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Archive for the ‘Autonomous Services’ Category



Lost Notifications? No Problem.

Sunday, December 7th, 2008

One of the most common questions I get on the topic of pub/sub messaging is what happens if a notification is lost. Interestingly enough, there are some who almost entirely write-off this pattern because of this issue, preferring the control of request/response-exception. So, what should be done about lost messages? The short answer is durable messaging. The long answer is design.

Durable Messaging

In order to prevent a message from being lost when it is sent from a publisher to a subscriber, the message is written to disk on the publisher side, and then forwarded to the subscriber, where it is also written to disk. This store-and-forward mechanism enables our systems to gracefully recover from either side being temporarily unavailable.

In my MSDN article on this topic, I outlined some problems with this approach. These problems are exacerbated for publishers. Imagine a publisher with 40 subscribers, publishing 10 messages a second, each containing 1MB of XML. If 10 of the subscribers are unavailable, that’s 100MB of data being written to the publisher’s disk every second, 6GB every minute. That’s liable to bring down a publisher before an administrator brews a cup of coffee.

Publishers have no choice but to throw away messages after a certain period of time.

Publisher Contracts

The whole issue of contracts and schema is considered one of the better understand parts of SOA. Unfortunately, the operational aspects of service contracts is hardly ever taken into account.

On top of the schema of the messages a service publishers, additional information is needed in the contract:

  1. How big will this message be?
  2. How often will it be published?
  3. How long will this message be stored if a subscriber is unavailable?

This first two pieces of information are important for subscribers to do load and capacity planning. The last one is the most important as it dictates the required availability and fault-tolerance characteristic of subscribers.

For Example

In the canonical retail scenario, when our sales service accepts an order, it publishes an order accepted event. Other services subscribed to this event include shipping, billing, and business intelligence.

While shipping and billing are highly available and able to keep up with the rate at which orders are accepted, the business intelligence service is not. BI has two main parts to it – a nightly batch that does the number crunching, and a UI for reporting off of the results of that number crunching. Some even do the reporting in a semi-offline fashion, emailing reports back to the user when they’re ready.

Furthermore, nobody’s going to invest in servers for making BI highly available.

And wasn’t the whole point of this publish/subscribe messaging to keep our services autonomous? That not all services have to have the same level uptime?

Houston, do we have a problem.?

Data Freshness

There is a glimmer of light in all this doom and gloom.

Not all services have the same data freshness requirements.

The business intelligence service above doesn’t need to know about orders the second they’re accepted. A daily roll-up would be fine, and an hourly roll-up bring us that much closer to “real time business intelligence”.

So, while BI is ready to accept the sales message schema, it would like a slightly different contract around it – less messages per unit of time, more data in each message.

From the operational perspective of the sales service, it would be cost effective to have less “online” subscribers. It could even take things a few steps further. Instead of using the regular messaging backbone for transmitting these hourly messages, it could use FTP. The data could even be zipped to take up even less space. Since the total data size is less than the corresponding online stream, is stored on cheaper, large storage, and the number of subscribers for this zipped, hourly update is fairly small, these messages can be kept around far longer.

If you’ve heard about consumer-driven contracts, this is it.

Note that we’re still talking about the same logical message schema.

Summary

It’s not that lost notifications aren’t a problem.

It’s that they feed the design process in such a way that the resulting service ecosystem is set up in such a way that notifications won’t get lost. I know that that sounds kind of recursive, but that’s how it works. Either subscribers take care of their SLA allowing them to process the online stream of events, or they should subscribe to a different pipe (which will have different SLA requirements, but maybe they can deal with those).

It make sense to have multiple pipes for the same logical schema.

It’s practically a necessity to make pub/sub a feasible solution.

 


Related Content

MSDN article on messaging and lost messages

Durable messaging dilemmas

Additional logic required for service autonomy

More in depth example on events and pub/sub between services

Consumer-Driven Contracts



SOA, EDA, and CEP a winning combo

Saturday, November 1st, 2008

jump in There’s been some discussion on the SOA yahoo group around the connection between SOA, EDA, and CEP (complex event processing) since Jack’s original post on the topic. I’ve been waiting for the right opportunity to jump in and it seems to have come.

Dennis asked this:

There are different design choices in a SOA, even when you already have identified the services. I have a simple example that I would like to share:

Imagine a order-to-cash process. One part of that process is to register an order. Suppose we have two services, Order Service and Inventory Service. The task is to register the order and make a corresponding reservation of the stock level. I would be pleased to have the groups view on the following 3 design options (A, B, C):

A.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The “process/application” sends another message (sync or async) to “reserveStock” on the the Inventory Service.

B.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The Order Service sends a message (sync or async) to “reserveStock” on the the Inventory Service.

C.
1. The “process/application” sends a message (sync or async) to “registerOrder” on the Order Service.
2. The Order Service publishes an “orderReceived” event.
3. The Inventory Service subscribes to the “orderReceived” event .

On the whole “already identified the services” thing – naming a service doesn’t mean much. It’s all about allocating responsibility, and until that’s been done, those “services” don’t give us very much information.

