Hi, I'm trying to write an implementation of the RPC spec (level 1, in OCaml). I found a few parts of the spec unclear - could someone clarify them for me?
It says: [ExportId] > The exporter chooses an ID before sending a capability over the wire. If > the capability is already in the table, the exporter should reuse the same ID. But later: [CapDescriptor] > senderHosted @1 :ExportId; > A capability newly exported by the sender. This is the ID of the new capability in the > sender's export table (receiver's import table). How can the exporter reuse the same ID, if it has to be newly exported? [Message] > This could be e.g. because the sender received an invalid or nonsensical > message (`isCallersFault` is true) or because the sender had an internal error > (`isCallersFault` is false). isCallersFault appears to be deprecated (`obsoleteIsCallersFault` appears much later). [Call.sendResultsTo] > When `yourself` is used, the receiver must still send a `Return` for the call, but sets the > field `resultsSentElsewhere` in that `Return` rather than including the results. When should `resultsSentElsewhere` be returned? Once the result is known? Or once the first takeFromOtherQuestion collects it? Can takeFromOtherQuestion be used more than once for a single source question? > The `Call` for bar'() has `sendResultsTo` set to `yourself`, with the value being the > question ID originally assigned to the bar() call. What does "the value" refer to here? `yourself` has type `Void`. > Vat B receives the `Return` for bar'() and sends a `Return` for bar(), with > `receivedFromYourself` set in place of the results. `receivedFromYourself` does not appear anywhere else in the spec. [Return.releaseParamCaps] > If true, all capabilities that were in the params should be considered released. Just to be sure: as if the sender had sent a release message for each one with `count=1`? [Payload] Why is it not possible to send exceptions in payloads? Should I export each broken capability as an export and then immediately send a Resolve for each one, resolving it to an exception? [Resolve] > When an export ID sent over the wire (e.g. in a `CapDescriptor`) is indicated to be a promise, > this indicates that the sender will follow up at some point with a `Resolve` message. If the > same `promiseId` is sent again before `Resolve`, still only one `Resolve` is sent. If the > same ID is sent again later _after_ a `Resolve`, it can only be because the export's > reference count hit zero in the meantime and the ID was re-assigned to a new export, therefore > this later promise does _not_ correspond to the earlier `Resolve`. It's not clear to me why it is useful for the receiver to know this. Presumably the sender can't reuse an export ID until the receiver explicitly releases it anyway. Should an implementation keep track of whether a resolve has arrived yet and behave differently based on this when it sees an export ID? > The sender promises that from this point forth, until `promiseId` is released, it shall > simply forward all messages to the capability designated by `cap`. Does something similar apply to Return messages? Might be worth mentioning it there too. [Disembargo] > Embargos are used to enforce E-order in the presence of promise resolution. That is, if an > application makes two calls foo() and bar() on the same capability reference, in that order, > the calls should be delivered in the order in which they were made. But if foo() is called > on a promise, and that promise happens to resolve before bar() is called, then the two calls > may travel different paths over the network, and thus could arrive in the wrong order. In > this case, the call to `bar()` must be embargoed, and a `Disembargo` message must be sent along > the same path as `foo()` to ensure that the `Disembargo` arrives after `foo()`. What does "this case" refer to? When exactly is an embargo needed, and when not? > There are two particular cases where embargos are important. Consider object Alice, in Vat A, > who holds a promise P, pointing towards Vat B, that eventually resolves to Carol. Could Carol be another promise here? Should Alice wait until the target is fully resolved before doing a disembargo, or do a disembargo for each step? [Accept] > This message is also used to pick up a redirected return -- see `Return.redirect`. `redirect` doesn't appear anyway else in this spec. I guess it's `Return.sendResultsTo.thirdParty`. [ Network-specific Parameters] > For interaction over the global internet between parties with no other prior arrangement, a > particular set of bindings for these types is defined elsewhere. (TODO(someday): Specify where > these common definitions live.) Do these definitions exist now? Thanks! -- You received this message because you are subscribed to the Google Groups "Cap'n Proto" group. To unsubscribe from this group and stop receiving emails from it, send an email to capnproto+unsubscr...@googlegroups.com. Visit this group at https://groups.google.com/group/capnproto.
