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Archive for the ‘Pub/Sub’ 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.



NServiceBus and RavenDB – better together!

Friday, July 22nd, 2011

Better togetherFor those of you who haven’t heard yet, the next version of NServiceBus will be making use of RavenDB as its default storage engine.

Update: We’re taking this integration even farther in version 4 in which we’re auditing all messages automatically and storing them to RavenDB as well as providing a nice UI for looking through that history.

What’s RavenDB?

For those of you who haven’t heard about RavenDB yet – it’s a transactional document database for .NET, and it’s been/being developed by my good friend and partner in crime Ayende Rahien (a.k.a Oren Eini). Although Ayende is known for his “not invented here” tendencies, when it comes to transactional document databases, well, you’d have been hard pressed to find one – especially with a decent .NET API.

NServiceBus Storage

In NServiceBus we have a variety of storage needs – from things like durable subscriptions so that you get fault-tolerant pub/sub, through persistence of long-running workflow (a.k.a saga) state, and durable timeouts so that your time-bound long-running processes never get stuck. None of these actually require relations – we were just using relational databases for storage because it was the easy answer everyone was going with.

Relational vs. Document

And these relational databases came with some downsides – developers had to “beg” their DBA to create the needed tables in the production databases, when that central database was down it prevented otherwise autonomous publishers and subscribers from going about their business, and it was very difficult to version the long-running workflows especially when newer versions required different state.

By moving to an embedded and transactional document DB, no longer do you need to bargain with the DBA, by keeping the storage together with the processing nodes you are back in parallel processing bliss, and the schema-less nature of the storage makes versioning those long-running workflows much easier.

Licensing

And I’m also happy to announce that an agreement has been reached with Hibernating Rhinos (Ayende’s company) so you won’t need to license RavenDB separately to get all of these benefits. RavenDB will be bundled with NServiceBus so licensing NServiceBus covers them both (for the storage needs mentioned above).

If you’re also thinking about using RavenDB as the storage for the rest of your system, you’ll be able to get a nice discount on the license when purchasing it together with NServiceBus. This will be going into effect with the release of NServiceBus 3.0 this October.

2018 Licensing Update

RavenDB licenses are no longer included as a part of NServiceBus as of version 7.

Companies Using NServiceBus

Interestingly enough, there’s been a big uptick in companies using NServiceBus with the introduction of licensing. Most of these companies are not the kind of big, stand-up-and-take-notice names that everybody likes to have on their roster.

What makes NServiceBus particularly attractive is that you can get a lot done without requiring some kind of dedicated BizTalk/WebSphere/Tibco expert. This has brought down the barrier for thousands of developers who just want to get on with the business of getting their app to market.

And when it comes to the big names, well, once they see how much faster they can get stuff done with NServiceBus as well as how robust and scalable it is in production, they don’t want any of their competitors to know about it!

Anyway, I’m glad to say that two companies have stepped forwards: Rackspace and Reuters. Hopefully we’ll get confirmation from one of the big banks soon that we can go public with them too.

Exciting times ahead.

Learn more.



The Danger of Centralized Workflows

Wednesday, July 13th, 2011

It isn’t uncommon for me to have a client or student at one of my courses ask me about some kind of workflow tool. This could be Microsoft Workflow Foundation, BizTalk, K2, or some kind of BPEL/orchestration engine. The question usually revolves around using this tool for all workflows in the system as opposed to the SOA-EDA-style publish/subscribe approach I espouse.

The question

The main touted benefit of these workflow-centric architectures is that we don’t have to change the code of the system in order to change its behavior resulting in ultimate flexibility!

Some of you may have already gone down this path and are shaking your heads remembering how your particular road to hell was paved with the exact same good intentions.

Let me explain why these things tend to go horribly wrong.

What’s behind the curtain

It starts with the very nature of workflow – a flow chart, is procedural in nature. First do this, then that, if this, then that, etc. As we’ve experienced first hand in our industry, procedural programming is fine for smaller problems but isn’t powerful enough to handle larger problems. That’s why we’ve come up with object-oriented programming.

I have yet to see an object-oriented workflow drag-and-drop engine. Yes, it works great for simple demo-ware apps. But if you try to through your most complex and volatile business logic at it, it will become a big tangled ball of spaghetti – just like if you were using text rather than pictures to code it.

