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Archive for the ‘Autonomous Services’ Category



[Podcast] Message Ordering: Is it Cost Effective?

Tuesday, January 1st, 2008

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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Asynchronous, High-Performance Login for Web Farms

Saturday, November 10th, 2007

Often during my consulting engagements I run into people who say, "some things just can’t be made asynchronous" even after they agree about the inherent scalability that asynchronous communications pattern bring. One often-cited example is user authentication – taking a username and password combo and authenticating it against some back-end store. For the purpose of this post, I’m going to assume a database. Also, I’m not going to be showing more advanced features like ETags to further improve the solution.

The Setup

Just so that the example is in itself secure, we’ll assume that the password is one-way hashed before being stored. Also, given a reasonable network infrastructure our web servers will be isolated in the DMZ and will have to access some application server which, in turn, will communicate with the DB. There’s also a good chance for something like round-robin load-balancing between web servers, especially for things like user login.

Before diving into the meat of it, I wanted to preface with a few words. One of the commonalities I’ve found when people dismiss asynchrony is that they don’t consider a real deployment environment, or scaling up a solution to multiple servers, farms, or datacenters.

The Synchronous Solution

In the synchronous solution, each one of our web servers will be contacting the app server for each user login request. In other words, the load on the app server and, consequently, on the database server will be proportional to the number of logins. One property of this load is its data locality, or rather, the lack of it. Given that user U logged in, the DB won’t necessarily gain any performance benefits by loading all username/password data into memory for the same page as user U. Another property is that this data is very non-volatile – it doesn’t change that often.

I won’t go to far into the synchronous solution since its been analysed numerous times before. The bottom line is that the database is the bottleneck. You could use sharding solutions. Many of the large sites have numerous read-only databases for this kind of data, with one master for updates – replicating out to the read-only replicas. That’s great if you’re using a nice cheap database like mySql (of LAMP), not so nice if you’re running Oracle or MS Sql Server.

Regardless of what you’re doing in your data tier, you’re there. Wouldn’t it be nice to close the loop in the web servers? Even if you are using Apache, that’s going to be less iron, electricity, and cooling all around. That’s what the asynchronous solution is all about – capitalizing on the low cost of memory to save on other things.

The Asynchronous Solution

In the asynchronous solution, we cache username/hashed-password pairs in memory on our web servers, and authenticate against that. Let’s analyse how much memory that takes:

Usernames are usually 12 characters or less, but let’s take an average of 32 to be sure. Using Unicode we get to 64 bytes for the username. Hashed passwords can run between 256 and 512 bits depending on the algorithm, divide by 8 and you have 64 bytes. That’s about 128 bytes altogether. So we can safely cache 8 million of these with 1GB of memory per web server. If you’ve got a million users, first of all, good for you 🙂 Second, that’s just 128 MB of memory – relatively nothing even for a cheap 2GB web server.

Also, consider the fact that when registering a new user we can check if such a username is already taken at the web server level. That doesn’t mean it won’t be checked again in the DB to account for concurrency issues, but that the load on the DB is further reduced. Other things to notice include no read-only replicas and no replication. Simple. Our web servers are the "replicas".

The Authentication Service

What makes it all work is the "Authentication Service" on the app server. This was always there in the synchronous solution. It is what used to field all the login requests from the web servers, and, of course, allowed them to register new users and all the regular stuff. The difference is that now it publishes a message when a new user is registered (or rather, is validated – all a part of the internal long-running workflow). It also allows subscribers to receive the list of all username/hashed-password pairs. It’s also quite likely that it would keep the same data in memory too.

