Udi Dahan   Udi Dahan – The Software Simplist
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Archive for the ‘Performance’ Category

Common CQRS Abuses

Sunday, February 26th, 2012

Abuse #1

“I’m using CQRS because I need to scale.”

While CQRS may be more scalable than other more traditional architectures, the use of asynchronous communication often complicates the user interaction model causing users to not see the changes they made to data in the UI until later. Trying to compensate for this (by writing even more code) digs one deeper into the complexity hole.

When I point to non-collaborative subdomains and state “You don’t need CQRS for that”, the reason is that in these areas you don’t tend to have much read/write contention. While multiple users/actors may be working in parallel, they don’t touch the same set of data (or do so only very rarely).

In these environments, all you need is a scalable data storage technology – something designed to scale-out (unlike most relational databases). This can take the form of NOSQL databases like HBase and Cassandra. Often all you need is the UI to query that directly and show the results, and the same goes for persisting the data back – possibly with some basic validation and calculation code on the side.

No commands, events, DTOs, publish/subscribe, domain model, etc.

As Ayende says – JFHCI, just f-ing hard code it.

You’d be surprised how much of your data this approach can apply to.

With the time you save on all the less important stuff, you’ll have more time to apply CQRS the right way for the high-value/high-complexity parts of your system.


Just a final note, as registration for my course in New York is coming to a close in 2 weeks, I wanted to let you all know that the price for the course will be going up this April, after the course in Sydney. The reason for this is that the courses I run myself (at the current rate) have been cannibalizing attendees from the partner companies I do the course with.

I’ll be providing significant discounts to independent consultants (and others paying their own way) to try to keep things fair. Hope to see you there.

Go to the registration page.

The Myth Of “Infinite Scalability”

Thursday, December 29th, 2011

globeScalability is a topic near and dear to my heart.

Many a client seeks me out for the first time for help in this area.

Usually the request is for an amount substantially smaller than infinity.

It’s usually on the discussion groups and in conference presentations that infinity is brought into it.

The basics

The first issue with scalability is the use of the word as an adjective: scalable.

“Is the system scalable?”

Or the similar verb use: “Does it scale?”

The problem here is the implication that there is a yes/no answer to the question.

Scalability is not boolean.

Linear Scalability

scalabilityWhen people talk about scalability, or a system being able to scale, they’re usually referring to a graph that looks something like this:

The red graph indicating a system that does not scale well, the green graph indicating one that does.

What is missing from this diagram are the labels of the axes.

The Y axis is Cost, Expense, or Money.
The X axis is usually the number of users (for internet-type companies).

Ultimately, scalability is a cost-function that will tell us how much it will cost to have the system support a certain number of users.

Linear scalability is when the cost of the next user is the same as the cost of the previous user. This means our system doesn’t have bottlenecks. This is what people usually mean when they say “infinite scalability”.

But there’s more

As many of the internet companies (and their investors) have realized over the years, there’s a difference between the number of users and the number of active users. It’s very easy to scale to a billion users when only 1000 of them are active at any given time.

To be more accurate, what we want is additional X-axes for things like total data managed by the system, number of requests per user, resource utilization per request, propagation speed (how quickly information entered by one user needs to be visible to others), and more.

Scalability is a multi-dimensional cost function, where part of an architects job is to figure out which dimensions are significant for the system/business, and what the expectation for growth is across each axis.

Preparing for “infinity”

Be careful not to optimize for only a single dimension – reality is a whole lot more complex.

There are so many other things to deal with as a system scales.

For example, do you really think you’re going to want your configuration entirely centralized? Putting everything in one place means easier management, yes, but it also means a mistake will instantly affect everyone. Is it worth the risk? Maybe instead of centralization, we could do with some automation that will allow a staged rollout of configuration changes with the ability to rollback.

The same goes for rolling out new versions, patches, and upgrades.

But that now means we may have multiple versions of the same system in production at the same time. How will that work? Will they all talk to the same database? How will we version the database then? If not, how will we handle state? Won’t this mean our code will have to be backwards compatible from one version to another? Isn’t that hard? Like, insanely hard?

