[Development] [QtCS] QtRemoteObjects Session Summary

[Simon Hausmann]

On Tuesday, June 09, 2015 01:23:29 PM Alan Alpert wrote:
> There was late-scheduled session on QtRemoteObjects at QtCS on
> Saturday. QtRemoteObjects is a playground module for object remoting
> of QObjects, and can be found at
> http://code.qt.io/cgit/playground/qtremoteobjects.git/ .
> 
> A brief overview of the existing features:
> -Exports QObjects remotely, creates a replicant on the client side you
> can interface with
> -Network nodes can serve as a registry to allow lookup by name.
> -Transport Layer Adapters
> -Bjoern is currently working on exporting of QAbstractItemModels
> 
> A brief overview of planned features:
> -QML API
> -Protocol Layer Adapters
> -Investigate merge with QWebChannel (sounds similar, of course no-one
> working on QWebChannel was present to confirm).
> 
> After it is established with those features and solid quality, it
> could be considered to promote it to an add-on module.
> 
> The idea with protocol adapters is that currently it only sends
> QDataStream output over the wire. I want to send more accessible
> JSON-RPC (like) data over the wire for working with node, and Attila
> had a demo of getting object data from Meteor (although that was
> custom hacked for demo purposes). Protocol adapters will allow you to
> serialize it in these other forms, and that is the mechanism by which
> we gain compatibility with other languages/frameworks without them
> needing explicit QRO support - We'll talk their language if there
> exists compatible semantics. They'll probably remain separate from the
> transport adapters, but we'll see how the implementation develops.
> I'll start looking into the implementation this month.
> 
> The question was raised "How is this not CORBA?". I'm not that
> familiar with CORBA (just read the wikipedia article today), but my
> suspicion is that we've cut out a lot of the boiler-plate by
> leveraging existing Qt Meta-object information. 

I don't suspect that this is the CORBA criticism. I mean, it's a fair point 
that you often do end up with boiler plate translating type systems, but 
that's mostly a matter of good IDL bindings.

No, the criticism I would have regarding CORBA or at least the way you may 
quickly end up using it is that it hides something that shouldn't be hidden: 
Failures.

When working with "remote" services things _will_ go wrong. There will be 
issues with your connection, there will be issues with the remote side not 
responding (having crashed for example), there will be many such issues. You 
don't see them when developing the system, but during real world deployment 
you run into them, and then it's too late. During development you haven't 
encountered those issues, so your code that calls methods and reads/writes 
properties is written as if it works locally, because it did during your 
testing. Systems like CORBA encourage developers to write code as if the 
method calls were local, completely ignoring the nature of unreliable 
transports and services.

I feel that mistake is made over and over again in many similar designs. It is 
perhaps one area where the web development is doing better. It's my (perhaps 
incorrect?) impression that web services tend to be accessed using more 
explicit code instead of using transparent method calls or property access. 
The XMLHttpRequest API is one example here, and the promises based fetch() API 
makes this even better IMHO.


The other aspect that I think is crucial in a distributed system is 
compatibility. This is why systems like thrift and protocol buffers exist: 
They provide type safety (!) and make it really easy to maintain wire 
compatibility. JSON is also "okay" with this, but it has the huge disadvantage 
that usually json formats are "untyped".


In my opinion a system that we promote to Qt developers should learn from 
these "mistakes" in the past: The API should encourage developers to write 
fault tolerant code (because distributed systems _do_ fail) and it should be 
easy to maintain wire compatibility (just like we do maintain binary 
compatibility).



Simon