The same message can be used to publish both single updates, and returning the full list when using NServiceBus. Let’s define the message:

[Serializable]
public class UsernameInUseMessage : IMessage
{
    private string username;
    public string Username
    {
        get { return username; }
        set { username = value; }
    }

    private byte[] hashedPassword;
    public byte[] HashedPassword
    {
        get { return hashedPassword; }
        set { hashedPassword = value; }
    }
}

And the message that the web server sends when it wants the full list:

[Serializable]
public class GetAllUsernamesMessage : IMessage
{

}

And the code that the web server runs on startup looks like this (assuming constructor injection):

 

public class UserAuthenticationServiceAgent

    public UserAuthenticationServiceAgent(IBus bus) 
    { 
        this.bus = bus;
        bus.Subscribe(typeof(UsernameInUseMessage)); 
        bus.Send(new GetAllUsernamesMessages());
    }

}

And the code that runs in the Authentication Service when the GetAllUsernamesMessage is received:

 

public class GetAllUsernamesMessageHandler : BaseMessageHandler<GetAllUsernamesMessage>
{
    public override void Handle(GetAllUsernamesMessage message)
    {
        this.Bus.Reply(Cache.GetAll<UsernameInUseMessage>());
    }
}

 

And the class on the web server that handles a UsernameInUseMessage when it arrives:

 

public class UsernameInUseMessageHandler : BaseMessageHandler<UsernameInUseMessage>
{
    public override void Handle(UsernameInUseMessage message)
    { 
        WebCache.SaveOrUpdate(message.Username, message.HashedPassword); 
    }
}

When the app server sends the full list, multiple objects of the type UsernameInUseMessage are sent in one physical message to that web server. However, the bus object that runs on the web server dispatches each of these logical messages one at a time to the message handler above.

So, when it comes time to actually authenticate a user, this the web page (or controller, if you’re doing MVC) would call:

public class UserAuthenticationServiceAgent
{
    public bool Authenticate(string username, string password)
    {
        byte[] existingHashedPassword = WebCache[username];
        if (existingHashedPassword != null)
            return existingHashedPassword == this.Hash(password);

        return false;
    }
}

 

When registering a new user, the web server would of course first check its cache, and then send a RegisterUserMessage that contained the username and the hashed password.

[Serializable]
[StartsWorkflow]
public class RegisterUserMessage : IMessage
{
    private string username;
    public string Username
    {
        get { return username; }
        set { username = value; }
    }

    private string email;
    public string Email
    {
        get { return email; }
        set { email = value; }
    }

    private byte[] hashedPassword;
    public byte[] HashedPassword
    {
        get { return hashedPassword; }
        set { hashedPassword = value; }
    }
}

 

When the RegisterUserMessage arrives at the app server, a new long-running workflow is kicked off to handle the process:

public class RegisterUserWorkflow :
    BaseWorkflow<RegisterUserMessage>, IMessageHandler<UserValidatedMessage>
{
    public void Handle(RegisterUserMessage message)
    {
        //send validation request to message.Email containing this.Id (a guid)
        // as a part of the URL
    }

    /// <summary>
    /// When a user clicks the validation link in the email, the web server
    /// sends this message (containing the workflow Id)
    /// </summary>
    /// <param name="message"></param>
    public void Handle(UserValidatedMessage message)
    {
        // write user to the DB

        this.Bus.Publish(new UsernameInUseMessage(
            message.Username, message.HashedPassword));
    }
}

That UsernameInUseMessage would eventually arrive at all the web servers subscribed.

Performance/Security Trade-Offs

When looking deeper into this workflow we realize that it could be implemented as two separate message handlers, and have the email address take the place of the workflow Id. The problem with this alternate, better performing solution has to do with security. By removing the dependence on the workflow Id, we’ve in essence stated that we’re willing to receive a UserValidatedMessage without having previously received the RegisterUserMessage.

Since the processing of the UserValidatedMessage is relatively expensive – writing to the DB and publishing messages to all web servers, a malicious user could perform a denial of service (DOS) attack without that many messages, thus flying under the radar of many detection systems. Spoofing a guid that would result in a valid workflow instance is much more difficult. Also, since workflow instances would probably be stored in some in-memory, replicated data grid the relative cost of a lookup would be quite small – small enough to avoid a DOS until a detection system picked it up.