 

Business Services

If we were to view this example in light of business services, and look at the business events that make up this process, maybe we’d get a different perspective.

Three business services: Sales, Inventory, and Shipping.

In Sales, many applications and people may operate, including the person and the application he used to submit the order. When the order is submitted and goes through all the internal validation stuff, Sales raises an OrderTentativelyAccepted event.

Inventory and Orders

Inventory, which is subscribed to this event, checks if it has everything in stock for the order. For every item in the order on stock, it allocates that stock to the order and publishes the InventoryAllocatedToOrder event for it. For items/quantities not in stock, it starts a long running process which watches for inventory changes.

When an InventoryChanged event occurs, it matches that against orders requiring allocation – if it finds one that requires stock, based on some logic to choose which order gets precedence, it publishes the InventoryAllocatedToOrder event.

Sales, which is subscribed to the InventoryAllocatedToOrder event, upon receiving all events pertaining to the order tentatively accepted, will publish an OrderAccepted event.

Orders and Shipping

When Inventory receives the OrderAccepted event, it generates the pick list to bring all the stock from the warehouses to the loading docks, finally publishing the PickListGenerated event containing target docks.

When Shipping receives the PickListGenerated event, it starts the yard management necessary to bring the needed kinds of trucks to the docks.

 

What else is possible

I could go on, talking about things like the maximum amount of time stock of various kinds can wait to be loaded on trucks, subscribing to earlier events to employ all kinds of optimization and prediction algorithms, having a Customer Care service notifying the customer about what’s going on with their order (probably different for different kinds of customers and preferred communication definitions). Obviously, we’d need a Billing service to handle the various kinds of billing procedures, whether or not the customer has credit, pays upon delivery, etc.

It turns out that many business domains map very well to this join of SOA and EDA.

 

What an ESB is for

When we have these kinds of business services primarily publishing events and subscribing to those of other services, you don’t need much else from your “enterprise service bus”. All sorts of transformation, routing, and orchestration capabilities don’t come into play at all.

In all truthfullness, those bits of functionality are really just a historical artifact of their broker heritage.

Don’t get me wrong, sometimes a broker is a nice thing to have – behind a service boundary in order to perform some complex integration between existing legacy applications.

Just keep that stuff in its place – not between services.

 

Complex Event Processing

We can look at how Sales transitions an order from being tentatively accepted to being accepted as requiring event correlation around InventoryAllocatedToOrder events. This isn’t exactly “complex” in its own right. If there were some kind of CEP engine that did this for us out of the box, it might be a possible technology choice for implementing this logic within our service.

As we add more concerns, like time, we may find new ways to make use of this engine. For instance, if the time to provide the order to the customer is approaching, we may choose to split the order into two – accepting one for which we have all the stock allocated, and leaving the second as tentatively accepted.

 

Summary

While it is difficult to move forward on service responsibility without discussing the events it raises and those it subscribe to, the whole issue of CEP can be postponed for a while.

Although there aren’t many who would say that EDA is necessary for driving down coupling in SOA, or that SOA won’t likely provide much value without EDA, or that SOA is necessary for providing the right boundaries for EDA, it’s been my experience that that is exactly the case.

CEP, while being a challenging engineering field, and managing the technical risks around it necessary for a project to succeed in some circumstances, and really shines when used under the SOA/EDA umbrella, it should not be taken by itself and used at the topmost architectural levels.

 


Related Content

SOA and Enterprise Processes

How client interaction fits with SOA

Time and SOA

Durable Messaging for Fault-Tolerant Services

And if you’re wondering about how to handle all that complexity inside services (different kinds of billing, periodic tests for electronics inventory, etc), you might like listening to this podcast about business components.



Additional Logic Required For Service Autonomy

Wednesday, October 22nd, 2008

Of the tenets of Service Orientation, the tenet of Autonomy is one that many understand intuitively. Interestingly enough, many in that same intuitive category don’t see pub/sub as a necessity for that autonomy.

Watch that first step

Although sometimes described as the first step of an organization moving to SOA, web-service-izing everything results in synchronous, blocking, request/response interaction between services. The problem being that if one service were to become unavailable, all consumers of that service would not be able to perform any work. With the deep service “call stacks” this architectural style condones, the availability and performance of the entire organization will be dictated by the weakest link.

 weak link

So, while I’d agree that many organizations do need to take this step, I’d caution against going into production at this step.

Pub/Sub Considered Helpful

When services interact with each other using publish/subscribe semantics we don’t have that technical problem of blocking. Subscribers cache the data published to them (either in memory or durably depending on their fault-tolerance requirements) thus enabling them to function and process requests even if the publisher is unavailable.

Consider the following scenario:

Let’s say we have an e-commerce site, a part of our Sales service responsible for selling products. Another service, let’s call it merchandising, is responsible for the catalog of products, and how much each product costs. Sales is subscribed to price update events published by Merchandising and saves (caches) those prices in its own database. When a customer orders some products on the site, Sales does not need to call Merchandising to get the price of the product and just uses the previously saved (cached) price. Thus, even if Merchandising is unavailable, Sales is able to accept orders. This is a big win as our merchandising application is not nearly as robust as our sales systems.