And that’s one of the fundamental fallacies about these tools – you are still writing code. The fact that it doesn’t look like the rest of your code doesn’t change that fact. Changing the definition of your workflow in the tool IS changing your code.

On productivity

Sometimes people mention how much more productive it would be to use these tools than to write the code “by hand”. Occasionally I hear about an attempt to have “the business” use these tools to change the workflows themselves – without the involvement of developers (“imagine how much faster we could go without those pesky developers!”).

For those of us who have experienced this first-hand, we know that’s all wrong.

If “the business” is changing the workflows without developer involvement, invariably something breaks, and then they don’t know what to do. They haven’t been trained to think the way that developers have – they don’t really know how to debug. So the developers are brought back in anyway and from that point on, the business is once again giving requirements and the devs are the one implementing it.

Now when it comes to developer productivity, I can tell you that the keyboard is at least 10x more productive than the mouse. I can bang out an if statement in code much faster than draggy-dropping a diamond on the canvas, and two other activities for each side of the clause.

On maintainability

Sometimes the visualization of the workflow is presented as being much more maintainable than “regular code”.

When these workflows get to be to big/nested/reused, it ends up looking like the wiring diagram of an Intel chip (or worse). Check out the following diagram taken from the DailyWTF on a customer friendly system:

stateModel

The bigger these get, the less maintainable they are.

Now, some would push back on this saying that a method with 10,000 lines of code in it may be just as bad, if not worse. The thing is that these workflow tools guide developers down a path where it is very likely to end up with big, monolithic, procedural, nested code. When working in real code, we know we need to take responsibility for the cleanliness of our code using object-orientation, patterns, etc and refactoring things when they get too messy.

Here is where I’d bring up the SOA/pub-sub approach as an alternative – there is no longer this idea of a centralized anything. You have small pieces of code, each encapsulating a single business responsibility, working in concert with each other – reacting to each others events.

Productivity take 2: testing and version control

If you’re going to take your most complex and volatile business logic and put it into these workflow tools, have you thought about how your going to test it? How do you know that it works correctly? It tends to be VERY difficult to unit-test these kinds of workflows.

When a developer is implementing a change request, how do they know what other workflows might have been broken? Do they have to manually go through each and every scenario in the system to find out? How’s that for productivity?

Assuming something did break and the developer wants to see a diff – what’s different in the new workflow from the old one, what would that look like? When working with a team, the ability to diff and merge code is at the base of the overall team productivity.

What would happen to your team if you couldn’t diff or merge code anymore?
In this day and age, it should be considered irresponsible to develop without these version control basics.

In closing

There are some cases where these tools might make sense, but those tend to be much more rare than you’d expect (and there are usually better alternatives anyway). Regardless, the architectural analysis should start without the assumption of centralized workflow, database, or centralized anything for that matter.

If someone tries to push one of these tools/architectures on you, don’t walk away – run!



Integration: How and Where

Friday, April 8th, 2011

integrationOne of the topics that comes up a lot in the context of an Enterprise Service Bus (ESB) is that of integration. Unfortunately, many people take their ideas of reuse and design their integration as being done from a single place – both logical and physical. That unfortunately creates a bottleneck for all integration activities, where some are likely to be higher priority than others.

The ugly truth

You don’t need an ESB for integration.

There. I’ve said it.

Most of the ESB products on the market, in focusing on integration, are addressing the wrong problem.

What integration is about

There are three primary components to integration – data format translation, protocol bridging, and logic. Let’s take the Agile “simplest solution that could possibly work” motto and apply it to these elements:

  1. Data format translation
    • Use something like MapForce (from Altova, the guys behind XMLSpy).
    • At under $1200 for a dev license and handling mapping to/from XML, EDI, flat files, and relational databases, you can host the resulting mapping in any endpoint, as XSLT or even Java/C# – no need to have “the bus” do this for you.
  2. Protocol bridging
    • Use something like /n software
    • At about $1500 for a dev license, you get solutions for FTP, HTTP, SMTP, POP, IMAP, LDAP, DNS, RSS, SMS, Jabber, SOAP, WebDav, etc and, again – you can host the DLLs in any endpoint you want. Don’t need expensive buses for this
  3. Logic
    This is all you – no technology can do this for you. If anything, the most important thing is to get all the ugly mapping and protocol bridging stuff out of the way and let you focus on your logic. This is Single Responsibility Principle (un)common sense.

