Via David McGhee’s Q&A with Dr. Don Ferguson, but read the whole thing.

Q: Could you tell you your thoughts or preference for a distributed or centralized ESB?

DON: there is no such thing as a centralized ESB.

This is the problem with a lot of the products that call themselves ESBs. They are centralized brokers which may be clustered for availability. But they are in no way an implementation of the Bus Architectural Pattern. Please check this before cutting a check to your vendor.

Also, understand that if you do security related things in your ESB, possibly as a part of your routing rules, that if the security infrastructure is centralized that means your ESB is too. Even if it really was distributed to begin with.

Buyer beware.

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.

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.

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.

So many gems in Steve Jones’ recent post SOA isn’t about technology.

business process isn’t everything.

one interface = one process = what a load of crap interfaces you have.

… with “services exposing legacy and BPM orchestrating services”. Its pretty amazing how this view just happens to match the product vendors stacks…

Its bollocks

BPM as the language of business is, IMO, snakeoil.

Starting with BPM is as silly as starting with WSDL.

I’ve talked about the BPM, SOA divide in this podcast, but from a more solutions architecture perspective. As well as this podcast on “Services as the interface to business processes”.

The main problem I have with BPM is that there’s this expectation that the steps of each process will actually be implemented the way that they were drawn. The way the business views its processes is, first of all, much different from the way the BPM analysts do. Second of all, who said these analysts have the ability to design systems that will be functionally correct, reliable, scalable, and all that. So, why in the name of X are we giving them tools to automate that? Software architects don’t even have these kinds of tools, although they do have the requisite knowledge.

I’m with Steve. Bollocks!

Willam Brogden has an article up on SearchWebServices.com on How Web Services can use JavaSpaces. I don’t want all the Microsoft folks tuning out now that they’ve heard the “J” word, so let me just say that there are technologies out there for .NET too.

A “JavaSpace” is really just a space, which is, at the end of the day, a queryable distributed in-memory hashtable. Something many of us are already doing for caching. The reason you shouldn’t be doing this yourself is simple. While keeping a single hashtable in memory on a single computer and synchronizing it against changes to your database is simple, doing that in a highly available manner across multiple servers is not. Vendors providing solutions in this space include:

• Gigaspaces
• IBM ObjectGrid
• Alachisoft’s NCache
• GemStone
• Tangosol’s Coherence

But there are others as well. Bottom line: don’t develop one of your own. Do a proof of concept with your short list of vendors and go from there.

The article sums it up nicely like this:

Although JavaSpaces servers are not trivial to set up, they are much easier than any other type of grid computing server. Furthermore, the simplicity of the interface makes the learning curve easier. The greatest advantage of the JavaSpaces approach is the ease with which additional workers can be added to the grid.

It should be clear from the example that there is a lot of extra communication traffic in a JavaSpaces solution so the only reason to use JavaSpaces or any other form of grid computing in support of a Web service is a requirement for computing power or special resources that are not feasible to supply on the server directly.

I have this to add to it. Whereas most traditional systems keep the idea of message-based communication and data caching separate, spaces allow you to kill two birds with one stone. Even if you don’t go the whole Space-Based Architecture route, you’ll find that spaces will fit nicely in your distributed architecture toolkit – I know I did.

From a somewhat old email discussion I had with Benjamin Carlyle:

> A while ago I tried to explain the difference between SOA and REST is
> that SOA is based on “MEST” – MESsage Transfer. The main thing is the
> message – which is a statement of intent, and includes the relevant
> data for that intent. For instance, a message like
> ChangeCustomerAddressMessage is itself the intent; the data it
> carries, the customer ID and the new address completes the picture.

Ahh. This puts your name in context for me. I did some reading on MEST some time ago. I think that there are probably a few misunderstandings on both sides of the fence still about what the other is all about. I have recently been reading “Software Factories”, ISBN 0-471-20284-3. It takes the view of a service being a software component. Code usable in different software environments that has enough self-description to be accessed from multiple languages and runtimes. This sets up services as platform-independent objects.

I think I understand this evolution and the corresponding object patters that are encoded into WS standards. Essentially, SOA attempts to make objects more accessible. REST is quite a different beast. Its primary function is to allow communication to evolve.

> The main difference between this style and REST is that where REST is
> focusing on one resource, SOA/MEST style accept that fact that as a
> result in the change of address, the customer may now be in a “rich
> neighbourhood”, and his customer rep now has over X people in rich
> neighborhoods, so the customer rep’s status changes. This cascading of
> changes I believe is done in orchestration/choreography type code outside of the resource in REST.
> Such complex business logic is best implemented using OO models which
> span objects/entities/resources.