Improved Bandwidth & Latency

The bottom line is that you’re getting much more out of your web tier this way, rather than hammering your data tier and having to scale it out much sooner. Also, notice that there is much less network traffic this way. Not such a big deal for usernames and passwords, but other scenarios built in the same way may need more data. Of course, the time it takes us to log a user in is much shorter as well since we don’t have to cross back and forth from the web server (in the DMZ) to the app server, to the db server.

The important thing to remember in this solution is doing pub/sub. NServiceBus merely provides a simple API for designing the system around pub/sub. And publishing is where you get the serious scalability. As you get more users, you’ll obviously need to get more web servers. The thing is that you probably won’t need more database servers just to handle logins. In this case, you also get lower latency per request since all work needed to be done can be done locally on the server that received the request.

ETags make it even better

For the more advanced crowd, I’ll wrap it up with the ETags. Since web servers do go down, and the cache will be cleared, what we can do is to write that cache to disk (probably in a background thread), and "tag" it with something that the server gave us along with the last UsernameInUseMessage we received. That way, when the web server comes back up, it can send that ETag along with its GetAllUsernamesMessage so that the app server will only send the changes that occurred since. This drives down network usage even more at the insignificant cost of some disk space on the web servers.

And in closing…

Even if you don’t have anything more than a single physical server today, and it acts as your web server and database server, this solution won’t slow things down. If anything, it’ll speed it up. Regardless, you’re much better prepared to scale out than before – no need to rip and replace your entire architecture just as you get 8 million Facebook users banging down your front door.

So, go check out NServiceBus and get the most out of your iron.



Creating Stateful Services with NServiceBus

Thursday, October 4th, 2007

On one of the projects I’m consulting on they needed some special behavior to handle the following scenario:

Since the service needs to perform all request processing in near-real-time, it caches all data from the DB in memory (yes, that’s a lot of memory). Since the service needs to handle multiple requests concurrently, we’re using multiple threads (so far, so good). The problem is that we don’t want the service to handle messages received until it’s finished caching everything. Also, we don’t want that check to show up in every message handler (important when you have lots of message types).

This is actually quite easy to do with NServiceBus. Here’s how:

Have a thread-safe class, let’s call it Loader, for the API to the caching. Something along the lines of:

If (!Loader.HasCachedEverything)
  Loader.CacheEverything();

Obviously, the Loader will have internal logic for checking if it has already started loading things from the DB, so that it won’t do the same thing twice.

OK, now on to the interesting stuff.

We’d like to have the above code run no matter which kind of message we’ve received, so we just write a “generic” message handler – which handles “IMessage” like so:

public class CachingMessageHandler : BaseMessageHandler<IMessage>
{
  public void Handle(IMessage message)
  {
    If (!Loader.HasCachedEverything)
    {
      Loader.CacheEverything();
      this.bus.HandleCurrentMessageLater();
      this.CanContinue = false;
    }
  }
}

When the message handler calls “HandleCurrentMessageLater”, the bus puts the current message in the back of the queue. If you’ve configured a transactional transport, this will be safe even in the case of a server restart.

Also, notice the “CanContinue = false”. This tells the bus that the message should not be passed on to any other message handlers, even if there are those that are configured to handle it.

We’ll also package this class up by itself, keeping it separate from the core logic of the service – making it easier to version these cross cutting concerns and the service logic. Let’s put it in “CrossCuttingConcerns.dll”

The final thing needed in order to achieve the behavior described above is to configure this message handler to run before any other handler. This is done in the config file of the process, under the “bus” object, in the “MessageHandlerAssemblies” property like so:

        <property name="MessageHandlerAssemblies">
          <list>
            <value>CrossCuttingConcerns</value>
            <value>ServiceLogic</value>
          </list>
        </property>

This is similar to the way HttpHandlers are (were?) configured in IIS – the order of the handlers defines the order in which the bus dispatches messages to them.

And that’s it.

We’re done.

If you have any questions you’d like to ask about NServiceBus, please feel free to send them my way: Questions@NServiceBus.com.

And just in closing I’d like to say that I don’t necessarily think you should be creating stateful services, but that there’s a time and place for everything.



