func (r *Range) InternalRangeLookup(args *proto.InternalRangeLookupRequest, reply *proto.InternalRangeLookupResponse) {
if err := engine.ValidateRangeMetaKey(args.Key); err != nil {
reply.SetGoError(err)
return
}
rangeCount := int64(args.MaxRanges)
if rangeCount < 1 {
reply.SetGoError(util.Errorf(
"Range lookup specified invalid maximum range count %d: must be > 0", rangeCount))
return
}
// We want to search for the metadata key just greater than args.Key. Scan
// for both the requested key and the keys immediately afterwards, up to
// MaxRanges.
metaPrefix := args.Key[:len(engine.KeyMeta1Prefix)]
nextKey := engine.NextKey(args.Key)
kvs, err := r.engine.Scan(nextKey, engine.PrefixEndKey(metaPrefix), rangeCount)
if err != nil {
reply.SetGoError(err)
return
}
// The initial key must have the same metadata level prefix as we queried.
if len(kvs) == 0 {
// At this point the range has been verified to contain the requested
// key, but no matching results were returned from the scan. This could
// indicate a very bad system error, but for now we will just treat it
// as a retryable Key Mismatch error.
err := proto.NewRangeKeyMismatchError(args.Key, args.Key, r.Meta)
reply.SetGoError(err)
log.Errorf("InternalRangeLookup dispatched to correct range, but no matching RangeDescriptor was found. %s", err)
return
}
// Decode all scanned range descriptors, stopping if a range is encountered
// which does not have the same metadata prefix as the queried key.
rds := make([]proto.RangeDescriptor, len(kvs))
for i := range kvs {
if err = gogoproto.Unmarshal(kvs[i].Value, &rds[i]); err != nil {
reply.SetGoError(err)
return
}
}
reply.Ranges = rds
return
}
