VMWare Fusion XP VM Losing DNS !

October 25, 2008

I’ve been running OSX Vmware Fusion 1.x and XP SP2 & SP3 for over a year and it’s been ROCK SOLID ! I run a web-connected OSX host, and a XP VM VPN’d into a corporate network all day, every day and I have not had a single problem. Until this week…when my calm seas were interrupted…

Out of the blue I notice witin the XP SP3 VM was failing to resolve DNS queries. OK. Why? I basically ran a number of diagonstics, checked driver versions and everything checked out in terms of VM integrity.No idea. The worst kind of problem…

I checked out the net and located numerous threads deliberating over the XP “Unable to flush DNS cache” type of errors, with long and elaborate threads falling into the detail of comparing router firmware versions and other such infinate variables. Eject..Eject…

It was only when I ran the VMWare packet sniffer on the OSX host I could see that the XP VM was requesting DNS, and the resoponses from the OSX host were being dispatched. From my understanding of low level IP it appeared that all was performing as expected. However I then started thinking about reasons why UDP packets were being neglected by the XP VM’s IP stack….BINGO !

I then checked out the Windows XP Event Viewer under the Security event list, and there I see all my DNS responses (from the OS X host) arriving back at my XP VM as UDP packets, all being summarily discarded by my failed/corrupted firewall. Couple of minutes later, having run the ‘support’ utility from the firewall supplier, the flood-gates openend and UDP/DNS was back in business.

Symptoms I encountered in XP:

  1. DNS resolution (i.e. ping http://www.xyz.com) within XP VM failing but direct direct addressing worked ok (i.e. ping 123.123.123.123)
  2. nslookup in the console returned ‘no response from server’ errors in response to queries.
  3. Right-clicking on the network connection icon in XP, and executing Repair proceeded through all steps apart from the final DNS cache at which point Unable to repair connection was returned.
  4. ipconfig /registerdns failed with a non-specific error

In hindsight the symptoms all point to UDP return-path and firewall but verifying the request path with the OSX VMWare vmnet-sniffer utility (located in /Library/Application Support/VMWare Fusion folder) made this a whole lot simpler.

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Where a Semantic Contract Fits…

October 10, 2008

I’ve been posting about the rise of the informal semantic contract relating to web-services and the deficiencies of XML Schema in adequately communicating the capability of anything other than a trivial service. Formalising a semantic contract by enriching a baseline structural contact (WSDL/XSD) with semantic or content-based constraints, effectively creates a smaller window of well-formedness, through which a consumer must navigate the well-formedness of their payload in issuing a request. Other factors such as incremental implementation of a complex business service ‘behind’ the generalised service interface compound the need for a semantic contract.

To clarify the relationship between structural and semantic, I happened upon a great picture which I’ve annotated…


Validating XML with Schema in Ruby

October 10, 2008

I posted a long time back about my troubles in finding a way of performing schema validation in ruby (see Ruby and Xml Schema). At that time I was using REXML and only able to perform well-formedness checks based on basic structural integrity, but had no way to take an XSD and validate an instance document.

I’m pleased to say that there is a way to do this now, namely libxml-ruby. It’s available as a gem (gem install libxml-ruby) and the process is pretty simple:

  document = LibXML::XML::Document.file(@xml_filename)
  schema = LibXML::XML::Schema.new(@xsd_filename)
  result = document.validate_schema(schema) do |message,flag|
    log.debug(message)
    puts message
  end

I’ve found this to be a very neat piece of code for dealing with the kind of schema integrity checking I’m looking for, and as I blend this with a number of other java-based parses using the Ruby Java Bridge I get a pretty good, consistent perspective on validity.


The Rise of the Semantic Contract

October 9, 2008

This is my stake in the ground for now. SOA in the market-place places total emphasis on 2 things. Web Services as a basis for communication and Re-use as a basis for convincing the boss to put some cash into your middleware bunker..no…play-pen…err…seat of learning.

In addition the militant splinter-groups of the new-wave of RESTafarians (of whom I am an empathising skeptic on this specific 🙂 point about service specification) call for the death of WSDL and the reliance on (WSDL-lite) WADL in the case of the less extremist, but plain-old inference from sample instance documents in the case of the hard-core….

I am finding myself sailing down the no-mans land between these two polarised viewpoints, and see the need for specification in the more complex end of the interface spectrum, but similarly don’t see how specifications help when decoding the specification is harder than inferring from samples when interface contracts are relatively intuitive. So there we have a basic mental picture of my map of  the universe.

Now I’m getting to the point.

I’m now convinced that SOA’s push for re-use established through WSDL everywhere, but equally the more recent RESTafarian voices relating to unspecification both have flaws when we are attempting to open up a generalised interface into a service endpoint capable of dealing with a range of entity variants (say product types for example).

My view here is that in the SOA landscape, the static and limited semantic capability of XMLSchema, and in the RESTian lanscape, the inability of humans to infer correctness without a large number of complex instance document snapshots, leads me to the conclusion that there is a vast, yawning, gaping, chasm of understanding in what constitutes an effective contract in locking down the permissible value permutations – aka the semantic contract.

I’ve seen MS-Word. I’ve seen MS-Excel. I’ve seen bleeding-eyes-on-5-hour-conference-calls-relating-to-who-means-what-when-we-say-customer, and best of all I’ve seen hardwired logic constructed in stove-pipes behind re-usable interfaces, aka lipstick on the pig.

I reckon the semantic contract – the contract locking down the permissible instances is far more important than the outer structural contract who’s value decays as the level of re-use and inherent complexity of the interface increases. In addition there are likely to be multiple iterations/increments of a semantic contract within the context of a structural contract as service functionality is incremented over successive iterations – adding product support incrementally to an ordering service of example. This leads to to the notion of the cable cross-section:

In the SOA context…WSDL drives tooling to abstract me from the structural contract. But the formation of the semantic contract as the expression of what the provider is willing to service via that re-usable and loose structural contract is the key to effective integration.

If we don’t pay this enough respect we’ll be using our system testing to mop-up the simple, avoidable instance-data related problems that could be easily avoided if we’d formalised the semantic contract earlier in the development lifecycle…

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