Yet, there are scenarios where data freshness requirements prevent this.

Too Much of a Good Thing?

Technically, the above story is accurate. There is nothing technically preventing Sales from accepting orders. Yet consider a scenario where Merchandising is down or unavailable for an extended period of time. While this may not be entirely likely for two servers in the same data center, consider physical kiosks which customers can use to buy products. Those kiosks may not receive updates for days. Should they accept orders?

That’s really a question to the business. If pricing data is stale for a time period greater than X, do not sell that item. The value of X may even be different for different kinds of products. Keep in mind that this issue only arose since we architected our services to be fully autonomous. In a synchronous systems architecture, this issue would not come up. As such, it is our responsibility as architects to go digging for these requirements as well as explaining to the business what the tradeoffs are.

In order to have more up to date data, we need to invest in more available hardware, networks, and infrastructure. This needs to be balanced against the predicted increase in revenue that more up to date (read higher) prices would give us.

You Can Get What You Pay For

Beyond the additional cost of writing that additional logic, and the perceived increased complexity, another difference to note between this architectural style and the synchronous/traditional one is that it puts control of spending back in the hands of business.

In a synchronous architecture, in order to achieve required performance and availability, all systems need to be performant requiring across the board investments in servers, networks, and storage. Without investing everywhere, the weakest link is liable to undo all other investments. In other words, your developers have made your investment choices for you. Scary, isn’t it.

A more prudent investment strategy would prefer spending on services that give the biggest bang for the buck, better known as return on investment. A pub/sub based architecture allows investing in data-freshness where it makes the most sense. For example, in sales of high profit products to strategic customers rather than inventory management of raw materials for products slated to be decommissioned.

That sounds a lot like IT-Business Alignment.

Maybe there’s something to this SOA thing after all…


Read more about:

7 Questions for Service Selection

7 Questions around data freshness 

Event-Driven Architecture and Legacy Applications

Autonomous Services and Enterprise Entity Aggregation

Or listen to a podcast describing Business Components, the connection of pub/sub and SOA.



Services Don’t Serve

Saturday, August 23rd, 2008

imageAnother prominent SOA practitioner and blogger, Steve Jones, shows that, when you’re identifying your top level business services you shouldn’t be thinking about who’s going to consume them.

“We have three high level business services: Engagement, Management, [and] Production. […] they represent different operational ambitions. Engagement is all about quantity and then filtering. Management is about the quality and Production is about realising the benefits.”

Services are not about “are you being served?”

They’re not about re-use, and barely about use. Events are what it’s all about.

Each service has its own responsibility and does what it needs to do, business-wise, to achieve its goals. Whether it’s about increasing the number of leads, ensuring high-profile clients get good service, or maximizing equipment utilization, services take responsibility.

I know I harp on this a lot.

It’s because it’s that important.



Command Query Separation and SOA

Monday, August 11th, 2008

One of the common questions I receive from people starting to use nServiceBus is how one-way messaging fits with showing the user a grid (or list) of data. Thinking about publish/subscribe usually just gets them even more confused. Trying to resolve all this with Service Oriented Architecture leaves them wondering – why bother?

client server

In regular client-server development, the server is responsible for providing the client with all CRUD (create, read, update, and delete) capabilities. However, when users look at data they do not often require it to be up to date to the second (given that they often look at the same screen for several seconds to minutes at a time). As such, retrieving data from the same table as that being used for highly consistent transaction processing creates contention resulting in poor performance for all CRUD actions under higher load.

A Scalable Solution

One of the common answers to this question is for the server/service to publish a message when data changes (say, as the result of processing a message) and for clients to subscribe to these messages. When such a notification arrives at a client, the client would cache the data it needs. Then, when the user wants to see a grid of data, that data is already on the client. Of course, this solution doesn’t work so well for older client machines (like some point of service devices) or if there are millions of rows of data.

The thing is that this solution is one implementation of a more general pattern – command query separation (CQS).

Command Query Separation

Wikipedia describes CQS as a pattern where "… every method should either be a command that performs an action, or a query that returns data to the caller, but not both. More formally, methods should return a value only if they are referentially transparent and hence possess no side effects."

Martin Fowler is less strict about the use of CQS allowing for exceptions: "Popping a stack is a good example of a modifier that modifies state. Meyer correctly says that you can avoid having this method, but it is a useful idiom. So I prefer to follow this principle when I can, but I’m prepared to break it to get my pop."

So, how does separating commands from queries and SOA help at all in getting data to and from a UI? The answer is based on Pat Helland’s thinking as described in his article Data on the Inside vs. Data on the Outside.

Services Cross Boxes

The biggest lie around SOA is that services run.

Let that sink in a second.

Sure services have runnable components, but that’s not why they’re important.

I’ll skip the books of background and cut to the chase:

Services communicate with each other using publish/subscribe and one-way messaging. Services have components inside them. Inside a service, these components can communicate with each using synchronous RPC, or any other mechanism. Also, these components can reside on different machines.

