/// <summary> Almost thread-safe version of <see cref="FetchAsync"/>. </summary>
/// <remarks>
/// You may call <see cref="TryGetNext"/> while the task is running, but NOT
/// any other method. The function returned by the task is not thread-safe.
///
/// This is intended for use in a "fetch in thread A, process in thread B"
/// pattern:
/// - call BackgroundFetchAsync, store into T
/// - repeatedy call TryGetNext until either T is done or no more events
/// - call the result of T, store into M
/// - if M is false or the last call to TryGetNext returned an event, repeat
/// </remarks>
public async Task <Func <bool> > BackgroundFetchAsync(CancellationToken cancel = default(CancellationToken))
{
const int maxBytes = 1024 * 1024 * 4;
var read = await Storage.ReadAsync(Position, maxBytes, cancel);
if (read.Events.Count == 0)
{
return(() => false);
}
return(() =>
{
Position = read.NextPosition;
if (read.Events.Count == 0)
{
return false;
}
foreach (var e in read.Events)
{
_cache.Enqueue(e);
_lastSequence = e.Sequence;
}
return true;
});
}
/// <see cref="IStorageDriver.ReadAsync"/>
public async Task <DriverReadResult> ReadAsync(long position, long maxBytes, CancellationToken cancel = new CancellationToken())
{
// Read data up to the requested position (if not already available)
var cachePos = _cache.GetPosition();
while (position >= cachePos)
{
var r = await _source.ReadAsync(cachePos, maxBytes, cancel);
var w = await _cache.WriteAsync(cachePos, r.Events, cancel);
if (r.NextPosition == cachePos)
{
break;
}
if (!w.Success)
{
throw new InvalidOperationException("Failed writing to cache stream.");
}
if (w.NextPosition != r.NextPosition)
{
throw new InvalidDataException(
$"Position mismatch after cache copy: {cachePos} -> ({r.NextPosition}, {w.NextPosition}.");
}
cachePos = w.NextPosition;
}
return(await _cache.ReadAsync(position, maxBytes, cancel));
}
public async Task <DriverReadResult> ReadAsync(long position, long maxBytes, CancellationToken cancel = new CancellationToken())
{
var sw = Stopwatch.StartNew();
try
{
return(await Inner.ReadAsync(position, maxBytes, cancel));
}
finally
{
Trace.WriteLine("ReadAsync " + sw.ElapsedMilliseconds);
}
}
public Task <DriverReadResult> ReadAsync(long position, long maxBytes, CancellationToken cancel = new CancellationToken()) => Wrapped.ReadAsync(position, maxBytes, cancel);