I think this is a common misunderstanding. Each resource demarcates a subset of an application’s state. Resources are likely to overlap. When one resource is updated, it is likely that other resources will also change. This is not well ennunciated by REST proponents who are far more focused on the other side of the exchange, that between client and server. In REST, your ChangeCustomerAddressMessage sent to object A would be a PUT of a customer address record over the top of the old one, at resource http://example.com/customer1.

I see REST as a tweak to the object model. Instead of having a base-class that defines methods and data to be transferred, REST decouples the definition of methods and data. The intention is that the set of methods, the set of data definitions, and the set of objects
(resources) can evolve independently of each other. This tweak does not change existing capabilities of the object model. Objects work pretty-much as before, but their interface to clients and the definition of their interface is modified.

The effect of the decoupling is that methods can be understood separately to data and to objects. For example, I can determine what a GET or PUT request is going to do to a resource regardless of the object the method is applied to. As an intermediatary I can operate as a caching proxy, or apply security policy appropriately. I see this like the jump to java beans. Some programmers were downright offended that they had to name their methods with “get” and “set” in order to get the architecture working, however the advantages are now so clear that the issue simply doesn’t come up anymore.

REST is about building a communicatons infrastructure that scales to a large number of participants who want different things from their architecture over time. At a technical level there are significant performance advantages to be gained by caching and other features, but at a social level it makes the definition of new interfaces simpler and allows them to change over time without disrupting the architecture as a whole. New methods can be introduced over time. New content types can be introduced over time. Old features can be supported simultaneously to new.

> It is this model of message/document based communication that isn’t
> very well supported by WSDL. Support for publish/subscribe semantics
> is also vary light in the WS world. This is the main reason why I see
> SOA as something outside of WS. I don’t see this as a “nod” to REST
> but a coherent architectural style that been employed long before the web.

Support for pub/sub is also pretty light in the REST world, something I have mind to do something about 🙂

> As for scalability, it’s very easy to scale the publishing of static
> html pages, it is an entire other thing to scale complex business
> logic that may have huge working sets of data. To tell you the truth,
> I’d be interested in seeing the design of such a system in terms of
> REST, specifically in how transactions are handled.

Transactions in REST are usually handled by demarcating all of the state that will be updated in a single resource, and PUTting a representation of that state to the resource. REST is usually applied at a chunky enough level that this makes sense. There is always the expectation in REST that objects or RDBMS or some other back-end technology does the work of making the updates.

There are other ways to handle transactions in REST, but they usually don’t come up. Again, it is a matter of scale and the level of abstraction. REST is a client-facing communications model. It isn’t necessarily useful for communication between services within the same tier. It may also be that the tier that presents a REST world view hides further tiers that use distributed object technologies.

> Anyway, that’s where I’m coming from. I think that the communications
> semantics of SOA – send a message, are just about as simple as it
> gets. It is, if anything, more minimalistic than REST.

I think that you are right about minimalism, at least after a fashion.
With WS-* or even corba, it is easy enough to make two programs work like one in an enclosed environment. REST is really about a larger-scale communications system where not all of the participants are part of the same organisation. It is about forging agreement on the larger scale, not an easy thing to do. However, I think that REST scales down better than SOA scales up. It is no harder to define special-purpose XML formats than it is to define a base-class, although defining all appropriate resources is probably starting to make life too difficult.

My organisation integrates disparate systems. It is about 50% of the work we do on a job. In this environment REST is a valuable tool in making things work with each other over long periods of time, and exposing the data of one system to another for viewing and update.

Orignal message:
> I’m quite interested in expanding my knowledge of SOA best practices.
> As I noted in the entry you refer to that my list is derived from a
> presentation by Sun Microsystems, the slides of which may be found href=”http://au.sun.com/events/developer/downloads/download.html?s_dLi
> nk=SOA _JBI_BPEL.pdf”>here. I often hear that SOA is an
> architectural style rather than a name for the WS-* stack, however it
> is hard to come to terms with what those principles are given the
> variety of views on the subject.
>
> It seems that many SOA proponents who are familiar with REST take a
> number of their particular constraints from REST itself. If those
> individuals are to be taken as the litmus test, SOA is generally a
> superset of REST that allows for less uniformity. In other words it
> looks like a version of REST that doesn’t scale up to such big network
> sizes. Vendors seem to see SOA differently, the message-bus focus I
> referred to in my entry.
>
> I would also like to note that the main focus of the title was on the
> principle difference that I percieve there to be between SOA and REST:
> the nature of the uniform interface. Even if you take your web
> services and try to construct a uniform interface, you do so as a
> base-class that encapsulates both verbs and content types. REST
> decouples these. I think the same is possible as document-oriented
> processing under web servcies, however this again seems to be a nod to
> REST rather than an alternative to REST practice.
>
> Do you have a specific list of constrains you yourself like to point
> to as the meaning of SOA?