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Archive for the ‘Web Services’ Category

A Queue Isn’t An Implementation Detail

Monday, May 25th, 2009

It’s hard to believe that this continues to pop up even as WCF is reaching its fourth version (emphasis mine):

“A common complaint is that the first call on a client object takes some disproportionately large amount of time, usually ten seconds or more, while successive calls are instantaneous. There are many reasons why this might happen so there’s no generic resolution for this problem.” — Nicholas Allen

The thing is that there IS a generic solution to this problem.

It’s queued messaging.

The only thing is that you have to give up talking to your services as if they were regular objects – calling methods on them and expecting a response. In other words, designing a distributed systems isn’t like designing a regular OO system just with some WCF sprinkled on top.

Even when trying to do fire and forget messaging on top of WCF (void method calls with the OneWay attribute), the underlying channel can still block your thread, as Nick mentioned.

A queue isn’t an implementation detail.
It’s the primary architectural abstraction of a distributed system.

Backwards-Compatibility: Why Most Versioning Problems Aren’t

Friday, April 10th, 2009


I’ve been to too many clients where I’ve been brought in to help them with their problems around service versioning when the solution I propose is simply to have version N+1 of the system be backwards-compatible with version N. If two adjacent versions of a given system aren’t compatible with each other, it is practically impossible to solve versioning issues.

Here’s what happens when versions aren’t compatible:

Admins stop the system from accepting any new requests, and wait until all current requests are done processing. They take a backup/snapshot of all relevant parts of the system (like data in the DB). Then, bring down the system – all of it. Install the new version on all machines. Bring everything back up. Let the users back in.

If, heaven-forbid, problems were uncovered with the new version (since some problems only appear in production), the admins have to roll back to the previous version – once again bringing everything down.

This scenario is fairly catastrophic for any company that requires not-even high availability, but pretty continuous availability – like public facing web apps.

If adjacent versions were compatible with each other, we could upgrade the system piece-meal – machine by machine, where both the old and new versions will be running side by side, communicating with each other. While the system’s performance may be sub-optimal, it will continue to be available throughout upgrades as well as downgrades.

This isn’t trivial to do.

It impacts how you decide what is (and more importantly, what isn’t) nullable.

It may force you to spread certain changes to features across more versions (aka releases).

As such, you can expect this to affect how you do release and feature planning.

However, if you do not take these factors into account, it’s almost a certainty that your versioning problems will persist and no technology (new or old) will be able to solve them.

Coming next… Units of versioning – inside and outside a service.

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.

Scaling Long Running Web Services

Wednesday, July 30th, 2008

While I was at TechEd USA I had an attendee, Will, come up and ask me an interesting question about how to handle web service calls that can take a long time to complete. He has a number of these kinds of requests ranging from computationally intensive tasks to those requiring sifting through large amounts of data. What Will was having problems with was preventing too many of these resource-intensive tasks from running concurrently (causing increased memory usage, paging, and eventually the server becoming unavailable).

For comparison later, here’s a diagram showing the trivial interaction:


One solution that he’d tried was to set up the web server to throttle those requests and keep a much smaller maximum thread-pool size for that application pool. The unfortunate side effect of that solution was that clients would get “turned away” by a not-so-pleasant Connection Refused exception.

Will had been to my web scalability talk and was curious about how I was using queues behind my web services. I’ve also heard this question from people just getting started with nServiceBus when looking at the Web Services Bridge sample. Here’s the code that’s in the sample and in just a second I’ll tell you why you shouldn’t do this:

public ErrorCodes Process(Command request)
    object result = ErrorCodes.None;
    IAsyncResult sync = Global.Bus.Send(request).Register(
        delegate(IAsyncResult asyncResult)
              CompletionResult completionResult = asyncResult.AsyncState as CompletionResult;
              if (completionResult != null)
                  result = (ErrorCodes) completionResult.ErrorCode;
    return (ErrorCodes)result;

Let me repeat, this is demo-ware. Do not use this in production.

