Develop Reference

Azure Service bus - pass parameter to message handler

In Azure Service Bus I need to listen for messages arriving from multiple subscriptions from different services busses at once.
To do this I created a list that contains objects with a connection string, a topic, a subscription name and some other information (the list is called 'jobs').
For each item in this list I am then creating a different task that creates the ServiceBusClient and the processor.
var jobs = GetAllServiceBusTopics();
Parallel.ForEach(jobs, async job =>
{
var client = new ServiceBusClient(job.Environment.ServiceBusConnectionString);
var options = new ServiceBusProcessorOptions();
var processor = client.CreateProcessor(job.Environment.TopicName, _subscriptionName, new ServiceBusProcessorOptions());
try
{
processor.ProcessMessageAsync += MessageHandler;
//Pass the job object somehow to the "MessageHandler" below.
processor.ProcessErrorAsync += ErrorHandler;
await processor.StartProcessingAsync();
Console.WriteLine("Wait for a minute and then press any key to end the processing");
Console.ReadKey();
Console.WriteLine("\nStopping the receiver...");
await processor.StopProcessingAsync();
Console.WriteLine("Stopped receiving messages");
}
finally
{
await processor.DisposeAsync();
await client.DisposeAsync();
}
});
And the handler that is called if a new message arrives:
static async Task MessageHandler(ProcessMessageEventArgs args)
{
//I need the "job" object from my loop above here.
}
How the concept generally works I learned on this website of Microsoft.
My first question:
Is this approach okay, or am I running in the wrong direction? Can I do it like this?
But even if this is okay, I have another more important task:
I need to pass the "job" object from my loop somehow to the message handler - as a parameter.
But I have currently no idea how to archvie this. Any proposals on this?

Is this approach okay, or am I running in the wrong direction? Can I do it like this?
Yes, you can do this. One thing to keep in mind is that you instantiate multiple ServiceBusClient instances, each causing a new connection to be established rather than using the same connection. I don't know how big the number of topics (jobs) might be but if it's large, you'll end up with connections starvation.
I need to pass the "job" object from my loop somehow to the message handler - as a parameter. But I have currently no idea how to archvie this. Any proposals on this?
That's not how ServiceBusProcessor is designed. It doesn't receive anything other than the incoming message that needs to be processed. If you need to have a job ID, that should be part of the message payload/metadata. If you need to know the entity it arrived from, you could add a subscription filter action to add a custom header with the identifier. An alternative approach would require wrapping the ServiceBusProcessor to retain the job ID/subscription identifier and use that in the event handler.

Solace topic subscription with correlationId

I subscribe to topics like that:
var topics = new string[]{
"TOP/IC/*/A/>",
"TOP/IC/*/B/>",
"ANY/*/STRU/CTURE"
// etc...
};
foreach(var t in topics)
{
var topic = ContextFactory.Instance.CreateTopic(t);
var sessionReturnCode = _session.Subscribe(topic, false); // _session is ISession
}
Incomming message has full topic name information. E.g. TOP/IC/ANY/A/N123/XFDJK985.
I would like to know from which subscription the message comes. Is it possible? (correlationId, whatever...). If yes, sample please.

When Solace .NET API, by default, Direct messages are dispatched to the same receive callback or delegate that is configured for the Session. However, it is possible to use special receive callbacks or delegates to handle Direct messages that are published to specific Topics. This functionality is also known as topic dispatching because messages with a specific Topic are dispatched to a specific callback or delegate.
More information is available here: https://docs.solace.com/Solace-PubSub-Messaging-APIs/API-Developer-Guide/Receiving-Direct-Message.htm

How to do error handling with EasyNetQ / RabbitMQ

I'm using RabbitMQ in C# with the EasyNetQ library. I'm using a pub/sub pattern here. I still have a few issues that I hope anyone can help me with:
When there's an error while consuming a message, it's automatically moved to an error queue. How can I implement retries (so that it's placed back on the originating queue, and when it fails to process X times, it's moved to a dead letter queue)?
As far as I can see there's always 1 error queue that's used to dump messages from all other queues. How can I have 1 error queue per type, so that each queue has its own associated error queue?
How can I easily retry messages that are in an error queue? I tried Hosepipe, but it justs republishes the messages to the error queue instead of the originating queue. I don't really like this option either because I don't want to be fiddling around in a console. Preferably I'd just program against the error queue.
Anyone?