[Podcast] Shared subscriptions between autonomous components

Sunday, September 9th, 2007

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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On rising data volumes

Monday, August 20th, 2007

Larry’s post Data Volumes Trumping Core Multiplication? Interesting Thought raises some interesting questions as to what will have a larger impact on the way we use program computers – rising data volumes or more cores:

It seems to me that nowadays we work more and more with data streams and not data sets. On a transaction-to-transaction basis, I think it’s an uncommon application that uses more data than can fit into several gigabytes of RAM (obvious exception: multimedia data).

While data stream processing is the heartbeat of many verticals, I’m seeing another trend there as well – the use of historical data as a part of that data stream processing. Some people have begun calling this Complex Event-Stream Processing (CEP), and the analysts are already beginning to eat it up. Regardless, the problem is that it is difficult to hold all historical data in memory so that when events arrive we can process them quickly.

So, my bottom line is that we’re being hit on multiple fronts – both the rate at which we need to process events and the amount of data required to process each event. Multiple cores help a bit, but probably not enough to discount scaling up to more machines. All this at the end of the day points out that we should not treat multiple cores any differently than multiple machines.

So, we either need languages to handle this (Erlang for one) or possibly frameworks (NServiceBus is my contribution). All I know is that Layered (Tiered) Architectures won’t cut it.



Don't EDA between existing systems

Thursday, August 16th, 2007

In Nick Malik’s great post, EDA: Avoiding coupling on the name he describes additional “handshakes” to be used to avoid the following problems:

Let’s say I have a system to handle a call center for financial services or telco. When a customer calls on the phone and asks to be enrolled in “Heavily Advertised Program ABC,” there may need to be three or four systems that interact to make that real.

Harry asks me to consider using a ‘logical name’ of the receiver. The sender contacts a logical end point, the addressing infrastructure turns that into a physical end point, and we still have decoupling.

Honestly, I like it but I think it is insufficient. What if we need to contact 20 downstream systems in a complex workflow, but I don’t want a single “orchestration coordinator” to be a bottleneck (or single point of failure). I don’t want to hand the orchestration off from my app to a central orchestration hub.

Let me propose a different approach.

When we use SOA/EDA (the same thing as far as I’m concerned), the top-level building block used is the Service. A service may make use of a number of existing systems to perform its work. The business-level events that we publish (and subscribe to) are done by the service, not the existing systems.

If there’s any orchestration/workflow that needs to be done as a result of a service receiving an event, it is done entirely internal to that service. Inter-service orchestrations don’t really exist, as in there is no orchestration coordinator that is not in a service. And the orchestration coordinators within a service don’t touch other services’ back-end systems – if anything, they publish other business level events.

Be aware: when just starting out on an SOA, you’ll find that multiple Services make use of the same backend systems. This may be necessary, but not a desirable state to stay in for too long since it embodies the most insidious and invisible kind of inter-service coupling there is.

I want to go back to Nick’s original question:

So what if no one picks the message up? Is that an error?

The answer is mu.

If a service publishes a business-event (message) and no other services currently care, that’s fine. It’s not an error. Actually, you’d probably have some kind of infrastructure “queue” where messages that haven’t been received more than X time get sent to, so that the event isn’t “lost”. On the other hand, within a service – if an existing system sends out a message that needs to arrive at another system, and that message doesn’t arrive or isn’t picked up “in time”, that is an error.

This is one of the advantages SOA brings to the table in terms of EDA (again, the same as far as I’m concerned). You get simple messaging semantics between services, while within the “sphere of control” of a service you need, and more importantly can do more complex messaging and orchestration.

Bottom line: you need higher abstractions than your existing systems to employ EDA effectively.

You might also want to check out my podcast on this topic: SOA, ESB, and Events.



[Podcast] Can We Do Away with Services and Just Leave the Messaging?

Saturday, August 11th, 2007

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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[Podcast] Using WCF for Entity and Activity Services to Implement Business Services

Tuesday, July 17th, 2007

This week we return to the topic of Entity, Activity, and Process services and compares their usages as top-level SOA elements and as implementation details of the application architecture inside a business service.