What’s happening is that in this web service call we’re putting a message in a queue for some other process/machine to process. When that processing is complete, we’ll get a message back in our local queue (which you don’t see) which is correlated to our original request, firing off the callback. We block the web method from completing (using the WaitOne call) thus keeping the HTTP connection to the client open.

The problem here is that we’re wasting resources (the HTTP connection and the thread) while waiting for a response which, as already mentioned, can take a long time. In B2B or other server to server integration environments there are all sorts of middleware solutions that help us solve these problems, however in Will’s case browsers needed to interact with this web service. All he had was HTTP.

HTTP Solutions

Another attendee who was listening in (sorry I don’t remember your name) said that he was solving similar problems using polling but that he was having scalability problems as well.

What often surprises my clients when we deal with these same issues is that I do suggest a polling based solution, but one that still uses messaging, and this is what I described to Will:

Since we can’t actually push a message to a browser over HTTP from our server when processing is complete, the browser itself will be responsible for pulling the response. We still don’t want to leave costly resources like HTTP connections open a long time, however if the browser is going to polling for a response, we’ll need some way to correlate those following requests with the original one. What we’re going to do is use the Asynchronous Completion Token pattern, and later I’ll show how to optimize it for web server technology.

Basic Polling


When the browser calls the web service, the web service will generate a Guid, put it in the message that it sends for processing, and return that guid to the browser. When the processing of the message is complete, the result will be written to some kind of database, indexed by that guid. The browser will periodically call another web method, passing in the guid it previously received as a parameter. That web method will check the database for a response using the guid, returning null if no response is there. If the browser receives a null response, it will “sleep” a bit and then retry.

One of the problems with this solution is that polling uses up server resources – both on the web server and our DB; threads, memory, DB connections. A better solution would decrease the resource cost of the polling. Let’s use the fundamental building blocks of the web to our advantage – HTTP GET and resources:

REST-full Polling

Instead of using a guid to represent the id of the response, let’s consider the REST principle of “everything’s a resource”. That would mean that the response itself would be a resource. And since every resource has a URI, we might as well use that URI in lieu of the guid. So, instead of our web service returning a guid, let’s return a URI – something like:


As you can see, the guid is still there. So, what’s different?


What’s different is that instead of having the processing code write the response to the database, it writes it to a resource. This can be done by writing some XML to a file on the SAN in the case of a webfarm. Also, the browser wouldn’t need to call a web service to get the response, it would just do an HTTP GET on the URI. If the it gets an HTTP 404, it would sleep and retry as before. The reason that the SAN is needed is that, as the browser polls, it may have its requests arrive at various web servers so the response needs to be accessible from any one of them.

Just as an aside, it would be better to free the processing node as quickly as possible and have something else write the response to the SAN. That would be done simply by sending a message from the processing node that would be handled by a different node that all it did was write responses to disk.

The reason that the URI makes a difference is that serving “static” resources is something that web servers do extremely efficiently without requiring any managed resources (like ASP.NET threads). That’s a big deal.

We’re still using HTTP connections for the polling but that’s something whose effect can be mitigated to a certain degree.

Timed REST-full Pollingimage

Since various requests can take varying amounts of time to process, it’s difficult to know at what rate the browser should poll. So, why don’t we have the web service tell it. As a part of the response to the original web service call, instead of just returning a URI, we could also return the polling interval – 1 second, 5 seconds, whatever is appropriate for the type of request. This value could easily be configurable [RequestType, PollingInterval].

An even more advanced solution would allow you to change these values dynamically. The advantage that would be gained would be that your operations team could better manage the load on your servers. When a large number of users are hitting your system, you could decrease the rate at which your servers would be polled, thus leaving more HTTP connections for other users.

Scaling and Adaptive Polling

You’d probably also want to scale out the number of processing nodes behind your queue. The nice thing is that you could change the polling interval as you scale the various processing nodes per request type providing better responsiveness for the more critical requests. Once we add virtualization, things get really fun:

We had separate queues per request type, so that we could easily see the load we were under for each type of request. That way, we could scale out the processing nodes per request type as well as change the polling interval. By virtualizing our processing nodes, and writing scripts to monitor queue sizes, we had those scripts automatically provisioning (and de-provisioning) nodes as well as changing the polling interval of the browsers.