Digging deeper into the logic

The interesting thing about most of the protocol stuff mentioned above is that they’re inherently unreliable and also their performance at runtime is unknowable due to the total load on the target server.

We wouldn’t want any of the above situations to cause our integration to “get stuck”. As such, it is best we think of our integration logic as a long-running process that manages other endpoints which do the actual protocol bridging and data transformations.

This is one of the areas where NServiceBus, with its sagas can actually help a lot. Just like the other pieces of integration mentioned above, these sagas can run on any endpoint.

You could alternatively look at other technologies like BizTalk and Business Process Execution Language (BPEL) engines, though many of those are designed to be physically centralized (just like many of the ESBs out there).

By the way, if you do want to use NServiceBus together with BizTalk, Michael Stephenson has published some great white papers on getting the two to work together. His latest is about integrating NServiceBus into BizTalk’s RFID processes – check it out.

Putting it all together

When we take all of these pieces and look at them in a cohesive architecture, here’s what it can look like:

Distributed Integration

As you can see, integration is physically distributed across multiple endpoints.

Not only that, but the integration logic is kept separate from the protocol bridging and data transformation, enabling independent versioning of each. Just as important, it makes it much easier to unit test that the integration logic is correct as we don’t have to simulate the target technologies.

Costs

As you can see, you can get integration capabilities just as powerful as if you went with something like BizTalk, but without creating a single point of failure in your architecture. In terms of costs, it’s also quite a bit cheaper.

For a high availability BizTalk deployment, you’ll be paying over $40,000 per CPU for the Enterprise Edition, not including the extra $7000 per CPU for SQL Server Standard Edition (clustered). For a clustered 4-CPU mid-size deployment, you’d be in the area of $200,000.

For the distributed integration solution above, you’d be paying around $2700 in dev licenses (for /n software and Altova MapForce), and $500 per core for NServiceBus Standard Edition. The reason there’s per-core licensing for NServiceBus is that in a virtualized environment, you’ll be provisioning virtual cores to your virtual machines. No reason to pay for a quad-core CPU when all you’re using is a single core. You can also use any number of cores running NServiceBus Express Edition at no cost, so a mid-size deployment with say 8 cores running Standard Edition, and another 24 cores running Express Edition would cost (with the volume discount) an additional $3800 – a total cost of $6,500.

BizTalk: $200,000       NServiceBus: $6,500

‘Nuff said.

In closing

We’ve been bitten by centralized architectures before.

By having our integration distributed, we can version, upgrade, and scale each piece independently.

You’ve seen how we can use simple, lightweight, and inexpensive technologies to create distributed integration solutions just as powerful and robust as the centralized ESB vendor-offerings out there, but at a tiny fraction of the cost.

Next time the topic of integration is brought up, you’ll know not to be suckered in by re-branded EAI brokers.

Learn more about NServiceBus.



Bus and Broker Pub/Sub Differences

Thursday, March 24th, 2011

differencesOne of the things which often confuses people using NServiceBus for the first time is that it only allows an endpoint to subscribe to a given event from a single other publishing endpoint. The rule that there can only be a single publisher for a given event type is one of the things that differentiates buses from brokers, though both obviously allow you to have multiple subscribers.

Brokers

Message brokers, more broadly known and used on the Java platform, don’t come with this constraint. For example, when using ActiveMQ, you can have any number of endpoints come to the broker and publish a message under a given topic.

So where’s the problem?

It’s all about accountability.

Let’s say you’ve subscribed to a given topic, and have received two events – one telling you that the price of bananas next week will be $1/kg and another telling you that it’ll be $2/kg.

Which one is right?

Especially given that those events may have been published by any other endpoint via the broker.

Is it first one wins? Last one wins? How about first one sent vs. first one received? Ditto for last. As a subscriber, can you really be held accountable for having the logic to choose the right one? Shouldn’t this responsibility have fallen to the publishing side?

This is one of the big drawbacks of the broker, hub and spoke architecture. No responsibility. No single source of truth – unless everybody’s going to some central database, in which case – what’s the point of all this messaging anyway?

Buses

The Bus Architectural Style is all about accountability. If you are going to publish an event, you are accountable for the correctness of the data in that event – there is no central database that a subscriber can go to “just in case”. And the only way that you can be held accountable, is if you have full responsibility – ergo, you’re the only one who can publish that type of event.