This is broader than just scaling out a service. There can be service components running on the client as well as the server.

SOA & CQS

Combining these two concepts together, here’s what comes out:

In this solution there are two services that span both client and server – one in charge of commands (create, update, delete), the other in charge of queries (read). These services communicate only via messages – one cannot access the database of the other.

The command service publishes messages about changes to data, to which the query service subscribes. When the query service receives such notifications, it saves the data in its own data store which may well have a different schema (optimized for queries like a star schema).

The client component which is in charge of showing grids of data to the user behaves the same as it would in a regular layered/tiered architecture, using synchronous blocking request/response to get its data – SOA doesn’t change that.

Composite Applications

Although the client side components of both the command and query services are hosted in the same process, they are very much independent of each other. That being said, from an interoperability perspective (the one that most people attribute to SOA), all of the client-side components will likely be developed using the same technology – although there are already ways to host Java code in .NET and vice-versa.

Of course, once we talk about web UI’s things are a bit different – but still similar. While web-server-side there may be a level of independence, for browser side inter-component communications we’re still likely to target javascript. There, I’ve managed to say something technical supporting mashups and SOA without lying through my teeth.

On the Microsoft side with the recent release of the Composite Application Guidance & Library (pronounced "prism") I hope that more of these principles will be reaching the "smart client". The command pattern is especially critical in maintaining the separation while enabling communication to still occur so I’m glad that, as one of the Prism advisors, I was able to simplify that part (Glenn still has nightmares about that rooftop conversation).

Publish / Subscribe

In the "scalable solution" section up top I mentioned how publish/subscribe to the smart client is really just one implementation of CQS and SOA. So, how different is it really?

smart client pub/sub

Well, there will probably be a different technology mapping. Instead of a star-schema OLAP product, we might simply store the published data in memory on the client. That is, if you designed your components to be technology agnostic.

In terms of the use of nServiceBus, the same component is going to be subscribing to the same type of message – all that’s different is that now every client will be having data pushed to them rather than this occurring server-side only.

You could have the same code deployed differently in the same system – stronger clients subscribing themselves, weaker ones using a remote server. Web servers would probably be considered stronger clients. This kind of flexible deployment has proven to be extremely valuable for my larger clients. The added benefit of enabling users to work (view data) even while offline (somewhere there’s no WIFI) is just icing on the cake.

A Word of Warning

Once the client starts receiving notifications, and handling those on a background thread (as it should) the code becomes susceptible to deadlocks and data races. Juval does a good job of outlining some of those with respect to the use of WCF. Prism doesn’t provide any assurances in this area either.

Summary

NServiceBus is not designed to be used for any and all types of communication in a given architecture. In the examples above, nServiceBus handles the publish/subscribe but leaves the synchronous RPC to existing solutions like WCF. Not only that, but synchronous RPC does have its place in architecture, just not across service boundaries. In all cases, data is served to users from a store different from that which transaction processing logic uses.

Command Query Separation is not only a good idea at the method/class level but has advantages at the SOA/System level as well – yet another good idea from 20 years ago that services build upon. Making use of CQS requires understanding your data and its uses – SOA builds on that by looking into data volatility and the freshness business requirements around it.

Finally, designing the components of your services in such a way that their dependency on technology is limited buys a lot of flexibility in terms of deployment and, consequently, significant performance and scalability gains.

Simple, it is. Easy, it is not.



7 Simple Questions for Service Selection

Friday, May 16th, 2008

“So, which services do I need?”

This innocuous question comes up a lot. Usually I get this question after a short problem domain description. One of these came up on the nServiceBus discussion groups. Ayende took it and ran with it turning it into a nice blog post, An exercise in designing SOA systems. I’ve been meaning to write something myself. Bill put up a response already in his Service Granularity Example. So, I’m late to the party, again, but here we go.

It’s almost impossible to know, right away, which services are appropriate.

So, I’m going to focus more on the process of getting there, rather than describing the solution itself.

The domain deals with a placement agency placing physicians in positions at hospitals. doctor

1. So, what does it actually do?

In Ayende’s post, he describes several services, but I’d rather look at them as use cases: registering an open position, registering a candidate, verifying their credentials, etc. It’s worth going through this requirements process. It doesn’t necessarily translate immediately to services, but there’s value in it.

2. What does it do it to?

We should also be looking at the data model, an entity relationship diagram (ERD) , where we see that we may have placed a certain physician at a number of positions. It’s also important for us to know about under which circumstances a physician finished their employment at a previous position before, say, trying to place them at a position in the same hospital or chain of hospitals. Don’t go thinking that this what the database schema will look like, it’s all about understanding connections between various bits of data.

3. When does that happen?

The next step is to map the uses cases above to the entities in the ERD, which entity is used in which use case. It’s also important to differentiate between entities (or even more importantly, specific fields of entities) that are used in a read-only fashion within a given use case. For instance, when registering a new position, we’ll want to check that against other open positions in the same hospital so we don’t end up registering the same position twice. Also, we might want to suggest verified physicians whose credentials match the position’s requirements. Data we wouldn’t be interested in might be which other physicians we placed at that hospital.