This had the enormous benefit of the system automatically shifting resources to provide the appropriate relative allocation for the current load as its macroscopic make-up changed.


Will was well-pleased with the solution which, although more complicated than what he had originally tried, was flexible enough to meet his needs. As opposed to pure server-based solutions, here we make more use of the browser (writing our own Javascript) instead of putting our faith in some Ajax-y library. That’s not to say that you couldn’t wrap this up into a library – in essence, it is a kind of messaging transport for browser to server communication allowing duplex conversations.

In fact, what could be done is to return multiple responses to the browser over a long period of time. In the response that comes back to the browser could be an additional URI where the next response will be. This can be used for reporting the status of a long running process, paging results, and in many other scenarios.

And, one parting thought, could this not be used for all browser to web service communication?

[Podcast] Highly Scalable Web Architectures

Thursday, June 19th, 2008

For those people who couldn’t come to TechEd USA and didn’t see my talks on how to build highly scalable web architectures, you’re in luck – Craig, the man behind the Polymorphic Podcast sat down with me and we chatted about what the problems, common solutions, and effective tactics there are in this space. For those of you who were at TechEd and still didn’t come to my talk – what were you thinking?!


Check it out.

Some of this stuff is a bit counter-intuitive (and not readily supported by the tools available in Visual Studio) so please, do feel free to ask questions (in the comments below).


Friday, March 28th, 2008

Ted says it really well, and let me add a big +1.

Note to those who didn’t attend the session: you didn’t hear me say it, so I’ll repeat it: I hate WSDL almost as much as I hate Las Vegas. Ask me why sometime, or if I get enough of a critical mass of questions, I’ll blog it. If you’ve seen me do talks on Web Services, though, you’ve probably heard the rant: WSDL creates tightly-coupled endpoints precisely where loose coupling is necessary, WSDL encourages schema definitions that are inflexible and unevolvable, and WSDL intrinsically assumes a synchronous client-server invocation model that doesn’t really meet the scalability or feature needs of the modern enterprise. And that’s just for starters.

I hate WSDL.

I still hate Vegas more, though.

image Web Services, and WSDL by connection have taken hold of the industry like cancer – inhibiting the minds of otherwise intelligent developers and architects. Whenever I get the “Web Services Question” (Does X support Web Services – where X is some design pattern, tool, and sometimes nServiceBus), I have to suppress an urge to groan – I’ve got the question that many times. The other day I was at a client and Sam, their head architect asked me that question. I gave my stock response:

“When you say ‘Web Services’, are you referring to SOAP or WSDL, and is HTTP a necessary component too?”

See how good I got at the suppressing thing?

Sam conceded that Web Services over TCP is OK too, so I pressed on with:

“What about UDP? FTP? MSMQ? Is it still ‘Web Services’ then? Is the rule then that ‘Web Services’ == SOAP?”

At that point, Sam was beginning to get a little flustered.

“And what’s so great about SOAP? Is it the interoperability? Because that’s just because it’s based on XSD.”

He didn’t know how to reply. Instead, he walked away from the whiteboard and sat down. I didn’t let up:

“And what if we want to do something other than Request/Response? How about one request with many responses? How about many requests and one response? And why does this decision need to be rigid? Shouldn’t we just be able to decide programmatically how many responses we want to return? Wouldn’t that flexibility be better than creating huge response structures for web methods to return?”

image Sam made his last stand:

“Look, we can’t go and do something different from the rest of the industry. Everybody else is doing Web Services. It’s not like the technology doesn’t work.”

I gave way, a little:

“If you want, we can offer two interfaces. One, the flexible, robust, scalable XSD over messaging based solution. The second, an icky, synchronous Web Services facade which calls into our first interface.

I’m not saying that the technology doesn’t work – but both of us know that every problem has multiple solutions, some are fragile and error prone like WS, others are more elegant and have decades of knowledge behind them like messaging.

