package capnp-rpc-unix

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Cap'n Proto is a capability-based RPC system with bindings for many languages


Dune Dependency






This package contains some helpers for use with traditional (non-Unikernel) operating systems.

Published: 27 Feb 2023


OCaml Cap'n Proto RPC library

Copyright 2017 Docker, Inc. Copyright 2019 Thomas Leonard. See for details.

API documentation



Cap'n Proto is a capability-based RPC system with bindings for many languages. Some key features:

  • APIs are defined using a schema file, which is compiled to create bindings for different languages automatically.

  • Schemas can be upgraded in many ways without breaking backwards-compatibility.

  • Messages are built up and read in-place, making it very fast.

  • Messages can contain capability references, allowing the sender to share access to a service. Access control is handled automatically.

  • Messages can be pipelined. For example, you can ask one service where another one is, and then immediately start calling methods on it. The requests will be sent to the first service, which will either handle them (in the common case where the second service is in the same place) or forward them until you can establish a direct connection.

  • Messages are delivered in E-Order, which means that messages sent over a reference will arrive in the order in which they were sent, even if the path they take through the network gets optimised at some point.

This library should be used with the capnp-ocaml schema compiler, which generates bindings from schema files.


RPC Level 2 is complete, with encryption and authentication using TLS and support for persistence.

The library has unit tests and AFL fuzz tests that cover most of the core logic. It is used as the RPC system in ocaml-ci.

The default network provided supports TCP and Unix-domain sockets, both with or without TLS. For two-party networking, you can provide any bi-directional byte stream (satisfying the Mirage flow signature) to the library to create a connection. You can also define your own network types.

Level 3 support is not implemented yet, so if host Alice has connections to hosts Bob and Carol and passes an object hosted at Bob to Carol, the resulting messages between Carol and Bob will be routed via Alice. Until that is implemented, Carol can ask Bob for a persistent reference (sturdy ref) and then connect directly to that.


To install, you will need a platform with the capnproto package available (e.g. Debian >= 9). Then:

opam install capnp-rpc-unix

(note: if you are using opam < 2.1, direct install is not possible, so do the following):

opam depext -i capnp-rpc-unix

Structure of the library

The code is split into several packages:

  • capnp-rpc contains the logic of the Cap'n Proto RPC Protocol, but does not depend on any particular serialisation. The tests in the test directory test the logic using a simple representation where messages are OCaml data-structures (defined in capnp-rpc/

  • capnp-rpc-lwt instantiates the capnp-rpc functor using the Cap'n Proto serialisation for messages and Lwt for concurrency.

  • capnp-rpc-net adds networking support, including TLS.

  • capnp-rpc-unix adds helper functions for parsing command-line arguments and setting up connections over Unix sockets. The tests in test-lwt test this by sending Cap'n Proto messages over a Unix-domain socket.

  • capnp-rpc-mirage is an alternative to -unix that works with Mirage unikernels.

Libraries that consume or provide Cap'n Proto services should normally depend only on capnp-rpc-lwt, since they shouldn't care whether the services they use are local or accessed over some kind of network.

Applications will normally want to use capnp-rpc-net and, in most cases, capnp-rpc-unix.


This tutorial creates a simple echo service and then extends it. It shows how to use most of the features of the library, including defining services, sending and receiving messages, and using encryption and authentication over network links.

When following the tutorial, you should be able to create all the files yourself, following the instructions. If you get stuck, you can find complete solutions in the examples directory.

A basic echo service

We start by writing a Cap'n Proto schema file:

interface Echo {
  ping @0 (msg :Text) -> (reply :Text);

This defines the Echo interface as having a single method called ping which takes a struct containing a text field called msg and returns a struct containing another text field called reply.

Save this as echo_api.capnp and compile it using capnp:

$ capnp compile echo_api.capnp -o ocaml
echo_api.capnp:1:1: error: File does not declare an ID.  I've generated one for you.
Add this line to your file:

Every interface needs a globally unique ID. If you don't have one, capnp will pick one for you, as shown above. Add the line to the start of the file to get:


interface Echo {
  ping @0 (msg :Text) -> (reply :Text);

Now it can be compiled:

$ capnp compile echo_api.capnp -o ocaml
echo_api.capnp --> echo_api.mli

The next step is to implement a client and server (in a new file) using the generated Echo_api OCaml module.

For the server, you should inherit from the generated Api.Service.Echo.service class:

module Api = Echo_api.MakeRPC(Capnp_rpc_lwt)

open Lwt.Infix
open Capnp_rpc_lwt

let local =
  let module Echo = Api.Service.Echo in
  Echo.local @@ object
    inherit Echo.service

    method ping_impl params release_param_caps =
      let open Echo.Ping in
      let msg = Params.msg_get params in
      release_param_caps ();
      let response, results = Service.Response.create Results.init_pointer in
      Results.reply_set results ("echo:" ^ msg);
      Service.return response

The first line (module Api) instantiates the generated code to use this library's RPC implementation.

The service object must provide one OCaml method for each method defined in the schema file, with _impl on the end of each one.

There's a bit of ugly boilerplate here, but it's quite simple:

  • The Api.Service.Echo.Ping module defines the server-side API for the ping method.

  • Ping.Params is a reader for the parameters.

  • Ping.Results is a builder for the results.

  • msg is the string value of the msg field.

  • release_param_caps releases any capabilities passed in the parameters. In this case there aren't any, but remember that a client using some future version of this protocol might pass some optional capabilities, and so you should always free them anyway.

  • Service.Response.create Results.init_pointer creates a new response message, using Ping.Results.init_pointer to initialise the payload contents.

  • response is the complete message to be sent back, and results is the data part of it.

  • Service.return returns the results immediately (like Lwt.return).