4. What just happened?

Another valuable perspective on the problem domain is the business process view – what are the interesting business events in the system and how they unfold over time. For instance, physician registered, position opened, physician’s credentials verified, and physician placed in position (or position filled by physician) are events that describe a different business perspective than use cases.image

5. How do I decide?

Once we know what events there are, we can start looking at what kind of decisions we might want to make when those events occur and what data we’d need to make those decisions. These decisions may be as simple as updating a database or sending an email to a user. They also may include more advanced logic like when the profitability of an agreement with a specific hospital chain changes, prefer placing physicians in positions in that chain over others.

6. How do I deal with all this information?

After we have all of this information, we can start looking for cohesive bunching across all of these axes using these rules:

  • Data that is modified by a use case gets published as an event.
  • Data that is required by a use case for read-only purposes, arrives as the result of subscribing to some event.

Look for rules that differentiate behaviour based on the properties of data. Look for a correlation to some business concept. For instance, physicians probably won’t be changing their specialization, and open positions often deal with a certain specialization. Therefore, specific data instances tied to two different specializations can be said to be loosely coupled.

7. Which property slices across the domain?image

Even though the ERD may not have made it clear, and the use cases didn’t show any particular break-down, nor did the events call out this point, the key to finding the way a business domain decomposes into services lies in decoupling specific data instances.

Actually, at this point we can clump autonomous components (mere technical bits) that handle a single message, into more granular business components.

If you think about it, it makes a lot of sense. The kind of credential checking you’d do for physicians specializing in brain surgery would likely be different than for general practitioners. The kind of information you’d store would, therefore, also be different.

But, which services do I need?

Quite frankly, I don’t have enough information to know.

But if we had continued this conversation, going through issues like transactional consistency, availability requirements, and other non-functional issues we could have  gotten there.

If there’s one thing that I hope you got out of this, it’s that the questions are what’s important. The iterative process of looking at the problem domain from various perspectives, incorporating the new-found knowledge, and asking more questions is what leads us to a solution. But we don’t stop there. We keep looking for characteristics which split services apart into business components, and for consistency requirements that brings autonomous components together into services.

It’s not easy, but by focusing on these simple questions, you can get to a coherent service oriented architecture.



Visual Cobol, Enterprise Processes, and SOA

Wednesday, April 23rd, 2008

There’s a fairly intense discussion going on these days amongst the SOA illuminati. In the hopes that people will see me standing beside them and conclude that I too know something, I’ve decided to chip in.

Jim brought the concept of cohesion to the regular SOA discussions around loose coupling in his post Anemic Service Model, which I think, all in all, is a very good idea.

Naïve Service Composition

image Jim first calls out a common anti-pattern that seems to have become quite rampant – I’d call it naïve service composition if only the things being composed could even be called services. And I think the tone being set is correct – a service needs to meet a stronger set of criteria than just being able to be composed. Multiple services sharing the same logical data store (in that the same actual rows/data elements are managed by multiple services) probably means there’s an encapsulation problem here. I agree with Jim sentiment here:

“On the one hand we’re inclined, and indeed encouraged by the SOA brigade, to think of this architecture as a good fit for purpose because it is very loosely coupled. Since every component or service is decoupled from every other component or service it should be possible to arrange and re-arrange them in a Lego-style in a myriad of useful ways. Building out “business services” from some more fundamental set of services is how the books tell us to do it. In fact we could even do that quite easily with point-and-[click] BPM tools, ruling out such overheads as developers and change management along the way. Right?”

MVC? There are, like, 6 of them!image

However, I disagree with some of the conclusions that Jim draws from that point. Jim states “build your services to implement business processes”, and that services are “just an instance of MVC”. I’m going to leave alone the MVC statement since there are like 6 documented kinds of MVC not including the Front Controller stuff that the web guys are now calling MVC. I’m going to focus on the business process advice. JJ also doesn’t seem to agree with this advice. As Savas has already taken issue with the tone of JJ’s response, I’ll keep my focus on the content.

Visual Cobol

First of all, in my previous conversations with Jim he had already denounced the procedural nature of composing higher-level business processes out of smaller services which implement small bits of common activities. Visual Cobol was how he described it. In JJ’s follow-up post, he called out the necessary aspect of autonomy that jives with Jim’s cohesion principle.

I’m a bit concerned about the way JJ tends to version what SOA means over time. It might make it impossible to have intelligent design discussions without tagging each sentence with “as SOA meant in 2006”. I acknowledge that the accepted meaning of SOA by various vendors has changed over the years. However, I’ve found that meanings rooted in decades of computer science tend to last and provide value that outlasts much of the industry-buzzword-bingo (SOA 2.0 anyone?).

Cohesion, Business Domains, and Business Processes

image My view of the original cohesion principles Steve discusses in his 2005 article Old Measures for New Services takes a business spin to Functional Cohesion:

A service should be responsible for one business domain.

