2008 May 29 - Thu

Evaluating Inter-Process Communication Frameworks

I'm reposting some comments regarding IPC frameworks that I made to the Boost-Users listserve today. It is in response to someone making unsubstantiated remarks regarding the relative merits of ACE and Boost, and another looking for some substatiated remarks. What follows are some substantiated remarks, based upon my personal experience with it and several other libraries.

I've started working on a number of distributed system projects. As a consequence, I started looking for distributed system libraries. References to ACE were most pervasive. I implemented a number of trial applications with the library. That was after plowing through relevant sections in the three primary ACE reference books. That was a good learning experience, if only to find out the various patterns in distributed architecture definition. I had the inter-process/inter-server communications (which only sent simple stuff) working well within ACE's Acceptor/Connector framework. ACE has a number of other patterns one can use. I was really impressed with the fact that the examples I used from the books worked as advertised, and I was able to bend them to my will.

ACE is based upon an interaction of classes, macros, and templates. One has to spend some time with the environment in order to become proficient with it. It has a large API. A number of lower level API's upon which higher level API's are based. For example the Acceptor/Connector uses constructs described earlier in the books.

Once I had my basic communications going, I realized I needed to get some form concrete messaging infrastructure in place. I had an impression that TAO, which is a layer above ACE, would be quite extravagant to implement, with it being an implementation of the CORBA specification. I wanted something a little lighter (a whole lot lighter actually).

As I worked through that project, I started hearing about ASIO, indirectly through some other libraries I was using. ASIO is now a member of Boost. I read a review somewhere that ASIO is a 'modern' replacement for ACE. If you want to get into real template structures and Boost oriented philosophy, I'd say that is a valid statement. I'd also say that ASIO is 'more to the point' and straight forward than is ACE, at least for the things I want to accomplish. But like ACE, ASIO is the basic communications infrastructure, no real messaging capability, which is what distributed computing is all about. ASIO turned out to be a little harder to get my head wrapped around as it uses a number of advanced C++ and Boost related idioms. For a run-of-the-mill C++ programmer, ACE would be better. For someone steeped in the power and obscurity of advanced C++, and is looking to advance their skill set, ASIO would be better.

I came across RCF - Interprocess communication for C++, which is a messaging framework riding atop of ASIO. Flexible, lightweight, and to the point. I worked through the examples and things worked as advertised. It has the encryption, publisher/subscriber, oneway/twoway idioms, and a few other nifty features.

At the same time I was doing that, seemingly coincidently, I learned a few more interesting facts. Going into this, I realized that I need a message dispatcher/proxy, some decent failover techniques, and some additional event handling for non-IPC related activities.

Someone suggested ICE from www.zeroc.com for an RCF-like solution, but working to a larger scale. I've heard that the library's originator is someone who spent much time on CORBA standards and redid the concept without the 'benefit' of committee involvement. I think the library has all the bases covered in terms of lightweight message handling, dispatching, resiliency, and higher level distributed processing philosophies. The drawback is that it will have a steeper learning curve than would an implementation using RCF. I like RCF, but I think I.m going to have to tilt towards ICE (itself, like RCF, developed and focused towards C++ in a multiple license environment).

On the non-IPC front, Qt's QCoreApplication looks to be a good substrate on which to build event driven daemons.

In the end, I think my solutions are going to involve:

• ZeroC's ICE for primary inter-process communications
• Qt QCoreApplication as a base for daemon development (which has built-in stuff for threads/locks, slots/signals)
• Wt, a C++ based web toolkit for distributed GUI development
• Boost Libraries to fill in all the holes
• a little legacy layer 3/4 ACE in one library I'm using, but with some work, I think I can convert the ACE stuff to ASIO

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