The problem you are running into with EasyNetQ/RabbitMQ is that it's much more "raw" when compared to other messaging services like SQS or Azure Service Bus/Queues, but I'll do my best to point you in the right direction.
Question 1.
This will be on you to do. The simplest way is that you can No-Ack a message in RabbitMQ/EasyNetQ, and it will be placed at the head of the queue for you to retry. This is not really advisable because it will be retried almost immediately (With no time delay), and will also block other messages from being processed (If you have a single subscriber with a prefetch count of 1).
I've seen other implementations of using a "MessageEnvelope". So a wrapper class that when a message fails, you increment a retry variable on the MessageEnvelope and redeliver the message back onto the queue. YOU would have to do this and write the wrapping code around your message handlers, it would not be a function of EasyNetQ.
Using the above, I've also seen people use envelopes, but allow the message to be dead lettered. Once it's on the dead letter queue, there is another application/worker reading items from the dead letter queue.
All of these approaches above have a small issue in that there isn't really any nice way to have a logarithmic/exponential/any sort of increasing delay in processing the message. You can "hold" the message in code for some time before returning it to the queue, but it's not a nice way around.
Out of all of these options, your own custom application reading the dead letter queue and deciding whether to reroute the message based on an envelope that contains the retry count is probably the best way.
Question 2.
You can specify a dead letter exchange per queue using the advanced API. (https://github.com/EasyNetQ/EasyNetQ/wiki/The-Advanced-API#declaring-queues). However this means you will have to use the advanced API pretty much everywhere as using the simple IBus implementation of subscribe/publish looks for queues that are named based on both the message type and subscriber name. Using a custom declare of queue means you are going to be handling the naming of your queues yourself, which means when you subscribe, you will need to know the name of what you want etc. No more auto subscribing for you!
Question 3
An Error Queue/Dead Letter Queue is just another queue. You can listen to this queue and do what you need to do with it. But there is not really any out of the box solution that sounds like it would fit your needs.

I've implemented exactly what you describe. Here are some tips based on my experience and related to each of your questions.
Q1 (how to retry X times):
For this, you can use IMessage.Body.BasicProperties.Headers. When you consume a message off an error queue, just add a header with a name that you choose. Look for this header on each message that comes into the error queue and increment it. This will give you a running retry count.
It's very important that you have a strategy for what to do when a message exceeds the retry limit of X. You don't want to lose that message. In my case, I write the message to disk at that point. It gives you lots of helpful debugging information to come back to later, because EasyNetQ automatically wraps your originating message with error info. It also has the original message so that you can, if you like, manually (or maybe automated, through some batch re-processing code) requeue the message later in some controlled way.
You can look at the code in the Hosepipe utility to see a good way of doing this. In fact, if you follow the pattern you see there then you can even use Hosepipe later to requeue the messages if you need to.
Q2 (how to create an error queue per originating queue):
You can use the EasyNetQ Advanced Bus to do this cleanly. Use IBus.Advanced.Container.Resolve<IConventions> to get at the conventions interface. Then you can set the conventions for the error queue naming with conventions.ErrorExchangeNamingConvention and conventions.ErrorQueueNamingConvention. In my case I set the convention to be based on the name of the originating queue so that I get a queue/queue_error pair of queues every time I create a queue.
Q3 (how to process messages in the error queues):
You can declare a consumer for the error queue the same way you do any other queue. Again, the AdvancedBus lets you do this cleanly by specifying that the type coming off of the queue is EasyNetQ.SystemMessage.Error. So, IAdvancedBus.Consume<EasyNetQ.SystemMessage.Error>() will get you there. Retrying simply means republishing to the original exchange (paying attention to the retry count you put in the header (see my answer to Q1, above), and information in the Error message that you consumed off the error queue can help you find the target for republishing.

