I wrote myself notes for this post months ago, but haven’t taken the time to write it down properly; a passionate discussion today with Rupert Benbrook prompted me to get back to this.
The WCF adapter does not support multi-part messages, and some would argue that’s a suggestion of the feature possibly being dropped in some future version of the product.
This small fact seems to have been glossed over by most – I, for one, haven’t heard this discussed anywhere, to the point that I’m left wondering if I’m really just “stuck in my ways”, but I honestly think that’s a poor decision.
Trying to search for some detail about this fact I found the following, rather old, but still very valid and informative, post; here are some of the key points made -
The reason support for multi-part message in WCF adapter is not being recommended is because WCF does not support it and the model proposed is that whoever wants to use something equivalent, need to aggregate the messages into one xml and send it as one single-part message.
(Tapas Nayak, Principal SDE, BizTalk Adapter Pack)
Multipart messages are a marginalized feature in BizTalk, the pipeline components like the flat-file and xml don’t understand them, same story for the majority of the adapters. So we decided to not support them in the WCF Adapter too
(John Taylor, Senior Technical Lead, BizTalk Server)
With regards to the first statement – I think that is somewhat down to interpretation – WCF did not change how web services work; WSDL is the same WSDL, specifications have not really changed; so – WCF is just a new (and of course – much better) implementation; this is an important basis for any discussion because it indicates that the WCF adapter could have taken the same path the SOAP adapter did.
Consider the following Service definition in WCF:
CompositeType GetDataUsingDataContract(CompositeType composite, CompositeType composite2, out CompositeType compositeOut);
or the equivalent web method in asmx:
public CompositeType GetDataUsingDataContract(CompositeType composite, CompositeType composite2, out CompositeType compositeOut)
the WSDL for both methods is pretty much identical, the operation definition looks as follows -
<wsdl:input wsaw:Action="http://tempuri.org/IService1/GetDataUsingDataContract" message="tns:IService1_GetDataUsingDataContract_InputMessage"/>
<wsdl:output wsaw:Action="http://tempuri.org/IService1/GetDataUsingDataContractResponse" message="tns:IService1_GetDataUsingDataContract_OutputMessage"/>
the messages are defined as so -
<wsdl:part name="parameters" element="tns:GetDataUsingDataContract"/>
<wsdl:part name="parameters" element="tns:GetDataUsingDataContractResponse"/>
And the schemas, in both cases, define the aggregated messages – with all the in/out parameters as required.
So – what is the difference? the way I see it is it is in the way the different adapters handle the messages as they pass through the adapter; the SOAP adapter has a layer in which, when receiving messages from the outside, breaks them to individual parts, delivering a multi-part message to the message box; similarly, when delivering messages from BizTalk to the service, it composes them, aggregating the various parts in the request message, to a single SOAP envelope; the WCF adapter does not include such functionality – it will take the message BizTalk provides and will wrap it with the SOAP envelope pretty much as is.
All my grievances about the SOAP adapter aside – I think its approach is a good one; it allowed the process to focus about the flow of the business process and avoided the need to define and construct artificial entities in the form of aggregated schemas, which often make very little sense (as the various parts can be largely unrelated from a process perspective, but still necessary for the service to perform its operation).
In my discussion with Rupert the same point was raised – the fact that SOAP supports one message in and one message out (i.e. no multipart messages), but in my view that’s a problematic argument – again – the whole point of the BizTalk architecture is that the send port can mask the transport mechanisms; ok – the SOAP adapter is not doing a particularly good job at that, but that’s hardly an excuse to make matters worse – I don’t want to have to know that I need an aggregated message to call this particular message because it uses the WCF adapter;
Moving on - I particularly disagree with the second statement mentioned - I think multipart messages are a very important feature in BizTalk, and would be surprised if any seasoned BizTalk developer would tell me they prefer not to use them (do let me know if you think I’m wrong); to start with – it allows one to define a type once and then use it multiple times; this makes maintenance infinitely better when types need to be changed (I thought it ought to be this schema, but found that this other schema is better); ok – so you have to be careful about how and where you re-use, to not introduce unwanted dependencies, but if you have large orchestrations which follow several steps, this is very useful.
Secondly, it allows one to easily link pieces of information that are only ‘somewhat-related’, especially important, in my view, when using pub/sub; having to pass several pieces of information from the publisher to the subscribers; the alternative, not surprisingly, would be exactly what is suggested above – create a schema defining the various pieces of information required and use that instead, but I think that’s a much larger maintenance overhead than a multi-part message definition; it is much easier to change a message definition that it is to change a schema; especially in large implementation with many different assemblies;
This was the point most passionately discussed with Rupert today – he argued that if a message is important enough to be published (with all its parts) it is important enough to create a schema for it; the point I was making, possibly not good enough, is that – to me, specifically in this area, the two are the same – I can define my aggregated message in a schema, that would have, say – 2 elements, each using a type defined in my other, existing schemas, or I could use a multi-part message, with two parts, each pointing at existing schemas; there’s very little difference in principal – I’m defining something – strongly typed and well defined, that is composed of two other, well known things, and I’m passing it around.
Obviously interoperability can only be achieved using standards like XSD, but that’s not a consideration at this point I’m publishing a message from BizTalk, which would be subscribed to by BizTalk, what’s wrong with describing it using BizTalk terms where conceptually there’s virtually no difference?
If there is no difference – why do I care? because multi-part messages are easier to create than aggregated schemas, and there’s less deployed artifact; I can simply put an assign shape and assign existing messages to the relevant parts; if I was to go down the aggregated schema route I would have to define the schema and create a map that would aggregate the messages; I could use code, or I could use xpath to assign the parts to the various elements, but I’d have to use a map or something to create the shell of the message first; seems like a lot of overhead for simply passing a couple of messages between processes.
I agree that many adapters don’t support multipart messages (the file and ftp adapters are the most prominent examples, outside the WCF adapter), and in many cases this makes perfect sense, but some do – SMTP and POP adapters, for example, use it very well; so does the SOAP adapter, of course.
There was one piece where we were in mostly in consensus, about the benefits of a single schema versus multi-part messages – when you wish to have a map in the port, I believe BizTalk would only consider the body part of the message, but again – to me that’s a shortfall in the product, not a proof that the existing feature is not useful; for both this and the pub/sub scenario I would have lover to be able to treat a multi-part message type in the same way a single-part message is treated – with a specific type that you can subscribe on (I can always set the message type manually, I guess, but would have been great to get out of the box support)
Similarly I agree that the built in xml and FF disassemblers don’t support multipart messages ( only in the sense that they will not disassemble them), but the model does – you can easily access, and parse, multipart messages from within a pipeline component, even a custom disassembler, so I can’t help wondering if the answer is in the following statement made by John Taylor, as quoted in that blog post -
The feature set for the WCF Adapter was potentially very large indeed and we were under some pressure to make cuts where we could.
and I think it’s a big shame.
I, for one, would be very disappointed if multi-part messages stop being supported, and I’m slightly worried – WCF is obviously taking more and more front stage, generally and with BizTalk specifically (even more so with the LOB adapter SDK and the BizTalk adapter pack that makes use of it), and as it does not support multi-part messages, it may well set the tone for future BizTalk developments.
Labels: BizTalk 2009