Please, can we park the whole “infinite scalability” thing?
It’s really not the most important concern – not by a long shot.

MySpace Architecture Considered Expensive

Friday, October 9th, 2009

I just finished listening to the Microsoft presentation on how they use the Concurrency & Coordination Runtime (CCR) in MySpace (the stated largest web site running .NET).

Some interesting numbers were stated in the talk.

  • Tens of thousands to hundreds of thousands of requests per second
  • Over 3 thousand web servers
  • Over a thousand mid-tier servers

No wonder most big web sites don’t run .NET. The Windows licenses would put them out of business.

Well, that is if you follow those same architectural practices.

I’ve written in the past of alternative architectural approaches that can scale to those levels at easily an order of magnitude less hardware (I think it’s closer to two OOMs) – here’s one of them on the topic of weather:

Building Super-Scalable Web Systems with REST.

By the way, the client quoted in that post is now well above 60 million users with only small incremental increases in hardware. Oh, and their running everything on Windows and .NET. The question is not “can it scale”, but rather “how much will it cost to scale”.

Architecture pays itself back faster than ever in the Web 2.0 world.

MSDN Magazine Smart Client Article

Saturday, March 28th, 2009


My article on “optimizing a large-scale Software+Services application” has been published in the April edition of MSDN Magazine.

Here’s a short excerpt:

“We had to juggle occasional connectivity, data synchronization, and publish/subscribe all at the same time. We learned that we couldn’t solve all problems either client-side or server-side, but rather that an integrated approach was needed since any changes on one side needed corresponding changes on the other side.”

Continue reading…

Building Super-Scalable Web Systems with REST

Monday, December 29th, 2008

I’ve been consulting with a client who has a wildly successful web-based system, with well over 10 million users and looking at a tenfold growth in the near future. One of the recent features in their system was to show users their local weather and it almost maxed out their capacity. That raised certain warning flags as to the ability of their current architecture to scale to the levels that the business was taking them.


On Web 2.0 Mashups

One would think that sites like Weather.com and friends would be the first choice for implementing such a feature. Only thing is that they were strongly against being mashed-up Web 2.0 style on the client – they had enough scalability problems of their own. Interestingly enough (or not), these partners were quite happy to publish their weather data to us and let us handle the whole scalability issue.

Implementation 1.0

The current implementation was fairly straightforward – client issues a regular web service request to the GetWeather webmethod, the server uses the user’s IP address to find out their location, then use that location to find the weather for that location in the database, and return that to the user. Standard fare for most dynamic data and the way most everybody would tell you to do it.

Only thing is that it scales like a dog.

Add Some Caching

The first thing you do when you have scalability problems and the database is the bottleneck is to cache, well, that’s what everybody says (same everybody as above).

The thing is that holding all the weather of the entire globe in memory, well, takes a lot of memory. More than is reasonable. In which case, there’s a fairly decent chance that a given request can’t be served from the cache, resulting in a query to the database, an update to the cache, which bumps out something else, in short, not a very good hit rate.

Not much bang for the buck.

If you have a single datacenter, having a caching tier that stores this data is possible, but costly. If you want a highly available, business continuity supportable, multi-datacenter infrastructure, the costs add up quite a bit quicker – to the point of not being cost effective (“You need HOW much money for weather?! We’ve got dozens more features like that in the pipe!”)

What we can do is to tell the client we’re responding to that they can cache the result, but that isn’t close to being enough for us to scale.

Look at the Data, Leverage the Internet

When you find yourself in this sort of situation, there’s really only one thing to do:

In order to save on bandwidth, the most precious commodity of the internet, the various ISPs and backbone providers cache aggressively. In fact, HTTP is designed exactly for that.

If user A asks for some html page, the various intermediaries between his browser and the server hosting that page will cache that page (based on HTTP headers). When user B asks for that same page, and their request goes through one of the intermediaries that user A’s request went through, that intermediary will serve back its cached copy of the page rather than calling the hosting server.