The client implementation is similar, but uses Api.Client instead of Api.Service. Here, we have a builder for the parameters and a reader for the results. Api.Client.Echo.Ping.method_id is a globally unique identifier for the ping method.

module Echo = Api.Client.Echo

let ping t msg =
  let open Echo.Ping in
  let request, params = Capability.Request.create Params.init_pointer in
  Params.msg_set params msg;
  Capability.call_for_value_exn t method_id request >|= Results.reply_get

Capability.call_for_value_exn sends the request message to the service and waits for the response to arrive. If the response is an error, it raises an exception. Results.reply_get extracts the reply field of the result.

We don't need to release the capabilities of the results, as call_for_value_exn does that automatically. We'll see how to handle capabilities later.

With the boilerplate out of the way, we can now write a to test it:

open Lwt.Infix

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let () = begin
    let service = Echo.local in service "foo" >>= fun reply -> "Got reply %S@." reply;

Here's a suitable dune file to compile the schema file and then the generated OCaml files:

 (name main)
 (libraries lwt.unix capnp-rpc-lwt logs.fmt)
 (flags (:standard -w -53-55)))

 (targets echo_api.mli)
 (deps    echo_api.capnp)
 (action (run capnp compile -o %{bin:capnpc-ocaml} %{deps})))

With this, you can now delete the generated files from your source directory:

$ rm echo_api.mli

The service is now usable:

$ opam install capnp-rpc-lwt

(note: or $ opam depext -i capnp-rpc-lwt for opam < 2.1)

$ dune exec ./main.exe
Got reply "echo:foo"

This isn't very exciting, so let's add some capabilities to the protocol...

Passing capabilities

Let's update the schema:


interface Callback {
  log @0 (msg :Text) -> ();

interface Echo {
  ping      @0 (msg :Text) -> (reply :Text);
  heartbeat @1 (msg :Text, callback :Callback) -> ();

This version of the protocol adds a heartbeat method. Instead of returning the text directly, it will send it to a callback at regular intervals.

The new heartbeat_impl method looks like this:

    method heartbeat_impl params release_params =
      let open Echo.Heartbeat in
      let msg = Params.msg_get params in
      let callback = Params.callback_get params in
      release_params ();
      match callback with
      | None -> "No callback parameter!"
      | Some callback ->
        Service.return_lwt @@ fun () ->
        Capability.with_ref callback (notify ~msg)

Note that all parameters in Cap'n Proto are optional, so we have to check for callback not being set (data parameters such as msg get a default value from the schema, which is "" for strings if not set explicitly).

Service.return_lwt fn runs fn () and replies to the heartbeat call when it finishes. Here, the whole of the rest of the method is the argument to return_lwt, which is a common pattern.

Capability.with_ref x f calls f x and then releases x (capabilities are ref-counted).

notify callback msg just sends a few messages to callback in a loop:

let (>>!=) = Lwt_result.bind		(* Return errors *)

let notify callback ~msg =
  let rec loop = function
    | 0 ->
      Lwt.return @@ Ok (Service.Response.create_empty ())
    | i ->
      Callback.log callback msg >>!= fun () ->
      Lwt_unix.sleep 1.0 >>= fun () ->
      loop (i - 1)
  loop 3

Exercise: create a Callback submodule in and implement the client-side Callback.log function (hint: it's very similar to ping, but use Capability.call_for_unit because we don't care about the value of the result and we want to handle errors manually). If you get stuck, the solution can be found in the examples/v2 directory.

To write the client for Echo.heartbeat, we take a user-provided callback object and put it into the request:

let heartbeat t msg callback =
  let open Echo.Heartbeat in
  let request, params = Capability.Request.create Params.init_pointer in
  Params.msg_set params msg;
  Params.callback_set params (Some callback);
  Capability.call_for_unit_exn t method_id request

Capability.call_for_unit_exn is a convenience wrapper around Callback.call_for_value_exn that discards the result.

In, we can now wrap a regular OCaml function as the callback:

open Capnp_rpc_lwt

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let callback_fn msg = "Callback got %S@." msg

let run_client service =
  Capability.with_ref (Echo.Callback.local callback_fn) @@ fun callback ->
  Echo.heartbeat service "foo" callback

let () = begin
    let service = Echo.local in
    run_client service

Step 1: The client creates the callback:

Step 2: The client calls the heartbeat method, passing the callback as an argument:

Step 3: The service receives the callback and calls the log method on it:

Exercise: implement Callback.local fn (hint: it's similar to the original ping service, but pass the message to fn and return with Service.return_empty ())

And testing it should give (three times, at one second intervals):

$ dune exec -- ./main.exe
Callback got "foo"
Callback got "foo"
Callback got "foo"

Note that the client gives the echo service permission to call its callback service by sending a message containing the callback to the service. No other access control updates are needed.

Note also a design choice here in the API: we could have made the Echo.heartbeat function take an OCaml callback and wrap it, but instead we chose to take a service and make do the wrapping. The advantage to doing it this way is that may one day want to pass a remote callback, as we'll see later.

This still isn't very exciting, because we just stored an OCaml object pointer in a message and then pulled it out again. However, we can use the same code with the echo client and service in separate processes, communicating over the network...