If you say bananas are going to cost $1/kg next week, that’s that. Subscribers will not hear from anybody else on that topic.

Now, this is not to say that you can’t have more than one physical publishing endpoint.

You see, buses differentiate between the logical and the physical. Brokers tend to assume that the physical hub-and-spoke topology is also the logical.

In a bus, while there can only be one logical endpoint publishing a given type of event, that endpoint can be physically scaled out across multiple machines. It is the responsibility of the bus to provide infrastructure facilities to allow for that to happen in such a way that to subscribers, it still appears as if there is really only one publishing endpoint.

The same is true about the subscriber – one logical subscribing endpoint may be scaled out across multiple machines.

Product Mix-ups

Unfortunately, there are many broker-style technologies out there that are being marketed under the banner of the Enterprise Service Bus. While some products have the ability to be deployed in both a centralized and distributed fashion (sometimes called “federated” or “embedded” mode), many do not enforce the “single publishing logical-endpoint per event-type” rule.

Without this constraint, it is just too easy to make mistakes.

NServiceBus

By enforcing this constraint, we see the same kind of question appear on the discussion group time and time again:

“I have an Audit event that I’d like all of my machines to publish, and have one machine subscribe to them all, but NServiceBus won’t let me. How do I make NServiceBus support this scenario?”

And the answer is the same every time:

“You should have all the machines Send the Audit message (configured to go to the single machine handling that message), and not Publish. It is not an event until its been handled by the endpoint responsible for it.”

The semantics of the message matter a lot.

When looking at Service-Oriented Architecture, these messages are the contract and, as any lawyer will tell you, contracts need to be explicit and the intentions really need to be spelled out – otherwise the contract is practically worthless.

In closing

Friction is sometimes a good thing – it prevents us from making mistakes. It keeps cars on the road. And because that’s not enough friction, we introduce curbs as well.

If you’re looking for a service bus technology for your next project, check that it’ll give you the friction that you need to keep everybody safe. Really check what it is that the vendors are offering you – more often than not, it’s some ESB lipstick on a broker pig.

To learn more about how NServiceBus supports this kind of publish/subscribe, click here.



Webcast on NServiceBus with Andreas Öhlund

Sunday, March 6th, 2011

Tomorrow (March 8th) Andreas will be presenting a webcast on NServiceBus for the European Virtual ALT.NET group.

This will actually be part 2 of his previous presentation – you can find the recording of part 1 here.

This time Andreas will be showing publish/subscribe communication as well as the use of sagas – get the full details here.



Polymorphism and Messaging

Thursday, January 13th, 2011

polymorphismOne of the questions that came up from my NServiceBus – .NET Service Bus Smackdown post was about the Polymorphic Message Dispatch and Polymorphic Message Routing features. People wanted to know what those are, why they’re important, and if other technologies (specifically WCF and BizTalk) support them.

Messaging Basics

First of all, when building a system using messaging, you don’t have methods that are invoked on some remote object (a.k.a “service”) to which you pass parameters. Instead, you use some generic piece of infrastructure (in the world of Java, this is most commonly a Message Broker) to send a message where a message can be thought of as a serializable class. Here’s an example of a message:

public class UserCreated : IMessage
{
    public Guid UserId { get; set; }
    public string Name { get; set; }
}

This message would be published using NServiceBus like this:

bus.Publish<UserCreated>( m =>
{
    m.UserId = Guid.NewGuid();
    m.Name = "John Smith";
});

This can be contrasted with RPC models like WCF where you need to define a “service” that has methods on it, where those methods accepts parameters. Sometimes developers try to make this more generic by having a single “service” with one method on it named something like “Process” where the parameter it accepts is of the type “object”, or they introduce generics like this: Process<T>(T message);

This is where Polymorphic Message Dispatch comes in:

Polymorphic Message Dispatch

While you can pull of the WCF generics thing, one thing that is more difficult (without writing your own dispatch model) is to have a pipeline of classes which can be invoked based on their relationship to the type passed in. Using NServiceBus, both of the following message handlers will be invoked when UserCreated arrives:

public class Persistence : IHandleMessages<UserCreated>
{
    public void Handle(UserCreated message) { }
}

public class Audit : IHandleMessages<IMessage>
{
    public void Handle(IMessage message) { }
}