Also, users located in the same geographic region by and large go through the same intermediaries when calling a remote site.

Leverage the Internet

The internet is the biggest, most scalable data serving infrastructure that mankind was lucky enough to have happen to it. However, in order to leverage it – you need to understand your data and how your users use it, and finally align yourself with the way the internet works.

Let’s say we have 1,000 users in London. All of them are going to have the same weather. If all these users come to our site in the period of a few hours and ask for the weather, they all are going to get the exact same data. The thing is that the response semantics of the GetWeather webmethod must prevent intermediaries from caching so that users in Dublin and Glasgow don’t get London weather (although at times I bet they’d like to).

REST Helps You Leverage the Internet

Rather than thinking of getting the weather as an operation/webmethod, we can represent the various locations weather data as explicit web resources, each with its own URI. Thus, the weather in London would be http://weather.myclient.com/UK/London.

If we were able to make our clients in London perform an HTTP GET on http://weather.myclient.com/UK/London then we could return headers in the HTTP response telling the intermediaries that they can cache the response for an hour, or however long we want.

That way, after the first user in London gets the weather from our servers, all the other 999 users will be getting the same data served to them from one of the intermediaries. Instead of getting hammered by millions of requests a day, the internet would shoulder easily 90% of that load making it much easier to scale. Thanks Al.

This isn’t a “cheap trick”. While being straight forward for something like weather, understanding the nature of your data and intelligently mapping that to a URI space is critical to building a scalable system, and reaping the benefits of REST.

What’s left?

The only thing that’s left is to get the client to know which URI to call. A simple matter, really.

When the user logs in, we perform the IP to location lookup and then write a cookie to the client with their location (UK/London). That cookie then stays with the user saving us from having to perform that IP to location lookup all the time. On subsequent logins, if the cookie is already there, we don’t do the lookup.

BTW, we also show the user “you’re in London, aren’t you?” with the link allowing the user to change their location, which we then update the cookie with and change the URI we get the weather from.

In Closing

While web services are great for getting a system up and running quickly and interoperably, scalability often suffers. Not so much as to be in your face, but after you’ve gone quite a ways and invested a fair amount of development in it, you find it standing between you and the scalability you seek.

Moving to REST is not about turning on the “make it restful” switch in your technology stack (ASP.NET MVC and WCF, I’m talking to you). Just like with databases there is no “make it go fast” switch – you really do need to understand your data, the various users access patterns, and the volatility of the data so that you can map it to the “right” resources and URIs.

If you do walk the RESTful path, you’ll find that the scalability that was once so distant is now within your grasp.

An Answer of Scale

Wednesday, August 13th, 2008

To the question of scale Ayende brings up, I thought I’d tap my concept map.

First of all, I wanted to address the relationship between various topics related to scalability:

performance topics

And on the connection between scalability and throughput:

 scalability topics

The important message here is that the scalability of a system is a cost function that gives throughput as a function of recurring costs and one time costs – servers and other hardware, and the join of buy & build:

Did you write your own locking/transaction mechanism on top of an open source distributed cache or did you buy a license for a space-based technology?

Also, don’t forget that people need to administer all the servers that you have. Those people cost money (easily100K per year). Maybe, because you haven’t invested in management or monitoring tools you need one person for every two servers. This will influence the breakdown of up front costs and recurring costs. Also, the level of availability you require will impact this as well.

In my experience, architects don’t consider often enough the operations environment in their "scalability calculations".

What this means is that there’s no such thing as technically "not being able to scale".

Rather, that the cost (up front + recurring) of supporting higher throughput grows faster than the function of revenue per user/request/whatever.

Sometimes, the solution is just to find ways to make more money per customer.

For more technical solutions, take a look at the difference between capacity and scalability and how the competing consumer pattern helps scale out.

Scalability, it’s all about the money.

Oh, I almost forgot, I also had a great conversation with Carl and Richard about scaling web sites that’s now up on the .NET Rocks site. Enjoy.