If we jump off from this point, we’ll see that certain business processes which occur entirely in one business domain are fully encapsulated, whereas those macro-processes which cross many domains (like Order to Cash) cross multiple services – they do not become a service since that would break the “one business domain” rule. Given that services are loosely coupled, avoiding temporal coupling leads to services raising events. Thus, macro-processes are really just a series of events of various services where each service does its own internal business processes.

Enterprise Processes >> Business Processes

I think that maybe some of the difficulty in discussing concrete SOA guidance has to do with granularity. I’ve started calling those macro-processes something different from business processes, and that may just bring me full circle to Jim’s guidance.

An Enterprise Process is any process which involves multiple business domains.

Under that definition, a service may be responsible for multiple business processes in the same business domain. But still, one business process is usually not a service by itself.

Business Components & Autonomous Components to the Rescue

image Finally, by introducing the additional levels of decomposition of business components and autonomous components I’ve found that we can focus the discourse on one concern at a time. My presentation on the topic can be found here. The 30 second pitch is this:

Business domains are inherently partitionable – data and rules. A business component represents one partition. An example of this is the domain of Sales being partitioned by strategic and non-strategic customers. Although the data structure might be similar or the same, the actual rows/data elements are not shared. Rules around discounts are different.

Within a business component, different activities should not interfere with each other. An autonomous component represents one activity. In our example, reporting on orders from strategic customers should not interfere with accepting their orders. As such, those activities should have different messages coming in on different endpoints. Each endpoint could have different characteristics, like durability. Losing a request for a report when a server restarts isn’t a big deal, however not a good idea for orders.

For more information you could check out these episodes from my podcast:

Business and Autonomous Components in SOA

Using Autonomous Components for SLAs in SOA

Questions and comments are always welcome.



Sundblad Mistaken on Services

Sunday, March 16th, 2008

The brilliant guys at 2xSundbland have launched their architect academy and it looks quite promising. I haven’t yet taken the trial lesson, but its in the queue. I have taken a look at the articles they have on the site as well, and they’re quite good. I especially like the Software Architecture vs. Software Engineering one. There is one topic in that article where I beg to differ, and it’s around services. The article (on page 7) describes the following scenario:

Typically, in such an environment [SOA], services tend to be parts of multiple systems. For example, consider a Products service! It might start its life as part of a sales system. Later it might be involved in a purchasing system, a product development system, a marketing system, a warehousing system, and perhaps in several other systems too. This process may take years, and it really never ends. The service is the same, but its responsibilities and its external exposure are increased with each system it’s enrolled in.

One of the core tenets of SOA that all vendors and analysts agree upon is that there should be loose coupling between services. If you were to design such a product service, it’s clear that changing part of its interface could break almost every system in the enterprise. That doesn’t sound like loose coupling to me.

If there’s one place that is the source of loose coupling – it’s the business. Warehousing is viewed by the business as being fairly independent of Marketing. While Sales might make use of data created in Product Development, business wouldn’t want any problems in IT related to Product Development to inhibit Sales ability to accept orders. That is another kind of loose coupling – the ability of one service to make use of “not-accurate-up-to-the-millisecond” data created by another service. That’s known as loose “temporal” coupling, as in loose coupling in the dimension of time.

Loosely-Coupled Services

So, in the example described we’d see the following services:

  • Sales
  • Purchasing
  • Product Development
  • Marketing
  • Warehousing / Inventory

Product data would flow between the services but each would have a very different internal view of it.

  • Product Development would be more interested in managing the schedule and risk around a product’s development.
  • Marketing would probably be more focused on its relation to competing products and pricing.
  • Purchasing would be maintaining data as to which suppliers are being used to supply raw materials for the production of the product.
  • Sales would be looking at actually accepting orders and giving discounts.
  • Warehousing would be focused on the storage and transportation aspects of the product.

As you can see, there is very little overlap in the data between these services even on something similar like product data. The logic of each service around the management of its data would be even more different. This leads to services with a high level of autonomy.

There Be Dragons…

Without starting at this business-level loose coupling, I doubt that any technical effort will succeed. That is to say every time I’ve seen this style implemented it has failed, but that’s no proof. Conversely, every time that we did start our SOA efforts by identifying the clear business fracture lines, we were able to maintain loose coupling all the way down. That is not to say that it always will succeed, but the logic is sound.

I suppose that the difference between my view on SOA and Sundblad’s stems from the fact that they put systems at a higher level of abstraction than services, and I put services on top. Regardless, I do agree with their views about architecture and engineering and consider them quite valuable.



NServiceBus implements Erlang Concurrency

Friday, February 8th, 2008

Going over the concurrency features of Erlang, the language famed for nine 9’s of uptime, I find that nServiceBus covers almost every single one.

Here’s the core list from Joe Armstrong’s book, Programming Erlang:

“In Erlang:

  • Creating and destroying processes is very fast.
  • Sending messages between processes is very fast.
  • Processess behave the same way on all operating systems.
  • We can have very large numbers of processes.
  • Processes share no memory and are completely independent.
  • The only way for processes to interact is through message passing.”

