/// <summary>Estimate the number of keys in the specified range.</summary>
/// <param name="db">Database used for the operation</param>
/// <param name="beginInclusive">Key defining the beginning of the range</param>
/// <param name="endExclusive">Key defining the end of the range</param>
/// <param name="onProgress">Optional callback called everytime the count is updated. The first argument is the current count, and the second argument is the last key that was found.</param>
/// <param name="cancellationToken">Token used to cancel the operation</param>
/// <returns>Number of keys k such that <paramref name="beginInclusive"/> <= k > <paramref name="endExclusive"/></returns>
/// <remarks>If the range contains a large of number keys, the operation may need more than one transaction to complete, meaning that the number will not be transactionally accurate.</remarks>
public static async Task <long> EstimateCountAsync([NotNull] IFdbDatabase db, Slice beginInclusive, Slice endExclusive, IProgress <FdbTuple <long, Slice> > onProgress, CancellationToken cancellationToken)
{
const int INIT_WINDOW_SIZE = 1 << 8; // start at 256 //1024
const int MAX_WINDOW_SIZE = 1 << 13; // never use more than 4096
const int MIN_WINDOW_SIZE = 64; // use range reads when the windows size is smaller than 64
if (db == null)
{
throw new ArgumentNullException("db");
}
if (endExclusive < beginInclusive)
{
throw new ArgumentException("The end key cannot be less than the begin key", "endExclusive");
}
cancellationToken.ThrowIfCancellationRequested();
// To count the number of items in the range, we will scan it using a key selector with an offset equal to our window size
// > if the returned key is still inside the range, we add the window size to the counter, and start again from the current key
// > if the returned key is outside the range, we reduce the size of the window, and start again from the previous key
// > if the returned key is exactly equal to the end of range, OR if the window size was 1, then we stop
// Since we don't know in advance if the range contains 1 key or 1 Billion keys, choosing a good value for the window size is critical:
// > if it is too small and the range is very large, we will need too many sequential reads and the network latency will quickly add up
// > if it is too large and the range is small, we will spend too many times halving the window size until we get the correct value
// A few optimizations are possible:
// > we could start with a small window size, and then double its size on every full segment (up to a maximum)
// > for the last segment, we don't need to wait for a GetKey to complete before issuing the next, so we could split the segment into 4 (or more), do the GetKeyAsync() in parallel, detect the quarter that cross the boundary, and iterate again until the size is small
// > once the window size is small enough, we can switch to using GetRange to read the last segment in one shot, instead of iterating with window size 16, 8, 4, 2 and 1 (the wost case being 2^N - 1 items remaning)
// note: we make a copy of the keys because the operation could take a long time and the key's could prevent a potentially large underlying buffer from being GCed
var cursor = beginInclusive.Memoize();
var end = endExclusive.Memoize();
using (var tr = db.BeginReadOnlyTransaction(cancellationToken))
{
#if TRACE_COUNTING
tr.Annotate("Estimating number of keys in range {0}", FdbKeyRange.Create(beginInclusive, endExclusive));
#endif
tr.SetOption(FdbTransactionOption.ReadYourWritesDisable);
// start looking for the first key in the range
cursor = await tr.Snapshot.GetKeyAsync(FdbKeySelector.FirstGreaterOrEqual(cursor)).ConfigureAwait(false);
if (cursor >= end)
{ // the range is empty !
return(0);
}
// we already have seen one key, so add it to the count
#if TRACE_COUNTING
int iter = 1;
#endif
long counter = 1;
// start with a medium-sized window
int windowSize = INIT_WINDOW_SIZE;
bool last = false;
while (cursor < end)
{
Contract.Assert(windowSize > 0);
var selector = FdbKeySelector.FirstGreaterOrEqual(cursor) + windowSize;
Slice next = Slice.Nil;
FdbException error = null;
try
{
next = await tr.Snapshot.GetKeyAsync(selector).ConfigureAwait(false);
#if TRACE_COUNTING
++iter;
#endif
}
catch (FdbException e)
{
error = e;
}
if (error != null)
{
// => from this point, the count returned will not be transactionally accurate
if (error.Code == FdbError.PastVersion)
{ // the transaction used up its time window
tr.Reset();
}
else
{ // check to see if we can continue...