And the question that this answers is:

Hi Udi,

We’ve been having some discussions about how to implement our latest project using SOA and this is what we came up with:

Every activity is a method, which is broken into a class ( Think separation of concerns ), so we get high reusability.
On top of those classes we have decided to put a WCF layer, so you can expose the method as a web method. If an activity needs data, then it will access a entity service via WCF. To make the activities useful for the business we put them in a class which we have called processes ( a process layer). A process contains one or more activities and is able to call other processes and activities aren’t allowed to call processes or other activities. On top of each process we have decided to put a WCF so the UI can access them.

So it’s pretty close to what you wrote about in the Microsoft Architecture Journal except that we don’t have direct call to the entity services, we wrap it up in an activity before the call, which is wrapped in a WCF-host. Much like the definitions in Ontology and Taxonomy of Services in a Service-Oriented Architecture

I would love to hear your comments and thoughts about this architecture.

With thanks, Ingo

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No such thing as a centralized ESB

Saturday, June 30th, 2007

Via David McGhee’s Q&A with Dr. Don Ferguson, but read the whole thing.

Q: Could you tell you your thoughts or preference for a distributed or centralized ESB?

DON: there is no such thing as a centralized ESB.

This is the problem with a lot of the products that call themselves ESBs. They are centralized brokers which may be clustered for availability. But they are in no way an implementation of the Bus Architectural Pattern. Please check this before cutting a check to your vendor.

Also, understand that if you do security related things in your ESB, possibly as a part of your routing rules, that if the security infrastructure is centralized that means your ESB is too. Even if it really was distributed to begin with.

Buyer beware.



[Podcast] Asynch Communication and the User Experience

Thursday, June 21st, 2007

In this podcast we answer questions about how web presentation layers can communicate asynchronously with services, Service Level Agreements with respect to asynchronous user interaction, and if using compiled DLLs for message schemas creates tight coupling.

Download via the Dr. Dobbs’ site

Or download directly here.

And here’s the original question:

Hi Udi,

I thought I’d ask a question about an implementation that seems to be bouncing off my head. We have been thinking about services that can, at the business owners discretion based on SLA expectations, be synchronous, async but within a “human” timeframe ie, the gui could still wait and just fail gracefully if the response took too long, or async where the client must check back later.

Client abc is using application xyz and sends a message via the bus to do something. Logically, client abc is waiting for a response even though application xyz sent it asynchronously. Client abc may wait with the gui in front of them, with a progress bar running or a little ajaxy polling widget. They may wait for an email or an message on their web site for long running operations. How would you arrange that the response to this particular message would get routed to only client abc? If the answer is you shouldn’t think that way, then what would a subscriber to that message type, as shown in your example, do with the client abc’s response? Imagine a password reset example where the result is a temporary password but the service consults or updates multiple systems as a result of the request.

You also use a shared message type library. As we have been studying WCF, we have focused mostly on the features that would help us allow service consumers and suppliers move independently. We have been looking at message versioning and loose coupling techniques. We have settled on the idea that a shared message library would make our message too brittle, once again forcing each consumer to sync up with each service update. What is your take on that?

Philip

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Humble yet uncompromising, Udi is a pleasure to interact with.”

Glenn Block Glenn Block, Senior Program Manager - WCF at Microsoft
“I have known Udi for many years having attended his workshops and having several personal interactions including working with him when we were building our Composite Application Guidance in patterns & practices. What impresses me about Udi is his deep insight into how to address business problems through sound architecture. Backed by many years of building mission critical real world distributed systems it is no wonder that Udi is the best at what he does. When customers have deep issues with their system design, I point them Udi's way.”

