In this podcast we revisit the topic of REST and how to make it work for process-centric enterprise systems. After describing the basic advantages and pitfalls of plain resource thinking, we’ll look at how mapping messaging concepts to resources provides solutions for transactional, multi-resource processing.

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Additional References

• Podcast: “REST vs Web Services Debate”
• REST Definition
• Roy Fielding’s Thesis: “Architectural Styles and the Design ofNetwork-based Software Architectures”
• ETag Definition

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In this podcast we’ll be discussing the issues around supporting different kinds of service-level agreements in an SOA, and how using message prioritization helps and hurts. We’ll also be looking at alternative solutions more distributed in nature,which make use of specific deployment and configuration schemes that result in a more robust system.

Symon asks:

Hi Udi,

I don’t know if this is really an NServiceBus question or a more general architecture question, so if you want to treat it as an Ask Udi question feel free. :)  My question is about handling message prioritization between services in an SOA architecture using an ESB-  in this case NServiceBus.

We’ve been running along nicely getting the hang of using the NServiceBus but have just hit a point in our application where it’s become clear that we’re going to need to prioritize messages between services according to an arbitrary set of rules.  To that end we’ve created a “controller” that determines a message’s priority according to a set of rules and that controller should forward the message to other services for handling according to that priority.  Where we’re stuck is figuring out the best way to handle the prioritization of the forwarded message.

We touched on the idea of having several endpoints representing low, medium or high priority and shunting the message to the appropriate endpoint, however since the receiving service consists of multiple workers located on other machines we need to have a dispatcher for each of these endpoints.  Since we’re using MSMQ this seems to be causing an explosion of queues and the solution seems to require a lot of overhead in terms of set up, but it *does* look like it might be flexible as long as we come up with a smart dispatcher.

Is this an “ideal” way to handle message prioritization?  I know there is a notion of priority in MSMQ messages, but this isn’t exposed in NServiceBus as far as I can tell.

Any suggestions?

Cheers,

Symon

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Additional References

• Podcast: “Shared Subscriptions between Autonomous Components”
• Podcast: “Space-Based Architecture for the Web”
• Info: “Master-Worker Pattern”

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In this podcast we’ll be discussing the issues around multi-threaded processing of messages by a service, specifically that the processing of message received second may be finished before that of the first. This scenario tends to rear its ugly head at higher levels of load and is critical for correctness in high-scalability environments.

Our long time listener Bill asks:

Hi Udi,

I have a question around processing of messages in proper order. When leveraging multiple threads to process messages in a message queue, it is possible for the second message in the queue to get processed before the first - especially if the first message is considerably larger than the second. I have taken a lot of care to make sure that messages are sent in the correct order, only to find that the receiving system can process them out of order anyway.

Consider a Policy Created notification, which must come before a Policy Approved notification. If both messages are sitting in the queue when the receiving service starts up, the approval message can be processed before the creation message. How can I make sure that message ordering is respected by the receiving system? I am using WCF/MSMQ as the underlying transport by the way. The only way I have found so far is to limit the receiving service to a single thread, which is by no means desirable.

Best Regards,

Bill

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Additional References

• Blog post: In-Order Messaging a Myth?
• Blog post: Handling Messages out of Order

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In this podcast we’ll be discussing certain methodologies for migrating an architecture from N-Tier to SOA. We’ll see what parts can be used almost unchanged, and which N-Tier concepts have no place in this new, service-oriented world.

Ketan asks:

Hello Udi,

I am Ketan, working as Analyst Programmer in India. I want some information regarding SOA Architecture. I have visited several blogs and sites and I do have enough knowledge regarding technical terms of SOA. I have worked a lot in 3-tier mechanism a lot and now want to switch on SOA Architecture in Visual Studio 2005.

I read your blog post How to migrate to SOA and liked the contents and am interested in doing it. Can you please be more precise about how to migrate from 3-tier to SOA? Actually, I have read whole content of above link and you have explained enough. But, still I want you to keep me out from dark. You have described it functionally, but I want some technical description of this process. Please help me in this.

If you can provide me some example/application (in which SOA Architecture has been implemented), then it will be very helpful to me.

Thanks in advance. Waiting for your favorable reply.

Ketan

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Additional References

• Blog post on migrating from layers to services
• Podcast on “Does SOA mean the end of OO”?

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In this podcast we’ll look at the issues around versioning and SOA and how the asynchronous nature of messaging obviates the need for previously accepted interface-based versioning practices.

Jarrod asks:

Hi Udi,

I have some questions around versioning and SOA:

Over the years I’ve been fairly adamant about the practice of versioning interfaces should a method change be needed.

Hence, IDog and IDog2

Generally I would also say that a method addition would need a new interface as much as a change to an existing method or property (sometimes requiring completely deprecating the old interface).

With SOA being used more and contract first designs coming down the pipe…an addition of a method or operation to a contract in SOA does not mean its a breaking change.