await tr.OnErrorAsync(error.Code).ConfigureAwait(false);
}
// retry
tr.SetOption(FdbTransactionOption.ReadYourWritesDisable);
continue;
}
//BUGBUG: GetKey(...) always truncate the result to \xFF if the selected key would be past the end,
// so we need to fall back immediately to the binary search and/or get_range if next == \xFF
if (next > end)
{ // we have reached past the end, switch to binary search
last = true;
// if window size is already 1, then we have counted everything (the range.End key does not exist in the db)
if (windowSize == 1)
{
break;
}
if (windowSize <= MIN_WINDOW_SIZE)
{ // The window is small enough to switch to reading for counting (will be faster than binary search)
#if TRACE_COUNTING
tr.Annotate("Switch to reading all items (window size = {0})", windowSize);
#endif
// Count the keys by reading them. Also, we know that there can not be more than windowSize - 1 remaining
int n = await tr.Snapshot
.GetRange(
FdbKeySelector.FirstGreaterThan(cursor), // cursor has already been counted once
FdbKeySelector.FirstGreaterOrEqual(end),
new FdbRangeOptions()
{
Limit = windowSize - 1
}
)
.CountAsync()
.ConfigureAwait(false);
counter += n;
if (onProgress != null)
{
onProgress.Report(FdbTuple.Create(counter, end));
}
#if TRACE_COUNTING
++iter;
#endif
break;
}
windowSize >>= 1;
continue;
}
// the range is not finished, advance the cursor
counter += windowSize;
cursor = next;
if (onProgress != null)
{
onProgress.Report(FdbTuple.Create(counter, cursor));
}
if (!last)
{ // double the size of the window if we are not in the last segment
windowSize = Math.Min(windowSize << 1, MAX_WINDOW_SIZE);
}
}
#if TRACE_COUNTING
tr.Annotate("Found {0} keys in {1} iterations", counter, iter);
#endif
return(counter);
}
}
private static async Task <List <Slice> > GetBoundaryKeysInternalAsync([NotNull] IFdbReadOnlyTransaction trans, Slice begin, Slice end)
{
Contract.Requires(trans != null && end >= begin);
#if TRACE_COUNTING
trans.Annotate("Get boundary keys in range {0}", FdbKeyRange.Create(begin, end));
#endif
trans.WithReadAccessToSystemKeys();
var results = new List <Slice>();
int iterations = 0;
var options = new FdbRangeOptions {
Mode = FdbStreamingMode.WantAll
};
while (begin < end)
{
FdbException error = null;
Slice lastBegin = begin;
try
{
var chunk = await trans.Snapshot.GetRangeAsync(KeyServers + begin, KeyServers + end, options, iterations).ConfigureAwait(false);
++iterations;
if (chunk.Count > 0)
{
foreach (var kvp in chunk.Chunk)
{
results.Add(kvp.Key.Substring(KeyServers.Count));
}
begin = chunk.Last.Key.Substring(KeyServers.Count) + (byte)0;
}
if (!chunk.HasMore)
{
begin = end;
}
}
catch (FdbException e)
{
error = e;
}
if (error != null)
{
if (error.Code == FdbError.PastVersion && begin != lastBegin)
{ // if we get a PastVersion and *something* has happened, then we are no longer transactionnal
trans.Reset();
}
else
{
await trans.OnErrorAsync(error.Code).ConfigureAwait(false);
}
iterations = 0;
trans.WithReadAccessToSystemKeys();
}
}
#if TRACE_COUNTING
if (results.Count == 0)
{
trans.Annotate("There is no chunk boundary in range {0}", FdbKeyRange.Create(begin, end));
}
else
{
trans.Annotate("Found {0} boundaries in {1} iteration(s)", results.Count, iterations);
}
#endif
return(results);
}