Space-Based Architecture (or SBA for short) just might be in your future if your building large-scale distributed systems. By focusing on high-throughput and low latency, SBA joins messaging and in-memory data caching and adds a good measure of load partitioning. However, with the entire industry enamoured with SOA, what place is left for SBA?

Before going too far ahead, you might want to take a look at my previous post “Space-Based Architectural Thinking, or listen to my podcast Space-Based Architecture for the Web. There’s also a 30 minute webcast online describing SBA more fully here. I’m also going to try to stay away from things concerning Jini this time after already discussing the connection between Jini and SOA, and the tradeoffs between two general approaches: Tasks and Spaces vs Message and Handlers.

OK, so the issue of state-management is a big one. Everybody wants to work stateless, because it scales. The only problem is that the business processes that we are automating are long running, meaning that there are external systems or people involved. This makes these processes inherently stateful. So, we need a way to scale statefully – SBA gives us that. For some background on the “Shared Nothing Architecture”, I suggest reading this post on inter-process SOA and this one as well.

Availability also has to be handled, not only in terms of having enough servers online to handle the required load but in having all the data required to process each request be accessible. This has often been handled by the database using ACID transactions – durability being that which solved availability issues, but also hurting latency the most. The problem with saving the state of our long-running business processes/workflows in the database is the load and the responsiveness requirements. In many verticals – telcos, financial, and defense to name a few, we need millisecond level latency on each stage of the workflow. This is what leads SBA to the in-memory, replicated data grid.

Note that SBA only intends to take these workflows out of the database, and not anything else – especially not Master Data. The lifetime of these workflows is incredibly short compared to that of master data like customers and products. It will have much different backup strategies as well. In terms of load, these workflows will be heavy on reads and writes together in the same transactions, but quite low in terms of just reads. If we have workflows that perform work in parallel, we easily end up with concurrency requirements that make DBAs cringe under the barrage of short transactions.

If you’re worried that Workflow Foundation (WF) won’t scale because of the above, you needn’t be. You can (more or less easily) replace the persistence mechanism of WF with your own, saving your workflow instances to an in-memory replicated data grid.

By enabling the objects in the grid to call back into logic on your servers, you have, in essence, done messaging and more. The added benefit that SBA receives from this is a unification of technology between caching and messaging. This translates directly to savings when it comes time to cluster each of those technology’s environments.

Finally, if we can find an attribute in the incoming stream of messages that creates a nice even distribution, we can then partition our load between our servers by that key. This will work up to the point where the load per key increases beyond a single server’s capacity, and then we have to look at re-partitioning, a non-trivial problem. However, if we put objects in our grid that represent the master data, and tie them to our workflow instances with both of those tied to the key of our load, a smart infrastructure can make sure all that data is already resident on the server that is handling that piece of the load. That decreases latency even more since we no longer have to pay network roundtrips to collect all the data needed before we can process it. That’s a substantial advantage for the above verticals.

But all of this has nothing to do with SOA.

Sure, it’ll change how we implement our Services internally, but it has no impact on their interfaces or the top-level service decomposition. In the Java community, the word “service” is often used to describe the logic of a system. Great significance is placed on keeping these “services” simple, as in Plain-Old Java Objects. The fact of the matter is that the logic of the system should be simple and independent of other concerns like data access and communcations (a la Web Services), but that does not make it a service, not in the SOA sense.

For more information on what Services in SOA are like, check out this podcast on Business and Autonomous Components in SOA. Actually, SBA will probably have the biggest impact on the way autonomous components will handle service-level agreements.

So, it appears that even with SOA, SBA has its place. The former dealing with business level agility, the latter dealing with all the technical aspects of supporting that agility. If you’re tasked with the designing the architecture of a scalable, available, high-throughput, low-latency distributed system, I’d strongly advise you to look at SBA – the technical value is overwhelming. Even if you don’t utilize all elements of SBA and choose the Master Worker Pattern instead of load partitioning, you’ll find the technologies supporting SBA to be quite flexible in that respect.

Will Space-Based Architectures be a part of your future? I don’t know for sure, but they’re a most welcome part of my present.