So, do you continue to create a new interface and version the name within an SOA environment? Even if its just one method that is being added?

A service contract, in general…with just a single method in my opinion is a bad design. Sure, there are exceptions to that but I can just see changes made to the service contracts over the course of a few years with many 1-2 method interfaces.

Day 1 : 12 methods on interface IDog

6 months later : 1 new method, create IDog2 which implements IDog

15 months later : 2 new methods, create IDog3 which implements IDog2

Some can argue that if you’re having to add methods like this, the original design was flawed. That may be…or you could just be in a volatile business environment in which SOA must adapt.

So my main questions are:

1) Do you think its the best practice to continue to version interfaces for non-breaking method additions in an SOA environment in which you also have control of the consumers (internal consumers)?

2) Do you think its the best practice to continue to version interfaces for non-breaking method additions in an SOA environment in which you do not have control of the consumers (external consumers)?

3) If the answer to the above is yes, is a single method on a new interface acceptable?

4) If you know of a problem in an existing interface on a method and have control of the consumers - do you “fix” the method and redeploy to the consumers? Or do you deprecate the entire interface and begin anew?

I’m interested in seeing your responses

Jarrod

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Additional References

• Podcast on Autonomous Services and Pub/Sub
• Blog post on “The Perils of WSDL first”

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In this podcast we’ll look at the issues around Durable Services, what makes them stateful or stateless, as well as how WCF and WF can be used to implement them. Finally, we’ll compare solutions based on .NET 3.5 and on NServiceBus–covering aspects such as transactions and persistent time-outs.

Suresh asks:

Hi Udi,

I’ve been reading about the coming “durable services” that will be available with the next version of WCF. I also have been listening to your podcasts and reading your blog posts about NServiceBus where you talk about long-running workflows. It sounds like both of these technologies are trying to solve the same problem.

Do durable services do away with long-running workflow? If so, does that mean we don’t need Workflow Foundation either? If not, what is the connection between them.

If you could shed some light on the matter that would be great.

Suresh

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Additional References

• Blog post on Workflow Services in Orcas
• Blog post on Orcas Durable Services
• Blog post on .NET Framework 3.5 Beta1 is Live on the Web

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In this podcast we’ll look at various patterns involved in creating MVC-based Smart Clients which communicate using asynchronous messaging and how to avoid threading problems there.

Neil asks:

Hi Udi,

We’re building a smart client application that uses WCF for full-duplex communications with our server. This is the asynchronous communication you talk about in your podcast. The smart-client is based on the MVC pattern, where model objects raise events when they’re changed so that the views can update themselves.

What’s started happening recently is that the smart-client has been freezing-up on us intermittently. We don’t know how to debug this and are wondering if its an architectural problem.

Any help you can give would be most appreciated.

Neil

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Additional References

• Blog post on Occasionally Connected Systems Architecture
• Blog post on ObjectBuilder synchronization features needed for pub/sub-ing Smart Clients

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In this podcast we’ll try to describe some of the pitfalls of trying to split a domain model between multiple services, as well as how SOA side-steps the “single version of the truth” issue found in reporting.

Rishi asks:

Hi Udi,
First of all, thanks for all the posts and info you share, it is very insightful compared to loads n’ loads of marketing bull and vendor whitepapers. Ok, I have two questions for you.

1. I want to create a SOA-based LOB application/platform and I generally understand the ‘tenets of services’ and reasons for the same. However, as you may well know, there is a lot of interdependency between business constructs (such as a customer with an order, or a delivery with a product construct). Now, should we share these constructs within a tightly-coupled and domain-based “system” which exposes a number of services to the outside world and use the constructs directly inside the so-called “system” boundary. Or on the opposite spectrum have a number of autonomous and independent services that expose, own, and share certain business constructs or at least part of the data that represents them as a whole. For example, an order service will need to interact with a customer construct, which primarily/essentially is (or should be?) owned by a customer profile service - in this context, with clear, direct, and immediate inter-dependency what is preferable? Where do we draw the enclosing line?

2. In a line of business application, reporting and shaping data are a given necessity. Now, with autonomous and independent services which define business constructs in their contextual ways, how do we practically shape, combine, and filter data across various services? We certainly can’t do joins over multiple services in a practical way? And as some suggest, if we are to maintain duplicate or master data elsewhere, it just seems very messy to me - just consider having 3 versions of an order services, with 2 versions of support services, and ‘n’ number of other services to comb data from. Further, for reporting and also analytical purposes we really need to have business data structured in a well-defined manner, which for all purposes needs to be the “single version of the truth”. I can’t see granular services, with all its tenets, sitting nicely with such other requirements of the business - and yet I am not even asking them to be real-time. How do we meet such business requirements in your view with SOA?

Hope the above makes sense!