But we can do both if you like. How’s that?”

image And it was agreed. The entire system would be built on one-way messaging patterns using XSD in cases where interoperability was required. And WS would be layered on, like a tiny little pig on top of a gigantic lipstick … thing – hmm, that metaphor isn’t really working – well, you get the idea.

I hate WSDL. Never been to Vegas, though.

QCon London 2008 Recap

Thursday, March 20th, 2008

Well QCon was a blast.

NServiceBus Tutorial

I gave a full day tutorial on nServiceBus and we had a full house! The tutorial was about 90% how to think about distributed systems, and 10% mapping those concepts onto nServiceBus. I made an effort to cram about 3 days of a 5 day training course I give clients into one day, but I think I was only about 85% successful. People didn’t have the time needed to let things really sink in and ask questions, but the lively forums and skype conversations available will probably do the trick.

Jim Webber after looking at the unit testing features of nServiceBus had this to say:

“Oh my God – you’ve created testable middleware! It’ll never catch on. The vendors won’t have it.”

To which I replied that several vendors were already coming on board with their own implementations of transports and saga persistence. I have absolutely no intention, desire, or (quite frankly) the ability to write an enterprise-class middleware runtime. All I hope to do with nServiceBus is to make it so that developers use what’s out there in one, middleware-product-agnostic way that will make their code more robust and flexible.

MEST & Mark – REST & Stefan

It was also great finally meeting the head MESTian, Mark Little, who also happens to work for Redhat as SOA Technical Development Manager and Director of Standards in the JBoss division. It was interesting to see the difference between how I went about messaging in nServiceBus (full peer-to-peer including pub/sub) whereas most of the Java world has the messaging infrastructure handled by something database-like in a deployment/networking kind of perspective. If that’s the way things are done, then I can definitely appreciate the advantages of Space-Based Architectures.

And I even got to steal Stefan Tilkov‘s RESTful ear for an hour or so before I had to jet back home. It looks like we MESTians and RESTians can be one big happy family. I’m guessing that our despise of WS connects us all at a deeper level 🙂

Core Design Principles

I also gave a talk about core design principles, “Intentions & Interfaces – making patterns concrete”, and it went over very well especially considering that that was the first time that I gave that talk. You can find the slides here. From the feedback I heard after the talk, I think many people were surprised how many different parts of a system can be designed this way, and how flexible it is without making the code any more complex. The message was this:

Make Roles Explicit

Despite its simplicity, that leads to IEntity, IValidator<T> where T : IEntity, (which I wrote about a year ago – generic validation) and with a bit of Service Locator capabilities, you can add a line of code to your infrastructure that will validate all entities before they’re sent from the client to the server.

It leads to IFetchingStrategy<T> for improved database loading performance (also a year old – better DDD implementation and the NHibernate implementation).

It’s also how nServiceBus does message handling – IMessage, IMessageHandler<T> where T: IMessage, ISaga<T> where T : IMessage.

San Francisco?

Just a quick shout to my readers in the San Francisco area, if you’d be interested in hearing these talks/tutorials, give the organizers of QCon a shout and they’ll bring me out. That’s actually what got me to London – one of the attendees of a talk I gave at Oredev in Sweden last November missed my tutorial there so he put in a request and that did it. (Thanks Jan, I appreciate it!)

If you’re in a different part of the world and you’d like to have me give one of these talks, or other ones (I have a fair amount of material on Domain Models/DDD and Occasionally Connected Smart Clients), I’d be happy to make the trip and see you there as well.

[Podcast] REST + Messaging = Enterprise Solutions

Sunday, March 16th, 2008

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



Download via the Dr. Dobb’s site

Or download directly here.

Additional References

Want more?

Check out the “Ask Udi” archives.

Got a question?

Send Udi your question to answer on the show.

Distributed Architecture on ARCast.TV Rapid Response

Monday, January 14th, 2008

A while ago, me and Ron Jacobs (virtually) got together and did a couple “rapid responses” to questions on the MSDN architecture forums, and I just noticed that they’re online. The really great thing is that there are transcripts! For your convenience, I’ve included them here.