Let's put a network connection between the client and the server. Here's the new (the top half is the same as before):

open Lwt.Infix
open Capnp_rpc_lwt

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let callback_fn msg = "Callback got %S@." msg

let run_client service =
  Capability.with_ref (Echo.Callback.local callback_fn) @@ fun callback ->
  Echo.heartbeat service "foo" callback

let secret_key = `Ephemeral
let listen_address = `TCP ("", 7000)

let start_server () =
  let config = Capnp_rpc_unix.Vat_config.create ~secret_key listen_address in
  let service_id = Capnp_rpc_unix.Vat_config.derived_id config "main" in
  let restore = Capnp_rpc_net.Restorer.single service_id Echo.local in
  Capnp_rpc_unix.serve config ~restore >|= fun vat ->
  Capnp_rpc_unix.Vat.sturdy_uri vat service_id

let () = begin
    start_server () >>= fun uri -> "Connecting to echo service at: %a@." Uri.pp_hum uri;
    let client_vat = Capnp_rpc_unix.client_only_vat () in
    let sr = Capnp_rpc_unix.Vat.import_exn client_vat uri in
    Sturdy_ref.with_cap_exn sr run_client

You'll need to edit your dune file to add a dependency on capnp-rpc-unix in the (libraries ... line and also:

$ opam depext -i capnp-rpc-unix

Running this will give something like:

$ dune exec ./main.exe
Connecting to echo service at: capnp://sha-256:3Tj5y5Q2qpqN3Sbh0GRPxgORZw98_NtrU2nLI0-Tn6g@
Callback got "foo"
Callback got "foo"
Callback got "foo"

Once the server vat is running, we get a "sturdy ref" for the echo service, which is displayed as a "capnp://" URL. The URL contains several pieces of information:

  • The sha-256:3Tj5y5Q2qpqN3Sbh0GRPxgORZw98_NtrU2nLI0-Tn6g part is the fingerprint of the server's public key. When the client connects, it uses this to verify that it is connected to the right server (not an imposter). Therefore, a Cap'n Proto vat does not need to be certified by a CA (and cannot be compromised by a rogue CA).

  • is the address to which clients will try to connect to reach the server vat.

  • eBIndzZyoVDxaJdZ8uh_xBx5V1lfXWTJCDX-qEkgNZ4 is the (base64-encoded) service ID. This is a secret that both identifies the service to use within the vat, and also grants access to it.

The server side

The let secret_key = `Ephemeral line causes a new server key to be generated each time the program runs, so if you run it again you'll see a different capnp URL. For a real system you'll want to save the key so that the server's identity doesn't change when it is restarted. You can use let secret_key = `File "secret-key.pem" for that. Then the file secret-key.pem will be created automatically the first time you start the service, and reused on future runs.

It is also possible to disable the use of encryption using Vat_config.create ~serve_tls:false .... That might be useful if you need to interoperate with a client that doesn't support TLS.

listen_address tells the server where to listen for incoming connections. You can use `Unix path for a Unix-domain socket at path, or `TCP (host, port) to accept connections over TCP.

For TCP, you might want to listen on one address but advertise a different one, e.g.

let listen_address = `TCP ("", 7000)	(* Listen on all interfaces *)
let public_address = `TCP ("", 7000)	(* Tell clients to connect here *)

let start_server () =
  let config = Capnp_rpc_unix.Vat_config.create ~secret_key ~public_address listen_address in

In start_server:

  • let service_id = Capnp_rpc_unix.Vat_config.derived_id config "main" creates the secret ID that grants access to the service. derived_id generates the ID deterministically from the secret key and the name. This means that the ID will be stable as long as the server's key doesn't change. The name used ("main" here) isn't important - it just needs to be unique.

  • let restore = Restorer.single service_id Echo.local configures a simple "restorer" that answers requests for service_id with our Echo.local service.

  • Capnp_rpc_unix.serve config ~restore creates the service vat using the previous configuration items and starts it listening for incoming connections.

  • Capnp_rpc_unix.Vat.sturdy_uri vat service_id returns a "capnp://" URI for the given service within the vat.

The client side

After starting the server and getting the sturdy URI, we create a client vat and connect to the sturdy ref. The result is a proxy to the remote service via the network that can be used in exactly the same way as the direct reference we used before.

Separate processes

The example above runs the client and server in a single process, which is convenient for testing. To run them in separate processes we just need to split into separate files and add some command-line parsing to let the user pass the URL.

Edit the dune file to build a client and server:

 (names client server)
 (libraries lwt.unix capnp-rpc-lwt logs.fmt capnp-rpc-unix)
 (flags (:standard -w -53-55)))

 (targets echo_api.mli)
 (deps    echo_api.capnp)
 (action (run capnp compile -o %{bin:capnpc-ocaml} %{deps})))

Here's a suitable

open Lwt.Infix
open Capnp_rpc_net

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let cap_file = "echo.cap"

let serve config = begin
    let service_id = Capnp_rpc_unix.Vat_config.derived_id config "main" in
    let restore = Restorer.single service_id Echo.local in
    Capnp_rpc_unix.serve config ~restore >>= fun vat ->
    match Capnp_rpc_unix.Cap_file.save_service vat service_id cap_file with
    | Error `Msg m -> failwith m
    | Ok () -> "Server running. Connect using %S.@." cap_file;
      fst @@ Lwt.wait ()  (* Wait forever *)

open Cmdliner

let serve_cmd =
  let doc = "run the server" in
  let info = "serve" ~doc in
  Cmd.v info Term.(const serve $ Capnp_rpc_unix.Vat_config.cmd)

let () =
  exit (Cmd.eval serve_cmd)

The cmdliner term Capnp_rpc_unix.Vat_config.cmd provides an easy way to get a suitable Vat_config based on command-line arguments provided by the user.

And here's the corresponding

open Capnp_rpc_lwt

let () =
  Logs.set_level (Some Logs.Warning);
  Logs.set_reporter (Logs_fmt.reporter ())

let callback_fn msg = "Callback got %S@." msg

let run_client service =
  Capability.with_ref (Echo.Callback.local callback_fn) @@ fun callback ->
  Echo.heartbeat service "foo" callback

let connect uri = begin
    let client_vat = Capnp_rpc_unix.client_only_vat () in
    let sr = Capnp_rpc_unix.Vat.import_exn client_vat uri in
    Capnp_rpc_unix.with_cap_exn sr run_client

open Cmdliner

let connect_addr =
  let i = [] ~docv:"ADDR" ~doc:"Address of server (capnp://...)" in
  Arg.(required @@ pos 0 (some Capnp_rpc_unix.sturdy_uri) None i)

let connect_cmd =
  let doc = "run the client" in
  let info = "connect" ~doc in
  Cmd.v info Term.(const connect $ connect_addr)

let () =
  exit (Cmd.eval connect_cmd)

To test, start the server running:

$ dune exec -- ./server.exe \
    --capnp-secret-key-file key.pem \
    --capnp-listen-address tcp:localhost:7000
Server running. Connect using "echo.cap".