Now some might say that WCF, BizTalk, and the .NET Service Bus allow you to do auditing in their own internal pipeline, and that’s true. The place where this becomes more powerful is when you need to build V2 of your system, and the publisher now publishes a slightly different event – that a user was created as a part of a campaign, requiring the subscriber to register statistics about the campaign. Of course, this event also means that a user was created. Here’s how you’d do that with NServiceBus:

public class UserCreatedFromCampaign : UserCreated
{
    public Guid CampaignId { get; set; }
}

//publisher code
bus.Publish<UserCreatedFromCampaign>( m =>
{
    m.UserId = Guid.NewGuid();
    m.Name = "John Smith";
    m.CampaignId = theCampaignId;
}

//subscriber code
public class Statistics : IHandleMessages<UserCreatedFromCampaign>
{
    public void Handle(UserCreatedFromCampaign message) { }
}

The important part is what you don’t see – since UserCreatedFromCampaign inherits from UserCrated, the Persistence handler we had from V1 will also be invoked, and so will the Audit handler of course. You don’t have to make your new code call the old code like you would in a method based dispatch model. This makes sure that the coupling in your service layer code remains constant over time as you grow the functionality of your system.

This was one of the main benefits mentioned by Rackspace in their use of NServiceBus (here):

“The main benefit NServiceBus has brought us so far is developer scalability due to lower coupling and higher consistency in our code.”

But, when looking at the above scenario, we can obviously expect that all sorts of things can happen in relation to campaigns – it is a separate concern, and thus should be handled by a separate subscriber. And this bring us to…

Polymorphic Message Routing

The challenge that we have here is that we no longer have a hierarchy where something clearly belongs on top of something else. We have users created and activities happening related to campaigns – that may happen in any combination. By having separate subscribers, we could then introduce new handlers/subscribers to our environment without touching or taking down any of the other subscribers. Here’s what the subscribers would look like:

public class Persistence : IHandleMessages<UserCreated>
{
    public void Handle(UserCreated message) { }
}

public class Statistics : IHandleMessages<CampaignActivityOccurred>
{
    public void Handle(CampaignActivityOccurred message) { }
}

But if each of the above messages were a class, how could we define a message which inherited from both?

Before answering that, we need to understand why the publisher wouldn’t just publish both of the above messages. You see, the publisher can’t make any assumptions about its subscribers – it could be that one of them has logic that correlates across both of these messages that could end up counting the occurrence as happening twice rather than once, possibly charging the account associated with the campaign twice. Publishing two messages results in two transactions when there really should have been one.

So, here’s how to define messages so that we can have multiple inheritance:

public interface UserCreated : IMessage
{
    Guid UserId { get; set; }
    string Name { get; set; }
}

public interface CampaignActivityOccurred : IMessage
{
    Guid CampaignId { get; set; }
    Guid ActivityId { get; set; }
}

public interface UserCreatedFromCampaign 
                 : UserCreated,
                   CampaignActivityOccurred 
{
}

And when the publisher publishes UserCreatedFromCampaign, the event would be routed to both the UserCreated subscriber and the CampaignActivityOccurred subscriber. The power of this approach is felt as we handle new requirements around purchases made related to a campaign. Now we can have another event which inherits from CampaignActivityOccurred and not have to worry since the existing subscriber will be routed those messages automatically.

Since WCF doesn’t have publish/subscribe capabilities, we might as well move along.

Not to throw a burning match on an ocean of oil, but REST doesn’t really support this either.

Not Content-Based Routing

This may sound like the content-based router pattern from EIP (CBR), but it’s not. The important difference is that there isn’t some part of the routing that depends on the structure of the messages. The major drawback of CBR is that it creates a central place in your system that needs to be changed any time *syntactic* changes happen to message structure *in addition to* to changes in the subscribers.

Now, this is where the BizTalk guys would say that “that’s why we can do message transformations”, and then the subscribers wouldn’t need to be changed. However, can we really know when getting a requirement that the change is syntactic and not semantic? I mean, it’s quite common that changes to message structures happen together with changes to processing logic.

You may be beginning to get the feeling that more and more logic is being sucked out of the subscribers into some monolithic black hole that is likely going to be unmaintainable and quite slow.

This is one of the main differences between using a bus and a broker – a bus supports the correct distribution of logic keeping the system loosely coupled; brokers are useful integration engines when you absolutely can’t change the applications being integrated. Enterprise Application Integration (EAI) brokers don’t usually make good Enterprise Service Bus (ESB) technology.

