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Re: [virgo-dev] Re: Running integration tests with maven

Steve and I discussed this and I agree with everything below. Breaking people's development environment in another timezone and not being available to provide a fix for several hours is something we must strive to avoid. I'd be interested in the perspective of seasoned Maven users on how these requirements can be addressed with Maven, although it is important to note that Virgo's modular GR structure may not be typical.

Glyn
On 16 Jul 2010, at 10:49, Steve Powell wrote:

> (My apologies for a long post.)
> 
> In discussing the build mechanisms we currently use, and how they might be improved, I would like to be explicit
> about the advantages and disadvantages of the present dependency control. At least IMO.
> 
> These might be viewed as potential requirements of inter-Git Repository dependency management.
> 
> [I shall use the terms GR for a Git Repository and GRuB for a Git Repository Build (a potential bug, perhaps?).]
> 
> The present build system has fixed version dependencies between GRuBs until they are explicitly updated (either
> manually with update_dependency.rb, or by means of Ripplor, etc.).
> 
> The primary advantage of this is that a GR can be worked on (and pushed) and not affect the rest of the build tree.
> Even though a GRuB is successful, it doesn't follow that the GRuBs that depend on it will succeed.
> 
> When we want to use the dependency on a successful GRuB in another GR, we can either make a manual update_dependency.rb
> change (and push the GR), or, more commonly, perform a "Ripple" of builds.
> 
> If we make every GRuB automatically use the most recent (successful) GRuBs it depends on, we gain the advantage that
> we do not have to perform the dependency updates (no more Ripplor).  However, we make the set of GRs more brittle,
> subject to subtle breakage. This sort of breakage is infuriating when it happens in the same room. If the cause is
> potentially half-way round the world in another time-zone then solving it will be harder.
> 
> When running a "ripple" of builds between GRuBs the complete (sub) tree is built locally before imposing the
> changes on every one else (before they are pushed). This means that the impact of a GRuB can be detected
> before the rest of the tree is broken.
> 
> The drawbacks seem to be that ripples are time-consuming, and can be invalidated due to parallel changes (this is
> essentially the optimistic locking syndrome), which make them even longer. Furthermore, if a ripple detects a problem
> caused by a GRuB dependency, the cure may involve other people and further delay.
> 
> Incidentally, the full power of a Ripple (or update_dependency) is rarely needed: often the Ripple actually just brings
> everything 'up-to-date'. If that is all it did (essentially a manual step to perform the automatic dependency management
> under discussion), then it would suffice.
> 
> Under the automatic system, it is possible for users to perform full tree builds locally before pushing their single changes
> to a GR.  This would make a Ripple unnecessary. However, the overhead of this (about an hour-and-a-half minimum
> elapsed time) is high enough to mean that people (including me) just won't do it.  Also, I will not do it when it is most
> necessary: either when I think there is no impact, so "it can't hurt", or when I want to use the dependency in a later 'GR'
> urgently and I 'cannot afford' to wait.  These are the times that chaos can ensue.
> 
> In Summary:
> A system that retains the protection of a private, manual, dependency update--or at least meant that
> updates to GRs that broke the tree were not adopted automatically--would be preferable to one that
> took the latest of everything all the time.  Latest isn't necessarily greatest.
> 
> Regards,
> Steve Powell
> [W:+44-2380-111-528; M: +44-7815-838-558; H:+44-1962-775-598]
> 
> Cannelloni n. A narrow boat for one. [SPD]
> Verisimilitude adj. Extremely evenly masticated. [SPD]
> Decrepit v. To muck-out Royal stables. [SPD]
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