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Unikraft Cloud

Run an Erlang app

In this guide we create and deploy a simple Erlang-based HTTP web server. To run this example, follow these steps:

  1. Install the kraft CLI tool and a container runtime engine, e.g. Docker.

  2. Clone the examples repository and cd into the examples/http-erlang26.2/ directory:

Terminal window
git clone https://github.com/kraftcloud/examples
cd examples/http-erlang26.2/

Make sure to log into Unikraft Cloud by setting your token and a metro close to you. We use fra0 (Frankfurt, 🇩🇪) in this guide:

Terminal window
# Set Unikraft Cloud access token
export UKC_TOKEN=token
# Set metro to Frankfurt, DE
export UKC_METRO=fra0

When done, invoke the following command to deploy this application on Unikraft Cloud:

Terminal window
kraft cloud deploy -p 443:8080 -M 512 .

The output shows the instance URL and other details:

Terminal window
[●] Deployed successfully!
 │
 ├────────── name: erlang-sw2bp
 ├────────── uuid: 1c4a8a51-fb61-45fc-87b8-26d192a7c2bc
 ├───────── state: starting
 ├──────── domain: https://patient-field-ck629j2u.fra0.kraft.host
 ├───────── image: erlang@sha256:d99feefa7973ba43f726356497f54c34a16421aa25a27fa547d2c1add418204e
 ├──────── memory: 512 MiB
 ├─────── service: patient-field-ck629j2u
 ├── private fqdn: erlang-sw2bp.internal
 ├──── private ip: 172.16.3.3
 └────────── args: /usr/bin/wrapper.sh /usr/bin/erl -noshell -s http_server

In this case, the instance name is erlang-sw2bp and the URL is https://patient-field-ck629j2u.fra0.kraft.host. They are different for each run.

Use curl to query the Unikraft Cloud instance of the Erlang-based HTTP web server:

Terminal window
curl https://patient-field-ck629j2u.fra0.kraft.host
Hello, World!

At any point in time, you can list information about the instance:

Terminal window
kraft cloud instance list
NAME               FQDN                                    STATE    STATUS              IMAGE                                 MEMORY   ARGS                                                     BOOT TIME
erlang-sw2bp       patient-field-ck629jsu.fra0.kraft.host  running  since 35secs        erlang@sha256:d99feefa7973ba43f72...  512 MiB  /usr/bin/wrapper.sh /usr/bin/erl -noshell -s http_se...  404.04 ms

When done, you can remove the instance:

Terminal window
kraft cloud instance remove erlang-sw2bp

Customize your Application

To customize the application, update the files in the repository, listed below:

  • http_server.erl: the actual Erlang HTTP server
  • Kraftfile: the Unikraft Cloud specification
  • Dockerfile: the Docker-specified application filesystem
-module(http_server).
-export([start/0]).


start() ->
spawn(fun () -> {ok, Sock} = gen_tcp:listen(8080, [{active, false}]),
                loop(Sock) end).


loop(Sock) ->
{ok, Conn} = gen_tcp:accept(Sock),
Handler = spawn(fun () -> handle(Conn) end),
gen_tcp:controlling_process(Conn, Handler),
loop(Sock).


handle(Conn) ->
gen_tcp:send(Conn, response("Hello, World!\n")),
gen_tcp:close(Conn).


response(Str) ->
B = iolist_to_binary(Str),
iolist_to_binary(
  io_lib:fwrite(
     "HTTP/1.0 200 OK\r\nContent-Type: text/html\r\nContent-Length: ~p\r\n\r\n~s\r\n",
     [size(B), B])).

The following options are available for customizing the application:

  • If only updating the implementation in the http_server.erl source file, no other change is required.

  • If new files are added, these have to be copied in the application filesystem, using the COPY command in the Dockerfile.

  • If a new Erlang source files is added, update the cmd line in the Kraftfile and replace http_server.erl to run that file when creating the instance.

  • More extensive changes may require expanding the Dockerfile with additional Dockerfile commands.

Learn More

Use the --help option for detailed information on using Unikraft Cloud:

Terminal window
kraft cloud --help

Or visit the CLI Reference.