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A simple emacs async HTTP server

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An evented IO webserver in Emacs Lisp.


Elnode will not run properly on anything less than Emacs 24. Elnode requires Emacs 24's lexical binding as it makes extensive use of closures.


Elnode is a great for these things:

  • nice simple server with few dependancies (just Emacs and cat basically)
  • prototyping webapps
  • browser testing
  • asynchronous apps, like chat apps


Elnode is packaged in marmalade.

For dealing with package repositories check out the Emacs Wiki but the short version is to add the following to your .emacs or your .emacs.d/init.el:

   '("marmalade" . ""))

And then do:

M-x list-packages

find Elnode in the list and press i or ENTER to install it.

If you don't want to use packages you can just install elnode.el on your load-path somewhere and:

(require 'elnode)

Install from this repository

Installing from the sources is complex and requires the dependancies declared in the file recipes/elnode.

The recipe file is used by elpakit or other tools to build the package.

elpakit can help build elnode, and help with running tests. Install elpakit from marmalade and then you can build Elnode with elpakit by doing:

M-x elpakit-make-multi

in the Elnode directory.

You can build the Elnode package and run the Elnode tests on that package with the following lisp:

(elpakit-test (list elnode-directory) 'elnode-tests 'elnode)

Where elnode-directory specifies your local Elnode repository directory.

The list can include more repository directories which will be combined into a single package archive.

Out of the box

When Elnode initializes it automatically starts a webserver and a Wiki engine.

If you:

M-x customize-group

you can alter a number of variables pertaining to the default configuration, including the directory used to keep files.

By default the package installs files in your .emacs.d - it uses a directory called elnode for the Wiki root and the webroot. Both are configurable with Elnode config variables.

You can also just ignore the built in stuff completely and write your own servers.

What Elnode servers are running?

Elnode tracks the servers an Emacs instance is running and you can see the view of that with:

M-x list-elnode-servers

The list shows TCP ports and handlers and you can press return on a handler and move to it's source code definition.

You can kill a server by hitting "k" on it.

How does it work?

The simplest thing that Elnode does is let you start a webserver on a directory:

M-x elnode-make-webserver [RET] 
Serve files from: [enter directory] [RET]
TCP Port (try something over 8000): 8009 [RET]

and there will be a webserver started on port 8009 serving files from whatever directory you specified.

By default Elnode starts that server on the host specified by elnode-init-host which is a variable you can customize:

M-x customize-variable [RET] elnode-init-host [RET]

Take care though, you don't want to expose your Emacs session to the Internet.

You can also use the prefix key to specify the host for just this one server:

C-u M-x elnode-make-webserver [RET] 
Docroot: [enter directory] [RET]
Port: 8009 [RET]

Basic Elnode for programmers

Elnode's power is most visible to programmers though.

You can define a handler function:

(defun my-test-handler (httpcon)
  "Demonstration function"
  (elnode-http-start httpcon 200 '("Content-type" . "text/html"))
  (elnode-http-return httpcon "<html><b>HELLO!</b></html>"))

And then start the server:

(elnode-start 'my-test-handler :port 8010 :host "localhost")

You can also start the server interactively... with:

M-x elnode-start

it interactively asks for the handler function and a port.

Stopping the server

If you can remember the port you started your server on then you'll be able to stop it, like:

(elnode-stop 8010)

You can also stop interactively:

M-x elnode-stop


Mapping paths to handlers

elnode-hostpath-dispatcher takes a-list of path/handler mappings:

   '(("^\\(.*\\)" . elnode-wikiserver)
     ("^\\(.*\\)" . user-admin)
     ("^.*//\\(.*\\)" . elnode-webserver)))

(defun root-handler (httpcon)
  (elnode-hostpath-dispatcher httpcon my-app-routes))

(elnode-start 'root-handler :port 8009)

This will create a server on port 8009 being handled by root-handler which will root the requests to the appropriate handler.

Any request for the host with the path /wiki/ will be sent to the Elnode Wiki engine.

Any request for the host with the path /admintool/ will be sent to the user-admin handler, presumably that is defined somewhere.

Any other request will be sent to the default Elnode webserver.

Elnode itself uses a hostpath dispatcher on the default Elnode server. This can actually be configured with the variable elnode-hostpath-default-table, so you can actually change the default behaviour of the Elnode default server just with Emacs config.

The use of regexs in Elnode's mapping is supported by other tools. Sub-expressions are capturable in mapping support routines such as elnode-docroot-for.

When a handler is called by elnode-hostpath-dispatcher then the parts of the match are available through the function elnode-http-mapping. So we could code the user-admin handler like this:

##! emacs-lisp
(defun user-admin (httpcon)
  (let ((username (elnode-http-mapping httpcon 1)))
    (user-admin-send-admin-page httpcon username)))

The (elnode-http-mapping httpcon 1) accesses the first sub-expression of the regex that caused the match:

     ("^\\(.*\\)" . user-admin)

so, everything AFTER the admintool/.

Some tools in Elnode do this for you, so you don't have to. Again, look at elnode-docroot-for.

Serving files

There are several helpers for serving files with Elnode. You can serve directories of files directly by making a webserver handler. A function elnode-webserver-handler-maker can make webservers:

##! emacs-lisp

(setq my-webserver
      (elnode-webserver-handler-maker "~/my-webroot"))

(elnode-start my-webserver :port 8010)

The Elnode webserver also produces index pages and can be configured with a number of variables:

  • elnode-webserver-index-page-template defines the page template used for the index
  • elnode-webserver-index-file-template defines the template for each file in the index, normally it's just an A tag ponting to the file.