Durable Messaging Dilemmas

Thursday, July 17th, 2008

I’ve received some great feedback on my MSDN article and some really great questions that I think more people are wondering about, so I think I’ll try to do a post per question and see how that goes.

Libor asks:

“Would you recommend using durable messaging for systems where there are similar requirements with respect to data reliability as you had – ie. not losing any messages? If so, then why didn’t the final version of your solution use it? If not, can you explain why?”

The answer is, as always, it depends, but here’s on what it depends:

When designing a system, we need to take a good, hard look at how we manage state, and what properties that state has. In a system of reasonable size we can expect various families of state with respect to their business value, data volatility, and fault-tolerance window. Each family needs to be treated differently. While durable messaging may be suitable for one, it may be overkill or underkill for another.

So, here’s what we’re going to be looking at:

  1. Business Value
  2. Data Volatility
  3. Fault-Tolerance Window

Business Value

When talking about business value, I want to talk about what it means “not losing any messages”. The question is under what conditions will the messages not be lost, or rather, what are the threshold conditions where messages may start getting lost. If all our datacenters are nuked, we will lose data. It’s likely the business is OK with that (as much as can be expected under those circumstances). If a single server goes down, it’s likely the business would not be OK with losing messages containing financial data. However if a message requesting the health of a server were to get lost under those same conditions, that would probably be alright. In other words, what does that message represent in business terms.

Data Volatility

Data volatility also has an impact. Let’s say that we’re building a financial trading system. The time that it takes us to respond to an event (message) that the cost of a certain financial instrument has changed, and the message that we send requesting to buy that security is critical. Let’s say that has to be done in under 10ms. Now, some failure has occurred preventing our message from reaching its destination for 20ms. What should we do with that message? Should we keep it around, making sure it doesn’t get lost? Not in this domain. On the contrary, that message should be thrown away as its “business lifetime” has been exceeded. Furthermore, even during that original period of 10ms, the use of durable messaging may make it close to impossible to maintain our response times.

Fault-Tolerance Window

These two topics feed into the third and more architectural one – fault-tolerance window: what period of time do we require fault tolerance, and with respect to how many (and what kind of) faults? This will lead us into an analysis of to how many machines do we need to copy a message before we release the calling thread. We’d also look at in which datacenters those machines reside. This will also impact (or be impacted by) the kinds of links we have to these datacenters if we want to maintain response times. These numbers will need to change when the system identifies a disaster – degrading itself to a lower level of fault-tolerance after a hurricane knocks out a datacenter, and returning to normal once it comes back up.

Re-Evaluating Durable Messaging

Durable messaging may be used at various points in each part of the solution, but we need to look at message size, the rate those messages are being written to disk, how fast the disk is, how much available disk we have (so we don’t make things worse in the case of degraded service), etc. Companies like Amazon also take into account disk failure rates, replacement rates (disks aren’t replaced immediately you know), and many other factors when making these decisionsimage


Our job as architects when designing the system is to find that cost-benefit balance for the various parts of the system according to these very applicative parameters. No, it’s not easy. No, cloud computing will not magically solve all of this for us. But, we are getting more technical tools to work with, operations staff is getting better at working with us in the design phase, and our thought processes more rigorous in dealing with the scary conditions of the real world.

To your question, Libor, as to why we didn’t eventually use durable messaging in our solution, the answer is that we solved the overall state management problem by setting up an applicative protocol with our partners which was resilient in the face of faults by using idempotent messages that could be resent as many times as necessary. You can read more about it here. This solution isn’t viable for other kinds of interactions but was just what we needed to get the job done.

Hope that helps.

NServiceBus Performance

Wednesday, May 21st, 2008

I’ve gotten this question several times already but now companies are beginning to look for performance comparisons in making decisions around the use of nServiceBus. It’s often compared to straight WCF, BizTalk, and now Neuron ESB. In Sam’s recent post he posts to a case study of Neuron doing 28 million messages an hour. That’s far more than I’ve ever heard quoted for BizTalk.


