/// <summary>
/// Runs the sample using the specified Event Hubs connection information.
/// </summary>
///
/// <param name="connectionString">The connection string for the Event Hubs namespace that the sample should target.</param>
/// <param name="eventHubName">The name of the Event Hub, sometimes known as its path, that she sample should run against.</param>
///
public async Task RunAsync(string connectionString,
string eventHubName)
{
// An Event Hub consumer is associated with a specific Event Hub partition and a consumer group. The consumer group is
// a label that identifies one or more consumers as a set. Often, consumer groups are named after the responsibility
// of the consumer in an application, such as "Telemetry" or "OrderProcessing". When an Event Hub is created, a default
// consumer group is created with it, called "$Default."
//
// Each consumer has a unique view of the events in the partition, meaning that events are available to all consumers
// and are not removed from the partition when a consumer reads them. This allows for different consumers to read and
// process events from the partition at different speeds and beginning with different events without interfering with
// one another.
//
// When events are published, they will continue to exist in the partition and be available for consuming until they
// reach an age where they are older than the retention period.
// (see: https://docs.microsoft.com/en-us/azure/event-hubs/event-hubs-faq#what-is-the-maximum-retention-period-for-events)
//
// Because events are not removed from the partition when consuming, a consumer must specify where in the partition it
// would like to begin reading events. For example, this may be starting from the very beginning of the stream, at an
// offset from the beginning, the next event available after a specific point in time, or at a specific event.
// We will start by creating a client using its default set of options.
await using (var client = new EventHubClient(connectionString, eventHubName))
{
// With our client, we can now inspect the partitions and find the identifier
// of the first.
string firstPartition = (await client.GetPartitionIdsAsync()).First();
// In this example, we will create our consumer for the first partition in the Event Hub, using the default consumer group
// that is created with an Event Hub. Our consumer will begin watching the partition at the very end, reading only new events
// that we will publish for it.
await using (EventHubConsumer consumer = client.CreateConsumer(EventHubConsumer.DefaultConsumerGroupName, firstPartition, EventPosition.Latest))
await using (EventHubProducer producer = client.CreateProducer(new EventHubProducerOptions {
PartitionId = firstPartition
}))
{
// Because our consumer is reading from the latest position, it won't see events that have previously
// been published. Before we can publish the events, we will need to ask the consumer to perform an operation,
// because it opens its connection only when it needs to. The first receive that we ask of it will not see
// any events, but will allow the consumer to start watching the partition.
//
// Because the maximum wait time is specified as zero, this call will return immediately and will not
// have consumed any events.
await consumer.ReceiveAsync(1, TimeSpan.Zero);
// Now that the consumer is watching the partition, let's publish the events that we would like to
// receive.
EventData[] eventsToPublish = new EventData[]
{
new EventData(Encoding.UTF8.GetBytes("Hello, Event Hubs!")),
new EventData(Encoding.UTF8.GetBytes("Goodbye, Event Hubs!"))
};
await producer.SendAsync(eventsToPublish);
Console.WriteLine("The event batch has been published.");
// Because publishing and receiving events is asynchronous, the events that we published may not be immediately
// available for our consumer to see. We will iterate over the available events in the partition, which should be
// just the events that we published.
//
// When a maximum wait time is specified, the iteration will ensure that it returns control after that time has elapsed,
// whether or not an event is available in the partition. If no event was available a null value will be emitted instead.
// This is intended to return control to the loop and avoid blocking for an indeterminate period of time to allow event
// processors to verify that the iterator is still consuming the partition and to make decisions on whether or not to continue
// if events are not arriving.
//
// For this example, we will specify a maximum wait time, and won't exit the loop until we've received at least one more
// event than we published, which is expected to be a null value triggered by exceeding the wait time.
//
// To be sure that we do not end up in an infinite loop, we will specify a fairly long time to allow processing to complete
// then cancel.
CancellationTokenSource cancellationSource = new CancellationTokenSource();
cancellationSource.CancelAfter(TimeSpan.FromSeconds(30));
TimeSpan maximumWaitTime = TimeSpan.FromMilliseconds(250);
List <EventData> receivedEvents = new List <EventData>();
Stopwatch watch = Stopwatch.StartNew();
await foreach (EventData currentEvent in consumer.SubscribeToEvents(maximumWaitTime, cancellationSource.Token))
{
receivedEvents.Add(currentEvent);
if (receivedEvents.Count > eventsToPublish.Length)
{
watch.Stop();
break;
}
}
// Print out the events that we received.
Console.WriteLine();
Console.WriteLine($"The following events were consumed in { watch.ElapsedMilliseconds } milliseconds:");
foreach (EventData eventData in receivedEvents)
{
// The body of our event was an encoded string; we'll recover the
// message by reversing the encoding process.
string message = (eventData == null) ? "<< This was a null event >>" : Encoding.UTF8.GetString(eventData.Body.ToArray());
Console.WriteLine($"\tMessage: \"{ message }\"");
}
}
}
// At this point, our client, consumer, and producer have passed their "using" scope and have safely been disposed of. We
// have no further obligations.
Console.WriteLine();
}
/// <summary>
/// Runs the sample using the specified Event Hubs connection information.
