Service oriented architecture centres heavily on the concepts of service providers and consumers. It’s easy with request/reply web services to fall into the lazy habit of thinking of the provider as being the “server” side of the request/reply interaction. The consumer requests information from the provider, which the provider – naturally – provides! But this is wrong.
What happens in an N-tier architecture where there may be many “servers” in the stack? What happens with JMS-based services using a one-way message exchange pattern (MEP)? If one application is using SOAP/JMS to send a message to another application, which is the consumer and which is the provider?
On the face of it, you might say the “sender” is the “provider” and the “receiver” is the “consumer”, but that ignores the fact that there are two types of one-way MEP – “one-way out” and “one-way in.” (Actually there are many types of MEP and they differ slightly depending on the version of WSDL you use, see the WSDL standard for more confusing details).
We really need to look beyond the technology to find the answer and the Web Services Glossary gives a clue. It splits the model into an “agent” (software or process) that operates on behalf of an “entity” (person or organization). Specifically a Provider Agent and a Requester Agent operate on behalf of a Provider Entity and a Requestor Entity respectively.
So the “provider” of a Web service is basically the person or organization responsible for that service. It is the person or organization that you contact to get permission to use the service, or obtain the WSDL, or give your credit card details for charging.
An example will help to clarify the relationship between provider and consumer in one-way MEPs. Suppose a service provides alert notifications. Multiple consumers subscribe to this service to receive alerts on subjects that are important to them. At the messaging level, the provider puts a message onto a JMS Topic and multiple consumers receive the message. This is a “one-way out” MEP.
Another service might be a central audit service where multiple agents send messages via a JMS Queue representing steps in a distributed process. This is commonly used for “track and trace” in distributed workflows. In this case, the message senders are not responsible for the audit system, they are “users” or “consumers” of the service. This is a “one-way in” MEP.
In summary, service providers and consumers can be confusing in an N-tier architecture or with one-way MEPs. The fundamental consideration is more “business” than “technical”. Who is the organization or person responsible for the service? Then the way consumers interact with them determines the MEP that is being used.
One of the original and fundamental tenets of the SOAP standard was that the SOAP message is independent of the underlying transport. Ostensibly you could use SOAP over HTTP, JMS, email, FTP etc. but the reality is that a standard binding has only ever existed for SOAP over HTTP. To paraphrase Henry Ford – “you can have any SOAP transport you like, as long as its HTTP”.
While HTTP is undoubtedly a good choice for SOAP – given its ubiquity – there is at least one other transport which demands attention. This is the JMS transport which is widely used inside the firewall of many organizations. Of all the companies that I work with, their SOA infrastructure heavily relies on JMS transports inside the firewall, with HTTP transports outside the firewall or to selected service end-points such as web pages. Of course my experience has significant selection effects, but nevertheless JMS is an important transport in many SOAs. Testament to this is that every major web-services product vendor (save Microsoft) supports SOAP over JMS (and even Microsoft now has SOAP over MSMQ as an important part of WCF).
The fly in the ointment is that there has never been a standardized binding for SOAP over JMS and as a result there is little interoperability between SOAP/JMS solutions provided by different platform vendors. If you happen to have any combination of different web-service platforms in your organization, then they cannot easily communicate with each other using SOAP over JMS without performing some unnatural acts.
Some of issues that need to be considered with a SOAP binding to JMS are:
- How do you represent the message content – text or binary? Most vendors have chosen a text message representation, but that has problems with multi-byte encodings, so other vendors have gone with a byte message representation.
- What headers do you define and what should their names be? How do you use the standard JMS headers? different vendors have different naming conventions and semantics.
- In the WSDL description, how do you represent the connection details to the JMS provider?
- How do your service endpoints manage the different message exchange patterns that are available with message-oriented transports?
Each of the vendors went their own way on many of these issues and as far as interoperability was concerned they basically ceded the field to HTTP. They made life difficult for large organizations with heterogeneous platforms and in my opinion didn’t do themselves any favours on the way. (Actually SOAP-encoding interoperability was so broken for a while that noone noticed the JMS issues…so maybe it wasn’t so bad).
Subsequently it was great to see some of the vendors get together a couple of years ago to agree on a standard SOAP binding for JMS that addresses most of the important considerations. The result was a Member Submission to W3C in September last year. My understanding is that this submission was previously circulated through most of the vendor community so hopefully it has general agreement on the technical details.
This has now taken its first steps to standardization with the initiation of a SOAP-JMS Binding Working Group who aim to publish a recommendation by April next year. Hopefully vendor support of the binding will be hot on its trail.
Note that the standard binding won’t address the fact that different JMS implementations do not interoperate. For example, a TIBCO JMS client will not be able to talk to a Websphere JMS provider because JMS is an API standard, not a wire-protocol standard. What the SOAP/JMS binding standard does mean is that once you have settled on a standard JMS provider for your services, you could define your service description in standard WSDL and your service provider (say Websphere or TIBCO or WSO2) and your service consumer (say TIBCO or WebLogic or Axis) would be able to communicate directly using SOAP over JMS “out of the box”.
Its been eight years (almost to the day) since SOAP 1.1 came out with the HTTP binding. Wouldn’t it be great if a standard JMS binding could be achieved within the decade! It’s been a very long wait. The JMS binding should have happened a lot sooner and I can’t say the “wait has been worth it” but it does fill an important hole in the Web-Services standards.
So what do we do in the meantime? You can eschew JMS altogether and stick with HTTP, but that requires another lot of hard work. You can stick with one and only one service platform, but that is difficult in large heterogeneous organizations – which is where SOA is supposed to provide maximum benefit. Or you can continue to do what many SOA implementers have done and deal with SOAP directly at the JMS layer – effectively using SOAP as plain-old-XML over JMS. I wrote more about this approach recently.
Another thing you can do in the meantime is ask your vendor when will they support the new SOAP/JMS binding?