By the way, if you’re looking for more Q&A style info, check out the Ask Udi podcast. If you have a pressing question and need a shorter turn around time than the month or so it usually takes me for the podcast, send me an email to OnlineConsultation@UdiDahan.com.

Number 1

Ron: Hey, welcome back to ARCast Rapid Response. This is your host Ron Jacobs and today I’m looking at the MSDN architecture forum where I see this message from “theking2.” Yeah? OK, so “king,” he says, he’s building a distributed architecture that has a number of external systems. These external systems interface through a telnet connection and so they accept commands and return results as ACKS or NACKS.

Typically these systems have limited resources for the number of simultaneous sessions you can open, so, five to fifty depending on the system. What he did to get around this, was, he created some Enterprise Services objects and some pooled objects that set up these connections and then he has some Web services. The Web services are going to receive an incoming message. They’re going to call these pooled COM+ objects and they’re going to make the telnet calls to the external systems. Sounds interesting.

He says, after a year of production it has become apparent that some of the external systems are not performing very well. He says the bulk of the requests, but not all, to the external systems can be done asynchronously. So, he’s opting for a message queue-based solution using pseudosynchronous calls whenever a direct response is needed.

So, the question is, at what layer would message queuing make most sense?

So, should the clients, this Web service that receives the message — should it do a queue? Put a message in the queue and then the COM+ objects would pop off or they have some central Web services that would pop it. So, the central Web services or these Enterprise Service objects? Or maybe just a communication at the top of the telnet. He says this is the first time when he’s using message queuing.

On the line with me I have Udi Dahan, the Software Simplist from Israel.

Udi, this is a very interesting application and my first gut reaction is, does it really matter where you put the queuing?

Udi Dahan: Well, actually I took a look at it as well and I’d have to say that it does because the problem that he’s trying to solve isn’t that clear. We know that there is some sort of performance problem but we’re not quite sure where it is. We know that there are long and varying latencies in the responses but we’re not really quite sure why.

While we know that their external system is a bit slow but our choice of where to put the queue will probably have an impact, obviously on the development model of the clients and the Web services as well as how those external systems would work. So, I’d have to say that choosing the correct place to put the queue is important.

Ron: Well, let me interject something here because what you said just made me think. Now, if the problem is that these external systems are slow and limited number of connections, the first question we ought to ask is, does queuing help this situation at all?

Udi: Well, that’s probably a good first step. I mean every single time someone comes with a solution and then says, “OK, what’s the problem,” it’s always a good thing to check that solution first.

It looks like the problem that he has here has to deal with or the reason that he wants to use a queue is to do some kind of load leveling. He’s getting too many requests or at too high a rate from his clients and external Web services and external web applications more than his back end systems can use. So, using a queue as a load leveling mechanism is definitely the right way to go. So, from that perspective I think that putting a queue somewhere in there is a good idea.

Ron: OK. So then if you put a queue, it seems to me that it’s not going to make that much difference which layer you put the queue, would it?

Udi: Well, it might for the main reason that you really have to look at where his bottleneck is and that’s his back end systems. The bottleneck also has to do with the number of connections that can be opened and the number of sessions that can be opened. The place that I’d be looking at doing that is probably between those pooled COM+ objects and his central Web services for the main reason that that really gives a nice encapsulation in terms of the Web services towards both his organization’s internal services if they are other Web services, web applications or clients and everybody else that’s going on out there while keeping that abstraction out of the way.

So, the choice of using pooled COM objects is one of the ways he does the load leveling now. One of the problems he has is that it doesn’t seem to be doing that much for him because the switches and knobs that are available in COM+ in order to do that load leveling aren’t that great. What I’d be looking at in his situation is to put a queue in there but on the back side of that queue, not talking directly to the external system but doing something with WCF.

WCF has an incredible amount of switches and knobs in order to do the load throttling and the number of threads that are open. He could also do that on a large number of URIs in order to sort of split up the load from that perspective allowing him to cache results quite a bit better. So, that’s where I’d be looking at too. Just throw away those COM+ objects, put WCF in there, use the MSMQ binding and start configuring things from there.