In this podcast we answer questions about how to use autonomous components to unify disparate building blocks like servers, middleware, and databases in order to handle the technical complexity of complying with detailed service-level agreements. Reuse of business logic, database schemas, and messaging topics between autonomous components are discussed as well.

Download via the Dr. Dobbs’ site.

Or download directly here.

And here’s this week’s question:

Hi Udi,

Thanks again for your continued assistance. I was very much interested by your advice to consolidate each of the services related to each product family into a single service, but as autonomous components.

From your description of autonomous components from a prior podcast, it seems that they are much the same as services – in that they communicate only via loosely coupled messaging, and can have their own databases. Would you say that the main difference between autonomous components is that different autonomous components within a service may in fact share business logic and databases? If so, it would seem that combining these services into a single service with 3 autonomous components would be a matter of definition, rather than an architectural shift. Any information you could provide to clarify this distinction would be fantastic.

Something else that’s been playing on my mind of late – is whether or not you would consider a topic as having to belong to a specific service. That is, would you say it is bad practice to have multiple services publish on a common topic? I suppose if we have multiple services publishing on a common topic, then they should be defined as autonomous components, belonging to a single larger service – in which case that common topic would belong to that new service.

As usual your advice is always extremely helpful. Please keep those podcasts coming!

Best Regards,
Bill

Additional References

• Podcast on Message Schemas between multiple Publishers and Subscribers
• Podcast on Business and Autonomous Components in SOA

In this podcast we answer questions about how to solve dependencies between systems that subscribe to events in SOA. We’ll also get into the pitfalls of employing distributed transaction when reusing existing systems even behind service boundaries.

Download via the Dr. Dobbs’ site.

Or download directly here.

And the original question was:

Udi:

I have a question regarding publishing events that relate to data changes. I found the article you wrote in the Arch Journal #8 very helpful. I think striving for autonomy is very important. The scenario I was thinking about is how can you ensure synchronization across subscribers of a particular event.

For example, System A publishes an event when customer information is updated. There are several systems that subscribe to this event. Two of the systems, System B and System C, need to be sync regarding customer information. System B uses operations from System C using the customer data. Using your example, System B has a process that runs for all “Preferred Customers”, and it uses processes on System C. However, System C may not have process the event to and may have a customer as preferred.

I have several thoughts, but would like to get your thoughts on this scenario. Are there any best practices or patterns?

Phil

Additional References:

• Autonomous Services and Enterprise Entity Aggregation (Article in Microsoft Architecture Journal)
• Podcast on Autonomous Services and Pub/Sub

You can find more episodes like this in the Ask Udi archives.

I see this all too often.

A company has some legacy (read “good”) code in one platform (let’s just use Java for this example). This company has now decided to standardize on the other platform (Microsoft’s happy). The company doesn’t want to throw away and/or rewrite the assets they already have from the previous platform.

What do they do?

Web Services, of course!

Obviously this has to be a part of the “grand” SOA effort currently under way. Here’s a chance to build some reusable services, right?

The only problem is that in order for things to work right, they really must have a chatty interface, and flow transaction context between these “services”, and all the other things I describe as anti-patterns.

So, what’s the solution?

Well, Web Services isn’t the only kind of interop out there. Take a look at what the guys from MainSoft are doing. Not only does it work, it’s been working for years now. You can go the other way too – run .NET code on JVM based application servers. And there are more solutions out there. There’s JNBridge (with some nifty online demos), and EZ JCOM who also do COM interop.

Bottom line, there are good technical solutions out there for reusing and interop-ing with assets from multiple platforms. Therefore, now that we’ve slain the holy cow of interop, there is absolutely no good reason or justification to create tightly coupled services.

Period.

In Nick Malik’s follow up post The value of intermediation, part 2, he describes “composable services” as follows:

I would suggest that a service, when composable, (a) provides information in a manner that can be readily reused without requiring multiple calls to other services for interpretation, and/or (b) provides functionality that can be executed to produce specific semantics in an encapsulated manner without requiring references to multiple other services and without unknown or undesirable side effects.

I woud state that a service that does not comply with the above “suggestions” is not a service in the SOA sense. I don’t care much for the “composable” or “reusable” qualifiers that are being placed on the term “service”. I believe that it creates fractured definitions that don’t improve understanding.