In nServiceBus, we don’t create or destroy processes – that’s a Windows issue. Instead, we just do messaging with endpoints. If there’s a process behind that endpoint, and it responds, then other interesting things can occur.

In the continued list:

  • Message passing is asynchronous.
  • Processes can monitor each other.
  • It is possible to selectively receive messages.
  • Remote processes appear largely the same as local processes.

All of this is part of the design philosophy of nServiceBus. While I have yet to see a carrier-grade implementation of nServiceBus, we are enjoying very impressive system-wide uptimes in production. Oh, and the programming model is still plain-old .NET, so you don’t have to learn any new languages or environments (even though I think that you might learn something – I know I did).



Sagas and Unit Testing – Business Process Verification Made Easy

Monday, February 4th, 2008

Sagas have always been designed with unit testing in mind. By keeping them disconnected from any communications or persistence technology, it was my belief that it should be fairly easy to use mock objects to test them. I’ve heard back from projects using nServiceBus this way that they were pleased with their ability to test them, and thought all was well.

Not so.

The other day I sat down to implement and test a non-trivial business process, and the testing was far from easy. Now as developers go, I’m not great, or an expert on unit testing or TDD, but I’m above average. It should not have been this hard. And I tried doing it with Rhino.Mocks, TypeMock, and finally Moq. It seemed like I was in a no-mans-land, between trying to do state-based testing, and setting expectations on the messages being sent (as well as correct values in those messages), nothing flowed.

Until I finally stopped trying to figure out how to test, and focused on what needed to be tested. I mean, it’s not like I was trying to build a generic mocking framework like Daniel.

Here’s an example business process, or actually, part of one, and then we’ll see how that can be tested. By the way, there will be a post coming soon which describes how we go about analysing a system, coming up with these message types, and how these sagas come into being, so stay tuned. Either that, or just come to my tutorial at QCon.

On with the process:

1. When we receive a CreateOrderMessage, whose “Completed” flag is true, we’ll send 2 AuthorizationRequestMessages to internal systems (for managers to authorize the order), one OrderStatusUpdatedMessage to the caller with a status “Received”, and a TimeoutMessage to the TimeoutManager requesting to be notified – so that the process doesn’t get stuck if one or both messages don’t get a response.

2. When we receive the first AuthorizationResponseMessage, we notify the initiator of the Order by sending them a OrderStatusUpdatedMessage with a status “Authorized1”.

3. When we get “timed out” from the TimeoutManager, we check if at least one AuthorizationResponseMessage has arrived, and if so, publish an OrderAcceptedMessage, and notify the initator (again via the OrderStatusUpdatedMessage) this time with a status of “Accepted”.

And here’s the test:

    public class OrderSagaTests 
    { 
        private OrderSaga orderSaga = null; 
        private string timeoutAddress; 
        private Saga Saga;     

        [SetUp] 
        public void Setup() 
        { 
            timeoutAddress = "timeout"; 
            Saga = Saga.Test(out orderSaga, timeoutAddress); 
        }     

        [Test] 
        public void OrderProcessingShouldCompleteAfterOneAuthorizationAndOneTimeout() 
        { 
            Guid externalOrderId = Guid.NewGuid(); 
            Guid customerId = Guid.NewGuid(); 
            string clientAddress = "client";     

            CreateOrderMessage createOrderMsg = new CreateOrderMessage(); 
            createOrderMsg.OrderId = externalOrderId; 
            createOrderMsg.CustomerId = customerId; 
            createOrderMsg.Products = new List<Guid>(new Guid[] { Guid.NewGuid() }); 
            createOrderMsg.Amounts = new List<float>(new float[] { 10.0F }); 
            createOrderMsg.Completed = true;     

            TimeoutMessage timeoutMessage = null;     

            Saga.WhenReceivesMessageFrom(clientAddress) 
                .ExpectSend<AuthorizeOrderRequestMessage>( 
                    delegate(AuthorizeOrderRequestMessage m) 
                    { 
                        return m.SagaId == orderSaga.Id; 
                    }) 
                .ExpectSend<AuthorizeOrderRequestMessage>( 
                    delegate(AuthorizeOrderRequestMessage m) 
                    { 
                        return m.SagaId == orderSaga.Id; 
                    }) 
                .ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return m.OrderId == externalOrderId && destination == clientAddress; 
                    }) 
                .ExpectSendToDestination<TimeoutMessage>( 
                    delegate(string destination, TimeoutMessage m) 
                    { 
                        timeoutMessage = m; 
                        return m.SagaId == orderSaga.Id && destination == timeoutAddress; 
                    }) 
                .When(delegate { orderSaga.Handle(createOrderMsg); });     

            Assert.IsFalse(orderSaga.Completed);     

            AuthorizeOrderResponseMessage response = new AuthorizeOrderResponseMessage(); 
            response.ManagerId = Guid.NewGuid(); 
            response.Authorized = true; 
            response.SagaId = orderSaga.Id;     

