package capnp-rpc-lwt

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


Dune Dependency






Bug fixes and diagnostics:

  • Don't GC imports with callbacks (@talex5, #210).

  • Improve "Already resolved!" error message (@talex5, #209). Show the state of the promise we were trying to set, and what we tried to set it to.

  • Reject attempts to send after disconnecting (@talex5, #208).

  • Unix: don't leak FDs if Network.connect fails (@talex5, #206). Also, use Lwt to open connections so we don't block the whole process.

New functions:

  • Add Sturdy_ref.with_cap and with_cap_exn convenience functions (@talex5, #207). Using the plain connect functions, it's easy to forget to release the live-ref at the end.

Build changes:

  • capnp-rpc-mirage: adapt test to tcpip 5.0.0 API (@hannesm, #205).

  • Upgrade to dune 2 (@talex5, #204).

  • Switch tests to build in native code and use the test stanza (@avsm, #203).


  • Show how to make a connection directly (@talex5, #202). The new test-bin/ example shows how a parent process can spawn a child and communicate with it directly over a socketpair.


  • Update for x509 0.11.0 API changes (@talex5, #196).

  • Update to new mirage network API (@Cjen1, #198).

  • Add echo benchmark test (@Cjen1, #197).

  • Estimate message sizes to improve performance (@talex5, #200). By default, capnproto allocates 8k buffers, but most capnp-rpc messages are much smaller than this.


  • Fix application logging, to use library's log (@Cjen1, #195).

  • Expose the endpoint logger (@Cjen1, #195).

  • Only enable debug logging for capnp libraries in the calc example. TLS now generates a lot of messages at debug level (@talex5, #200).


  • Port to latest interfaces for x509 (0.10+), mirage-crypto, alcotest (1.0+), and mirage-types post the -lwt package merge (@avsm #190, review by @talex5 @hannesm)

  • Convert many info-level log messages to debug level (@talex5 #193)


Breaking changes:

  • The networking parts of capnp-rpc-lwt have been split off into a new capnp-rpc-net package (@talex5, #182). This means that libraries that provide or consume capnp-rpc services can just depend on capnp-rpc-lwt, without pulling in a full TLS stack. Only applications should need to depend on capnp-rpc-net (which will probably happen automatically via capnp-rpc-unix or capnp-rpc-mirage). If you get compile errors in your code due to this change, you probably just need to open Capnp_rpc_net.

Other changes:

  • Add Prometheus metrics for connections (@talex5, #183). capnp-rpc-net now reports the current number of active connections, the total number of messages received, and the total number of messages enqueued, sent and dropped (from which you can work out the current number of queued messages).

  • Adapt to x509 0.8.x API changes (@hannesm, #176).

  • In the tutorial, say where to put the Callback module (@talex5, #177).

  • "No client certificate found" should not be fatal (@talex5, #178).

  • Remove unused dependency on mirage-flow-unix from opam file (@talex5, #179).


Breaking changes:

  • Wrap errors with the `Capnp tag to make it easier to compose with other types of error (#172, #173).

  • Prefix all command-line options with capnp- (#163). e.g. --listen-address is now --capnp-listen-address. The old names were confusing for applications that supported other protocols too (e.g. a web server).

New features:

  • Add Capability.with_ref convenience function (#170). This automatically calls dec_ref when done.

  • Add Unix Cap_file module to load and save Sturdy_refs (#165). In particular, this ensures that saved cap files get a mode of 0o600, since they contain secrets.

  • Export cmdliner network address parsing (#165). This is useful if you don't want to use the default option parsing. For example, if you want to make Cap'n Proto an optional feature of your program.

  • Upgrade from uint (which is deprecated) to the newer stdint (#166, #168). The latest version of uint is just a wrapper around stdint, so this shouldn't break anything if you are using the latest version.

  • Put cmdliner options in their own man-page section (#163). Use Capnp_rpc_unix.manpage_capnp_options to control where in your man-page they appear.

  • Enable SO_KEEPALIVE for TCP connections (#167). For use with Docker's libnetwork, try something like this in your stack.yml:

      - 'net.ipv4.tcp_keepalive_time=60'

Bug fixes:

  • Close listening socket when shutting down a vat (#171).

  • Don't mark secret keys as executable (#164).

  • Update README example to use dune (#162).

Build changes:

  • Replace topkg with dune-release (#169)

  • Update opam email address and fix missing bound (#161).

  • Update the dune files to allow duniverse / vendored builds (#165).

  • Fix the crossed-calls unit test (#171).

  • Force all capnp-rpc subpackages to have the same version (#173).


  • Update uint.uint32 to uint (@Cjen1, #159).

  • Update to new x509 API (@talex5, #158).

  • Require base64 >= 3.0.0 for capnp-rpc-mirage too (@talex5, #157).

  • Put test sockets in temporary directory (@talex5, #156).


  • Update for various upstream API changes, switch to the opam 2 metadata format, and convert from jbuilder to dune (@talex5, #152).

  • Adjust to mirage-stack / mirage-protocols changes (Nick Betteridge, #151).

    • update mirage/network for upgraded Ipaddr

    • update Dockerfile to use opam2, apt-get update, and newer opam-repository

  • Update dependencies from opam-repository (@talex5, #148).


  • Updates for new x509 API and for OCaml 4.06 (#143).

  • Add some diagrams to the tutorial (#134).

  • Add FAQ: how can I import a sturdy ref that I need to start my vat? (#137)

Build updates:

  • Add build dependency on conf-capnproto (#146). Projects using the schema compiler themselves should also now add this dependency instead of relying on capnp to pull it in.