In Closing

NServiceBus has all sorts of features you didn’t know you needed until you saw what life could be like when you had them. Most of these features don’t have snazzy drag-and-drop demos that make people ooh-and-aah and TechEds and PDCs, but they’re really necessary to avoid finding yourself in yet another big-ball-of-mud code base telling your manager/customer (again) that it would be faster to rebuild the system from scratch than to implement that new requirement in the old one.

Take NServiceBus for a spin and see for yourself.



NServiceBus 2.5 Released

Friday, December 31st, 2010

Just before we usher in the new year, I’m happy to announce the release of NServiceBus version 2.5.

Go to the NServiceBus website

Yes, there’s a new logo, and the website’s been redesigned.
It’s been a long time coming – the previous version (2.0) was released in March.

I’m really quite excited about this version as it rolls up all the bug fixes and enhancements that customers have asked for as they ran version 2.0 under the most severe types of production environments. The next thing that is a big deal that many have been asking for is a licensed version of NServiceBus – that is, the ability to purchase a commercial license and receive support.

We all know how managers like having a throat to choke.

And now they’ll have one – NServiceBus Ltd is the company that will be providing licensing, services, and support for all customers’ NServiceBus needs. After more than 33,000 downloads and over 1000 developers in the community, the demand has really grown. Who would’ve thought all this would happen when I started NServiceBus 4 years ago (before it even had a name).

Why NServiceBus is better than WCF for your distributed systems

This question comes up repeatedly for people hearing about NServiceBus for the first time.

The answer is simple – reliability.

A system built with NServiceBus is so much more reliable to all kinds of production conditions than WCF that it’s hardly a fair comparison at all. While WCF can be configured to provide something kind of close to the same level of reliability, you need to do a fair amount of spelunking through the various options of netMsmqBinding to get it right.

The second reason to use NServiceBus instead of WCF is publish/subscribe.

The ability to make use of events and the observer pattern not just to achieve loose coupling within a single process, but across many processes, machines, and sites. Can you imagine going back to programming without events? Shudder. But that’s exactly what it’s like to use WCF in your distributed system. NServiceBus brings you the best of object-oriented programming but in a distributed and reliable infrastructure.

Don’t wait any longer

Take NServiceBus for a spin.

But things may look a bit different after you do…

RedPillBluePill

http://www.NServiceBus.com

And have a happy New Year.



The Known Unknowns of SOA

Monday, November 15th, 2010

rumsfeldOne of the better known analysts in the enterprise software area, JP Morgenthal, wrote this post about the relationship between SOA, BPM, and EA. In it he defines SOA as follows:

“SOA is a practice that focuses on modeling the entities, and relationships between entities, that comprise the business as a set of services. This can be done on a small or large scale. Typically, the relationships in this model represent consumer/provider relationships.”

I have some serious concerns about the ramifications of this definition/description.

First of all, when reading “entities”, many people will interpret that to mean the entities found in Entity Relationship Diagrams [ERD] or in Object Oriented Analysis & Design [OOAD]. In both, these entities are identified as the “nouns” of the domain. Examples of these ERD/OOAD-type entities include things like Customer, Order, and Product.

These are almost always the wrong place to start for identifying services in SOA.

Second, on the consumer/provider relationship: on the one had, this fits very well with how web services can consume (or call) other web services. However, the downsides of using web services as services in SOA is becoming well enough known that even in the same post we see this warning:

“Web Services is not SOA, it is merely a standardized approach to accessing functionality on remote systems.”

But the question remains, if a producer/consumer relationship is OK for SOA-type services, why doesn’t that hold for web services? And the answer is… it depends on the type of producer/consumer relationship. The typical relationship is one of synchronous calls from consumer to producer, this is not OK for SOA-type services either.

You see, this synchronous producer/consumer implies a model where services are not able to fulfill their objectives without calling other services. In order for us to achieve the IT/Business alignment promised by SOA, we need services which are autonomous, ie. able to fulfill their objectives without that kind of external help.

Instead, we need to look for a more loosely coupled producer/consumer relationship – like publish/subscribe, where the producer emits events, and the consumer subscribes and handles those events. The reason that this kind of relationship doesn’t hurt autonomy is that it disconnects services on the dimension of time. In order for a service to be able to make a decision autonomously without synchronously calling any other service, using only information provided by events it received in the past, it must be strongly aligned with the business.

