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tsujigiri / axiom


a micro-framework for web applications in Erlang


Axiom Build Status Flattr Button

Axiom is a micro-framework for building web applications in Erlang. It is inspired by Sinatra and built on top of Cowboy.

Getting Started

Axiom is built to make creating web applications fast and easy. A minimal application would look like this:

-export([start/0, handle/3]).

start() ->

handle(<<"GET">>, [<<"hi">>], _Request) ->
    <<"Hello world!">>.

This handles requests for GET /hi and returns "Hello world!".

The third argument given to the handler is of type cowboy_req:req(). Use the cowboy_req module, if you need anything out of the request.

The return value can be a binary string or iolist. So, this also works:

handle(<<"GET">>, [<<"hello">>, Who], _Request) ->
    [<<"Hello ">>, Who, <<"!">>].

If you want to specify a response status code and/or headers, use a tuple with either the status code and body or status code, headers and body, in these respective orders.


{418, <<"<h1>I'm a teapot!</h1>">>}


{418, [{<<"X-Foo">>, <<"bar">>}], <<"<h1>I'm a teapot!</h1>">>}

As a third option a cowboy_req:req() can be returned. In this case, to set the response headers and body, use the cowboy_req:set_resp_header/3 and cowboy_req:set_resp_body/2 functions. To set the status code, use axiom:set_resp_status/2. These functions return a new cowboy_req:req() to be used further and to be returned from YourHandler:handle/3.

The full spec of YourHandler:handle/3 is expected to look like this:

handle(Method, Path, Req) -> Body | Req | {Status, Body} | {Status, Headers, Body}.

    Method = binary(),
    Path = [PathSegment]
    PathSegment = binary()
    Req = cowboy_req:req()
    Body = iodata()
    Status = non_neg_integer()
    Headers = [Header]
    Header = {binary(), binary()}

Request parameters

To get the request parameters out of the request, you can use the two handy functions axiom:params(Req) and axiom:param(Name, Req). The first returns a proplist of all parameters, the second one returns the named parameter's value. Keys and values are binary strings.


axiom:start/1 has a bigger brother called axiom:start/2, taking a proplist as the second argument. Possible properties and their defaults are as follows:

    {nb_acceptors: 100},        % acceptor pool size
    {host, '_'},                % host IP
    {port, 7654},               % host port
    {public, "public"}          % custom path for static files

Static Files

Static files are served via the cowboy_static handler. By default, every file in the ./public directory and all its subdirectories will be made accessible via URL path the same as file's relative path. E.g. the file ./public/about.html can be accessed via GET /about.html. Note: Currently, if the contents of the ./public subtree change, Axiom needs to be restarted to reflect the change.

You can specify a custom directory via the public option.

When you use this feature, it is advisable to start Erlang with the +A n flag. This will start n async threads. Rule of thumb is to use your machine's number of CPU cores.


You can redirect requests with redirect/2:

handle(<<"GET">>, [<<"foo">>], Req) ->
  Req1 = axiom:redirect("/bar", Req),

handle(<<"GET">>, [<<"bar">>], Request) ->
    <<"<h1>Welcome back!</h1>">>.


Axiom comes with Django template support via erlydtl. To make use of it in your application, create a directory named templates and in it, create a template, e.g. my_template.dtl:

<h1>Hello {{who}}</h1>

In your handler, specify the template to be rendered:

handle(<<"GET">>, [<<"hello">>], _Request) ->
    axiom:dtl(my_template, [{who, "you"}]).

For convenience, the second argument, a proplist of parameters, can have atoms, lists or binaries as keys. That way request parameters can be put in there, without you having to convert them first.

The templates are compiled into modules when rebar compile is called.

To see what else erlydtl can do for you, take a look at its project page.


Axiom comes with a basic session handler and ets based session store. To use it, add this tuple to the configuration proplist:

{sessions, []}

In your handler you can then use axiom_session:set(Key, Value, Request) and axiom_session:get(Key, Request).

To set attributes for the cookie, storing the session ID, add some parameters to the session configuration in a tuple with the key cookies:

{sessions, [{cookies, [param()]}]}

Possible parameters are:

param() = {max_age, integer()} |
          {local_time, calendar:datetime()} |
          {domain, binary()} |
          {path, binary()} |
          {secure, true | false} |
          {http_only, true | false}

The default session store is the axiom_session_ets module. You can use your own by adding a store tuple to the sessions tuple:

{sessions, [{store, my_session_store, []}]}

For implementation details take a look into the axiom_session_ets module.


The functions before_filter/1 and after_filter/1 can be implemented to deal with the cowboy_req:req() before and after YourHandler:handle/3. When implemented, these are called no matter which handle function matches the request.

In your handler module:

before_filter(Req) ->
    % do stuff

after_filter(Req) ->
    % do more stuff


Not Found

To overwrite Axiom's response to 404 errors, just create a catch-all handler:

handle(_Method, _Path, _Req) ->
    {404, <<"nope.">>}.

Note that you have to take care of the status code yourself, as otherwise the default of 200 is sent back to the client.

Internal Server Error

To handle these yourself, you can implement a function named error/1. The argument is the cowboy_req:req() object, otherwise it works like your Handler:handle/3 function.


To send a chunked reply, call axiom:chunk/2 for each chunk:

chunk(Data::iodata(), Req::cowboy_req:req()) -> {ok, Req1::cowboy_req:req()}.

The returned cowboy_req:req() object has to be given as an argument to subsequent calls to chunk and as the return value of your Handler:handle/3 function.

To stream data with a Content-Type other than text/html, use chunk/3, which has an additional parameter, to be set to the type you want:

chunk(Data::iodata(), Req::cowboy_req:req(), Type::binary()) -> {ok, Req1::cowboy_req:req()}.


handle(<<"GET">>, [<<"stream">>], Req) ->
    {ok, Req1} = axiom:chunk(<<"Hello">>, Req, <<"text/plain">>),
    {ok, Req2} = axiom:chunk(<<" world">>, Req1),
    {ok, Req3} = axiom:chunk(<<"!">>, Req2),


To use it in your OTP application, add this to your rebar.config:

{lib_dirs, ["deps"]}.
{deps, [
    {'axiom', "0.1.0", {git, "git://", {tag, "v0.1.0"}}}

then, as usual:

rebar get-deps
rebar compile


Please take a look at the LICENSE file! (tl;dr: it's the MIT License)


To express your appreciation you are welcome to Flattr Button, flip a few bits of internet change into my hat (bitcoin:1BWapzSP6rmhhggkwN3xUToQo64j7Rjhvk) or buy me a beer. :beer: