Web Service Messaging Nirvana

Web Services and specifically SOAP has got a bad rap in recent years, believed to be yet another over-hyped remote procedure call (RPC) protocol, and not cool enough in light of more current hip approaches like REST. Some of this flak is justified and the SOAP community does itself no favours when it releases a newer version of a spec (SOAP 1.2) which goes out of its way to state that SOAP is no longer an acronym. What next? TAFKAS? (The Artist Formerly Know As SOAP)

Anyway, I digress. I blogged in the past about two main areas where SOAP can be used. As you might guess, the main area that interests me is Messaging.

Over the last few years, I've been on a quest to explore how a JavaEE architect and developer can better implement messaging based web services and service consumers in a more loosely coupled and less error prone way. My motivation has been driven by a reaction to the traditional RPC based Web Services programming model that many have used and still use today and all the problems that such a model brings.

Over time I have constructed a list of 8 key values or principles that I believe are necessary for achieving "Web Services Messaging Nirvana". In most cases, these principals are based more on architectural and development approach, than the specific technology or SOAP toolkit used. However, as always, the two can't be completely divorced from each other..

Here's my 8 key values:

1. Promote Interface/Contract-First Development

• “Implementation-First” leads to tight coupling between the code in the service provider and the code in the service consumer
• "Implementation First" leads to a strong dependency on the initial SOAP toolkit used for implementation, which is then difficult to migrate off and leads to the difficulties of vendor lock-in

2. Enable a Message-Passing Programming Model

• RPC programming leads to lots of find grained 'chatty' services where brittle pairings exist between provider and consumer which are resistant to change
• RPC clients tend to assume endpoints are implementations when in reality, they may just be messaging intermediaries (eg. a service bus)

3. De-couple “Service Interface Definition” from “Message Document Definition”

• In-lining XML type definitions inside a WSDLs prevents re-use of the message structure definitions across multiple web services and other components in the system as a whole (import external Schemas instead)
• The message document structures may already be pre-defined in the organisation (eg. industry-wide standard schemas), so the ability to share these definitions across many web services is required‏

4. Promote Postel’s Law: “Be conservative in what you do; be liberal in what you accept from others”

• In other words, allow developers to create services with strongly defined interfaces, yet forgiving implementations
• Service intermediaries and end-points may only care about using part of the message content - they should not concern themselves about trying to validate other parts of a message
• Enable minor interface version additions without necessarily requiring an upgrade or change to the service provider or service consumer

5. Avoid the need for manually generated static stubs and skeletons

• These are invariably an unnecessary time-consuming addition to the development and build process
• Interface modification always requires re-generation of stubs, skeletons and Java types, forcing service providers and consumers to need to always be corrected, re-compiled and re-deployed.
• A requirement for Skeletons and Stubs is unlikely when generated Java types are not being used in the implementation

6. Separate data from the code that operates on it

• Object-oriented programming is great for service implementations but not for de-coupling disparate elements of the higher level distributed systems that are based on the principle of passing messages or events around
• Enable message data to be passed between systems without one system caring about or pre-empting how the other system may use and operate on the message’s content

7. Provide the developer with a choice of XML Tools to use

• SOAP Toolkits which force messages to be marshalled into a strongly typed Java graph of objects, results in one and only one way that the programmer can operate on the data
• If only part of a message is required or performance is key, a developer should have the option to use a SAX parser, a Streaming parser or just an XPath based Java API to query and operate on the data
• If XQuery or XSLT offers some developers a better abstraction and quicker development process then don't prevent a developer from being able plug in this option
• Retain the choice of being able to generate strongly types Java objects for situations where the developer needs or prefers it, but enable the developer to choose the technology (eg. JAXB, XmlBeans)‏

