api.fabaccess-api/resource.capnp

@0x8c2f829df1930cd5;

using CSharp = import "programming_language/csharp.capnp";
$CSharp.namespace("FabAccessAPI.Schema");

using Persistent = import "/capnp/persistent.capnp".Persistent;
using Value = import "/capnp/schema.capnp".Value;

using User = import "user.capnp".User;
using L10NString = import "utils.capnp".L10NString;
using UUID = import "utils.capnp".UUID;

struct Resource {
    # BFFH's smallest unit of a physical or abstract "thing".
    # A resource can be as simple and physical as a table, as complex as a PCB production line or as
    # abstract as "people with specific know-how are present".

    uuid @0 :UUID;
    # An stable, globally unique descriptor for a resource. Two resources with the same UUID are
    # (almost¹) guaranteed to be the same instance, and the UUID of a resource will survive through
    # server restarts, renaming, reconfiguration etc.
    #
    # [¹]: UUID are 128-bit integer. A collision is *possible*, just *very* unlikely. If you
    # generate 1 billion UUID every second for the next 100 years you have a chance for a collision
    # of about 50%.


    id @1 :Text;
    # Every resource in BFFH has a human-readable "name" that is locally unique, but not persistent.
    # That is a resource called "hello" today may be called "bye" tomorrow and a resource called
    # "hi~~" may not be the same resource as the resource called "hi~~" yesterday.  This name is
    # canonical and thus identifying. There is exactly *one* valid representation of this name at
    # any given point in time. Thus this name can also not be translated.

    name @2 :L10NString;
    # A resource may also have a human-meaningful name that is designed to be shown to users. This
    # name does not have to be unique or identifiable or canonical, its main use is to be
    # human-meaningful. For example a "name" could be the translations:
    # - (en, "Prusa SL1 SLA-Printer")
    # - (de, "Prusa SL1 SLA-Drucker")
    # - (es, "Impresora 3D de SLA Prusa SL1")

    description @3 :L10NString;
    # A resource may have a description attached to tell an user some more information on a resource
    # in a free-form format.
    # Similar to the human-meaningful name this description can be translated.

    notify @4 :Notify;
    # Readonly access to the state of a resource.
    # A resource can have "state". State are values attached to a resource that describe a specific
    # state that users or administrators want this resource to be in. Usually this state consists of
    # a number of primitive values encoding for example "turned on" or "turned off".
    # Users with the required permissions may change this state.

    claimable @5 :Claimable;
    # Writeable access to the state of a resource.
    # Resources are semaphores. They allow writeable access for n ∈ ℕ\{0} clients, depending on the
    # exact resource in question. In some cases n ≔ 1, and the only write access is exclusive.
    # "Claims" model this by requiring a client to first assert a claim, thus reserving a semaphore
    # slot or failing if no more are available, and then using this claim to write to a resources
    # state.

    interestable @6 :Interestable;
    # Sometimes clients are not just interested in the state of a resource but rather want a
    # resource to stay in a specific state. e.g. somebody working in a makerspace wants the space to
    # stay open, even though they themselves may not have permission to keep the makerspace open.
    # "Interest" represents this. Specifically right now it tells BFFH that the client wants at
    # least one `Claim` to remain.
}

struct Map(Key, Value) {
    # A serialized key-value map represented as a list of (k,v) tuples.

    entries @0 :List(Entry);

    struct Entry {
        key @0 :Key;
        val @1 :Value;
    }
}
using State = Map(Text, Value);
# Update state provided to a resource via a claim is represented as a Map of human-readable
# identifiers to Cap'n Proto Values. These Values can be either primitive types such as Uint8,
# Float64 or more complex types such as structs, lists, or enums.
# The resulting state of a resource, which is the output of whatever internal logic the resource
# implements, is also represented in this form, but the keys and also values may be different.
#
# Later on very common cases (use, register, return, etc.) can get shortcut functions in the Claim
# interface that pre-emptively check permissions and ability (so you get the respective cap iff the
# resource supports that update and if you're allowed to do that) but these functions only serve to
# make the update more efficient than calling `update` with the string identifier and dynamic typed
# value but do the exact same serverside as an `update` call would. This way we can make future
# versions of the API more efficient and easier to use while not breaking compatibility with old
# clients.
# 
# TODO: This has the potential problem that a newer client can not distinguish between a server
# using an old version of the API and a client simply not being allowed to call a specific shortcut
# method because in both cases that cap will be a nullptr. Could be solved by making `Claim` a
# struct and indicating which shortcut methods it knows of. 
# Not sure if this is a big problem, we optimize for old clients and up-to-date servers.
#
# TODO: We should provide a number of sensible implementations for common complex `Value` types such
# as "colour", "temperature", etc. and define identifiers for common values.