Karl Wannenmacher Karl Wannenmacher, Senior Lead Expert at Frequentis AG
“I have been following Udi’s blog and podcasts since 2007. I’m convinced that he is one of the most knowledgeable and experienced people in the field of SOA, EDA and large scale systems.
Udi helped Frequentis to design a major subsystem of a large mission critical system with a nationwide deployment based on NServiceBus. It was impressive to see how he took the initial architecture and turned it upside down leading to a very flexible and scalable yet simple system without knowing the details of the business domain. I highly recommend consulting with Udi when it comes to large scale mission critical systems in any domain.”

Simon Segal Simon Segal, Independent Consultant
“Udi is one of the outstanding software development minds in the world today, his vast insights into Service Oriented Architectures and Smart Clients in particular are indeed a rare commodity. Udi is also an exceptional teacher and can help lead teams to fall into the pit of success. I would recommend Udi to anyone considering some Architecural guidance and support in their next project.”

Ohad Israeli Ohad Israeli, Chief Architect at Hewlett-Packard, Indigo Division
“When you need a man to do the job Udi is your man! No matter if you are facing near deadline deadlock or at the early stages of your development, if you have a problem Udi is the one who will probably be able to solve it, with his large experience at the industry and his widely horizons of thinking , he is always full of just in place great architectural ideas.
I am honored to have Udi as a colleague and a friend (plus having his cell phone on my speed dial).”

Ward Bell Ward Bell, VP Product Development at IdeaBlade
“Everyone will tell you how smart and knowledgable Udi is ... and they are oh-so-right. Let me add that Udi is a smart LISTENER. He's always calibrating what he has to offer with your needs and your experience ... looking for the fit. He has strongly held views ... and the ability to temper them with the nuances of the situation.
I trust Udi to tell me what I need to hear, even if I don't want to hear it, ... in a way that I can hear it. That's a rare skill to go along with his command and intelligence.”

Eli Brin, Program Manager at RISCO Group
“We hired Udi as a SOA specialist for a large scale project. The development is outsourced to India. SOA is a buzzword used almost for anything today. We wanted to understand what SOA really is, and what is the meaning and practice to develop a SOA based system.
We identified Udi as the one that can put some sense and order in our minds. We started with a private customized SOA training for the entire team in Israel. After that I had several focused sessions regarding our architecture and design.
I will summarize it simply (as he is the software simplist): We are very happy to have Udi in our project. It has a great benefit. We feel good and assured with the knowledge and practice he brings. He doesn’t talk over our heads. We assimilated nServicebus as the ESB of the project. I highly recommend you to bring Udi into your project.”

Catherine Hole Catherine Hole, Senior Project Manager at the Norwegian Health Network
“My colleagues and I have spent five interesting days with Udi - diving into the many aspects of SOA. Udi has shown impressive abilities of understanding organizational challenges, and has brought the business perspective into our way of looking at services. He has an excellent understanding of the many layers from business at the top to the technical infrstructure at the bottom. He is a great listener, and manages to simplify challenges in a way that is understandable both for developers and CEOs, and all the specialists in between.”

Yoel Arnon Yoel Arnon, MSMQ Expert
“Udi has a unique, in depth understanding of service oriented architecture and how it should be used in the real world, combined with excellent presentation skills. I think Udi should be a premier choice for a consultant or architect of distributed systems.”

Vadim Mesonzhnik, Development Project Lead at Polycom
“When we were faced with a task of creating a high performance server for a video-tele conferencing domain we decided to opt for a stateless cluster with SQL server approach. In order to confirm our decision we invited Udi.

After carefully listening for 2 hours he said: "With your kind of high availability and performance requirements you don’t want to go with stateless architecture."

One simple sentence saved us from implementing a wrong product and finding that out after years of development. No matter whether our former decisions were confirmed or altered, it gave us great confidence to move forward relying on the experience, industry best-practices and time-proven techniques that Udi shared with us.
It was a distinct pleasure and a unique opportunity to learn from someone who is among the best at what he does.”

Jack Van Hoof Jack Van Hoof, Enterprise Integration Architect at Dutch Railways
“Udi is a respected visionary on SOA and EDA, whose opinion I most of the time (if not always) highly agree with. The nice thing about Udi is that he is able to explain architectural concepts in terms of practical code-level examples.”