Most projects which bandy about the SOA acronym aren’t actually made up of services – they’re made up of XML over HTTP functions calling other XML over HTTP functions, eventually calling XML over HTTP databases. You can layer as much XML and HTTP as you want on top of it, but at the end of the day, most projects are just functions calling functions calling databases – in other words, procedural programming in the large, and no amount of SOAP will wash away the stink.

Here’s a different definition of services for SOA that may communicate a bit better what it’s all about:

A service is the technical authority for a specific business capability.
Any piece of data or rule must be owned by only one service.

What this means is that even when services are publishing and subscribing to each other’s events, we always know what the authoritative source of truth is for every piece of data and rule.

Also, when looking at services from the lense of business capabilities, what we see is that many user interfaces present information belonging to different capabilities – a product’s price alongside whether or not it’s in stock. In order for us to comply with the above definition of services, this leads us to an understanding that such user interfaces are actually a mashup – with each service having the fragment of the UI dealing with its particular data.

Ultimately, process boundaries like web apps, back-end, batch-processing are very poor indicators of service boundaries. We’d expect to see multiple business capabilities manifested in each of those processes.

I know that this may be more confusing than the traditional web services approach but, to paraphrase Donald Rumsfeld, it is better to know that you don’t know, than to not know that you don’t know 🙂



Race Conditions Don’t Exist

Tuesday, August 31st, 2010

crossing-the-finish-lineNot in the business world anyway.

The problem is that, as software developers, we’re all too quick to accept them at face value. We don’t question the requirements – in all fairness, it was never our job to do so. We were the ones that implemented them, preferably quickly.

For example

Let’s say we get the requirement the following requirements:

1. If the order was already shipped, don’t let the user cancel the order.
2. If the order was already cancelled, don’t let the user ship the order.

The race condition here is when we have two users who are looking at the same order, which is neither cancelled nor shipped yet, and each submits a command – one to ship the order, the other to cancel it.

In these cases, the code is simple – just an if statement before performing the relevant command.

So what’s the problem

A microsecond difference in timing shouldn’t make a difference to core business behaviors. Which means that we’ve actually got here is a bug in the requirements. Users are actually dictating solutions here rather than requirements.

Let’s ask our stakeholders, “why shouldn’t we let users cancel a shipped order? I mean, the users don’t want the products.”

And the stakeholders would respond with something like, “well, we don’t want to refund the user’s money then. Or, at least, not all their money. Well, maybe if they return the products in their original packaging, *then* we could give a full refund.”

And as we drilled deeper, “when do refunds need to be given? Right away, in the same transaction?”

The stakeholders would explain, “no, refunds don’t need to be given right away.”

It turns out we were missing the concept of a refund, as well as assuming that all things needed to be processed and enforced immediately. Once we dug into the requirements, we found that there is actually plenty of time to allow both transactions to go through. We just need to add some checks during shipping’s long-running process to see if the order was cancelled, and then to cut the process short.

So is everything a long-running process then?

That’s actually a fair question – long-running processes are a lot more common than at first appears.

What we’re seeing is that cancellation is now a command that has no reason to fail – just like CQRS tells us. When this command is performed, it publishes the OrderCancelled event, which the billing service subscribes to.

Billing then starts a long-running process (a saga, in NServiceBus lingo), also listening to events from the shipping process, ultimately making a decision when a refund should be given, and for how much.

Deeper business analysis

As we discuss matters more with our business stakeholders, we hear that most orders are actually cancelled within an hour of being submitted. It is quite rare for orders to be cancelled days later.

In which case, we could look at modeling the acceptance of an order as a long-running process itself.

When a user places an order, we don’t immediately publish an event indicating the acceptance of an order, instead a saga is kicked off – which opens up a timeout for an hour later. If a cancellation command arrives during that period of time, the user gets a full refund (seeing as we didn’t charge anything since billing didn’t get the accepted event to begin with), and the saga just shuts itself down. If the timeout occurs an hour later, and the saga didn’t get a cancel command, then the order is actually accepted and the event is published.

Yes, sagas are everywhere, once you learn to see with business eyes, and no race conditions are left.

In closing

Any time you see requirements that indicate a race condition, dig deeper.

What you’re likely to find are some additional business concepts as well as the introduction of time and the creation of long-running business processes. The implementation at that point will pivot from being trivial if-statements to being richer sagas.

Keep an eye out.



   


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