Ron: There’s a lot of stuff in the message, but I think his core concern is performance. He mentions pseudosynchronous calls. I think by that he means, a message comes in to the web service, he’s going to drop something on the queue and then hold that message response until he gets a response back from a queue. So, it’s sort of synchronous but sort of not synchronous. So, in effect he’s kind of waiting on a queue instead of waiting on the pooled object to make this outbound telnet call.

I could agree if you said, “Well, look, our big problem is that we keep getting time outs because when we go to get a COM+ object from the pool, COM+ waits for a while and then it says, “Hey, there’s no object available’ and it returns an error,” then the queue is definitely going to help that problem. But in terms of the sheer through put or performance of the system, this is not going to help at all. It’s going to still be the same performance.

Now if you said, “Oh look, we can do some of this work kind of at a later point in time, ” well queuing doesn’t allow you to time shift the work. Right? So, if you said, “Look we can rethink this solution.” So you get a message in, we stuff something in a queue that we’ll deal with later, and then very quickly return a response like some kind of a number like, hey “your transaction number blah, blah, blah, will be processed later, it’s queued for processing, ” whatever.

I mean that introduces a lot of complexity in the system but it clearly would provide better response at the Web service layer. What do you think?

Udi: Well I think that at the most basic level, his throughput is dictated by his back end systems. From what he seems to be describing, every single request that is going through there, has to hit that back end system. If he has a limited number of back end systems that are supporting a limited number of connections, that’s going to limit his throughput no matter what technology he puts in front of that. So that’s at the core level. You just can’t get away from that.

The one thing that I would agree with you in your description there is the choice of using those COM+ objects. I mean COM+ was a great technology when it came out. The problem occurs, of course, when we start getting into larger and larger delays around the response time and we start getting all sorts of time out exceptions and things like that. So in that respect, I definitely say you know, take a step back from there.

But in terms of everything that he has around there, the queue isn’t going to make the back end system run any faster. What it will do is definitely complicate his system because he’s taking something that used to be synchronous and making it asynchronous. Writing Web services in order to handle that, I mean just adding a bunch of threads in order to listen to queues is not going to make things any simpler.

However, what it might do is to improve the resource usage of those Web services, OK? So instead of having those Web services have a bunch of threads open, waiting for the response coming back from those COM+ pooled objects, those threads could be relinquished and really just be triggered back up when a response comes back from the queue.

So I don’t see an improvement in the kind of solution that MSMQ or queuing would put in there in terms of the latency — how long it would take for a response to get back. However, I do see an improvement in terms of the resource usage of all the other players in the system.

Ron: I would agree with that. I would just say though that if you make the Web server that is hosting these Web services more resource efficient, maybe all you’re going to do is enable it to get more requests in queue the more quickly. Ultimately, this solution I think is going to solve a lot of problems related to time outs and server busy errors and that sort of thing, thread contentions, but not likely to increase overall performance.

But I definitely agree though. I would move this solution forward to WCF. I used to be on the COM+ team. COM+ was rolled into WCF so that it would have similar capabilities for pooling, instancing behavior, transactional support, those sorts of things. I would definitely move that forward into WCF.

OK! So great answer, Udi. Thank you so much for being on this ARCast Rapid Response.

Number 2

Ron: Hey this is Ron Jacobs back with another ARCast.TV Rapid Response. Today I’m joined by Udi Dahan, the Software Simplist from Israel.

Udi, I’m looking at the MSDN Architecture Forum and here’s a question from “blast.” Blast says he’s looking for where to put business rules. He’s developing a WinForm application. He uses data sets as the data layer, he says. He’s thinking about business rules and where to put them.

He says obviously, the more organized and centralized business rules are, the better. He’s tempted to put the business rules in the UI layer especially with the type data set. It makes a lot of sense there but not all rules belong on the client. He says some rules belong on the server, perhaps in a trigger.