8. Integrate with the host container's Security Framework for Web Services Security

• For an application server based production environment, the server's Security Framework is already likely to be plugged into the organisation's larger security infrastructure (inc. LDAP, Kerberos, PKI and other elements) to provide full support for authentication, access control, single sign-on, identity propagation, encryption and signing. This infrastructure needs to be leveraged by the SOAP Toolkit, not ignored
• The SOAP Toolkit needs to delegate to, or at least share its security context with, the host container, especially for its support of WS-Security and SAML

In the past, with J2EE 1.2, 1.3 and 1.4 compatible application servers, these values have been very hard to achieve with the application server's built-in SOAP support (ie. JAX-RPC - oriented around the concept of RPC, skeletons, stubs, and generated Java types) or with a Third-Party SOAP Toolkit (eg Apache Axis - little integration with the container's underlying Security Framework).

As a result, on these platforms, I've struggled to be able to get anywhere near achieving my goal of hitting the 8 key values of my Web Service quest.

JavaEE 1.5 introduced the JAX-WS API which is the newer Web Services toolkit for JavaEE developers. The focus of this standard, in my opinion, is still largely based on RPC and generated Java types. However, it does offer a glimmer of hope in form of the option of using a 'dnyamic' server side API (Provider interface) and client-side API (Dispatcher interface).

To my eyes the Provider/Dispatcher facility could finally give me a chance of achieving the 8 key values in a real Web Services based solution.

Version 10.0 of WebLogic was the first version to introduce JAX-WS, but at the time lacked integration with WebLogic's Security Framework and WS-Security and SAML support, thus lessening its relevance to many solutions where securing web services was a key requirement. Many users continued to use WebLogic's JAX-RPC stack due to the need to leverage Weblogic security.

Things have now changed and the JAX-WS implementation in version 10.3 of WebLogic should address this. I hope to blog soon about my experiments with JAX-WS Providers/Dispatchers in WebLogic 10.3 and my quest to realise my 8 key values of Web Services Messaging Nirvana.



Song for today: Burn Baby by Mother Tongue

Web Services: RPC, REST and Messaging

[Originally posted on my old BEA Dev2Dev blog on February 25, 2008]

Choosing a model for interoperable communication in the enterprise

For the implementation of Web Services in the enterprise environment, I've seen many different technologies used. Recently, in my spare moments, I've reflected on this and have come to the conclusion that all these technologies tend to fit one of three models (or hybrids of these models).

I would summarise these three models as:

1. Remote Procedure Calls (RPC). A client-server based remotable pattern where a subset of an existing system's local functions is exposed pretty much 'as-is' over the wire to client programs.
2. Resource-oriented Create-Read-Update-Delete (CRUD). A client-server based resource-oriented pattern where the server-side provides a representation of a set of resources (often hierarchical) and exposes Create, Read, Update and Delete capabilities for these resources to client programs.
3. Messaging (eg. as commonly seen with Message Oriented Middleware and B2B). Messages or documents are passed asynchronously between peer systems in either, but not always both, directions.

Sometimes its hard to distinguish between these models and where the boundaries lie. In fact, I don't think there are boundaries, only grey areas and all three models lie in the same spectrum. I've attempted to visualise this spectrum in the diagram below.

Depending on where your implementation lies in the spectrum, the different highlighted characteristics should manifest themselves. 

 

In the Web Services world, we may typically implement these three models using one of the following three approaches:

1. Remote Procedure Calls: SOAP using a synchronous RPC programming approach and, typically, generated 'skeletons/stubs' and some sort of Object-to-XML marshalling technology
2. Resource-oriented Create-Read-Update-Delete: REST or 'RESTful Web Services' or ROA, re-using World-Wide-Web based approaches and standards like HTTP and URIs
3. Messaging: SOAP using an asynchronous Message/Document passing approach where invariably the documents are defined by schemas and, often, the use of message-level (rather than transport-level) security elements is required

The confusing thing is the fact that SOAP can happily and naturally satisfy two of these three models (ie. both RPC and Messaging), albeit that these two models are polar opposites which causes much confusion and is probably part of the reason why SOAP gets a bad name in some quarters.

