/// <summary>
/// Tries to claim ownership of the specified partition.
/// </summary>
///
/// <param name="partitionId">The identifier of the Event Hub partition the ownership is associated with.</param>
/// <param name="completeOwnershipEnumerable">A complete enumerable of ownership obtained from the stored service provided by the user.</param>
///
/// <returns>The claimed ownership. <c>null</c> if the claim attempt failed.</returns>
///
private async Task <PartitionOwnership> ClaimOwnershipAsync(string partitionId,
IEnumerable <PartitionOwnership> completeOwnershipEnumerable)
{
// We need the eTag from the most recent ownership of this partition, even if it's expired. We want to keep the offset and
// the sequence number as well.
var oldOwnership = completeOwnershipEnumerable.FirstOrDefault(ownership => ownership.PartitionId == partitionId);
var newOwnership = new PartitionOwnership
(
InnerClient.EventHubName,
ConsumerGroup,
Identifier,
partitionId,
oldOwnership?.Offset,
oldOwnership?.SequenceNumber,
DateTimeOffset.UtcNow,
oldOwnership?.ETag
);
// We are expecting an enumerable with a single element if the claim attempt succeeds.
var claimedOwnership = (await Manager
.ClaimOwnershipAsync(new List <PartitionOwnership> {
newOwnership
})
.ConfigureAwait(false));
return(claimedOwnership.FirstOrDefault());
}
/// <summary>
/// Performs load balancing between multiple <see cref="EventProcessor{T}" /> instances, claiming others' partitions to enforce
/// a more equal distribution when necessary. It also manages its own partition pumps and ownership.
/// </summary>
///
/// <param name="cancellationToken">A <see cref="CancellationToken"/> instance to signal the request to cancel the operation.</param>
///
/// <returns>A task to be resolved on when the operation has completed.</returns>
///
private async Task RunAsync(CancellationToken cancellationToken)
{
while (!cancellationToken.IsCancellationRequested)
{
Stopwatch cycleDuration = Stopwatch.StartNew();
// Renew this instance's ownership so they don't expire.
await RenewOwnershipAsync().ConfigureAwait(false);
// From the storage service provided by the user, obtain a complete list of ownership, including expired ones. We may still need
// their eTags to claim orphan partitions.
var completeOwnershipList = (await Manager
.ListOwnershipAsync(InnerClient.FullyQualifiedNamespace, InnerClient.EventHubName, ConsumerGroup)
.ConfigureAwait(false))
.ToList();
// Filter the complete ownership list to obtain only the ones that are still active. The expiration time defaults to 30 seconds,
// but it may be overriden by a derived class.
IEnumerable <PartitionOwnership> activeOwnership = completeOwnershipList
.Where(ownership => DateTimeOffset.UtcNow.Subtract(ownership.LastModifiedTime.Value) < OwnershipExpiration);
// Dispose of all previous partition ownership instances and get a whole new dictionary.
InstanceOwnership = activeOwnership
.Where(ownership => ownership.OwnerIdentifier == Identifier)
.ToDictionary(ownership => ownership.PartitionId);
// Some previously owned partitions might have had their ownership expired or might have been stolen, so we need to stop
// the pumps we don't need anymore.
await Task.WhenAll(PartitionPumps.Keys
.Except(InstanceOwnership.Keys)
.Select(partitionId => RemovePartitionPumpIfItExistsAsync(partitionId, PartitionProcessorCloseReason.OwnershipLost)))
.ConfigureAwait(false);
// Now that we are left with pumps that should be running, check their status. If any has stopped, it means an
// unexpected failure has happened, so try closing it and starting a new one. In case we don't have a pump that
// should exist, create it. This might happen when pump creation has failed in a previous cycle.
await Task.WhenAll(InstanceOwnership
.Where(kvp =>
{
if (PartitionPumps.TryGetValue(kvp.Key, out PartitionPump pump))
{
return(!pump.IsRunning);
}
return(true);
})
.Select(kvp => AddOrOverwritePartitionPumpAsync(kvp.Key, kvp.Value.SequenceNumber)))
.ConfigureAwait(false);
// Find an ownership to claim and try to claim it. The method will return null if this instance was not eligible to
// increase its ownership list, if no claimable ownership could be found or if a claim attempt failed.
PartitionOwnership claimedOwnership = await FindAndClaimOwnershipAsync(completeOwnershipList, activeOwnership).ConfigureAwait(false);
if (claimedOwnership != null)
{
InstanceOwnership[claimedOwnership.PartitionId] = claimedOwnership;
await AddOrOverwritePartitionPumpAsync(claimedOwnership.PartitionId, claimedOwnership.SequenceNumber).ConfigureAwait(false);
}
// Wait the remaining time, if any, to start the next cycle. The total time of a cycle defaults to 10 seconds,
// but it may be overriden by a derived class.
TimeSpan remainingTimeUntilNextCycle = LoadBalanceUpdate - cycleDuration.Elapsed;
if (remainingTimeUntilNextCycle > TimeSpan.Zero)
{
// If a stop request has been issued, Task.Delay will throw a TaskCanceledException. This is expected and it
// will be caught by the StopAsync method.
await Task.Delay(remainingTimeUntilNextCycle, cancellationToken).ConfigureAwait(false);
}
}
}