interface Access {
    # Allow syncronous read access to a resource's output state. You're not given this capability
    # directly but instead Notify, Interest and Claim all extend it, allowing you to call these
    # methods on any of those.

    readOutput @0 State;

    # TODO: There should probably be a more efficient approach for reading state than "read *all*
    # state".
}

interface Notify extends(Access) {
    # The Notify interface allows clients to be informed about state changes asyncronously.  It is
    # mainly designed around the `register` function which allows a client to register a callback
    # that is called every time state changes happen to the resource in question.
    # 
    # Notify are ephermal. If the connection to the server is lost all `Notify` from that client are
    # unregistered.

    register @0 ( cb :Callback );
    # Register a given callback to be called on every state update. If this client already has a
    # callback registered for this resource the old callback is replaced.

    unregister @1 ();
    # Unregister this callback

    interface Callback {
        # This callback interface needs to be implemented on the client

        newState @0 State;
        # A server will call newState() with the updated output state. However a server will only
        # allow one in-flight call, so as long as the previous call to newState() hasn't completed
        # the server will drop intermediary updates as to not overload a client.
        # Specifically, example timeline:
        #    1. Update A 
        #    2. Server calls newState(A) 
        #    3. Update B 
        #    4. Update C 
        #    5. Call to newState(A) completes
        #    6. Server calls newState(C)
        # So Update B was never sent to the client but the client will eventually always end up with
        # the latest state.

        # TODO: There should probably be a more efficient approach here too, something along the
        # lines of server-side filtering.
    }
}

interface Interestable {
    # "Interest" right now it tells BFFH that the client wants at least one `Claim` to remain.

    register @0 ( cb :Callback ) -> ( handle :Handle );
    # Register a callback that BFFH will use to send notifications back to the client asyncronously.
    # This creates an "Interest" on this resource. Setting the callback to a `nullptr` will still
    # register an interest but the server will not be able to inform a client about an impeding
    # claim drop.

    blocking @1 ();
    # As an alternative to the `register`/`Callback` system you can also call `blocking` which will
    # — as the name suggests — block until the last claim is being dropped. This will register an
    # ephermal Interest that can not survive a disconnect.

    interface Callback {
        drop @0 ();
        # The last claim on the resource this Interest is registered is being dropped, invalidating
        # the Interest.
    }

    interface Handle {
        # A Handle back to the server side Interest registered. Destroying this capability will also
        # inform the server and remove the Interest again.
        # TODO: `extends (Persistance)` so that clients can `save` this capability and thus make the
        #       Interest survive disconnects.

    }
}


interface Claimable {
    # Having this capability set (i.e. not be a `nullptr`) means the user has at least writeable
    # access to a resource and the resource is claimable (n > 0).

    claim @0 () -> ClaimResponse;
    # Assert a claim on a resource.

    struct ClaimResponse {
        enum Error {
            # Error describing why a claim failed.

            exhausted @0;
            # There are no more free Claim slots

            locked @1;
            # The resource was locked

            precondition @2;
            # Some precondition was not met

            dependencies @3;
            # Resource failed to secure dependencies
        }

        union {
            failed :group {
                error @0 :Error;
                reason @1 :L10NString;
            }
            success @2 :Claim;
        }
    }
}

interface Claim extends(Access) {
    # TODO: extend Persistance. Claims and Interests need to be able to survive a connection loss,
    # which is exactly what `SturdyRef`/Persistance are designed to provide. The Persistance
    # interface only provides one method, `save`, returning a `SturdyRef`. A SturdyRef is a generic
    # and generally speaking opaque type that can be restored to a live capability using some sort
    # of `Restorer` service.
    # In this case the `Restorer` service could be `Claimable` / `Interestable` providing a 
    # `restore( ref: SturdyRef )` method.

    readInput @0 () -> State;
    # Get the current *input* state. This is not the output state that `Notify` or Actors get access
    # to but instead the currently stored input state of a resource.

    update @1 State -> UpdateResult;
    # Update the State of the claimed resource with the given one

    struct UpdateResult {
        enum Error {
            # Reason why the update failed 

            denied @0;
            # Update was denied beause user is missing an required permission

            precondition @1;
            # Some other precondition failed, e.g. because a required field is not set

            invalid @2;
            # The update is invalid, e.g. because an unknown field was set.

            typeError @3;
            # A field in the update has a known identifier but a bad type for that identifier
        }

        union {
            failed :group {
                error @0 :Error;
                field @1 :Text;
                reason @2 :L10NString;
            }
            success @3 :Void;
        }
    }
}