With the server still running in another window, run the client using the echo.cap file generated by the server:

$ dune exec ./client.exe echo.cap
Callback got "foo"
Callback got "foo"
Callback got "foo"

Note that we're using Capnp_rpc_unix.with_cap_exn here instead of Sturdy_ref.with_cap_exn. It's almost the same, except that it displays a suitable progress indicator if the connection takes too long.

Congratulations on finishing the main tutorial! You now know how to:

  • Define Cap'n Proto services and clients, independently of any networking.

  • Pass capability references in method arguments and results.

  • Stretch capabilities over a network link, with encryption, authentication and access control.

  • Configure a vat using command-line arguments.


Imagine we have a server with the following API:


interface Callback {
  log @0 (msg :Text) -> ();

interface Echo {
  ping      @0 (msg :Text) -> (reply :Text);
  heartbeat @1 (msg :Text, callback :Callback) -> ();
  getLogger @2 () -> (callback :Callback);

It's the same API we saw in the tutorial above, but extended with a logging service, which the client can get from the main echo service by calling getLogger.

The implementation of the new method in the service is simple - we export the callback in the response in the same way we previously exported the client's callback in the request:

    method get_logger_impl _ release_params =
      let open Echo.GetLogger in
      release_params ();
      let response, results = Service.Response.create Results.init_pointer in
      Results.callback_set results (Some service_logger);
      Service.return response

Exercise: create a service_logger that prints out whatever it gets (hint: use Callback.local) See examples/pipelining for the solution.

The client side is more interesting:

let get_logger t =
  let open Echo.GetLogger in
  let request = Capability.Request.create_no_args () in
  Capability.call_for_caps t method_id request Results.callback_get_pipelined

We could have used call_and_wait here (which is similar to call_for_value but doesn't automatically discard any capabilities in the result). However, that would mean waiting for the response to be sent back to us over the network before we could use it. Instead, we use callback_get_pipelined to get a promise for the capability from the promise of the getLogger call's result.

Note: the last argument to call_for_caps is a function for extracting the capabilities from the promised result. In the common case where you just want one and it's in the root result struct, you can just pass the accessor directly, as shown. Doing it this way allows call_for_caps to release any unused capabilities in the result automatically for us.

We can test it as follows:

let run_client service =
  let logger = Echo.get_logger service in
  Echo.Callback.log logger "Message from client" >|= function
  | Ok () -> ()
  | Error (`Capnp err) ->
    Fmt.epr "Server's logger failed: %a" Capnp_rpc.Error.pp err

This should print (in the server's output) something like:

$ dune exec ./main.exe
[client] Connecting to echo service...
[server] Received "Message from client"

In this case, we didn't wait for the getLogger call to return before using the logger. The RPC library pipelined the log call directly to the promised logger from its previous question. On the wire, the messages are sent together, and look like:

  1. What is your logger?

  2. Please call the object returned in answer to my previous question (1).

Now, let's say we'd like the server to send heartbeats to itself:

let run_client service =
  Capability.with_ref (Echo.get_logger service) @@ fun callback ->
  Echo.heartbeat service "foo" callback

Here, we ask the server for its logger and then (without waiting for the reply), tell it to send heartbeat messages to the promised logger (you should see the messages appear in the server's output).

Previously, when we exported our local callback object, it arrived at the service as a proxy that sent messages back to the client over the network. But when we send the (promise of the) server's own logger back to it, the RPC system detects this and "shortens" the path; the capability reference that the heartbeat handler gets is a direct reference to its own logger, which it can call without using the network.

These optimisations are very important because they allow us to build APIs like this with small functions that can be composed easily. Without pipelining, we would be tempted to clutter the protocol with specialised methods like heartbeatToYourself to avoid the extra round-trips most RPC protocols would otherwise require.

Guide to sturdy refs

A Capability.t is a "live" reference to a service, and is typically tied to a TCP connection. If the TCP connection fails, or the client or server is restarted, the capability becomes broken and can no longer be used. By contrast, a "sturdy ref" is an offline reference to a service, which can be saved as a URL in a file, emailed to someone, etc. A sturdy ref can be used to get a live ref when needed.

Hosting multiple sturdy refs

The Restorer.single restorer used in the tutorial above is useful for vats hosting a single sturdy ref. However, you may want to host multiple sturdy refs, perhaps to provide separate "admin" and "user" capabilities to different clients, or to allow services to be created and persisted as sturdy refs dynamically. To do this, we can use Restorer.Table.

examples/sturdy-refs is an example of this. It defines a simple API for a logging service:


interface Logger {
  log @0 (msg :Text) -> ();