The problem is further compounded with SOAP because the SOAP specification and accompanying collateral talks about two possible 'Styles' (ie. 'RPC' style and 'Document' style). However as I explained here, just because your SOAP Web Services are marked as 'Document' style in your WSDLs, it doesn't mean you are not doing remote procedure call based programming.

When faced with the REST zealot or the WS-* zealot, we probably need to bear this spectrum in mind. For the Web Services paradigm, there is not a 'one-size fits all' and specific requirements for a given situation should dictate which position in this spectrum best lends itself to satisfying the requirements. Also, the overlap between the models may be greater than shown in the diagram. For example, some would argue that REST can happily and more appropriately be used to fulfil what would otherwise be RPC oriented problems, in addition to solving Resource-oriented CRUD style problems.

Soundtrack for today: Purity by The God Machine

Tips for Web Services Interoperability

[Originally posted on my old BEA Dev2Dev blog on August 10, 2007]

I try to follow some simple rules to maximise interoperability when developing Web Services using WebLogic and/or AquaLogic Service Bus (ALSB). Most of the rules are pretty obvious, but perhaps one or two are not?. In case some of these rules are useful to others, I thought I'd share them, so here they are:

1. Use SOAP over HTTP. I've blogged here about why SOAP over JMS shouldn't be used if interoperability is a concern.


2. Conform to the WS-I Basic Profile 1.1 by using the free WS-I Test Tool. Test the WSDL and over-the-wire SOAP requests/responses for the created Web Services, for conformity using the tool available here (look for "Interoperability Testing Tools 1.1").


3. Expose Web Services using the "Document-Literal-Wrapped" style with the 'dotNetStyle' flag to help WS-I conformity and to be especially Microsoft product friendly. I partly covered this in the blog here


4. Use the WS-* standards judiciously. WebLogic implemented standards such as WS-Addressing, WS-Security, SAML and WS-ReliableMessaging are not necessarily implemented by other Web Services products/stacks or the specification version supported by these may be different.


5. Don't necessarily dismiss the use of WebLogic 'add-value' / 'non-standard' Web Services features at face-value
   
   
   • 'Buffered' Web Services are interoperable with other client Web Services stacks at the basic SOAP-HTTP level because the service consumer is not aware that the service implementation uses a JMS queue for buffering internally.
   • 'Callbacks' may be interoperable with non-WebLogic service consumers as long as the non-WebLogic consumers include the WS-Addressing 'Reply-To' header in the request and provide a web service endpoint to be asynchronously called back on for the specified 'Reply-To' URL
   • 'Asynchronous Requests/Responses' may be interoperable with non-WebLogic service providers as long as the non-WebLogic providers honour the received WS-Addressing 'Reply-To' header of the request, by sending the Web Service response asynchronously to the specified 'Reply-To' URL.
   • However, 'Conversational' Web Services are highly unlikely to be interoperable with non-WebLogic based service providers or consumers. The specification 'WS-Conversation' which the 'Conversational' feature would probably most clearly map to, doesn't really exist as a public specification and there is no indication that it ever will (an incomplete internal draft version has been dormant for a few years now).
6. For SOAP/HTTP Proxies created in ALSB, activate the "WS-I compliance enforcement" option (for the development phase of the project at least). When ALSB is used to act as an intermediary between Web Services consumers and providers, this ALSB option will help any Web Service non-conformities to be detected, so that they can be quickly rectified.

Note: ALSB also transparently converts between SOAP version 1.1 and SOAP version 1.2 inbound and outbound messages and ALSB is specifically tested by BEA for interoperability against third-party vendor toolkits such as Microsoft .NET and Apache Axis.

Soundtrack for today: Forensic Scene by Fugazi

RPC-Encoded. Document-Literal. Does it really matter?