Nick continues with assertions about the connection between SOA and Canonical Data Models:

I argue, passionately, that a service that does not leverage a Canonical data model (explicit or implicit) is not useful outside a small handful of very specific situations. Such a limited service provides some small benefits, but probably not more than a COM+ component would, and certainly not enough to justify all this interest in SOA. We get no business agility from this kind of service. Therefore, as an Enterprise Architect, you can create it, but I will not use it, nor will I look kindly on another application that does. Poor integration is just barely a half step up from no integration. Some would argue it is a step down.

Just to be clear, a Canonical Data Model is something that exists without dependence on any service. That is why I don’t agree with its use in SOA. Each service has its own message schema which makes use of its own data schema if you will. A data schema is the definition of structures that are used in more than one message type. The service controlls both of these schemas and decides when and how to version them, which versions to support on which endpoints, etc. It’s all part of the service’s autonomy. I’m not quite sure what an implicit canonical data model is.

I’m also unclear as to the “benefit” a service provides. Services are the way we model our business domain – they are therefore a part of our business. In my previous post on intermediation and SOA I gave an example of a Purchasing Service. This service does not really exist to provide something external to it with a “benefit”, it’s an inherent part of the structure of the business, much like the Purchasing Department of the company is. It participates in business processes but is not ruled by them. I really don’t see how you could compare that to any kind of technological component. In that respect, you don’t “use” a service. Neither do you really “integrate” them. Each service decides which processes it needs to be a part of, simply by choosing which events (of the EDA kind) it subscribes to. This leads to a kind of “distributed integration” that maintains service autonomy and has no single point of versioning or failure.

One of the comments quoted includes the statement “Intermediation greatly complicates any message exchange pattern other than request-response and pub-sub”. Like I described in my previous post, intermediation, if it even does occur, occurs behind service boundaries. Between services, you have the basic message exchange patterns of duplex request/response (one way messaging each time, with correlation between them) and pub/sub. You really don’t need anything else, and yesterday’s middleware already handled all of that for us. Most of the advanced ESB functionality isn’t needed between services.

The next example given discusses a banking scenario (well, more of an HR one really), which brings Nick to the following conclusion:

This process is neither pub-sub nor request-response. It is a long-running orchestration with compensating events. The process is NOT complicated by the ability to intermediate.

In fact, I would argue that nearly all valuable long running transactions MUST have the ability to intermediate in order to allow them to be composed, and recomposed, and orchestrated.

Long running anything, is a series of message exchanges that are tied together in some way. This could be as simple as having all the messages contain the same process ID in their body. “Compensating events” are just the result of business logic being run in a service, possibly changing its own state (updating those backend systems and applications), and sending out other messages. If there’s any intermediation of the kind Nick describes, it occurs within a service between its backend systems and applications. He also implicitly states that there is value in composing/orchestrating these “long running transactions”, apparently without having the service be involved. I don’t see it. Not the way I do my services. Once again, each service is responsible for itself – its part of the global “long running transaction”, ie business process. If one service were to choose to implement its cross-application workflows in a hard-coded manner, that should have no impact on any other services, nor should they be aware of it. That service may have been implemented poorly and as a result have difficulty responding to changing business conditions quickly, but that’s its own business. That implementation could easily be upgraded in the future, without changing the overall Service-Oriented Architecture.

Nick’s conclusion follows:

In conclusion, I don’t say that intermediation is a requirement of a service oriented architecture. But I do say that intermediation is a requirement of a service oriented architecture designed to deliver composability, and therefore, business agility.

Mine is a little different. When your SOA is made up of autonomous business-level services, you will have business agility. The implementations of some services may benefit through the use of intermediation, but it is not a top level concern.

This week’s question comes from Jayan, who asks:

Hi Udi,

I just went through your Blog which talks about not creating entity services, but instead creating a business service. I understood why you would want it to be a business service, although I am still struggling to fully define what a business service is.