Here is an example using it. The server writes out two cap files, for two different logger instances. Two different clients load these two files and send messages to the server:

let make_service ~config ~services name =
  let service = Logger.local name in
  let id = Capnp_rpc_unix.Vat_config.derived_id config name in
  Restorer.Table.add services id service;
  name, id

let start_server () =
  let config = Capnp_rpc_unix.Vat_config.create ~secret_key listen_address in
  let make_sturdy = Capnp_rpc_unix.Vat_config.sturdy_uri config in
  let services = Restorer.Table.create make_sturdy in
  let restore = Restorer.of_table services in
  let services = (make_service ~config ~services) ["alice"; "bob"] in
  Capnp_rpc_unix.serve config ~restore >|= fun vat ->
  services |> List.iter (fun (name, id) ->
      let cap_file = name ^ ".cap" in
      Capnp_rpc_unix.Cap_file.save_service vat id cap_file |> or_fail;
      Printf.printf "[server] saved %S\n%!" cap_file

let run_client cap_file msg =
  let vat = Capnp_rpc_unix.client_only_vat () in
  let sr = Capnp_rpc_unix.Cap_file.load vat cap_file |> or_fail in
  Printf.printf "[client] loaded %S\n%!" cap_file;
  Sturdy_ref.with_cap_exn sr @@ fun cap ->
  Logger.log cap msg

let () = begin
    start_server () >>= fun () ->
    run_client "./alice.cap" "Message from Alice" >>= fun () ->
    run_client "./bob.cap" "Message from Bob"
$ dune exec ./main.exe
[server] saved "alice.cap"
[server] saved "bob.cap"
[client] loaded "./alice.cap"
[server] "alice" says "Message from Alice"
[client] loaded "./bob.cap"
[server] "bob" says "Message from Bob"

Creating services dynamically

So far, we have been providing a static set of sturdy refs. We can also generate new services, with new sturdy refs, dynamically, and return them to clients. Let's allow holders of a logger to create nested sub-loggers. Then the admin can create alice.cap and bob.cap using the API, instead of hard-coding them, and Alice and Bob can create further delegate to other users as needed.


interface Logger {
  log @0 (msg :Text) -> ();
  sub @1 (label :Text) -> (logger :Logger);

To begin with, we'll just create live refs dynamically. We can use the new API like this:

let () = begin
    start_server () >>= fun root_uri ->
    let vat = Capnp_rpc_unix.client_only_vat () in
    let root_sr = Capnp_rpc_unix.Vat.import vat root_uri |> or_fail in
    Sturdy_ref.with_cap_exn root_sr @@ fun root ->
    Logger.log root "Message from Admin" >>= fun () ->
    let for_alice = Logger.sub root "alice" in
    let for_bob = Logger.sub root "bob" in
    Logger.log for_alice "Message from Alice" >>= fun () ->
    Logger.log for_bob "Message from Bob"
$ dune exec ./main.exe
[server] "root" says "Message from Admin"
[server] "root/alice" says "Message from Alice"
[server] "root/bob" says "Message from Bob"

The Persistence API

However, we'd like to be able to turn these live capabilities (for_alice and for_bob) into URLs that we can send to Alice and Bob over some other transport (e.g. ssh or email).

Cap'n Proto defines a standard Persistence API which services can implement to allow clients to request their sturdy ref.

On the client side, calling Persistence.save_exn cap will send a request to cap asking for its sturdy ref. For example, after getting a live capability, the admin can request the sturdy ref like this:

    (* The admin creates a logger for Alice and saves it: *)
    let for_alice = Logger.sub root "alice" in
    Persistence.save_exn for_alice >>= fun uri ->
    Capnp_rpc_unix.Cap_file.save_uri uri "alice.cap" |> or_fail;
    (* Alice uses it: *)
    run_client "alice.cap"

If successful, the client can use this sturdy ref to connect directly to the logger in future:

$ dune exec ./main.exe
[server] "root" says "Message from Admin"
[server] "root/alice" says "Message from Alice"

If you try the above, it will fail with Unimplemented: Unknown interface 14468694717054801553UL. To add support on the server side, we must tell each logger instance what its public address is and have it implement the persistence interface. The simplest way to do this is to wrap the Callback.local call with Persistence.with_sturdy_ref:

let rec local ~services sr label =
  let module Logger = Api.Service.Logger in
  Persistence.with_sturdy_ref sr Logger.local @@ object

Then pass the services and sr arguments when creating the logger:

  let root_id = Capnp_rpc_unix.Vat_config.derived_id config "root" in
  let root =
    let sr = Capnp_rpc_net.Restorer.Table.sturdy_ref services root_id in
    Logger.local ~services sr "root"

Persisting sturdy refs

Currently, each time we run the server we generate a new ID for Alice's logger (alice.cap will be different each time). For a real service, we will want to persist the IDs somehow.

Table.add is not a good choice for dynamic services because it requires all capabilities to be loaded into the table at start-up, which may be a performance problem.

Instead, we can create the table using Table.of_loader. When the user asks for a sturdy ref that is not in the table, it calls our load function to load the capability dynamically from storage (you can still use Table.add to register static services, as before).

A database such as sqlite3 is often a good choice for the dynamic services, but as Cap'n Proto is also a useful on-disk format, we'll just use that in this guide.

Here's the interface for a Db module that loads and saves loggers:

open Capnp_rpc_lwt
open Capnp_rpc_net

include Restorer.LOADER

type loader = [`Logger_beacebd78653e9af] Sturdy_ref.t -> label:string -> Restorer.resolution
(** A function to create a new in-memory logger with the given label and sturdy-ref. *)

val create : make_sturdy:(Restorer.Id.t -> Uri.t) -> string -> t * loader Lwt.u
(** [create ~make_sturdy dir] is a database that persists services in [dir] and
    a resolver to let you set the loader (we're not ready to set the loader
    when we create the database). *)

val save_new : t -> label:string -> Restorer.Id.t
(** [save_new t ~label] adds a new logger with label [label] to the store and
    returns its newly-generated ID. *)

There is a Capnp_rpc_unix.File_store module that can persist Cap'n Proto structs to disk. First, define a suitable Cap'n Proto data structure to hold the information we need to store. In this case, it's just the label:


struct SavedLogger {
  label @0 :Text;

struct SavedService {
  logger @0 :SavedLogger;

