public async Task <IQueuedTask> PeekAsync()
{
IQueuedTask peekedTask = null;
DateTimeOffset refNow = mTimestampProvider.GetNow();
CheckNotDisposedOrThrow();
//This simply returns the latest item on top of the queue,
// without acquiring any lock
string peekSql = $@"SELECT q.*
FROM {mOptions.Mapping.QueueTableName} as q
WHERE q.task_locked_until_ts < @t_now
ORDER BY q.task_priority ASC,
q.task_locked_until_ts ASC,
q.task_lock_handle_id ASC
LIMIT 1" ;
using (NpgsqlConnection conn = await OpenConnectionAsync())
using (NpgsqlCommand peekCmd = new NpgsqlCommand(peekSql, conn))
{
peekCmd.Parameters.AddWithValue("t_now",
NpgsqlDbType.TimestampTz,
refNow);
await peekCmd.PrepareAsync();
using (NpgsqlDataReader taskReader = await peekCmd.ExecuteReaderAsync())
{
if (await taskReader.ReadAsync())
{
peekedTask = await taskReader.ReadQueuedTaskAsync();
}
await taskReader.CloseAsync();
}
await conn.CloseAsync();
}
return(peekedTask);
}
private DateTimeOffset GenerateImmediatePastLockedTimestamp()
{
return(mTimestampProvider
.GetNow()
.AddSeconds(-ImmediatePastSecondsInterval));
}
public async Task <IQueuedTaskToken> DequeueAsync(params string[] selectTaskTypes)
{
NpgsqlConnection conn = null;
QueuedTask dequeuedTask = null;
QueuedTaskResult dequeuedTaskResult = null;
PostgreSqlQueuedTaskToken dequeuedTaskToken = null;
MonotonicTimestamp startDequeue;
DateTimeOffset refNow = mTimestampProvider.GetNow();
CheckNotDisposedOrThrow();
try
{
mLogger.DebugFormat("Begin dequeue task. Looking for types: {0}.",
string.Join <string>(",", selectTaskTypes));
startDequeue = MonotonicTimestamp
.Now();
conn = await OpenQueueConnectionAsync();
if (conn == null)
{
return(null);
}
using (NpgsqlTransaction tx = conn.BeginTransaction(IsolationLevel.ReadCommitted))
{
//1. Dequeue means that we acquire lock on a task in the queue
// with the guarantee that nobody else did, and respecting
// the priority and static locks (basically the task_locked_until which says
// that it should not be pulled out of the queue until the
// current abstract time reaches that tick value)
dequeuedTask = await TryDequeueTaskAsync(selectTaskTypes, refNow, conn, tx);
if (dequeuedTask != null)
{
//2. Mark the task as being "Processing" and pull result info
// The result is stored separately and it's what allows us to remove
// the task from the queue at step #2,
// whils also tracking it's processing status and previous results
dequeuedTaskResult = await TryUpdateTaskResultAsync(dequeuedTask, conn, tx);
if (dequeuedTaskResult != null)
{
await tx.CommitAsync();
dequeuedTaskToken = new PostgreSqlQueuedTaskToken(dequeuedTask,
dequeuedTaskResult,
refNow);
}
}
if (dequeuedTaskToken != null)
{
IncrementDequeueCount(MonotonicTimestamp.Since(startDequeue));
}
else
{
await tx.RollbackAsync();
}
}
}
finally
{
if (conn != null)
{
await conn.CloseAsync();
conn.Dispose();
}
}
return(dequeuedTaskToken);
}