Currently what we have requested is to have a single service, extract, transform and load master data. From your explanations it seems like this is a business service and not just an entity one. What one of the sales guys from an SOA company is saying that this is very possible and would be easy to do.. What I wanted to get your perspective is if this is the right thing to do. What we requested is below:

1. To create a business service that will extract, transform and load data (One service for user, one for customer and one for product?)
2. This service will then be called by the different applications we have (3 Java applications, 1 .Net Application and 1 Siebel Application as well as a host of VB/Excel Applications)
3. The service will be calling on different backend sources for data from SAP systems, Access DBs, Excel Files, Web Pages & Oracle Systems

They say that creating this would produce a.) a reusable service and b.) cost savings. Although I am still apprehensive because it seems as you mentioned in your podcast, each system would have slightly different set of rules for the data entity (and btw you are right).. would this still matter? The thing we want to eliminate is the replication of extracting and loading, although each system would transform the data in its own specific way..

Appreciate any perspective that you might have.

Thanks,

Jayan

Get it via the Dr. Dobb’s site here.

Or download directly here.

Additional References

Podcast on Master Data Management and SOA
Podcast on Business and Autonomous Components in SOA

Nick Malik has an interesting post on The value of intermediation in SOA where he starts out suggesting a couple of books that stand at the basis of much of today’s SOA thinking. I agree that far too few people seem to have read them.

In his previous post Is it service-oriented if the message cannot be intermediated, Nick defines intermediability as “SOA should give us the ability to intercept a message going from point A to point B, and react to that message without informing either end of that pipe.”. I’ll respond to this in due course.

Anyway, he continues on by saying “SOA [is] an architecture for Enterprise Application Integration.”

I can’t agree with that statement. The main reason is that EAI puts the application in the center, and that integrating existing applications one of the primary purposes of it. It is my assertion that in order to solve many of the problems that we are having today, we need to take a broader, business based view of the enterprise and model that with services. A service may be implemented with one or more applications. However, my experience has been that these services tend to use parts of existing applications, with multiple services using different parts of the same application. The reason for this is that the applications we have today, especially the ERP monoliths, do a lot, and at the same time, not everything. This is part of the reality that EAI tried to solve, but then got mired down in cross system hell. You just can’t solve poor business decomposition in the technology domain.

The value of putting services at the fore makes it possible to gradually phase out and evolve legacy applications, and migrate costly mainframe apps bit by bit without having these changes ripple out and break other services. The same is true for those systems’ data – backup strategies are defined at the service level, impacted primarily by their Service-Level Agreements.

While I whole-heartedly agree with what Nick has to say in terms of OO intermediation of the Dependency Injection variety, and that scaling up those same concepts in terms of messaging is the right way to go, I take issue with orchestration in the intermediation area. These “tactical changes” need to be done in the context of the top, business-level service strategy. That means that all logic belongs within a service. The “network” between services is just that, a “dumb” network – no business logic of any kind, just technological capabilities like knowing which physical server to route messages to.

In this spirit, I’d like to suggest an alternative solution to the example Nick gives. Here’s the scenario:

Let’s say that system 1 generates an invoice. It sends an event to the world saying “invoice here” and system 2 captures that message. System 2 asks for details about the invoice… perhaps it will place the information on a web site for internal support teams.

Let’s say that we are moving to a CRM solution in our internal support groups. We want to create the information in the CRM system related to the invoices that specific customers have been issued. We need to integrate these two systems. The existing web app needs to have a link to the CRM system’s data, to allow the user to move across easily.

And here is the solution he prescribes:

We can intercept the request for further information from the web app to the publisher. When the publisher responds with information about the invoice, we can insert the invoice in the CRM system, add a link to the CRM record for that invoice to the data structure, and resume our response to the web app. Assuming that our canonical schema has a field for ‘foreign key’, we have just integrated our CRM and web information portal… without changing either one.

Without getting into the business-level analysis of what the correct service decomposition might be, here’s what I suggest (although all of these “systems” might just end up within the same service, or having parts of them being used by multiple services).

First of all, have all information about the invoice available via the message only. This could be done by actually putting all the invoice data in the message, or by placing a URI instead where other systems can HTTP GET it from – REST style. This decreases coupling between the publisher and its subscribers. However, we haven’t solved the problem of our web apps getting access to the relevant data in the CRM system.