Using Cap'n Proto for this makes it easy to add extra fields or service types later if needed (SavedService.logger can be upgraded to a union if we decide to add more service types later). We can use this with File_store to implement Db:

open Lwt.Infix
open Capnp_rpc_lwt
open Capnp_rpc_net

module File_store = Capnp_rpc_unix.File_store
module Store = Store.Make(Capnp.BytesMessage)

type loader = [`Logger_beacebd78653e9af] Sturdy_ref.t -> label:string -> Restorer.resolution

type t = {
  store : Store.Reader.SavedService.struct_t File_store.t;
  loader : loader Lwt.t;
  make_sturdy : Restorer.Id.t -> Uri.t;

let hash _ = `SHA256

let make_sturdy t = t.make_sturdy

let save t ~digest label =
  let open Store.Builder in
  let service = SavedService.init_root () in
  let logger = SavedService.logger_init service in
  SavedLogger.label_set logger label; ~digest @@ SavedService.to_reader service

let save_new t ~label =
  let id = Restorer.Id.generate () in
  let digest = Restorer.Id.digest (hash t) id in
  save t ~digest label;

let load t sr digest =
  match File_store.load ~digest with
  | None -> Lwt.return Restorer.unknown_service_id
  | Some saved_service ->
    let logger = Store.Reader.SavedService.logger_get saved_service in
    let label = Store.Reader.SavedLogger.label_get logger in
    let sr = Capnp_rpc_lwt.Sturdy_ref.cast sr in
    t.loader >|= fun loader ->
    loader sr ~label

let create ~make_sturdy dir =
  let loader, set_loader = Lwt.wait () in
  if not (Sys.file_exists dir) then Unix.mkdir dir 0o755;
  let store = File_store.create dir in
  {store; loader; make_sturdy}, set_loader

Note: to avoid possible timing attacks, the load function is called with the digest of the service ID rather than with the ID itself. This means that even if the load function takes a different amount of time to respond depending on how much of a valid ID the client guessed, the client will only learn the digest (which is of no use to them), not the ID. The file store uses the digest as the filename, which avoids needing to check the ID the client gives for special characters, and also means that someone getting a copy of the store (e.g. an old backup) doesn't get the IDs (which would allow them to access the real service).

The main start_server function then uses Db to create the table:

let serve config = begin
    (* Create the on-disk store *)
    let make_sturdy = Capnp_rpc_unix.Vat_config.sturdy_uri config in
    let db, set_loader = Db.create ~make_sturdy "./store" in
    (* Create the restorer *)
    let services = Restorer.Table.of_loader (module Db) db in
    let restore = Restorer.of_table services in
    (* Add the root service *)
    let persist_new ~label =
      let id = Db.save_new db ~label in
      Capnp_rpc_net.Restorer.restore restore id
    let root_id = Capnp_rpc_unix.Vat_config.derived_id config "root" in
    let root =
      let sr = Capnp_rpc_net.Restorer.Table.sturdy_ref services root_id in
      Logger.local ~persist_new sr "root"
    Restorer.Table.add services root_id root;
    (* Tell the database how to restore saved loggers *)
    Lwt.wakeup set_loader (fun sr ~label -> Restorer.grant @@ Logger.local ~persist_new sr label);
    (* Run the server *)
    Capnp_rpc_unix.serve config ~restore >>= fun _vat ->
    let uri = Capnp_rpc_unix.Vat_config.sturdy_uri config root_id in
    Capnp_rpc_unix.Cap_file.save_uri uri "admin.cap" |> or_fail;
    print_endline "Wrote admin.cap";
    fst @@ Lwt.wait () (* Wait forever *)

The server implementation of the sub method gets the label from the parameters and uses persist_new to save the new logger to the database:

    method sub_impl params release_param_caps =
      let open Logger.Sub in
      let sub_label = Params.label_get params in
      release_param_caps ();
      let label = Printf.sprintf "%s/%s" label sub_label in
      Service.return_lwt @@ fun () ->
      persist_new ~label >|= function
      | Error e -> Error (`Capnp (`Exception e))
      | Ok logger ->
        let response, results = Service.Response.create Results.init_pointer in
        Results.logger_set results (Some logger);
        Capability.dec_ref logger;
        Ok response
$ dune exec -- ./main.exe serve --capnp-secret-key=server.key --capnp-listen-address unix:/tmp/demo.sock &
Wrote admin.cap

$ dune exec -- ./main.exe log admin.cap "Hello from Admin"
[server] "root" says "Hello from Admin"

$ dune exec -- ./main.exe sub admin.cap alice
Wrote "alice.cap"

$ dune exec -- ./main.exe log alice.cap "Hello from Alice"
[server] "root/alice" says "Hello from Alice"

$ dune exec -- ./main.exe sub alice.cap bob
Wrote "bob.cap"

$ dune exec ./main.exe log bob.cap "Hello from Bob"
[server] "root/alice/bob" says "Hello from Bob"

You should find that the loggers now persist even if the service is restarted.

Further reading


Why does my connection stop working after 10 minutes?

Cap'n Proto connections are often idle for long periods of time, and some networks automatically close idle connections. To avoid this, capnp-rpc-unix sets the SO_KEEPALIVE option when connecting to another vat, so that the initiator of the connection will send a TCP keep-alive message at regular intervals. However, TCP keep-alives are sent after the connection has been idle for 2 hours by default, and this isn't frequent enough for e.g. Docker's libnetwork, which silently breaks idle TCP connections after about 10 minutes.

A typical sequence looks like this:

  1. A client connects to a server and configures a notification callback.

  2. The connection is idle for 10 minutes. libnetwork removes the connection from its routing table.

  3. Later, the server tries to send the notification and discovers that the connection has failed.

  4. After 2 hours, the client sends a keep-alive message and it too discovers that the connection has failed. It establishes a new connection and retries.

