Re: Introducing GNUPaste (and guile-wiredtiger future)

[Amirouche Boubekki]

Héllo,

On 2017-12-14 19:37, Kristofer Buffington wrote:
> Hello!
>
> I am excited to share GNUPaste! This is a really simple web app
> similar to paste.lisp.org built with Guile. I have a linode running it
> from git on GuixSD.
>
> https://paste.freshbakedyams.com (Please use it!)
> Source: https://github.com/kristoferbuffington/gnupaste.git
>
> Currently the frontend uses twitter bootstrap + jquery and highlightjs
> from a CDN. It really doesn't need all that boilerplate. It will
> definitely change in the future. GNUPaste depends on guile-wiredtiger
> and guile-fibers to compile.

Thanks for considering guile-wiredtiger.

Basically, guile-wiredtiger is not compatible yet with fiber
in the general case, because fiber will spawn several threads
and several fibers in each thread (and I think that fibers
can be stolen by other threads but I am not sure).

The way wiredtiger works is that there is one
*connexion* (called *environment* in guile wiredtiger)
per database and one *session* per "thread" of execution
(called *context* in guile wiredtiger).

I changed the naming because they are different from the
original things. Both environment and context are backed
by fluids.

In a pre-fork thread model, one must use with-context [1]
after the fork to populate the current fuild with a specific
context.

[1] 
https://framagit.org/a-guile-mind/guile-wiredtiger/blob/master/wiredtiger/extra.scm#L328

The thing is that when using fibers, the thread of execution
is a fiber, hence simply said the wiredtiger extra abstraction
called context fails. Because context/session must not be
shared between different fibers even if they are executed in
the same thread.

Otherwise said, guile-wiredtiger extra abstraction context
and environment are handy in single thread context because
it avoids passing the context around. It's also handy in
simple multithread settings where you don't need to pass
environment around (to create a new session per thread).
But it fails in the advanced use case of guile fibers.

This won't trigger a crash under low traffic, but will fail
under load and advanced use of guile wiredtiger, like multiple
statments transactions. This can be mitigated by turning off
preemption in guile-fibers but again it's not perfect solution.

I failed to create an API that makes simple things simple and
complex things possible. There might be an escape if fibers
implemented fluids for fibers something like PEP 550 [2].
But anyway, it won't be perfect, so I will rework the current
databases in guile wiredtiger to completly avoid the use of
fluids and instead pass around database cnx and session.

[2] https://www.python.org/dev/peps/pep-0550/

I started doing this in culturia [3] but it's far from being
complete. Since I need to convert all databases grf3, feature-space
and ix to that style.

Also, it's a backward incompatible change.

[3] 
https://github.com/a-guile-mind/culturia.one/commit/15328e53fcb51d43f14de2f9f21d0b309237969a

At the end of the day, I don't think I want to maintain grf3,
feature-space and ix inside guile-wiredtiger because they are
much more than wiredtiger bindings. So I am pondering the fact
that I will drop those databases abstraction from guile wiredtiger
and focus on improving the core bindings (like actionable exceptions)
and maybe improve bindings coverage.

For the next release 0.8 of guile-wiredtiger, the abstractions grf3,
feature-space and ix will be deprecated. And after for 0.9 release
they will be moved to the example folder (or better, forked by
other people to be maintained separatly).

> Ultimately I want to be able to deploy with something like:
>
> $ guix system disk-image gnupaste-system.scm
>
> Then boot it up in a VPS.

That will be neat!

> Thanks!
> Kris

--
Amirouche ~ amz3 ~ http://www.hyperdev.fr