Neil Robbins Neil Robbins, Applications Architect at Brit Insurance
“Having followed Udi's blog and other writings for a number of years I attended Udi's two day course on 'Loosely Coupled Messaging with NServiceBus' at SkillsMatter, London.

I would strongly recommend this course to anyone with an interest in how to develop IT systems which provide immediate and future fitness for purpose. An influential and innovative thought leader and practitioner in his field, Udi demonstrates and shares a phenomenally in depth knowledge that proves his position as one of the premier experts in his field globally.

The course has enhanced my knowledge and skills in ways that I am able to immediately apply to provide benefits to my employer. Additionally though I will be able to build upon what I learned in my 2 days with Udi and have no doubt that it will only enhance my future career.

I cannot recommend Udi, and his courses, highly enough.”

Nick Malik Nick Malik, Enterprise Architect at Microsoft Corporation
You are an excellent speaker and trainer, Udi, and I've had the fortunate experience of having attended one of your presentations. I believe that you are a knowledgable and intelligent man.”

Sean Farmar Sean Farmar, Chief Technical Architect at Candidate Manager Ltd
“Udi has provided us with guidance in system architecture and supports our implementation of NServiceBus in our core business application.

He accompanied us in all stages of our development cycle and helped us put vision into real life distributed scalable software. He brought fresh thinking, great in depth of understanding software, and ongoing support that proved as valuable and cost effective.

Udi has the unique ability to analyze the business problem and come up with a simple and elegant solution for the code and the business alike.
With Udi's attention to details, and knowledge we avoided pit falls that would cost us dearly.”

Børge Hansen Børge Hansen, Architect Advisor at Microsoft
“Udi delivered a 5 hour long workshop on SOA for aspiring architects in Norway. While keeping everyone awake and excited Udi gave us some great insights and really delivered on making complex software challenges simple. Truly the software simplist.”

Motty Cohen, SW Manager at KorenTec Technologies
“I know Udi very well from our mutual work at KorenTec. During the analysis and design of a complex, distributed C4I system - where the basic concepts of NServiceBus start to emerge - I gained a lot of "Udi's hours" so I can surely say that he is a professional, skilled architect with fresh ideas and unique perspective for solving complex architecture challenges. His ideas, concepts and parts of the artifacts are the basis of several state-of-the-art C4I systems that I was involved in their architecture design.”

Aaron Jensen Aaron Jensen, VP of Engineering at Eleutian Technology
Awesome. Just awesome.

We’d been meaning to delve into messaging at Eleutian after multiple discussions with and blog posts from Greg Young and Udi Dahan in the past. We weren’t entirely sure where to start, how to start, what tools to use, how to use them, etc. Being able to sit in a room with Udi for an entire week while he described exactly how, why and what he does to tackle a massive enterprise system was invaluable to say the least.

We now have a much better direction and, more importantly, have the confidence we need to start introducing these powerful concepts into production at Eleutian.”

Gad Rosenthal Gad Rosenthal, Department Manager at Retalix
“A thinking person. Brought fresh and valuable ideas that helped us in architecting our product. When recommending a solution he supports it with evidence and detail so you can successfully act based on it. Udi's support "comes on all levels" - As the solution architect through to the detailed class design. Trustworthy!”

Chris Bilson Chris Bilson, Developer at Russell Investment Group
“I had the pleasure of attending a workshop Udi led at the Seattle ALT.NET conference in February 2009. I have been reading Udi's articles and listening to his podcasts for a long time and have always looked to him as a source of advice on software architecture.
When I actually met him and talked to him I was even more impressed. Not only is Udi an extremely likable person, he's got that rare gift of being able to explain complex concepts and ideas in a way that is easy to understand.
All the attendees of the workshop greatly appreciate the time he spent with us and the amazing insights into service oriented architecture he shared with us.”