I know this is an old post but - just in case it helps someone else - here is my self-answered question (I needed to ask it because existing help was not enough) that explains how I implemented retrying failed messages on their original queues. The following should answer your question #1 and #3. For #2, you may have to use the Advanced API, which I haven't used (and I think it defeats the purpose of EasyNetQ; one might as well use RabbitMQ client directly). Also consider implementing IConsumerErrorStrategy, though.
1) Since there can be multiple consumers of a message and all may not need to retry a msg, I have a Dictionary<consumerId, RetryInfo> in the body of the message, as EasyNetQ does not (out of the box) support complex types in message headers.
public interface IMessageType
{
int MsgTypeId { get; }
Dictionary<string, TryInfo> MsgTryInfo {get; set;}
}
2) I have implemented a class RetryEnabledErrorMessageSerializer : IErrorMessageSerializer that just updates the TryCount and other information every time it is called by the framework. I attach this custom serializer to the framework on a per-consumer basis via the IoC support provided by EasyNetQ.
public class RetryEnabledErrorMessageSerializer<T> : IErrorMessageSerializer where T : class, IMessageType
{
public string Serialize(byte[] messageBody)
{
string stringifiedMsgBody = Encoding.UTF8.GetString(messageBody);
var objectifiedMsgBody = JObject.Parse(stringifiedMsgBody);
// Add/update RetryInformation into objectifiedMsgBody here
// I have a dictionary that saves <key:consumerId, val: TryInfoObj>
return JsonConvert.SerializeObject(objectifiedMsgBody);
}
}
And in my EasyNetQ wrapper class:
public void SetupMessageBroker(string givenSubscriptionId, bool enableRetry = false)
{
if (enableRetry)
{
_defaultBus = RabbitHutch.CreateBus(currentConnString,
serviceRegister => serviceRegister.Register<IErrorMessageSerializer>(serviceProvider => new RetryEnabledErrorMessageSerializer<IMessageType>(givenSubscriptionId))
);
}
else // EasyNetQ's DefaultErrorMessageSerializer will wrap error messages
{
_defaultBus = RabbitHutch.CreateBus(currentConnString);
}
}
public bool SubscribeAsync<T>(Func<T, Task> eventHandler, string subscriptionId)
{
IMsgHandler<T> currMsgHandler = new MsgHandler<T>(eventHandler, subscriptionId);
// Using the msgHandler allows to add a mediator between EasyNetQ and the actual callback function
// The mediator can transmit the retried msg or choose to ignore it
return _defaultBus.SubscribeAsync<T>(subscriptionId, currMsgHandler.InvokeMsgCallbackFunc).Queue != null;
}
3) Once the message is added to the default error queue, you can have a simple console app/windows service that periodically republishes existing error messages on their original queues. Something like:
var client = new ManagementClient(AppConfig.BaseAddress, AppConfig.RabbitUsername, AppConfig.RabbitPassword);
var vhost = client.GetVhostAsync("/").Result;
var aliveRes = client.IsAliveAsync(vhost).Result;
var errQueue = client.GetQueueAsync(Constants.EasyNetQErrorQueueName, vhost).Result;
var crit = new GetMessagesCriteria(long.MaxValue, Ackmodes.ack_requeue_false);
var errMsgs = client.GetMessagesFromQueueAsync(errQueue, crit).Result;
foreach (var errMsg in errMsgs)
{
var innerMsg = JsonConvert.DeserializeObject<Error>(errMsg.Payload);
var pubInfo = new PublishInfo(innerMsg.RoutingKey, innerMsg.Message);
pubInfo.Properties.Add("type", innerMsg.BasicProperties.Type);
pubInfo.Properties.Add("correlation_id", innerMsg.BasicProperties.CorrelationId);
pubInfo.Properties.Add("delivery_mode", innerMsg.BasicProperties.DeliveryMode);
var pubRes = client.PublishAsync(client.GetExchangeAsync(innerMsg.Exchange, vhost).Result, pubInfo).Result;
}
4) I have a MessageHandler class that contains a callback func. Whenever a message is delivered to the consumer, it goes to the MessageHandler, which decides if the message try is valid and calls the actual callback if so. If try is not valid (maxRetriesExceeded/the consumer does not need to retry anyway), I ignore the message. You can choose to Dead Letter the message in this case.
public interface IMsgHandler<T> where T: class, IMessageType
{
Task InvokeMsgCallbackFunc(T msg);
Func<T, Task> MsgCallbackFunc { get; set; }
bool IsTryValid(T msg, string refSubscriptionId); // Calls callback only
// if Retry is valid
}
Here is the mediator function in MsgHandler that invokes the callback:
public async Task InvokeMsgCallbackFunc(T msg)
{
if (IsTryValid(msg, CurrSubscriptionId))
{
await this.MsgCallbackFunc(msg);
}
else
{
// Do whatever you want
}
}