So he’s asking where do you put your rules? How do you think about this problem, Udi?

Udi: Well, it looks like what he’s doing here is developing a two-tier client that is using WinForms and using datasets and speaking directly to the database. That in essence is part of his problem in that in terms of performance, he’d like to run more rules in the UI layer so that the user won’t be sending garbage to the database.

He also understands that because he’s building a multi-user system, there is a limited capability, in terms of concurrency, of actually having all the rules run correctly in order to make sure that everything is correct. So, his choice of an architecture, working two-tier is the main problem of why he has to fragment his business rules.

If he were to move towards a three-tier solution, that is put an application server between his smart client and the database, it would be a lot easier to put those business rules there. Now, once the business rules are out of the database, because again, we don’t have to deal with the concurrency issues once we have an application server and we’re using transactions there and we don’t have any disconnected problems, then what we can do is use those same DLLs, that same CLR code that runs the business rules, and deploy it client-side and use it there.

So, in terms of deployment, what we’d have is we’d have the same rules, both running client-side and server-side, whereas from a development perspective, we’d have them organized and centralized. That’s the way that I’d go about it.

Ron: Yeah, you know, I think conceptually I agree with you that a multi-tier solution would be a very good idea here. What I would probably think about conceptually, is breaking down rules into things that really ought to happen on the client-side. In particular, rules related to validation of data, so you know that you’ve got good and complete data before you ship it off to the server-side. Oftentimes you have to do that anyway because you have a button that shouldn’t be enabled until the data is valid, or something like that.

Udi: Absolutely.

Ron: Of course, we all know that if you have middle-tier web services, you must do validation both on the client-side and the server-side, because you must ensure that the valid data is received on the web server. So I agree with you that creating an assembly that you deploy on both sides is a good idea.

I would just expand on what you said a little bit and think about maybe on the server-side using a workflow foundation and business rules and workflow as a way to handle a lot of the heavier lifting, server-side validations and business rules that might require maybe sifting through more data or whatever kind of things, but server-side business rules that are more oriented towards business logic, and even if you have very, very data-intensive roles, then maybe some of those might even happen in the database. Don’t you agree?

Udi: Oh, absolutely. Absolutely. That’s something that I think often gets swept under the rug too much. Things like unique constraints and things like that are kinds of business rules. They protect the referential integrity and if we look at the alternatives, sometimes getting 10 million rows out of the database, in order to do some sort of unique email validation upon them, that’s just going to kill your performance.

There are certain things that it just makes sense to do them in the database, it’s just the best way to do it. The hard part, from a development perspective, is maintaining the coherence of your business rules. When you say, “OK, I want a single perspective, what are all the rules in my system?”

Even though we might try to keep it all CLR based, some of the things like unique constraints, like referential integrity, will be in the database. So, what I sometimes suggest to do is to have a separate solution, in terms of your development team, where you put all your business rules.

This includes both the SQL statements for defining your unique constraints and your referential integrity. Also put in that validation logic, your workflow that you’re going to be running server-side. If it’s AJAX controls and regular expressions that you’re going to be doing client-side in order to validate that data, absolutely make sure you have, from a development perspective, one place where you can go where you can see everything, because if you don’t do that [inaudible] can be running, and when things stop working, you won’t know how to debug it.

Ron: All right. Well, excellent answer. Udi, thank you so much.


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Bryan Wheeler, Director Platform Development at msnbc.com
Udi Dahan is the real deal.

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.

Beyond the training, we were able to spend some time with Udi discussing issues unique to our business domain. Because Udi is a rare combination of a big picture thinker and a low level doer, he can quickly hone in on various issues and quickly make good (if not startling) recommendations to help solve tough technical issues.” November 11, 2010

Sam Gentile Sam Gentile, Independent WCF & SOA Expert
“Udi, one of the great minds in this area.
A man I respect immensely.”

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“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.”

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“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.”

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“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.
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“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.
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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.”

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“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.”

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“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.”

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“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.”

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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.

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“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!”

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“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.
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“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.”

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“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.”

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“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.”

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“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.”

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“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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