/// <summary>
/// Add tablet locations to the cache. Already known tablet locations will
/// have their entry updated and expiration extended.
/// </summary>
/// <param name="tablets">The discovered tablets to cache.</param>
/// <param name="requestPartitionKey">The lookup partition key.</param>
/// <param name="requestedBatchSize">
/// The number of tablet locations requested from the master in the
/// original request.
/// </param>
/// <param name="ttl">
/// The time in milliseconds that the tablets may be cached for.
/// </param>
public void CacheTabletLocations(
List <RemoteTablet> tablets,
ReadOnlySpan <byte> requestPartitionKey,
int requestedBatchSize,
long ttl)
{
long expiration = _systemClock.CurrentMilliseconds + ttl;
var newEntries = new List <TableLocationEntry>();
if (tablets.Count == 0)
{
// If there are no tablets in the response, then the table is empty. If
// there were any tablets in the table they would have been returned, since
// the master guarantees that if the partition key falls in a non-covered
// range, the previous tablet will be returned, and we did not set an upper
// bound partition key on the request.
newEntries.Add(TableLocationEntry.NewNonCoveredRange(
Array.Empty <byte>(),
Array.Empty <byte>(),
expiration));
}
else
{
// The comments below will reference the following diagram:
//
// +---+ +---+---+
// | | | | |
// A | B | C | D | E | F
// | | | | |
// +---+ +---+---+
//
// It depicts a tablet locations response from the master containing three
// tablets: B, D and E. Three non-covered ranges are present: A, C, and F.
// An RPC response containing B, D and E could occur if the lookup partition
// key falls in A, B, or C, although the existence of A as an initial
// non-covered range can only be inferred if the lookup partition key falls
// in A.
byte[] firstLowerBound = tablets[0].Partition.PartitionKeyStart;
if (requestPartitionKey.SequenceCompareTo(firstLowerBound) < 0)
{
// If the first tablet is past the requested partition key, then the
// partition key falls in an initial non-covered range, such as A.
newEntries.Add(TableLocationEntry.NewNonCoveredRange(
Array.Empty <byte>(), firstLowerBound, expiration));
}
// lastUpperBound tracks the upper bound of the previously processed
// entry, so that we can determine when we have found a non-covered range.
byte[] lastUpperBound = firstLowerBound;
foreach (var tablet in tablets)
{
byte[] tabletLowerBound = tablet.Partition.PartitionKeyStart;
byte[] tabletUpperBound = tablet.Partition.PartitionKeyEnd;
if (lastUpperBound.SequenceCompareTo(tabletLowerBound) < 0)
{
// There is a non-covered range between the previous tablet and this tablet.
// This will discover C while processing the tablet location for D.
newEntries.Add(TableLocationEntry.NewNonCoveredRange(
lastUpperBound, tabletLowerBound, expiration));
}
lastUpperBound = tabletUpperBound;
// Now add the tablet itself (such as B, D, or E).
newEntries.Add(TableLocationEntry.NewCoveredRange(tablet, expiration));
}
if (lastUpperBound.Length > 0 &&
tablets.Count < requestedBatchSize)
{
// There is a non-covered range between the last tablet and the end
// of the partition key space, such as F.
newEntries.Add(TableLocationEntry.NewNonCoveredRange(
lastUpperBound, Array.Empty <byte>(), expiration));
}
}
byte[] discoveredlowerBound = newEntries[0].LowerBoundPartitionKey;
byte[] discoveredUpperBound = newEntries[newEntries.Count - 1].UpperBoundPartitionKey;
_lock.EnterWriteLock();
try
{
// Remove all existing overlapping entries, and add the new entries.
TableLocationEntry?floorEntry = _cache.FloorEntry(discoveredlowerBound);
if (floorEntry is not null &&
requestPartitionKey.SequenceCompareTo(floorEntry.UpperBoundPartitionKey) < 0)
{
discoveredlowerBound = floorEntry.LowerBoundPartitionKey;
}
bool upperBoundActive = discoveredUpperBound.Length > 0;
_cache.ClearRange(discoveredlowerBound, discoveredUpperBound, upperBoundActive);
foreach (var entry in newEntries)
{
_cache.Insert(entry);
}
}
finally
{
_lock.ExitWriteLock();
}
}