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authorsterni <sternenseemann@systemli.org>2021-02-21T11·57+0100
committersterni <sternenseemann@systemli.org>2021-04-01T18·50+0000
commit93a746aaaa092ffc3e7eb37e1df30bfd3a28435f (patch)
tree4197f3d5c7b5e2c4cdcb4be3dd1a17ed3ecc2ad7 /web/bubblegum/README.md
parent68f3ac64c4a2ae50dcb125f067692536f647e370 (diff)
feat(web/bubblegum): nix CGI programming framework r/2394
So here is what has been keeping me up at night: At some point I
realized that nix actually made a somewhat passable language for CGI
programming:

* That `builtins.getEnv` exists as one of the impurities of Nix is
  perfect as environment variables are the main way of communication
  from the web server to the CGI application.

* We can actually read from the filesystem via builtins.readDir and
  builtins.readFile with bearable overhead if we avoid importing the
  used paths into the nix store.

* Templating and routing are convenient to implement via indented strings
  and attribute sets respectively.

Of course there are obvious limitation:

* The overhead of derivations is probably much to great for them to be
  useful via IfD.

* Even without derivations, nix evaluation is very slow to the point
  were a trivial application takes between 100ms and 400ms to produce a
  response.

* We can't really cause effects other than producing a response which
  makes it not viable for a lot of applications. There are some ways
  around this:

  * With a custom interpreter we could have streaming and multiplexed
    I/O (using lazy lists emulated via attrsets) to cause such effects,
    but it would probably perform terribly.

  * We can use builtins.fetchurl to call other HTTP-based microservices,
    but only in very limited constraints, i. e. only GET, no headers,
    and only if the tarball ttl is set to 0 in the global nix.conf.

* Terrible error handling capabilities because builtins.tryEval actually
  doesn't catch a lot of errors.

To prove that it actually works, there are some demo applications,
which I invite you to run and potentially break horribly:

    nix-build -A web.bubblegum.examples && ./result
    # navigate to http://localhost:9000

The setup uses thttpd and executes the nix CGI scripts using
users.sterni.nint which automatically passed `depot`, so they can
import the cgi library.

Change-Id: I3a22a749612211627e5f8301c31ec2e7a872812c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2746
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
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+# //web/bubblegum
+
+`bubblegum` is a CGI programming library for the Nix expression language.
+It provides a few helpers to make writing CGI scripts which are executable
+using [//users/sterni/nint](../../users/sterni/nint/README.md) convenient.
+
+An example nix.cgi script looks like this (don't worry about the shebang
+too much, you can use `web.bubblegum.writeCGI` to set this up without
+thinking twice):
+
+```nix
+#!/usr/bin/env nint --arg depot '(import /path/to/depot {})'
+{ depot, ... }:
+
+let
+  inherit (depot.web.bubblegum)
+    respond
+    ;
+in
+
+respond "OK" {
+  "Content-type" = "text/html";
+  # further headers…
+} ''
+  <!doctype html>
+  <html>
+    <head>
+      <meta charset="utf-8">
+      <title>hello world</title>
+    </head>
+    <body>
+      hello world!
+    </body>
+  </html>
+''
+```
+
+As you can see, the core component of `bubblegum` is the `respond`
+function which takes three arguments:
+
+* The response status as the textual representation which is also
+  returned to the client in the HTTP protocol, e. g. `"OK"`,
+  `"Not Found"`, `"Bad Request"`, …
+
+* An attribute set mapping header names to header values to be sent.
+
+* The response body as a string.
+
+Additionally it exposes a few helpers for working with the CGI
+environment like `pathInfo` which is a wrapper around
+`builtins.getEnv "PATH_INFO"`. The documentation for all exposed
+helpers is inlined in [default.nix](./default.nix) (you should be
+able to use `nixdoc` to render it).
+
+For deployment purposes it is recommended to use `writeCGI` which
+takes a nix CGI script in the form of a derivation, path or string
+and builds an executable nix CGI script which has the correct shebang
+set and is automatically passed a version of depot from the nix store,
+so the script has access to the `bubblegum` library.
+
+For example nix CGI scripts and a working deployment using `thttpd`
+see the [examples directory](./examples). You can also start a local
+server running the examples like this:
+
+```
+$ nix-build -A web.bubblegum.examples && ./result
+# navigate to http://localhost:9000
+```