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From Jan Lehnardt <>
Subject Re: On Plugins and Extensibility
Date Mon, 25 May 2015 14:59:52 GMT

> On 22 May 2015, at 00:29, Paul Davis <> wrote:
> Hey everyone,
> So I've been meaning to write this email for sometime but have been
> kept busy with lots of super fun things that are super fun. Anyway, I
> just wanted to get this out there to start getting feed back from
> everyone involved.
> Also, while this is called "Plugin Proposal" it shouldn't be confused
> with the original couch_plugins use case. This is a lot lower level
> (and may be used by something like couch_plugins if we go there again
> in the future). Generally speaking this is just for cleaning up
> internals and for people that want to run either minimal installs of
> CouchDB or include the CouchDB in a larger Erlang application.
> Plugin Proposal
> ===============
> Background
> ----------
> As we've grown the code base to include more and more applications
> we're getting to the point where we've started adding various points
> of extension in various ways. The best existing example is the
> couch_stats application which loads stat/metric definitions from
> applications. Henning Diedrich has some unmerged worked which looks to
> follow a similar path for HTTP URL handlers. And Ilya Khlopotov has
> some work for providing vendor specific hooks.
> While each of these have some overlaps in their intended use case,
> they also share the fact that they've all implemented their own idea
> of extensibility in slightly different ways. That's not necessarily
> bad, but I think that we could reduce a lot of complexity if we take a
> step back and write a utility application that could then be used to
> support each of these features so that we can have both the
> extensibility as well as simplify the implementation of each
> individual feature.
> I'll start with a bit of background and then describe a general
> approach as well as show some hopefully explicit example snippets of
> how such a system might be used. Granted I haven't written out an
> entire implementation of this so I may be off the mark in some places.
> Bikeshed First
> --------------
> I have no idea what we'd call this. We could repurpose the
> couch_plugins app conceivably or make something new. For the the
> purposes of this document I'll call it couch_epi (for extensible
> plugin interface) and hopefully that's terrible enough someone will
> think of a better name for the actual application.

Yay bikeshed :)

couch_plugins was always meant to be a frontend to a more elaborate
system in the core as proposed and discussed here. Since couch_plugins
was a short-lived experiment, I don’t see any reason that we can give
the couch_plugins name to this, and if we ever have an HTTP endpoint
to install plugins, we can call the handling module couch_plugins_frontend
or somesuch.

As for the rest, I like where this is going, I’ll keep watching the
discussion with interest! Thanks Paul! :)