More controlled serving

If you need more control over serving files you can write handlers with elnode-docroot-for. This does a lot of complex work for you to map a directory tree to a webserver namespace.

This example shows how to use elnode-docroot-for

##! emacs-lisp

(defun elnode-org-handler (httpcon)
  (elnode-docroot-for "~/work/org"
      with org-file
      on httpcon
      do (with-current-buffer (find-file-noselect org-file)
           (let ((org-html
                  ;; This might throw errors so you could condition-case it
                  (org-export-as-html 3 nil nil 'string)))
             (elnode-send-html httpcon org-html)))))

The first argument is the directory of files which you want to serve, then with variable specifies the name of a variable to use in the body of the code which will be bound to the filename of the file the user wants. Then on httpcon specifies the HTTP connection to use and then do .... specifies the code to use.

elnode-docroot-for processes incomming requests on the httpcon you specify by checking the request matches a file in the directory you specify (it sends a 404 if it does not find one).

It also does last modified caching on the file and sends an HTTP 304 response if the file has not been updated since the last request.

If a matching file exists and the it is not cached then elnode-docroot-for runs the do code to send the response correctly.

Sending files

Elnode also has elnode-send-file for sending files to the response, along with elnode-docroot-for this makes a powerful simple webserver tool. elnode-send-file can be used to send any arbitary file:

##! emacs-lisp
(defun my-status-page (httpcon)
  (elnode-http-start httpcon 200 '("Content-type" . "text/html"))
  (elnode-send-file httpcon "~/static-status-file.html"))

A handler that will only ever respond with one static file. Of course, this isn't very interesting, combined with elnode-docroot-for it can be used to serve directories and the like, or you could work out the filename to be sent with some other method.

There is another use for elnode-send-file which is simple templating. You can pass parameters to elnode-send-file and it will template them into the file:

(defun my-templater(httpcon)
  (let ((hash (make-hash-table 
                 :test 'equal 
                 :data "username" "nicferrier")))
     (elnode-http-start httpcon 200 '("Content-type" . "text/html"))
         httpcon "~/my-template.html" 
         :replacements hash)))

The template file must have sections marked up like:

<<html &lt;!##E username E##!&gt; html>>

for each of the variables.

This makes for simple but quite powerful templating.

Really Really simple file sending

It's also possible to make send file functions automatically so if you want to map a handler that serves just one file in a dispatcher that's possible:

##! emacs-lisp
`(("^\\(.*\\)" . elnode-wikiserver)
  ("^.*//styles.css" . ,(elnode-make-send-file "~/mainstyles.css"))
  ("^.*//\\(.*\\)" . elnode-webserver))

It's also possible to use templating with this style of programming by passing a function returning the alist variable map as :replacements:

##! emacs-lisp
(defun my-templater ()
  '(("username" . "william occam")))

`(("^\\(.*\\)" . elnode-wikiserver)
  ("^.*//styles.css" . ,(elnode-make-send-file 
                           :replacements 'my-templater))
  ("^.*//\\(.*\\)" . elnode-webserver))

This makes templating and setting up very simple websites very easy indeed.

Accessing data in the HTTP request

There are a bunch of functions that do what you would expect about data in the HTTP request:

##! emacs-lisp

(elnode-http-method httpcon)
=> "POST"

(elnode-http-pathinfo httpcon)
=> "/wiki/blah.creole"

(elnode-http-query httpcon)
=> "a=10&b=20&c=the+quick+brown+fox"

(elnode-http-params httpcon)
=> (("a" . "10")("b" . "20")("c" . "the quick brown fox"))

(elnode-http-param httpcon "username")
=> "nicferrier"

(elnode-http-cookie httpcon "session-id")
=> "1213313"

(elnode-http-header httpcon "Date")
=> "Mon, Feb 27 2012 22:10:21 GMT"

(elnode-http-header httpcon 'date)
=> "Mon, Feb 27 2012 22:10:21 GMT"

(elnode-http-header httpcon 'date :time) ;; with convert flag set to :time
=> (20299 65357)

Note that Elnode generally can accept symbol's as well as strings to name things, if it can't it's a bug, please report it.

Also, Elnode can handle some conversions sometimes. I would like to respond to user demand about when and where to do that and what to do. Please give me feedback.

Elnode's raw data

Elnode stores most of it's internal state on the connection object and it's all accessible, interesting properties and how to access them:

##! emacs-lisp

(process-get httpcon :elnode-http-status)
=> "GET / HTTP/1.1"

(process-get httpcon :elnode-http-resource)
=> "/"

(process-get httpcon :elnode-http-version)
=> "1.1"

These are not supported by Elnode at all, there is no guarantee that the names of these properties won't change. If you feel that you want official support (ie: a function) then make an issue on the Elnode github.

To Do?

If you're playing with elnode but you can't think of anything to do with it...

  • make an elnode param handler that sanitzes input
    • one way to do that was found by aidalgol:
(require 'htmlize)
 "<a href='/blah?a=10&b=2'></a><script>call_func()</script>")
  • an elpa repository written with elnode
    • turn the package list into html
    • allow packages to be downloaded from elnode
    • upload of packages will require fixing the request management a little
  • an emacsclient with elnode
    • write a command line client that submits data to the server over HTTP
    • it should interact with the emacs user in the same way that emacs server does
    • why? because then a single Emacs could have just 1 server socket open for all sorts of different roles
  • alter elnode-webserver-handler-maker to do indexing better
    • take an optional index producing function?
    • take keyword flags that set the behaviour?
    • eg: :doindexes 't
  • browse-current-buffer
    • start an elnode server on some random port exposing the current buffer
    • automatically open a browser on the started server

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