/// </summary>
///
/// <param name="connectionString">The connection string for the Event Hubs namespace that the sample should target.</param>
/// <param name="eventHubName">The name of the Event Hub, sometimes known as its path, that she sample should run against.</param>
///
public async Task RunAsync(string connectionString,
string eventHubName)
{
// We will start by creating a client using its default set of options.
await using (var client = new EventHubClient(connectionString, eventHubName))
{
// With our client, we can now inspect the partitions and find the identifier of the first.
string firstPartition = (await client.GetPartitionIdsAsync()).First();
// In this example, we will make use of multiple consumers for the first partition in the Event Hub, using the default consumer group
// that is created with an Event Hub. Our initial consumer will begin watching the partition at the very end, reading only new events
// that we will publish for it.
await using (EventHubProducer producer = client.CreateProducer(new EventHubProducerOptions {
PartitionId = firstPartition
}))
{
// Each event that the initial consumer reads will have attributes set that describe the event's place in the
// partition, such as it's offset, sequence number, and the date/time that it was enqueued. These attributes can be
// used to create a new consumer that begins consuming at a known position.
//
// With Event Hubs, it is the responsibility of an application consuming events to keep track of those that it has processed,
// and to manage where in the partition the consumer begins reading events. This is done by using the position information to track
// state, commonly known as "creating a checkpoint."
//
// The goal is to preserve the position of an event in some form of durable state, such as writing it to a database, so that if the
// consuming application crashes or is otherwise restarted, it can retrieve that checkpoint information and use it to create a consumer that
// begins reading at the position where it left off.
//
// It is important to note that there is potential for a consumer to process an event and be unable to preserve the checkpoint. A well-designed
// consumer must be able to deal with processing the same event multiple times without it causing data corruption or otherwise creating issues.
// Event Hubs, like most event streaming systems, guarantees "at least once" delivery; even in cases where the consumer does not experience a restart,
// there is a small possibility that the service will return an event multiple times.
//
// In this example, we will publish a batch of events to be received with an initial consumer. The third event that is consumed will be captured
// and another consumer will use it's attributes to start reading the event that follows, consuming the same set of events that our initial consumer
// read, skipping over the first three.
int eventBatchSize = 50;
EventData thirdEvent;
await using (EventHubConsumer initialConsumer = client.CreateConsumer(EventHubConsumer.DefaultConsumerGroupName, firstPartition, EventPosition.Latest))
{
// The first receive that we ask of it will not see any events, but allows the consumer to start watching the partition. Because
// the maximum wait time is specified as zero, this call will return immediately and will not have consumed any events.
await initialConsumer.ReceiveAsync(1, TimeSpan.Zero);
// Now that the consumer is watching the partition, let's publish a batch of events.
EventData[] eventBatch = new EventData[eventBatchSize];
for (int index = 0; index < eventBatchSize; ++index)
{
eventBatch[index] = new EventData(Encoding.UTF8.GetBytes($"I am event #{ index }"));
}
await producer.SendAsync(eventBatch);
Console.WriteLine($"The event batch with { eventBatchSize } events has been published.");
// We will consume the events until all of the published events have been received.
var receivedEvents = new List <EventData>();
CancellationTokenSource cancellationSource = new CancellationTokenSource();
cancellationSource.CancelAfter(TimeSpan.FromSeconds(30));
await foreach (EventData currentEvent in initialConsumer.SubscribeToEvents(cancellationSource.Token))
{
receivedEvents.Add(currentEvent);
if (receivedEvents.Count >= eventBatchSize)
{
break;
}
}
// Print out the events that we received.
Console.WriteLine();
Console.WriteLine($"The initial consumer processed { receivedEvents.Count } events of the { eventBatchSize } that were published. { eventBatchSize } were expected.");
foreach (EventData eventData in receivedEvents)
{
// The body of our event was an encoded string; we'll recover the
// message by reversing the encoding process.
string message = Encoding.UTF8.GetString(eventData.Body.ToArray());
Console.WriteLine($"\tMessage: \"{ message }\"");
}
// Remember the third event that was consumed.
thirdEvent = receivedEvents[2];
}
// At this point, our initial consumer has passed its "using" scope and has been safely disposed of.
//
// Create a new consumer beginning using the third event as the last sequence number processed; this new consumer will begin reading at the next available
// sequence number, allowing it to read the set of published events beginning with the fourth one.
await using (EventHubConsumer newConsumer = client.CreateConsumer(EventHubConsumer.DefaultConsumerGroupName, firstPartition, EventPosition.FromSequenceNumber(thirdEvent.SequenceNumber.Value)))
{
// We will consume the events using the new consumer until all of the published events have been received.
CancellationTokenSource cancellationSource = new CancellationTokenSource();
cancellationSource.CancelAfter(TimeSpan.FromSeconds(30));
int expectedCount = (eventBatchSize - 3);
var receivedEvents = new List <EventData>();
await foreach (EventData currentEvent in newConsumer.SubscribeToEvents(cancellationSource.Token))
{
receivedEvents.Add(currentEvent);
if (receivedEvents.Count >= expectedCount)
{
break;
}
}
// Print out the events that we received.
Console.WriteLine();
Console.WriteLine();
Console.WriteLine($"The new consumer processed { receivedEvents.Count } events of the { eventBatchSize } that were published. { expectedCount } were expected.");
foreach (EventData eventData in receivedEvents)
{
// The body of our event was an encoded string; we'll recover the
// message by reversing the encoding process.
string message = Encoding.UTF8.GetString(eventData.Body.ToArray());
Console.WriteLine($"\tMessage: \"{ message }\"");
}
}
}
}
// At this point, our client, all consumers, and producer have passed their "using" scope and have safely been disposed of. We
// have no further obligations.
Console.WriteLine();
}