[Originally posted on my old BEA Dev2Dev blog on July 31, 2007]

In SOAP, there are two possible styles:

1. RPC. Implies a SOAP body structure which indicates service name, and multiple parameters and return values
2. Document. Implies a SOAP body which is a complex message document

In SOAP, there are two possible uses:

1. Encoded. Adheres to a set of rules for serialising a graph of typed objects using basic XML schema data-types, but as a whole, does not conform to a schema
2. Literal. Body content conforms to a specific XML schema

In most SOAP toolkits, the most common combinations of Style and Use are RPC-Encoded and Document-Literal. Additionally, RPC-Literal is becoming more prevalent although it is currently a lot less common. Document-Encoded doesn't really make sense and as a result I doubt you'll find it implemented in your favourite SOAP toolkit.

RPC-Encoded was the initial message format for SOAP, when SOAP was originally aimed at just the Remote Procedure Call programming model. Document-Literal was incorporated into the SOAP standard in time for SOAP 1.0. It was intended to enable XML documents (messages) to be passed as the full content of the SOAP body, usually with one input message part and one return message part.

Like most J2EE Application Servers, the core of WebLogic's Web Services support is based on the JAX-RPC 1.1 specification. JAX-RPC defines a Remote Procedure Call based programming model and API for developers who want to expose a set of Java methods remotely (JAX-RPC does not offer much in the way of support for adopting the alternative distributed computing model of "Messaging").

Given that JAX-RPC is based on the RPC programming model, then in terms of best practices, it's obvious then that we should be using RPC-Encoded (or RPC-Literal) as the preferred SOAP Style/Use for creating and exposing newly developed Web Services, isn't it?

Well, not necessarily....

The terminology of RPC versus Document SOAP Styles is very unfortunate when we start to consider Remote Procedure Call versus Document/Messaging distributed programming models. These terms imply that the RPC Style should be used for RPC programming models and that the Document style should be used for Document (Messaging) programming models. That is not the case at all. In practice, the SOAP Style has nothing to do with a programming model, it merely dictates how to translate a WSDL binding to a SOAP message. For example, WebLogic's JAX-RPC toolkit equally supports exposing the same Java methods remotely via either style. You can use either style with either programming model.

A SOAP Style/Use of Document-Literal provides two distinct advantages over RPC-Encoded:

1. WS-I Basic Profile precludes the use of "Encoded" as the SOAP Use. So, if promoting interoperability and openness is your concern then you wouldn't choose RPC-Encoded over Document-Literal* (and why else would you be using SOAP other than for interoperability?).


2. RPC-Encoded provides no real separation between the format of the SOAP body (eg. that could be defined by a Schema) and the transport protocol and invocation format of a SOAP operation (eg. defined by a WSDL). For Document-Literal, the SOAP body content conforms to one or more Schemas which can optionally be externalised from the WSDL (and then included into the WSDL via an 'import' statement). Why is this important? Well, Document-Literal can promote the re-use of the same XML Schemas across the many different Web Services you may need to expose and throughout the rest of your distributed application logic which may need to deal with the same XML data formats. As we know, re-use reduces development effort, helps avoid errors and promotes consistency.

In summary, the decision of using RPC-Encoded or Document-Literal really doesn't have a direct relation to whether one is adopting a Remote Procedure Call programming model rather than a Document/Message-passing programming model. In practice, regardless of programming model, Document-Literal offers practical advantages.

Footnote: There is a new alternative Web Services toolkit which can be used, based on JAX-WS 2.0. JAX-WS offers developers an alternative programming model based on Messaging in addition to the Remote Procedure Programming model which is also supported. JAX-WS is newly supported in WebLogic 10, with some restrictions.

* In fact, use WebLogic's 'Wrapped' option with "Document-Literal" to help further promote interoperability with both Microsoft based Web Services toolkits (which traditionally prefer the Document-Literal-Wrapped style) and Remote Procedure Call oriented client Web Services toolkits (which often expect to be able to include the 'remote operation' name within the SOAP requests they send, rather than using another mechanism such as WS-Addressing to identify the operation to invoke).