On some platforms, capnp-rpc-unix (>= 0.9.0) is able to reduce the timeout to 1 minute by setting the TCP_KEEPIDLE socket option. On other platforms, you may have to configure this setting globally (e.g. with sudo sysctl net.ipv4.tcp_keepalive_time=60).

How can I return multiple results?

Every Cap'n Proto method returns a struct, although the examples in this README only use a single field. You can return multiple fields by defining a method as e.g. -> (foo :Foo, bar :Bar). For more complex types, it may be more convenient to define the structure elsewhere and then refer to it as -> MyResults.

Can I create multiple instances of an interface dynamically?

Yes. e.g. in the example above we can use Callback.local fn many times to create multiple loggers. Just remember to call Capability.dec_ref on them when you're finished so that they can be released promptly (but if the TCP connection is closed, all references on it will be freed anyway). Using Capability.with_ref makes it easier to ensure that dec_ref gets called in all cases.

Can I get debug output?

First, always make sure logging is enabled so you can at least see warnings. The examples in this document enable some basic logging.

If you turn up the log level to Debug, you'll see lots of information about what is going on. Turning on colour in the logs will help too - see test-bin/ for an example.

Many references will be displayed with their reference count (e.g. as rc=3). You can also print a capability for debugging with Capability.pp.

CapTP.dump will dump out the state of an entire connection, which will show you what services you’re currently importing and exporting over the connection.

If you override your service’s pp method, you can include extra information in the output too. Use Capnp_rpc.Debug.OID to generate and display a unique object identifier for logging.

How can I debug reference counting problems?

If a capability gets GC'd with a non-zero ref-count, you should get a warning. For testing, you can use Gc.full_major to force a check.

If you try to use something after releasing it, you'll get an error.

But the simple rule is: any time you create a local capability or extract a capability from a message, you must eventually call Capability.dec_ref on it.

How can I import a sturdy ref that I need to start my vat?

Let's say you have a capnp service that internally requires the use of another capnp service:

Here, creating the Frontend service requires a sturdy ref for the Backend service. But this sturdy ref must be imported into the frontend vat. Creating the frontend vat requires passing a restorer, which needs Frontend!

The solution here is to construct Frontend with a promise for the sturdy ref, e.g.

let run_frontend backend_uri =
  let backend_promise, resolver = Lwt.wait () in
  let frontend = Frontend.make backend_promise in
  let restore = Restorer.single id frontend in
  Capnp_rpc_unix.serve config ~restore >|= fun vat ->
  Lwt.wakeup resolver (Capnp_rpc_unix.Vat.import_exn vat backend_uri)

How can I release other resources when my service is released?

Override the release method. It gets called when there are no more references to your service.

Is there an interactive version I can use for debugging?

The Python bindings provide a good interactive environment. For example, start the test service above and leave it running:

$ ./_build/default/main.exe
Connecting to server at capnp://insecure@

Note that you must run without encryption for this, and use a non-secret ID:

let config = Capnp_rpc_unix.Vat_config.create ~serve_tls:false ~secret_key listen_address in
let service_id = Restorer.Id.public "" in

Run python from the directory containing your echo_api.capnp file and do:

import capnp
import echo_api_capnp
client = capnp.TwoPartyClient('')
echo = client.bootstrap().cast_as(echo_api_capnp.Echo)

Importing a module named foo_capnp will load the Cap'n Proto schema file foo.capnp.

To call the ping method:"From Python").wait()
<$Results reader (reply = "echo:From Python")>

To call the heartbeat method, with results going to the server's own logger:

echo.heartbeat("From Python", echo.getLogger().callback).wait()
Service logger: "From Python"

To call the heartbeat method, with results going to a Python callback:

class CallbackImpl(echo_api_capnp.Callback.Server):
    def log(self, msg, _context): print("Python callback got %s" % msg)

echo.heartbeat("From Python", CallbackImpl())
Python callback got From Python
Python callback got From Python
Python callback got From Python

Note that calling wait_forever prevents further use of the session, however.

Can I set up a direct 2-party connection over a pre-existing channel?

The normal way to connect to a remote service is using a sturdy ref, as described above. This uses the NETWORK to open a new connection to the server, or reuses an existing connection if there is one. However, it is sometimes useful to use a pre-existing connection directly.

For example, a process may want to spawn a child process and communicate with it over a socketpair. The example shows how to do this:

$ dune exec -- ./test-bin/calc_direct.exe
parent: application: Connecting to child process...
parent: application: Sending request...
 child: application: Serving requests...
 child: application: 21.000000 op 2.000000 -> 42.000000
parent: application: Result: 42.000000
parent: application: Shutting down...
parent:   capnp-rpc: Connection closed
parent: application: Waiting for child to exit...
parent: application: Done

How can I use this with Mirage?

Note: capnp uses the stdint library, which has C stubs and might need patching to work with the Xen backend. explains why OCaml doesn't have unsigned integer support.

Here is a suitable

open Mirage

let main =
    ~packages:[package "capnp-rpc-mirage"; package "mirage-dns"]
    "Unikernel.Make" (random @-> mclock @-> stackv4 @-> job)

let stack = generic_stackv4 default_network

let () =
  register "test" [main $ default_random $ default_monotonic_clock $ stack]

This should work as the

open Lwt.Infix

open Capnp_rpc_lwt

module Make (R : Mirage_random.S) (C : Mirage_clock.MCLOCK) (Stack : Mirage_stack.V4) = struct
  module Mirage_capnp = Capnp_rpc_mirage.Make (R) (C) (Stack)

  let secret_key = `Ephemeral

  let listen_address = `TCP 7000
  let public_address = `TCP ("localhost", 7000)

  let start () () stack =
    let dns = Mirage.Network.Dns.create stack in
    let net = ~dns stack in
    let config = Mirage_capnp.Vat_config.create ~secret_key ~public_address listen_address in
    let service_id = Mirage_capnp.Vat_config.derived_id config "main" in
    let restore = Restorer.single service_id Echo.local in
    Mirage_capnp.serve net config ~restore >>= fun vat ->
    let uri = Mirage_capnp.Vat.sturdy_uri vat service_id in (fun f -> f "Main service: %a" Uri.pp_hum uri);
    Lwt.wait () |> fst


Conceptual model

An RPC system contains multiple communicating actors (just ordinary OCaml objects). An actor can hold capabilities to other objects. A capability here is just a regular OCaml object pointer.

