Try the new constraint solver!

[David Greenspan]

Hello all,

I've rewritten Meteor's version constraint solver to be faster and

more predictable, using real constraint satisfaction algorithms.  I'd

like you to try this new preview release and report any issues you

come across, using one of the following commands:

meteor --release METEOR@new-version-solver-2          # one-off

meteor update --release METEOR@new-version-solver-2   # sticky

The version solver is what decides which other packages to add,

remove, upgrade, or downgrade automatically when you add, remove, or

update a package in your app.  It's an often-invisible but crucial

part of the Meteor experience.  The old solver served us well, but it

would occasionally take a very long time to run, and it used heuristic

methods so it was difficult to reason about.

In one test of the new solver, a case that previously took 20 seconds

now takes around 1.5 seconds, and furthermore it finds a globally

optimal solution.  There is some further optimization to do to bring

down the constant factors, so you may actually find that the

"Selecting package versions" sometimes takes *longer* now with this

release.  That will be ironed out with subsequent preview releases.

In the mean time, it would be extremely helpful for you to try the new

solver before it is released.  (And if you run into a very slow case --

defined as more than a few seconds -- we can now turn it into a

reproducible benchmark.)

There are some new features, too!

* Better error messages when you have conflicts or an "unknown

  package".  We now show more information about what the constraints

  are and where they come from.

* Meteor will never upgrade or downgrade your top-level dependencies

  to semver-incompatible versions.  For example, adding a package

  won't downgrade an existing top-level package by a minor version.

  Or, if a local package is found to be older than the expected

  version as recorded in .meteor/versions, you'll get an error.  To

  override this behavior and make breaking changes to your package

  versions, pass the new command-line switch `--breaking`.

* Fixes issue #3653: "Downgrading packages for no reason".  This bug

  occurs when checking out and running a Meteor app on a new machine.

  Meteor would downgrade a package if it hadn't heard of the version

  in .meteor/versions.  Now this case will trigger a catalog refresh.

* The new cost function will choose "patched" versions of packages in

  some cases where it would previously choose older versions.  For

  example, if you add package A to your app, and package A uses

  package B...@1.0.0, the solver will consider versions of B like 1.0.1

  and 1.0.0_1 to be good initial versions for B.  After that, the

  behavior is the same as before, namely, the version of B will

  generally remain constant until a different version is asked for by

  a newer version of A or by another package, or you run `meteor

  update B`.

Thanks for reading, and I look forward to your feedback and to making

a few performance tweaks before releasing the new solver.

-- David

[Arunoda Susiripala]

Thanks.
This is great.

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[Paul Knobloch]

Seems to work faster.
But the meteor shell isn't working anymore with your version.

[Phil Cockfield]

20 seconds => 1.5 seconds.  Boom.  

You rock David!!!

Thanks man! :)

[Sashko Stubailo]

Wow, that's a solid improvement! Great work, Greenspan.

[Max Harris]

Hi Paul,

meteor shell --release METEOR@new-version-solver-2

[Max Harris]

This works well for me! Thanks!

- Max