> Requirements
> ------------
> The three major requirements I've thought of are:
>  # Automatically discoverable
>  # Minimize apps that need to be started for tests
>  # Support release upgrades
> === Automatically Discoverable ===
> The biggest thing here is that I don't want to require a change to a
> default.ini or similar to enable or disable specific functionality
> when we can already signify that by having the application present or
> not. This is both for groups that may want to add new Erlang
> applications to a release as well as anyone that wants to run a
> minimal/embedded Couch. These are both obviously advanced uses but I
> think are important given the number of ways that CouchDB is being
> used.
> === Minimize the apps that need to be started for tests ===
> This one I think should be obvious to anyone that's been writing unit
> tests lately. There are some often silly places where we require
> applications be started just to run some tests. For example, places
> where we may want to call a function that's been instrumented and
> requires couch_stats to have knowledge about the stat.
> === Support release upgrades ===
> This one is obviously fairly advanced and limited in its audience but
> its something I'd like to at least consider in the design. This comes
> into effect for things like couch_stats that use a text file for its
> extension method. The issue is that the release upgrade mechanics
> don't provide any sort of signal that is easily usable to indicate
> when this file has changed during an upgrade so we're left polling the
> file system which is less than optimal.
> === Other Things to Consider ===
> A couple other things I'd like to keep in mind while discussing this
> is that I'd also like to minimize the amount of boilerplate code and
> coupling to make support this system. It should be hopefully a matter
> of a few lines of code to enable the extensibility on either side of
> the interface.
> General Design
> ==============
> The general themes that I see between all of our current extensions is
> that they're all basically just bags of random bits of data that each
> feature that is then used by each feature to define some behavior.
> For instance, couch_stats is just a list of tuples with some names,
> metric types, and descriptions. The dynamic chttpd handlers are just a
> list of URL endpoints to an MFA. And the vendor specific plugins are
> just a collection of functions that we'd like to invoke at specific
> points.
> Given that, the easiest approach I see is to implement a module that
> can be placed into the supervision tree that connects the data to a
> central repository (hosted by couch_epi) that can then be queried by
> each feature.
> Data Centric Examples
> ---------------------
> For a concrete example, lets consider couch_stats. Any application
> that wants to record metrics through the standard couch_stats app
> could add an entry in its supervision tree with something like:
>    {
>        appname_stats,
>        {couch_epi_data_source, start_link, [
>            appname,
>            {epi_key, {couch_stats, definitions}}
>            {priv_file, "couch_stats.cfg"}
>        ]},
>        permanent,
>        5000,
>        worker,
>        dynamic
>    }
> Then we'd just implement couch_epi_data_source once that would read
> data from the specified file from the application's priv directory and
> track it in an ets table.
> When couch_stats wants to learn about all the installed data for its
> stat definitions it would then just do something like:
>    couch_epi:get({couch_stats, definitions})
> Which would return a list of {appname, Data} tuples or something
> similar. To ensure that couch_stats can react to changes in these
> values, we would also provide an API like such:
>     couch_epi:listen({couch_stats, definitions})
> And any process that called that function would get a message whenever
> the data for that key changed which it could use for its own nefarious
> purposes.
> For upgrades, instead of specifying {priv_file, FileName} we could
> specify {mfa, {Mod, Fun, Args}} which would be invoked. Then we could
> add a code_change function to that module that would allow us to call
> something like couch_epi:reload() which would re-run the load for that
> process's data source.
> Function Centric Examples
> -------------------------
> Hopefully its obvious that given the data centric approach we could do
> something quite similar for functions (given that an MFA is just a
> small bit of data that we can use to invoke any function).
> Though ovbiously we'd like to be able to have a bit more of a useful
> API for clients so that we don't require all function based extensions
> to have to reimplement that function invocation code.
> The first thing that would change would be to provide a different type
> of supervision tree entry to indicate this. Off the top of my head
> this would look something like such:
>    {
>        appname_funcs,
>        {couch_epi_functions, start_link, [
>            appname,
>            {module, appname_funcs_mod}
>        ]},
>        permanent,
>        5000,
>        worker,
>        dynamic
>    }
> Then any function exported by appname_funcs_mod (that wasn't a builtin
> function, though maybe even if so?) could be invoked by an API like
> such:
>    couch_epi:invoke(my_function_name, Arg1, Arg2, Arg3).
>    couch_epi:apply(my_function_name, Args).
> We could also add various helper utilities or an Options parameter
> that would handle things like ignoring all exceptions, letting
> exceptions bubble or other such things that any invocation point might
> desire.
> More Details
> ------------
> The final app would have something like such:
>    couch_epi.erl - API for features accessing extension data
>    couch_epi_data_source.erl - Module that is inserted into
> application supevision trees to provide data sources for extension
> points
>    couch_epi_functions.erl - Module that is inserted into application
> supervision trees to provide function invocations
>    couch_epi_server.erl - Handles registration requests from
> couch_epi_data_soruce and couch_epi_functions and stores that
> information in an ets table. Also has the list of pids registered to
> listen for updates and notifies them.
>    couch_epi_sup.erl - probably a single child for couch_epi_server.erl
>    couch_epi_util.erl - The usual collection of functions that don't
> quite fit anywhere else (if needed).
> This should be a rather simple application in general. One side wants
> to publish some data, and the other wants to use it and possibly be
> notified when a particular bit of data is changed. And then we'll also
> provide some API sugar around invoking functions.
> Conclusion
> ==========
> Hopefully that all makes sense to at least some people in parts. I've
> been thinking about this on and off over a few weeks so my thoughts
> are a bit jumbled as I try and remember the salient points. I figured
> I'd just try and start getting them out there so that other people can
> comment on things and or let me know that I've forgotten something
> obvious that cripples this entire approach.

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