The solution presents itself at the business level. The invoice is not “complete” without the appropriate CRM data. Therefore, it does not make sense for a service to publish it that way. Let’s call this service the Purchasing Service. It would handle the workflow of receiving the first system’s event, adding the invoice to the CRM system, and taking the resulting full invoice data and publishing that. All external systems like the web apps would see just the final event. Orchestration, if there even is such a thing, occurs within the service boundary. This technological level intermedation isn’t even a blip at the business level. We can also imagine other services, say a Sales Service, that would use the CRM system as well.

In summary, when moving to SOA, intermediation provides many technological benefits in getting data and behavior to work across existing systems and applications, however it’s laregly a NO-OP at the service level. After phasing out many of those existing applications behind the service boundaries, the same service-level interactions would persist. Your Service-Oriented Architecture would not be any different. That’s the technical agility aspect of SOA.

For a great description of what grid computing really is, read this.

What they say about the connection to SOA, though, requires some clarification. Here’s the quote:

There is a lot of talk going on about synergy between Grid Computing and SOA. It is however driven primarily by implementation concerns at this point rather than by any deeper considerations. Clearly, Grid Computing can deliver unchanged value without SOA, yet WS-* based implementation (such as Globus) can be beneficial in some cases (highly distributed heterogeneous environments that should only exist in unfortunate legacy-support situations).

The main thing that I want to call out is that “grids” don’t cross service boundaries – not at the logical level anyway. Although, even if you did share a single grid infrastructure between services implementations, you may have some problems maintaining service-level agreements, autonomy may be put in danger.

Just something to keep in mind.

I’ve looked over the example Guy put up in his blog on how to interact with external services from a workflow, and tried it out using asynchronous APIs with WCF. This is done quite simply by having our service accept an object as a parameter that it can call methods on (which has availability issues, but whatever). The object that we pass in is quite simply a reference to our own service. Anyway, I haven’t been able to get the SendActivity in WF to work with it. Bummer. I guess I’m “stuck” doing things the “old fashioned” way.

By having messages dealing with workflow contain the ID of the specific instance they refer to, we can do simple message to workflow mapping. When a message handler receives a message containing a workflow ID, it just goes to the workflow store and retrieves the object by its ID. Finally, it calls the Handle method on that object, and I’m done. Workflow classes are just state machines whose triggers are the arrival of a message.

Here’s some example code so you can see how simple it really is:

public class WorkflowMessageHandler : IMessageHandler<Stage1Msg>
{
	public void Handle(Stage1Msg msg)
	{
		using (IDBScope scope = this.DbServices.GetScope(TransactionOption.On))
		{
			IWorkflow wf = this.DbServices.Get(msg.WorkFlowID);
			wf.Handle(msg);

			scope.Complete();
		}
	}
}

You can see how easy it would be to take this and make it generic. Just define an IWorkflowMessage that inherits from IMessage and has a single property – WorkFlowID. Then we could have a BaseWorkflowMessageHandler<T> which would inherit from IMessageHandler<T> where T : IWorkflowMessage.

After that, it would be enough to have a class inherit from the base for a specific message and you’d be done, just like this:

public class Stage1Message : IWorkflowMessage { // WorkFlowID and other data };

public class WorkflowMessageHandler : BaseWorkflowMessageHandler<Stage1Msg> {}

I could even automate the creation of these message handlers given the set of messages that correspond to a workflow. I could then create all sorts of sexy designers on top of that.

But, seeing as there’s so little extra code to get long-running workflows to work with “asynchronous services”, I don’t think I’ll bother. I mean, why do I even need a “SendActivity”? It’s just a simple little call:

this.Bus.Send(msg);

It appears that clean designs don’t leave much to be draggy-dropped. Oh well. Your mileage may vary.

Patrick Logan asks about the Actor Model (see here for its definition).

Is the actor model on the list of foundational topics in CS programs today?

Is the actor model about “objects”? Why or why not?

Is the actor model relevant today? Why or why not?

Are today’s systems becoming more or becoming less like actor systems? Why or why not?

Here are my answers:

If the actor model isn’t in CS programs, it damn well should be.

I don’t think that the actor model is about objects, as I think it would obscure their relevance.

I think that the actor model is relevant today. I use it all the time. I just call them autonomous components. The only difference is that autonomous components don’t create other autonomous components.

Once again, I think that systems today can be modeled very well with actors – especially when it comes to systems that want to efficiently use multi-core CPUs.

What do you think?