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Careful with Content-Based Routing

Sunday, March 20th, 2011.

Every once in a while I get clients who ask me why NServiceBus doesn’t support content-based routing. My answer sometimes surprises them, “because it is a dangerous pattern that should usually be avoided”.

Content-Based Routing and ESBs

Since content-based routing often appears on feature lists for various ESBs, many people consider them to be a necessary part of systems built on SOA principles. The pattern also appears in the book Enterprise Integration Patterns, which apparently is also a convincing reason to use it, even though the book specifically states:

“When implementing a Content-Based Router, special caution should be taken to make the routing function easy to maintain as the router can become a point of frequent maintenance.”

That’s right – maintenance nightmare.

For Example

Let’s take a real-world example – a trading system where users can put orders for stock and for forex (money from other countries).

We’d start by creating a message that can be sent by a client:

public class PlaceOrder : IMessage
    public OrderTypeEnum OrderType { get; set; }
    public string Code { get; set; }
    public double Amount { get; set; }

public enum OrderTypeEnum

Clients would send messages by using code like the following:

Bus.Send<PlaceOrder>(m => {
    m.OrderType = OrderTypeEnum.Stock;
    m.Code = "MSFT";
    m.Amount = 300;
    m.SetHeader("AccountId", myAccountId);

The header is the way that clients identify themselves in the system.

Let’s say that the logic for handling the different types of orders is different, but also that we’d like the logic to be deployed to different endpoints. One reason we might want to do this is so that we can independently scale each of these endpoints. This is where it would appear we’d need some content-based routing – having some code that looks at the OrderType property and decides where to route based on its value.

Before we get into solutions, let’s make this more involved.

Not only do we want to route based on OrderType, but we want to use the account ID in the header to check in our database (or via a web service) if the account belongs to one of our VIP customers, and if so, it should be given higher priority.

Content-Based Routing and Brokers

We can see that if we were to go with a content-based routing solution, this would drive our architecture to a hub-and-spoke model where the hub becomes quite large and complex, as well as likely becoming a bottleneck in terms of performance.

This logically centralized place through which all communication flows defines the Broker architectural style – not that of a Bus. In the Bus architectural style, there is no logical (or physical) hub. You can think of it as a kind of peer-to-peer setup. Just like you wouldn’t want ethernet getting involved in applicative routing decisions, neither should your bus get involved.

Business-Topology Mapping Solutions

Instead, when following the Bus architectural style – we look at mapping stable business characteristics to bus-level topology. At one level, that would mean defining two different message types for our different types of trades:

public abstract class PlaceOrder
    public string Code { get; set; }
    public double Amount { get; set; }

public class PlaceStockOrder : PlaceOrder { }

public class PlaceForexOrder : PlaceOrder { }

Once we have two different message types, then we can configure the client to have those statically sent to different endpoints like this:

    <add Messages="Messages.PlaceStockOrder, Messages" Endpoint="Stock" />
    <add Messages="Messages.PlaceForexOrder, Messages" Endpoint="Forex" />

And when it comes to handling our VIP customers, the recommendation would be to have those customers be served by a different set of web servers – we wouldn’t want a sudden flux in regular customers stealing all the HTTP connections from our VIP customers. Then we’d statically configure our VIP front-end to talk to our VIP back-end like this:

    <add Messages="Messages.PlaceStockOrder, Messages" Endpoint="VIP_Stock" />
    <add Messages="Messages.PlaceForexOrder, Messages" Endpoint="VIP_Forex" />

And the logic to identify VIP customers would be done in the login screen which, from there, would direct the user to the appropriate web server farm.

Static vs. Dynamic

It may appear that the above statically configured solution is less flexible then the afore-mentioned hub-and-spoke content-based routing solution. And that’s probably correct. But the question is, are the business requirements (better called “objectives” in this case) likely to change? If we make the technological solution 10x more flexible than the business needs, but at the cost of maintainability (read time to market), we probably haven’t used the right tool for the job.

Many times we can find stable business objectives and align the topology of our solution with them. Not all that is dynamic and flexible is necessarily better than that which is static.

I’d go so far to say that if a solution makes heavy use of content-based routing, it is likely a more fragile solution as implementations of stable business objectives and volatile requirements are mixed up together.

In Closing

NServiceBus intentionally does not support content-based routing so as not to make it easy for developers to make architectural blunders that could require full-system rewrites a couple of years down the road.

If you want to learn more about these kinds of architectural principles, I suggest coming to my course. I’m afraid that New York City is already sold out, but I’ll be coming back to the US again around October. Stockholm, London, Sydney, and Oslo are now all open for registration.

Hope to see you there.