Here, I have implemented a Nuget package (EasyDeadLetter) for this purpose, which can be easily implemented with the minimum changes in any project.
All you need to do is follow the four steps :
First of all, Decorate your class object with QeueuAttribute
[Queue(“Product.Report”, ExchangeName = “Product.Report”)]
public class ProductReport { }
The second step is to define your dead-letter queue with the same QueueAttribute and also inherit the dead-letter object from the Main object class.
[Queue(“Product.Report.DeadLetter”, ExchangeName =
“Product.Report.DeadLetter”)]
public class ProductReportDeadLetter : ProductReport { }
Now, it’s time to decorate your main queue object with the EasyDeadLetter attribute and set the type of dead-letter queue.
[EasyDeadLetter(DeadLetterType =
typeof(ProductReportDeadLetter))]
[Queue(“Product.Report”, ExchangeName = “Product.Report”)]
public class ProductReport { }
In the final step, you need to register EasyDeadLetterStrategy as the default error handler (IConsumerErrorStrategy).
services.AddSingleton<IBus>
(RabbitHutch.CreateBus(“connectionString”,
serviceRegister =>
{
serviceRegister.Register<IConsumerErrorStrategy,
EasyDeadLetterStrategy>();
}));
That’s all. from now on any failed message will be moved to the related dead-letter queue.
See more detail here :
GitHub Repository
NuGet Package

Cant consume message from Topic in activemq

I am new to activemq. T want to ask a question about the topics of Activemq. I succeed to get a message from a queue. Also I can send message to topic/Queue, but I can't get a message from Topic.
I have tried using Java Code. The result is the same.
The following is my core code:
connection.ClientId = clientId;
connection.Start();
using (ISession session = connection.CreateSession())
{
ITopic topic = new Apache.NMS.Commands.Topic(topicName);
IDestination destination = SessionUtil.GetDestination(session, topicName,
DestinationType.Topic);
using (IMessageConsumer consumer = **session.CreateDurableConsumer**(topic, "news", null, false))
{
**consumer.Listener += new MessageListener(consumer_Listener);**
//**IMessage iMsg = consumer.Receive();**
// if (iMsg != null)//{
// ITextMessage msg = (ITextMessage)iMsg;
// return msg.Text;
// }
//else
//return iMsg;
}
}
I also using: IMessage iMsg = consumer.Receive();
IMsg　always null（topicname has messages. How can I consume topic's message?

The Messages would need to have been sent after the Topic consumer was created. A Topic is fire and forget, if there are no consumers then the message is discarded. Any consumer that comes online will only receive message sent after that time unless it is either a Durable Topic consumer or a Queue consumer.
In the case of a durable consumer you must have created an instance of it so there is a subscription record before those message were sent to the Topic. So I would guess your problem is that you didn't subscribe this consumer before and so the Broker was not storing any Messages for it.

I was so stupid about the phrase "using".Beacause I use "using" open connection and session. when the code block was excuted, the connnection/session is disappear. Now I dont use "using" block to cerate connection. just like normal code. It works. also I build "Global.asax" file. The program can listener Topic once started up. At the same time, I write a function to colse the connection.I tested. Once a message was sent to the topic, the Onessage() function would be exectued.
just resolve my problem.maybe you would have better answer.Thanks Tim.