Alexey Shestialtynov Alexey Shestialtynov, Senior .Net Developer at Candidate Manager
“I met Udi at Candidate Manager where he was brought in part-time as a consultant to help the company make its flagship product more scalable. For me, even after 30 years in software development, working with Udi was a great learning experience. I simply love his fresh ideas and architecture insights.
As we all know it is not enough to be armed with best tools and technologies to be successful in software - there is still human factor involved. When, as it happens, the project got in trouble, management asked Udi to step into a leadership role and bring it back on track. This he did in the span of a month. I can only wish that things had been done this way from the very beginning.
I look forward to working with Udi again in the future.”

Christopher Bennage Christopher Bennage, President at Blue Spire Consulting, Inc.
“My company was hired to be the primary development team for a large scale and highly distributed application. Since these are not necessarily everyday requirements, we wanted to bring in some additional expertise. We chose Udi because of his blogging, podcasting, and speaking. We asked him to to review our architectural strategy as well as the overall viability of project.
I was very impressed, as Udi demonstrated a broad understanding of the sorts of problems we would face. His advice was honest and unbiased and very pragmatic. Whenever I questioned him on particular points, he was able to backup his opinion with real life examples. I was also impressed with his clarity and precision. He was very careful to untangle the meaning of words that might be overloaded or otherwise confusing. While Udi's hourly rate may not be the cheapest, the ROI is undoubtedly a deal. I would highly recommend consulting with Udi.”

Robert Lewkovich, Product / Development Manager at Eggs Overnight
“Udi's advice and consulting were a huge time saver for the project I'm responsible for. The $ spent were well worth it and provided me with a more complete understanding of nServiceBus and most importantly in helping make the correct architectural decisions earlier thereby reducing later, and more expensive, rework.”

Ray Houston Ray Houston, Director of Development at TOPAZ Technologies
“Udi's SOA class made me smart - it was awesome.

The class was very well put together. The materials were clear and concise and Udi did a fantastic job presenting it. It was a good mixture of lecture, coding, and question and answer. I fully expected that I would be taking notes like crazy, but it was so well laid out that the only thing I wrote down the entire course was what I wanted for lunch. Udi provided us with all the lecture materials and everyone has access to all of the samples which are in the nServiceBus trunk.

Now I know why Udi is the "Software Simplist." I was amazed to find that all the code and solutions were indeed very simple. The patterns that Udi presented keep things simple by isolating complexity so that it doesn't creep into your day to day code. The domain code looks the same if it's running in a single process or if it's running in 100 processes.”

Ian Cooper Ian Cooper, Team Lead at Beazley
“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do.”

Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading.
Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project.
On the personal level - Udi is a great communicator and can persuade even the most difficult audiences (I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”

Adam Dymitruk Adam Dymitruk, Director of IT at Apara Systems
“I met Udi for the first time at DevTeach in Montreal back in early 2007. While Udi is usually involved in SOA subjects, his knowledge spans all of a software development company's concerns. I would not hesitate to recommend Udi for any company that needs excellent leadership, mentoring, problem solving, application of patterns, implementation of methodologies and straight out solution development.
There are very few people in the world that are as dedicated to their craft as Udi is to his. At ALT.NET Seattle, Udi explained many core ideas about SOA. The team that I brought with me found his workshop and other talks the highlight of the event and provided the most value to us and our organization. I am thrilled to have the opportunity to recommend him.”

Eytan Michaeli Eytan Michaeli, CTO Korentec
“Udi was responsible for a major project in the company, and as a chief architect designed a complex multi server C4I system with many innovations and excellent performance.”


Carl Kenne Carl Kenne, .Net Consultant at Dotway AB
“Udi's session "DDD in Enterprise apps" was truly an eye opener. Udi has a great ability to explain complex enterprise designs in a very comprehensive and inspiring way. I've seen several sessions on both DDD and SOA in the past, but Udi puts it in a completly new perspective and makes us understand what it's all really about. If you ever have a chance to see any of Udi's sessions in the future, take it!”

Avi Nehama, R&D Project Manager at Retalix
“Not only that Udi is a briliant software architecture consultant, he also has remarkable abilities to present complex ideas in a simple and concise manner, and...
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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