Cheers,
Rishi

PS: I use the word business construct purposefully, and it does not directly attribute to a well-defined programmable entity (in object-oriented way, that is).

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Additional References

• Ask Udi Podcast #000–Does SOA mean the end of OO?
• Ask Udi Podcast #009–Master Data Management and SOA
• Ask Udi Podcast #016–How does Extract, Transform, Load fit with SOA?

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This week we’re discussing scenarios involving the use of multiple autonomous components handling the same event. We also get into the topics of component hosting as well as solution development structure.

Our long-time listener Bill asks the following:

Hi Udi,

Thank you for your podcast clarifying the concept of autonomous components. It certainly cleared up a lot in terms of how you define an autonomous component and helped position the concept better in my mind.

I am considering building our Policy Administration service with 3 autonomous components as you previously suggested - one for each insurance product family. As you previously mentioned, these autonomous components could either share a database instance or have separate database instances, even though they may share schema (being a part of the same service).

What concerns me however is that there is some data that must be shared between these autonomous components. A perfect example is Region. If a new Region is defined somewhere in the enterprise, a RegionCreatedNotification is published onto the bus. If I have 3 autonomous components sharing the same database, is it best I just choose one to be the subscriber for this event? Or should I create a new separate queue to receive these notifications and put a message handler for it in a separate MessageHandlers assembly? Or should I subscribe all 3 autonomous components and have them each check if the Region has already been created in the database before inserting a new one?

Option (1) seems presumptuous because it assumes all 3 autonomous components are sharing the same database. Option (2) seems a bit strange because it is like creating a 4th autonomous component only to receive the RegionCreatedNotification messages. And Option (3) seems wasteful since there is redundant code and the system is doing extra unnecessary lookups. At the moment it looks like Option (2) is my best bet, but I’m very interested to get your take on it.

Another question is whether you would say that autonomous components should be hosted in different processes. Since I don’t need to run these autonomous components on separate servers at this stage, it would seem wasteful to create a service host for each component. I could always create separate service hosts at a later time if I needed to host one or more autonomous components on other servers.

And the final question I have is in terms of solution structure. I was contemplating the following structure:

ProductFamily1
———————-
Company.PolicyAdministration.ProductFamily1.Domain
Company.PolicyAdministration.ProductFamily1.MessageHandlers
Company.PolicyAdministration.ProductFamily1.Messages
Company.PolicyAdministration.ProductFamily1.Persistence
Company.PolicyAdministration.ProductFamily1.Persistence.Implementation
Company.PolicyAdministration.ProductFamily1.Etc…

ProductFamily2
———————-
Company.PolicyAdministration.ProductFamily2.Domain
Company.PolicyAdministration.ProductFamily2.MessageHandlers
Company.PolicyAdministration.ProductFamily2.Messages
Company.PolicyAdministration.ProductFamily2.Persistence
Company.PolicyAdministration.ProductFamily2.Persistence.Implementation
Company.PolicyAdministration.ProductFamily2.Etc…

ProductFamily3
———————-
Company.PolicyAdministration.ProductFamily3.Domain
Company.PolicyAdministration.ProductFamily3.MessageHandlers
Company.PolicyAdministration.ProductFamily3.Messages
Company.PolicyAdministration.ProductFamily3.Persistence
Company.PolicyAdministration.ProductFamily3.Persistence.Implementation
Company.PolicyAdministration.ProductFamily3.Etc…

Common
———————-
Company.PolicyAdministration.Common.Domain
Company.PolicyAdministration.Common.Persistence
Company.PolicyAdministration.Common.Messages
Company.PolicyAdministration.Common.ServiceHost
Company.PolicyAdministration.Common.Etc…

Do you have any thoughts on the above structure?

Once again thank you very much for your extremely valuable advice!

Best Regards,
Bill

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Additional References:

• Ask Udi Podcast #017 - Using Autonomous Components for SLAs in SOA
• Ask Udi Podcast #020 - How to structure .NET solutions and components

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This week we have a comment from a fellow SOA blogger, Jack van Hoof:

ESB’s are a lot about messaging and therefore a better name might be “Enterprise Data Bus”. It’s the asynchronous messaging that needs such an infrastructure with persistency and mediation facilities. All the WS-* standards are about messaging as well, leveraging the message itself to tell the infrastructure how it has to be handled.

I think WS-* will make it possible to have the ESB evolve from a vendor-product to a concept implemented in the operating systems an network devices that understand WS-*. Then you can leave the prefix “Enterprise” and we will be ready for an univeral asynchronous data bus over the Internet (or any other network you like). This will help breaking the current “services centric” idea of SOA into a “messages centric” perspective.

What are your thoughts?

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Additional References:

• Podcast on if you need an ESB for SOA
• Podcast on Enterprise Service Discovery
• Blog post on ESBs as a global dataspace
• Blog post about what the brave new world of WS-* brings us

Want more? Check out the “Ask Udi” archives.

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