Essentially, each object provides a call method, which takes:

  • some pure-data message content (typically an array of bytes created by the Cap'n Proto serialisation), and

  • an array of pointers to other objects (providing the same API).

The data part of the message says which method to invoke and provides the arguments. Whenever an argument needs to refer to another object, it gives the index of a pointer in the pointers array.

For example, a call to a method that transfers data between two stores might look something like this:

- Content:
  - InterfaceID: xxx
  - MethodID: yyy
  - Params:
    - Source: 0
    - Target: 1
- Pointers:
  - <source>
  - <target>

A call also takes a resolver, which it will call with the answer when it's ready. The answer will also contain data and pointer parts.

On top of this basic model the Cap'n Proto schema compiler (capnp-ocaml) generates a typed API, so that application code can only generate or attempt to consume messages that match the schema. Application code does not need to worry about interface or method IDs, for example.

This might seem like a rather clumsy system, but it has the advantage that such messages can be sent not just within a process, like regular OCaml method calls, but also over the network to remote objects.

The network is made up of communicating "vats" of objects. You can think of a Unix process as a single vat. The vats are peers - there is no difference between a "client" and a "server" at the protocol level. However, some vats may not be listening for incoming network connections, and you might like to think of such vats as clients.

When a connection is established between two vats, each can choose to ask the other for access to some service. Services are usually identified by a long random secret (a "Swiss number") so that only authorised clients can get access to them. The capability they get back is a proxy object that acts like a local service but forwards all calls over the network. When a message is sent that contains pointers, the RPC system holds onto the pointers and makes each object available over that network connection. Each vat only needs to expose at most a single bootstrap object, since the bootstrap object can provide methods to get access to any other required services.

All shared objects are scoped to the network connection, and will be released if the connection is closed for any reason.

The RPC system is smart enough that if you export a local object to a remote service and it later exports the same object back to you, it will switch to sending directly to the local service (once any pipelined messages in flight have been delivered).

You can also export an object that you received from a third-party, and the receiver will be able to use it. Ideally, the receiver should be able to establish a direct connection to the third-party, but this isn't yet implemented and instead the RPC system will forward messages and responses in this case.


To build:

git clone
cd capnp-rpc
opam pin add -ny .
opam depext -t capnp-rpc-unix capnp-rpc-mirage
opam install --deps-only -t .
make test

If you have trouble building, you can use the Dockerfile shown in the CI logs (click the green tick on the main page).


Running make test will run through the tests in test-lwt/, which run some in-process examples.

The calculator example can also be run across two Unix processes.

Start the server with:

$ dune exec -- ./test-bin/calc.exe serve \
    --capnp-listen-address unix:/tmp/calc.socket \
Waiting for incoming connections at:

Note that key.pem does not need to exist. A new key will be generated and saved if the file does not yet exist.

In another terminal, run the client and connect to the address displayed by the server:

dune exec -- ./test-bin/calc.exe connect capnp://sha-256:LPp-7l74zqvGcRgcP8b7-kdSpwwzxlA555lYC8W8prc@/tmp/calc.socket/

You can also use --capnp-disable-tls if you prefer to run without encryption (e.g. for interoperability with another Cap'n Proto implementation that doesn't support TLS). In that case, the client URL would be capnp://insecure@/tmp/calc.socket.


Running make fuzz will run the AFL fuzz tester. You will need to use a version of the OCaml compiler with AFL support (e.g. opam sw 4.04.0+afl).

The fuzzing code is in the fuzz directory. The tests set up some vats in a single process and then have them perform operations based on input from the fuzzer. At each step it selects one vat and performs a random (fuzzer-chosen) operation out of:

  1. Request a bootstrap capability from a random peer.

  2. Handle one message on an incoming queue.

  3. Call a random capability, passing randomly-selected capabilities as arguments.

  4. Finish a random question.

  5. Release a random capability.

  6. Add a capability to a new local service.

  7. Answer a random question, passing random-selected capability as the response.

The content of each call is a (mutable) record with counters for messages sent and received on the capability reference used. This is used to check that messages arrive in the expected order.

The tests also set up a shadow reference graph, which is like the regular object capability reference graph except that references between vats are just regular OCaml pointers (this is only possible because all the tests run in a single process, of course). When a message arrives, the tests compare the service that the CapTP network handler selected as the target with the expected target in this simpler shadow network. This should ensure that messages always arrive at the correct target.

In future, more properties should be tested (e.g. forked references, that messages always eventually arrive when there are no cycles, etc). We should also test with some malicious vats (that don't follow the protocol correctly).

Dependencies (13)

  1. lwt >= "5.6.1"
  2. mirage-crypto-rng-lwt >= "0.11.0" & < "1.0.0"
  3. dune >= "3.0"
  4. base64 >= "3.0.0"
  5. extunix
  6. logs
  7. fmt >= "0.8.7"
  8. astring
  9. cstruct-lwt
  10. cmdliner >= "1.1.0"
  11. mirage-flow < "4.0.0"
  12. capnp-rpc-net = version
  13. ocaml >= "4.08.0"

Dev Dependencies (4)

  1. asetmap with-test
  2. mdx with-test & >= "2.2.1" & < "2.4"
  3. alcotest-lwt >= "1.6.0" & with-test
  4. alcotest >= "1.6.0" & with-test

Conflicts (1)

  1. jbuilder

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