Soundtrack for today: Off To One Side by Come

The problem with using SOAP over JMS in SOA

[Originally posted on my old BEA Dev2Dev blog on March 28, 2007]

Sometimes I talk to people who seem to view the use of SOAP over JMS as the perfect combination to enable loosely coupled asynchronous shared services. However, when I dig deeper these people have invariably assumed that JMS is an 'over-the-wire' protocol, like HTTP. It is not.

Question: Why is this a problem?

Answer: Interoperability, plain and simple.

HTTP is a standard 'over-the-wire' protocol. HTTP belongs in the Application Layer of both the OSI model (layer 7) and the Internet Protocol Suite (layer 4 or 5). SOAP is a 'standard' (or W3C recommendation at least) transport agnostic protocol which uses an XML payload.

Due to the standard and technology agnostic nature of both SOAP and HTTP, many platforms and toolkits out there, written in different languages and on different operating systems, can interoperate using SOAP over HTTP by simply adhering to both of these standards (or at least the WS-I version of these standards).

However, JMS is not an 'over-the-wire' protocol. It is a Java API which requires that a client application uses a JMS provider library (JAR) provided by the vendor of the JMS Server hosting the services. This is analogous to requiring a JDBC driver for a particular vendor's database before a Java application can talk to that database. The actual 'over-the-wire' protocol used under the covers within the JMS provider library is not defined (it could be IIOP for example, or it could be some high speed non-standard vendor specific protocol).

As a result, in most cases, the only types of applications which can talk to a specific vendor's JMS Server are other Java based applications. It gets worse. If, for example, the JMS server vendor is IBM WebSphere and the service consumer is running within Oracle's Application Server, there may be problems even getting IBM's JMS client provider library working from within Oracle's Application Server in the first place, due to JMS implementation clashes. Some JMS Server vendors provide one or two non-Java based JMS libraries too (for example for C++ or .NET), but these are often limited in functionality and scope and often only support specific versions of specific platforms and operating systems.

In other words, the onus of interoperability, when using SOAP over JMS, is on the support of the vendor of the JMS server for all possible service consumer environments rather than the onus being on the service client's host environment support for standards. Vendors cannot scale to provide JMS support for all of the wide mix of programming languages, application servers and operating systems (including different versions) out there, so interoperability will take a big hit. Even for consumer applications that can use the JMS provider, one has to give the service consumer the provider library first before it can invoke services - not very loosely coupled I think.

As a result, an enterprise's design choice to use SOAP over JMS, as the default mechanism for interoperability for an enterprise's mix of heterogeneous systems, is likely to be fundamentally flawed in my opinion.

It is important to state that I am not saying that Message Oriented Middleware (MOM) does not have a place in a SOA framework. In fact, quite the opposite is true. To achieve capabilities such as asynchronous messaging, guaranteed delivery, only once delivery, and publish/subscribe mechanisms, MOMs are an essential part of the SOA fabric. That's why many vendor's ESB platforms are built on the underlying technology of Message Oriented Middleware. However, what I am saying is that JMS should not be the preferred API for exposing shared services to remote service clients. Using middleware such as an ESB for example, a service with an asynchronous interface can be exposed via a SOAP over HTTP interface, for example, where the ESB performs the switching between the consumer facing synchronous invocation protocol and the underlying internal asynchronous message passing mechanism which may or may not use JMS internally.

With the right organisation and governance in place, I believe it can be valid to decide to expose a shared service via SOAP/JMS in addition to SOAP/HTTP or another more 'open' protocol, where there are valid exceptional circumstances (eg. high performance requirements). However, it is probably best to treat these decisions on an exception by exception basis because the overhead of supporting two access methods for a service does have an additional overhead due to increased configuration, maintenance, and testing costs.

Is HTTP the perfect transport for SOAP, especially for asynchronous services? Not at all. However, if consumers can't invoke these services in the first place - that's worse.

Have I got something against JMS? Not at all. Its one of my favourite JavaEE APIs. I'm not talking about JavaEE here. I am talking about SOA.

Soundtrack for today: Happy Man by Sparklehorse