Configuration DSL step-by-step

Written by hannes
Classified under: mirageosbackground
Published: 2016-05-10 (last updated: 2021-11-19)

Sorry for being late again with this article, I had other ones planned, but am not yet satisfied with content and code, will have to wait another week.

MirageOS configuration

As described in an earlier post, MirageOS is a library operating system which generates single address space custom kernels (so called unikernels) for each application. The application code is (mostly) independent on the used backend. To achieve this, the language which expresses the configuration of a MirageOS unikernel is rather complex, and has to deal with package dependencies, setup of layers (network stack starting at the (virtual) ethernet device, or sockets), logging, tracing.

The abstraction over concrete implementation of e.g. the network stack is done by providing a module signature in the mirage-types package. The socket-based network stack, the tap device based network stack, and the Xen virtual network device based network stack implement this signature (depending on other module signatures). The unikernel contains code which applies those dependent modules to instantiate a custom-tailored network stack for the specific configuration. A developer should only describe what their requirements are, the user who wants to deploy it should provide the concrete configuration. And the developer should not need to manually instantiate the network stack for all possible configurations, this is what the mirage tool should embed.

Initially, MirageOS contained an adhoc system which relied on concatenation of strings representing OCaml code. This turned out to be error prone. In 2015 Drup developed Functoria, a domain-specific language (DSL) to organize functor applications, primarily for MirageOS. It has been introduced in a blog post. It is not limited to MirageOS (although this is the primary user right now).

Functoria has been included in MirageOS since its 2.7.0 release at the end of February 2016. Functoria provides support for command line arguments which can then either be passed at configuration time or at boot time to the unikernel (such as IP address configuration) using the cmdliner library underneath (and includes dynamic man pages, help, sensible command line parsing, and even visualisation (mirage describe) of the configuration and data dependencies).

I won't go into details about command line arguments in here, please have a look at the functoria blog post in case you're interested. Instead, I'll describe how to define a Functoria device which inserts content as code at configuration time into a MirageOS unikernel (running here, source). Using this approach, no external data (using crunch or a file system image) is needed, while the content can still be modified using markdown. Also, no markdown to HTML converter is needed at runtime, but this step is completely done at compile time (the result is a small (still too large) unikernel, 4.6MB).


Similar to my website post, this unikernel only has a single resource and thus does not need to do any parsing (or even call read). The main function is start:

let start stack _ =
  S.listen_tcpv4 stack ~port:80 (serve rendered) ;
  S.listen stack

Where S is a V1_LWT.STACKV4, a complete TCP/IP stack for IPv4. The functions we are using are listen_tcpv4, which needs a stack, port and a callback (and should be called register_tcp_callback), and listen which polls for incoming frames.

Our callback is serve rendered, where serve is defined as:

let serve data tcp =
  TCP.writev tcp [ header; data ] >>= fun _ ->
  TCP.close tcp

Upon an incoming TCP connection, the list consisting of header ; data is written to the connection, which is subsequently closed.

The function header is very similar to our previous one, splicing a proper HTTP header together:

let http_header ~status xs =
  let headers = (fun (k, v) -> k ^ ": " ^ v) xs in
  let lines   = status :: headers @ [ "\r\n" ] in
  Cstruct.of_string (String.concat "\r\n" lines)

let header = http_header
    ~status:"HTTP/1.1 200 OK"
    [ ("Content-Type", "text/html; charset=UTF-8") ;
      ("Connection", "close") ]

And the rendered function consists of some hardcoded HTML, and references to two other modules, and

let rendered =
    (String.concat "" [
        "<html><head>" ;
        "<title>1st MirageOS hackathon: 11-16th March 2016, Marrakech, Morocco</title>" ;
        "<style>" ; ; "</style>" ;
        "</head>" ;
        "<body><div id=\"content\">" ; ;
        "</div></body></html>" ])

This puts together the pieces we need for a simple HTML site. This unikernel does not have any external dependencies, if we assume that the mirage toolchain, the types, and the network implementation are already provided (the latter two are implicitly added by the mirage tool depending on the configuration, the first you'll have to install manually opam install mirage).

But wait, where do Style and Content come from? There are no ml modules in the repository. Instead, there is a and style.css in the data subdirectory.


We use the builtin configuration time magic of functoria to translate these into OCaml modules, in such a way that our unikernel does not need to embed code to render markdown to HTML and carry along a markdown data file.

Inside of, let's look again at the bottom:

let () =
  register "marrakech2016" [
      ~deps:[abstract config_shell]
      ( stackv4 @-> job )
      $ net

The function register is provided by the mirage tool, it will execute the list of jobs using the given name. To construct a job, we use the foreign combinator, which might have dependencies (here, a list with the single element config_shell explained later, using the abstract combinator), the name of the main function (Unikernel.main), a typ (here constructed using the @-> combinator, from a stackv4 to a job), and this applied (using the $ combinator) to the net (an actual implementation of stackv4).

The net implementation is as following:

let address addr nm gw =
  let f = Ipaddr.V4.of_string_exn in
  { address = f addr ; netmask = f nm ; gateways = [f gw] }

let server = address "" "" ""

let net =
  if_impl Key.is_xen
    (direct_stackv4_with_static_ipv4 default_console tap0 server)
    (socket_stackv4 default_console [Ipaddr.V4.any])

Depending on whether we're running on unix or xen, either a socket stack (for testing) or the concrete IP configuration for deployment (using if_impl and is_xen from our DSLs).

So far nothing too surprising, only some combinators of the functoria DSL which let us describe the possible configuration options.

Let us look into config_shell, which embeds the markdown and CSS into OCaml modules at configuration time:

type sh = ShellConfig

let config_shell = impl @@ object
    inherit base_configurable

    method configure i =
      let open Functoria_app.Cmd in
      let (>>=) = Rresult.(>>=) in
      let dir = Info.root i in
      run "echo 'let data = {___|' >" >>= fun () ->
      run "cat data/style.css >>" >>= fun () ->
      run "echo '|___}' >>" >>= fun () ->
      run "echo 'let data = {___|' >" >>= fun () ->
      run "omd data/ >>" >>= fun () ->
      run "echo '|___}' >>"

    method clean i = "rm -f"

    method module_name = "Functoria_runtime"
    method name = "shell_config"
    method ty = Type ShellConfig

Functoria uses classes internally, and we extend the base_configurable class, which extends configurable with some sensible defaults.

The important bits are what actually happens during configure and clean: execution of some shell commands (echo, omd, and rm) using the functoria application builder interface. Some information is as well exposed via the Functoria_info module.


We walked through the configuration magic of MirageOS, which is a domain-specific language designed for MirageOS demands. We can run arbitrary commands at compile time, and do not need to escape into external files, such as Makefile or shell scripts, but can embed them in our

I'm interested in feedback, either via twitter or via eMail.

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