Before giving some numbers, please keep in mind that high performance of system infrastructure does not necessarily by itself mean that the system above it is running that fast. For instance, you may have server heartbeats running really quickly but the time it takes to save a purchase order borders on a minute. So, please, take all benchmarks with a grain of salt, or two, or a whole shaker-full.

While I’m not at liberty to say on which specific domain/company these numbers were measured, I can say that we had the full gamut of “stateless services”, statefull services (sagas), number crunching, large data sets, many users, complex visualization, etc. Also, this wasn’t the largest installation of nServiceBus that I’m aware of, but its the one I have the most specific numbers for.


OK, so using the default nServiceBus distribution using MSMQ, on servers where the queue files themselves were on separate SCSI RAID disks, we were pumping around 1000 durable, transactionally processed messages per second, per server. That means that similar to the Neuron case, no messages would be lost in the case of a single fault per server per window (time to replace a failed disk set at 3 hours from failure, through detection, to replacement per site – but that’s more an operational staffing concern, not the technology itself).

So, that’s 3.6 million messages per hour per server, at full load. We had a total of 98 servers doing these kinds of processing, not including web servers, databases, etc. Keep in mind that web servers would be communicating with other servers using nServiceBus, but that would maybe be an unfair comparison to the Neuron numbers.

Server Breakdown

Anyway, the 48 number crunching servers (blade centers) we had were at full load, so we were pumping more than 170 million messages there. Keep in mind that those servers had a really fast backbone so weren’t held up by IO. Your environment may be different.

Another 30 (regular pizza boxes) were doing our sagas. Saga state was stored in a distributed in-memory “cache”, so once again IO wasn’t an issue for processing those messages. We were at about 70% utilization there, coming to just over 100 million messages an hour.

The last 20 were clustered boxes (fairly expensive) that handled the various nServiceBus distributor and timeout manager processes were at full load since they handled control messages for all the servers as well as dynamically routing the load. However, on those boxes we used much higher performance disks for the messages, since they had to feed everything else, capable of doing, on average, around 5000 messages a second. That adds up to 360 million messages an hour.

Unnecessary Durability

Later, we moved a bunch of messages that didn’t need all that durability and transactionality off the disks, pushing the total throughput over 1 billion messages an hour. That was about 100 million per hour durable, 900 million per hour non-durable. You can guess that we were left with plenty of IO to spare at that point while we weren’t yet pushing the limit of our memory.

One thing that’s important to understand is the size of the messages that didn’t require durability was less than 1MB, with most weighing in under 10KB. Also, since most of those messages were published, less state management was required around them, enabling us to further improve performance.


NServiceBus didn’t give us all that by itself. It was the result of skilled architects, developers, and operations staff working together for many iterations, deploying, monitoring, re-designing, etc. You need to understand your technology, your hardware, and your specific performance, availability, and fault-tolerance requirements if you want to get anywhere.

There’s no magic.

I didn’t see the number or kinds of servers involved in the Neuron case study so this wasn’t ever really a comparison. Nor or we talking about the same system here.

So, please, don’t base your decisions on arbitrary numbers. Spend some time setting up a scaled down version of your target architecture with all the relevant technologies and measure. Be aware that you want high performance end to end, not just of the messaging part. At times, it makes sense to actively throw away messages (of the non-durable, published kind) to help a server come online faster especially after a restart.

Thus ends the tale of another “benchmark”.

Scalability Article up on InfoQ

Thursday, April 10th, 2008

I’ve published a new article on performance and scalability on InfoQ:

Spectacular Scalability with Smart Service Contracts

In this article, I attempt to debunk some of the myths around stateless-ness as the key to scalability.

Here’s how it starts:

It was a sunny day in June 2005 and our spirits were high as we watched the new ordering system we’d worked on for the past 2 years go live in our production environment. Our partners began sending us orders and our monitoring system showed us that everything looked good. After an hour or so, our COO sent out an email to our strategic partners letting them know that they should send their orders to the new system. 5 minutes later, one server went down. A minute after that, 2 more went down. Partners started calling in. We knew that we wouldn’t be seeing any of that sun for a while.