  • Remove generics from persistent.capnp (#141) so that it compiles on systems with older capnproto compilers (e.g. Ubuntu 14.04).

  • unix/ uses Fmt.failwith, which requires fmt.0.8.4 (#139).

  • capnp-rpc-lwt requires Uri.with_userinfo, which is only in uri >= 1.6.0 (#138).

  • Move test-lwt to unix module (#133).

0.3 Unikernels

This release adds a new capnp-rpc-mirage package, which provides support for using the library within a MirageOS unikernel. See for details.

There are a few minor API changes:

  • Capnp_rpc_unix.Vat_config.derived_id ?name config is now Capnp_rpc_unix.Vat_config.derived_id config name. If you weren't passing a ~name argument before, use "main" to get the same ID.

  • Capnp_rpc_unix.Network's Socket_address module is now called Location.

  • There is an explicit network parameter in Network.connect, etc. This is needed to support Mirage, where the network isn't a global.

Bug fixes:

  • Fix race when reconnecting. We notified the user that the capability had broken while the old connection was still shutting down. If they immediately tried to reconnect, we tried to reuse the old connection. Now, we wait for it to be removed.

  • Fix handling of leaks in switchable. If we detected the ref-count was invalid, we tried to resolve to an error, but resolving now checks that the ref-count is valid first so this failed.

Documentation and examples:

  • Fixed ref-counting bug in calculator example. Also, changed the service ID to match what the C++ client expects. With these changes, the C++ client's tests pass when used with the OCaml service.


  • Also test answering questions with errors or with a promise from another question.

Code cleanups:

  • Use a better way to get the client certificate from a TLS connection (suggested by @hannesm).

  • Use Alcotest_lwt for unit-tests.

  • Move capnp:// URI handling to Capnp_rpc_lwt.Capnp_address. This allows it to be shared with the Mirage code.

  • Add Capnp_rpc_lwt.VAT_NETWORK with simpler signature than S.VAT_NETWORK.

  • The address sub-module of S.NETWORK is now available separately as S.ADDRESS.

0.2 Persistence, encryption and access control

This release brings support for RPC Level 2.

The API for implementing services and clients is mostly unchanged, but the APIs for setting up networking are very different. If you read the tutorial with the 0.1 release, you will probably want to read the new version again from the "Networking" point onwards.

The main change is that when connecting to a service you now give a URI of the form:


The client will connect to address, check the server's public key matches hash:digest, and then pass the (secret) service ID to get access to a particular service. The server will typically display the URI to use on start-up, or write it to a file.

The communications are encrypted using TLS. If you want to disable TLS, use the form capnp://insecure@address. This should only be needed for interoperability with non-TLS services, as the system will generate keys and certificates automatically, making secure use just as easy as the non-secure case.

The other major new feature is support for persistent services. In version 0.1 you could specify an offer argument when creating a vat, telling it a service to provide in response to bootstrap requests. Now, you pass a restore argument, which can restore different services depending on the service ID provided by the client.

The new Restorer.Table module provides a table-based lookup restorer, to which services can be added dynamically. If you have a lot of services and don't want to add them all at startup, you can use Restorer.Table.of_loader and provide your own function for loading services.

Other changes

Documentation changes:

  • The tutorial has been extended and a FAQ added.

  • The recommended layout of protocol files has changed. The Client sub-module is gone, and service becomes local.

  • The examples now have .mli files and there is a new example demonstrating persistence. The examples have been updated to the new layout convention.

Main API changes:

  • The Capnp_rpc_lwt.Capability module adds some useful functions:

    • broken creates a broken capability.

    • when_broken allows you to be notified when a capability breaks (e.g. because of a network failure).

    • wait_until_settled waits until a promise has resolved, if you don't want to pipeline (e.g. you want to send a large amount of data, so prefer to find out where the service is and avoid any forwarding).

    • equal tests if two capabilities designate the same service.

  • The new Capnp_rpc_lwt.Sturdy_ref module provides an abstraction for off-line capabilities. Sturdy_ref.connect can be used to get a live connection. If you try to connect to multiple services in the same vat, it will share a single connection automatically. Sturdy_ref.reader and Sturdy_ref.builder can be used for passing sturdy refs in messages.

  • The new Capnp_rpc_lwt.Restorer module is used to implement sturdy-refs at the hosting side.

  • The new Capnp_rpc_lwt.Persistence module provides support for the Cap'n Proto persistence protocol. Clients use to request a sturdy ref from a service, and services can use Persistence.with_sturdy_ref to answer such requests automatically.

  • The new Capnp_rpc_unix.Vat_config collects together all vat configuration in one place.

  • The new Capnp_rpc_unix.File_store can store Cap'n Proto structs in a directory. It can be useful when implementing persistence.

  • The new Capnp_rpc_lwt.Auth module provides support for generating and handling secret keys and fingerprints.

  • The new Capnp_rpc_lwt.Tls_wrapper provides support for doing TLS handshakes, with authentication and encryption.

In the core capnp-rpc package (which applications do not normally use directly):

  • The new cap#when_released method can be used to find out when a capability is no longer needed. The restorer uses this to remove dynamically loaded services from its table automatically.

  • The new when_broken helper adds a callback to call when a promise or far-ref becomes broken.

  • NETWORK_TYPES is now separate from CORE_TYPES.


  • Initial release. RPC Level 1 is fully implemented.


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