Re: Introducing GNUPaste (and guile-wiredtiger future)

[Kristofer Buffington]

Amirouche Boubekki <address@hidden> writes:

> 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.

I have noticed that I get "Resource busy" issues if multiple requests
are handled by the server simultaneously. I tried running (with-env ...)
in the request-handler instead, but it is the same problem. Multiple
environments can't open the database simultaneously either.

> 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.

IMHO I think feature-space, ix and grf3  don't really belong in
guile-wiredtiger.

> 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).

I really like the advanced abstractions. I'm not sure I'm knowledgeable
enough to fork and improve them yet.

>> 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

I really don't want to rely on a database server. It would be easy
enough to store pastes on the filesystem, maybe even with (guix store)
or use git like tekuti and get the benefit of revision history with
paste modifications.

Thanks for the update!
Kris