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  1. Daniel Marbach Says:

    Hy Udi
    Thanks for this great article. I have a question though. Isn’t in terms of NServiceBus the registry which handles the publishers and subscribers the hub? I know for me as client of nservicebus this is transparently handled but nonetheless this is the “single point” where the routing logic is placed.

  2. udidahan Says:


    There is no central “registry” – each publisher holds on to its own list of subscribers, and clients have configured on their own end where message types are to be sent.

  3. Jon Says:

    Hi Udi,

    If we decided that for some specific purpose that content based routing was the best way to go, what tool should we use? Is it possible to use NServiceBus? If so, how? Or are you saying that content based routing should never be used?

    Thanks for another informative post, Jon

  4. JDR Says:

    Great article as I’ve struggled with this distinction. To consider an extension of your example.. imagine you have many vendors that can fulfill the order. This is where I very often see a router/broker pattern as the number of permutations and abstraction to the client become a selling point. To follow your recipe, we would have a situation where there are many combinations and a requirement that the knowledge of which vendor should fulfill be percolated all the way to the client so they can create the proper message. I very much agree with your philosophy but I seeking the borders of when a broker will make sense.
    Thanks for the timely article.

  5. Dick Davies Says:

    I’d be interested in hearing your opinion on how thingslike AMQP topic
    exchanges fit into the picture (realise that’s not your focus, but
    they do seem to be a good way to address some issues you
    outlined back in http://bit.ly/pubsub-soa )

  6. Elias Rangel Says:

    Hi Udi,

    In this case the intention of each message can be described by the message type as you suggest: Forex, Stock, etc.

    What I don’t understand is how to handle a case where the intention is based on a property that is truly dynamic, or that at least has a set of values that could be hard to express as a type.

    For example, if you had a Purchase Order with an Address property, and you needed to route based on the city or zip code: how would you model that? how would you use NServiceBus to deal with it?

  7. udidahan Says:


    While you can implement your own content-based routing on top of NServiceBus, I’d question the architecture that gave rise to that need. In short, I’d strongly recommend against it.

  8. udidahan Says:


    You’d likely route to a single message type to a single integration endpoint, and that would take responsibility for calling the 3rd party vendor systems.

  9. udidahan Says:


    That sounds like a good topic for another post 🙂

  10. udidahan Says:


    In that case you’d probably publish a PurchaseOrderReceived event, where all subscribers would receive it, and only the one responsible for the given city/zip would actually process it.

  11. udidahan Says:


    Looks like you got your post:


  12. Paul Says:

    Udi, I think your analysis is based on preconceived notions of how components and services are implemented. My experience is that with an appropriately designed canonical object-schema (or XML schema), the hub/spoke pattern doesn’t emerge.

    If CBR culminates in hub/spoke then significant oversights have occurred along the way

  13. udidahan Says:


    I suppose it’s possible that CBR won’t result in a tightly coupled architecture – I haven’t seen it happen though. I agree that discipline and skill is critical and, assuming that, any tool (including CBR) can be wielded effectively.

  14. Aaron Says:

    My searching for an answer to this problem has come closest to an answer in this blog. But I’m stuck on one thing. I’ve a command queue in a multi tenanted app, commands need to be processed sequentially per customer. One big queue for all can’t be processed in parallel with out breaking the order. Load balancing processing of many individual customer queues is trending towards that complex hub and spoke model described here as best to be avoided… Does NService bus do that balancing?
    (I’m currently just using msmq directly as a service bus seemed like over kill to start of with.)

  15. udidahan Says:


    NServiceBus doesn’t do that out of the box, no. If you were using NServiceBus, what you could look at doing is using an “intercepting handler” to check to see if the current message is out of order, and then tell the bus to put it at the back of the queue, in essence re-ordering on the receiving end.

    Does that help?

  16. Aaron Says:


    Yeah though that seems like a step backwards… If I’ve got N messages queued up for a customer X and Message 1 takes a longer time, so the next worker bumps CustomerX.Message2, to the end of the queue, all down to N need to be cycled through until message 2 is back in front. But perhaps that’s a trivial cost?

    Further skim reading today, mostly of past CQRS blog articles here suggests I need to take a step backwards myself.

    In most cases the UI wont let a single user shotgun a barrage of commands. And since there is always an unavoidable risk of Fred, sending a command to adjust something Jane just cancelled anyway maybe order could be made irrelevant. Is that what I’m missing here? So it’s equating Jane beating Fred by a few seconds to click submit – to a Worker with Jane’s message beating a Worker with Fred’s message by some time fraction.

    It’s not a command queue but just a number of durable async services, is that a better way to look at it?