The system that was supposed to increase the profitability of orders from strategic partners crumbled. The then seething COO emailed the strategic partners again, this time to ask them to return to the old system. The weird thing was that although we had servers to spare, just a few orders from a strategic customer could bring a server to its knees. The system could scale to large numbers of regular partners, but couldn’t handle even a few strategic partners.

This is the story of what we did wrong, what we did to fix it, and how it all worked out.

Continue reading…

WCF Everywhere? Not on my watch.

Saturday, December 29th, 2007

silver bullets The other day I was at Juval’s presentation where the main message was WCF is a better .NET. In other words, if you use WCF on every one of your classes, you’ll benefit. I don’t know about you, but I’m quite wary of silver bullets – they tend to inflict quite a bit of pain when used indiscriminately. This post is my response to all the people who came up to me at the end of the presentation and wanted to know if I agreed with these far-reaching architectural statements.

oz First of all let me say that Juval is indeed a master presenter. The “looks like a class, walks like a class, quacks like a class” bit was excellent. I could tell that most people didn’t notice the speedy hands quickly deleting all attributes from the classes before the “looks like a class…” bit. At times, I got flashbacks from the Wizard of Oz – “pay no attention to the man behind the curtain”. If all attributes in WCF only went on the interfaces, then this might actually fly, but we all know that that’s not the case.

One of the interesting comparisons Juval made with WCF was the introduction of .NET. Few people in the audience seemed to remember (or maybe were just professionally younger than .NET’s 8 years), but when it came out .NET was marketed as being mainly about XML Web Services. Juval stated that this was done to play down the fact that .NET made the previous Windows programming technologies obsolete. He then drew the same conclusion about WCF – that it’s as much .NET 3.0 as .NET was the next version of MFC; besides being written in a language that resembles the previous technology, it’s really all different. I don’t think that anyone would argue the difference, but is it really a “plain .NET” killer?

The answer seemed to come around the overhead of WCF – yet Juval deftly deflected that issue with a demo showing WCF doing 200 calls a second. And everybody just bought it – I was shocked. That’s 5ms per call. If you actually take Juval’s advice and use WCF on all your classes, you’ve bought yourself one hell of a performance nightmare. Say you have around 20 of your objects involved in a sequence to handle a user action – not that many actually. With a 5ms lag per object interaction, that user action is going to take 100ms – not including any database or webservice stuff you might be doing. If you do that in a server environment, you’ll be doing roughly 10 concurrent users per core. And that’s not even doing any heavy calculations or anything. Moderately sized systems are running upwards of 1000 concurrent users – if they needed 100 cores (or dozens of servers) for that, I’m guessing that they’d be out of business.

Let’s cut this short – WCF everywhere doesn’t scale, doesn’t perform, isn’t maintainable, or testable either. In other words – don’t do it. I know Juval is a brilliant guy, and an amazing presenter – but I don’t believe he would be employing this with his own clients. This actually bears repeating. WCF is a fine technology for your application’s boundaries, but don’t be pushing it in.

Don’t do it.


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Bryan Wheeler, Director Platform Development at msnbc.com
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We brought him on site to give our development staff the 5-day “Advanced Distributed System Design” training. The course profoundly changed our understanding and approach to SOA and distributed systems.

Consider some of the evidence: 1. Months later, developers still make allusions to concepts learned in the course nearly every day 2. One of our developers went home and made her husband (a developer at another company) sign up for the course at a subsequent date/venue 3. Based on what we learned, we’ve made constant improvements to our architecture that have helped us to adapt to our ever changing business domain at scale and speed If you have the opportunity to receive the training, you will make a substantial paradigm shift.

If I were to do the whole thing over again, I’d start the week by playing the clip from the Matrix where Morpheus offers Neo the choice between the red and blue pills. Once you make the intellectual leap, you’ll never look at distributed systems the same way.

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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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