Bloomberg API request timing out

Having set up a ReferenceDataRequest I send it along to an EventQueue
Service refdata = _session.GetService("//blp/refdata");
Request request = refdata.CreateRequest("ReferenceDataRequest");
// append the appropriate symbol and field data to the request
EventQueue eventQueue = new EventQueue();
Guid guid = Guid.NewGuid();
CorrelationID id = new CorrelationID(guid);
_session.SendRequest(request, eventQueue, id);
long _eventWaitTimeout = 60000;
myEvent = eventQueue.NextEvent(_eventWaitTimeout);
Normally I can grab the message from the queue, but I'm hitting the situation now that if I'm making a number of requests in the same run of the app (normally around the tenth), I see a TIMEOUT EventType
if (myEvent.Type == Event.EventType.TIMEOUT)
throw new Exception("Timed Out - need to rethink this strategy");
else
msg = myEvent.GetMessages().First();
These are being made on the same thread, but I'm assuming that there's something somewhere along the line that I'm consuming and not releasing.
Anyone have any clues or advice?
There aren't many references on SO to BLP's API, but hopefully we can start to rectify that situation.

I just wanted to share something, thanks to the code you included in your initial post.
If you make a request for historical intraday data for a long duration (which results in many events generated by Bloomberg API), do not use the pattern specified in the API documentation, as it may end up making your application very slow to retrieve all events.
Basically, do not call NextEvent() on a Session object! Use a dedicated EventQueue instead.
Instead of doing this:
var cID = new CorrelationID(1);
session.SendRequest(request, cID);
do {
Event eventObj = session.NextEvent();
...
}
Do this:
var cID = new CorrelationID(1);
var eventQueue = new EventQueue();
session.SendRequest(request, eventQueue, cID);
do {
Event eventObj = eventQueue.NextEvent();
...
}
This can result in some performance improvement, though the API is known to not be particularly deterministic...

I didn't really ever get around to solving this question, but we did find a workaround.
Based on a small, apparently throwaway, comment in the Server API documentation, we opted to create a second session. One session is responsible for static requests, the other for real-time. e.g.
_marketDataSession.OpenService("//blp/mktdata");
_staticSession.OpenService("//blp/refdata");
The means one session operates in subscription mode, the other more synchronously - I think it was this duality which was at the root of our problems.
Since making that change, we've not had any problems.

My reading of the docs agrees that you need separate sessions for the "//blp/mktdata" and "//blp/refdata" services.

A client appeared to have a similar problem. I solved it by making hundreds of sessions rather than passing in hundreds of requests in one session. Bloomberg may not be to happy with this BFI (brute force and ignorance) approach as we are sending the field requests for each session but it works.

Nice to see another person on stackoverflow enjoying the pain of bloomberg API :-)
I'm ashamed to say I use the following pattern (I suspect copied from the example code). It seems to work reasonably robustly, but probably ignores some important messages. But I don't get your time-out problem. It's Java, but all the languages work basically the same.
cid = session.sendRequest(request, null);
while (true) {
Event event = session.nextEvent();
MessageIterator msgIter = event.messageIterator();
while (msgIter.hasNext()) {
Message msg = msgIter.next();
if (msg.correlationID() == cid) {
processMessage(msg, fieldStrings, result);
}
}
if (event.eventType() == Event.EventType.RESPONSE) {
break;
}
}
This may work because it consumes all messages off each event.

It sounds like you are making too many requests at once. BB will only process a certain number of requests per connection at any given time. Note that opening more and more connections will not help because there are limits per subscription as well. If you make a large number of time consuming requests simultaneously, some may timeout. Also, you should process the request completely(until you receive RESPONSE message), or cancel them. A partial request that is outstanding is wasting a slot. Since splitting into two sessions, seems to have helped you, it sounds like you are also making a lot of subscription requests at the same time. Are you using subscriptions as a way to take snapshots? That is subscribe to an instrument, get initial values, and de-subscribe. If so, you should try to find a different design. This is not the way the subscriptions are intended to be used. An outstanding subscription request also uses a request slot. That is why it is best to batch as many subscriptions as possible in a single subscription list instead of making many individual requests. Hope this helps with your use of the api.

By the way, I can't tell from your sample code, but while you are blocked on messages from the event queue, are you also reading from the main event queue while(in a seperate event queue)? You must process all the messages out of the queue, especially if you have outstanding subscriptions. Responses can queue up really fast. If you are not processing messages, the session may hit some queue limits which may be why you are getting timeouts. Also, if you don't read messages, you may be marked a slow consumer and not receive more data until you start consuming the pending messages. The api is async. Event queues are just a way to block on specific requests without having to process all messages from the main queue in a context where blocking is ok, and it would otherwise be be difficult to interrupt the logic flow to process parts asynchronously.