            Saga.ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return (destination == clientAddress && 
                                m.OrderId == externalOrderId && 
                                m.Status == OrderStatus.Authorized1); 
                    }) 
                .When(delegate { orderSaga.Handle(response); });     

            Assert.IsFalse(orderSaga.Completed);     

            Saga.ExpectSendToDestination<OrderStatusUpdatedMessage>( 
                    delegate(string destination, OrderStatusUpdatedMessage m) 
                    { 
                        return (destination == clientAddress && 
                                m.OrderId == externalOrderId && 
                                m.Status == OrderStatus.Accepted); 
                    }) 
                .ExpectPublish<OrderAcceptedMessage>( 
                    delegate(OrderAcceptedMessage m) 
                    { 
                        return (m.CustomerId == customerId); 
                    }) 
                .When(delegate { orderSaga.Timeout(timeoutMessage.State); });     

            Assert.IsTrue(orderSaga.Completed); 
        } 
    }

You might notice that this style is a bit similar to the fluent testing found in Rhino Mocks. That’s not coincidence. It actually makes use of Rhino Mocks internally. The thing that I discovered was that in order to test these sagas, you don’t need to actually see a mocking framework. All you should have to do is express how messages get sent, and under what criteria those messages are valid.

If you’re wondering what the OrderSaga looks like, you can find the code right here. It’s not a complete business process implementation, but its enough to understand how one would look like:

using System; 
using System.Collections.Generic; 
using ExternalOrderMessages; 
using NServiceBus.Saga; 
using NServiceBus; 
using InternalOrderMessages;     

namespace ProcessingLogic 
{ 
    [Serializable] 
    public class OrderSaga : ISaga<CreateOrderMessage>, 
        ISaga<AuthorizeOrderResponseMessage>, 
        ISaga<CancelOrderMessage> 
    { 
        #region config info     

        [NonSerialized] 
        private IBus bus; 
        public IBus Bus 
        { 
            set { this.bus = value; } 
        }     

        [NonSerialized] 
        private Reminder reminder; 
        public Reminder Reminder 
        { 
            set { this.reminder = value; } 
        }     

        #endregion     

        private Guid id; 
        private bool completed; 
        public string clientAddress; 
        public Guid externalOrderId; 
        public int numberOfPendingAuthorizations = 2; 
        public List<CreateOrderMessage> orderItems = new List<CreateOrderMessage>();     

        public void Handle(CreateOrderMessage message) 
        { 
            this.clientAddress = this.bus.SourceOfMessageBeingHandled; 
            this.externalOrderId = message.OrderId;     

            this.orderItems.Add(message);     

            if (message.Completed) 
            { 
                for (int i = 0; i < this.numberOfPendingAuthorizations; i++) 
                { 
                    AuthorizeOrderRequestMessage req = new AuthorizeOrderRequestMessage(); 
                    req.SagaId = this.id; 
                    req.OrderData = orderItems;     

                    this.bus.Send(req); 
                } 
            }     

            this.SendUpdate(OrderStatus.Recieved);     

            this.reminder.ExpireIn(message.ProvideBy - DateTime.Now, this, null); 
        }     

        public void Timeout(object state) 
        { 
            if (this.numberOfPendingAuthorizations <= 1) 
                this.Complete(); 
        }     

        public Guid Id 
        { 
            get { return id; } 
            set { id = value; } 
        }     

        public bool Completed 
        { 
            get { return completed; } 
        }     

        public void Handle(AuthorizeOrderResponseMessage message) 
        { 
            if (message.Authorized) 
            { 
                this.numberOfPendingAuthorizations--;     

                if (this.numberOfPendingAuthorizations == 1) 
                    this.SendUpdate(OrderStatus.Authorized1); 
                else 
                { 
                    this.SendUpdate(OrderStatus.Authorized2); 
                    this.Complete(); 
                } 
            } 
            else 
            { 
                this.SendUpdate(OrderStatus.Rejected); 
                this.Complete(); 
            } 
        }     

        public void Handle(CancelOrderMessage message) 
        {     

        }     

        private void SendUpdate(OrderStatus status) 
        { 
            OrderStatusUpdatedMessage update = new OrderStatusUpdatedMessage(); 
            update.OrderId = this.externalOrderId; 
            update.Status = status;     

            this.bus.Send(this.clientAddress, update); 
        }     

        private void Complete() 
        { 
            this.completed = true;     

            this.SendUpdate(OrderStatus.Accepted);     

            OrderAcceptedMessage accepted = new OrderAcceptedMessage(); 
            accepted.Products = new List<Guid>(this.orderItems.Count); 
            accepted.Amounts = new List<float>(this.orderItems.Count);     

            this.orderItems.ForEach(delegate(CreateOrderMessage m) 
                                        { 
                                            accepted.Products.AddRange(m.Products); 
                                            accepted.Amounts.AddRange(m.Amounts); 
                                            accepted.CustomerId = m.CustomerId; 
                                        });     

            this.bus.Publish(accepted); 
        } 
    } 
}

All this code is online in the subversion repository under /Samples/Saga.

Questions, comments, and general thoughts are always appreciated.



   


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“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.”

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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