  17. udidahan Says:


    If you’re dealing with a collaborative domain, then message ordering is too shallow a solution to consider for the problem domain. You’ll need to analyze much deeper business specific scenarios.

  18. Ajeeth Says:

    Hello Udi,

    Can I host multiple end points under a single process. How do I organize them in Visual Studio. I am getting an exception when I have multiple endpoint config file.


  19. udidahan Says:


    NServiceBus doesn’t currently allow multiple endpoints in a single process but it’s something we’re going to start work on next quarter.

  20. Ajeeth Says:

    Thanks Udi. Really excited to interact with you. So far the NService Bus journey is too good. You have made the job of a developer much easier !!. Good abstractions, no room for developer to make mistake. Really an awesome tool.

  21. udidahan Says:

    Glad to hear it Ajeeth!

  22. Luke McGregor Says:

    I get what you mean about wanting to avoid building a complex hub in the middle of the system and this sounds like a great thing to avoid. I like your example of using a more granular message type to be able to provide a more specific subscription rather than dynamically routing.

    But what should I do if building a more granular message type isn’t really an option. For example if your subscribers want to listen to only data about a specific (dynamic) list of clients. You cant really seperate this to a new message type at build time without making the publisher understand the concern of the subscriber (ie what clients they are interested in)

    What would you suggest should happen in this kind of scenario?


  23. udidahan Says:

    Hi Luke,

    The solution you’re looking for is what I describe as “data distribution” rather than “publish subscribe”. Although both have the option for one-to-many communication, the intention with data distribution is much more physical whereas with pub/sub it is more logical in nature.

    Also, it is much more common to see user-facing clients as the recipients of data distribution notifications while in pub/sub often it is more server to server in nature.

    In any case, this is something that I’ve wanted to do for some time with NServiceBus but via a separate API than the traditional pub/sub one, but we haven’t yet found the time to do it.

    You could try to extend the existing NServiceBus model to do this, but it wouldn’t be easy. If you need to do this today, I’d suggest finding something designed more specifically for content-based routing.

  24. Jack Says:

    Hello Udi,

    Can you elaborate on use cases that can’t be solved without content-based routing?

    In my particular scenario, I would like to distribute 1 task of each type (CPU/IO/Network) to each machine in a pool of workers. without content-routing I would need to have 3x the number of interfaces

    in addition to that, without content-based routing, how could I solve the notion of different locations? (i.e. tasks in europe region shouldn’t be routed to queues that USA region servers are listening to)

    How can I solve these issues with NServiceBus ?


  25. udidahan Says:


    Multi-region routing is usually handled at a higher level (DNS) with the machines in that region talking to each other except for explicit cross-region communication (done with the NServiceBus Gateway – http://docs.particular.net/nservicebus/gateway/ ).

    I’m not entirely sure about the first part of your question. Are you asking about how to optimize utilization of the machines so that they wouldn’t be operating on many tasks requiring the same type of resource (CPU/IO/Network) at the same time?

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always with a smile. Udi is indeed a top-league professional!”

Ben Scheirman Ben Scheirman, Lead Developer at CenterPoint Energy
“Udi is one of those rare people who not only deeply understands SOA and domain driven design, but also eloquently conveys that in an easy to grasp way. He is patient, polite, and easy to talk to. I'm extremely glad I came to his workshop on SOA.”

Scott C. Reynolds Scott C. Reynolds, Director of Software Engineering at CBLPath
“Udi is consistently advancing the state of thought in software architecture, service orientation, and domain modeling.
His mastery of the technologies and techniques is second to none, but he pairs that with a singular ability to listen and communicate effectively with all parties, technical and non, to help people arrive at context-appropriate solutions. Every time I have worked with Udi, or attended a talk of his, or just had a conversation with him I have come away from it enriched with new understanding about the ideas discussed.”

Evgeny-Hen Osipow, Head of R&D at PCLine
“Udi has helped PCLine on projects by implementing architectural blueprints demonstrating the value of simple design and code.”

Rhys Campbell Rhys Campbell, Owner at Artemis West
“For many years I have been following the works of Udi. His explanation of often complex design and architectural concepts are so cleanly broken down that even the most junior of architects can begin to understand these concepts. These concepts however tend to typify the "real world" problems we face daily so even the most experienced software expert will find himself in an "Aha!" moment when following Udi teachings.
It was a pleasure to finally meet Udi in Seattle Alt.Net OpenSpaces 2008, where I was pleasantly surprised at how down-to-earth and approachable he was. His depth and breadth of software knowledge also became apparent when discussion with his peers quickly dove deep in to the problems we current face. If given the opportunity to work with or recommend Udi I would quickly take that chance. When I think .Net Architecture, I think Udi.”

Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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