Cap'n Web: a new RPC system for browsers and web servers

Post by catern » Tue, Sep 23, 2025, 4:33 AM UTC

kentonv wrote: post #111

frollogaston wrote: post #99

Earlier quoted context omitted.

It's not a programming paradigm shift, more of a change to how runtimes work. We want to avoid the overhead of kernel threads in servers, and async/await on top of an event loop is a convenient way to do that, like in JS, Rust, and now Python. Meanwhile Go doesn't have async/await and never will because it doesn't need it; it does greenthreading instead. Java has that too now. Either way, your code waits on IO like b…

I disagree that async/await is purely about avoiding overhead of kernel threads. Kernel threads are actually not that expensive these days. You can have a server with 10,000 threads, no problem. The problem is synchronization becomes extremely hard to reason about. With event loop concurrency, each continuation (callback) becomes effectively a transaction, in which you don't need to worry about anything else modifyin…

Post by [deleted] » Tue, Sep 23, 2025, 4:37 AM UTC

keyle wrote: post #199

mdasen wrote: post #90

Earlier quoted context omitted.

In C#, there's expression trees which handle things like this and it's how Entity Framework is able to convert the lambdas it's given into SQL. This means that you can pass around code that can be inspected or transformed instead of being executed. Take this EntityFramework snippet: db.People.Where(p => p.Name == "Joe") `Where` takes an `Expression > predicate`. It isn't taking the `Func` itself, but an `Expression`…

C#, Swift, Dart, Rust... Python. Many languages take lambda/predicate/closure as filter/where. It generally unrolls as a `for loop` underneath, or in this case LINQ/SQL. C# was innovative for doing it first in the scope of SQL. I remember the arrival of LINQ... Good times.

Post by mpweiher » Tue, Sep 23, 2025, 5:23 AM UTC

Post by mdavidn » Tue, Sep 23, 2025, 5:56 AM UTC

mtrovo wrote: post #57

The section on how they solved arrays is fascinating and terrifying at the same time https://blog.cloudflare.com/capnweb-javascript-rpc-library/#... . > .map() is special. It does not send JavaScript code to the server, but it does send something like "code", restricted to a domain-specific, non-Turing-complete language. The "code" is a list of instructions that the server should carry out for each member of the arra…

Post by ignoramous » Tue, Sep 23, 2025, 6:02 AM UTC

prngl wrote: post #163

This is cool. There's an interesting parallel with ML compilation libraries (TensorFlow 1, JAX jit, PyTorch compile) where a tracing approach is taken to build up a graph of operations that are then essentially compiled (or otherwise lowered and executed by a specialized VM). We're often nowadays working in dynamic languages, so they become essentially the frontend to new DSLs, and instead of defining new syntax, we…

  Alice and Bob each maintain some state about the connection. In particular, each maintains an "export table", describing all the pass-by-reference objects they have exposed to the other side, and an "import table", describing the references they have received. 

  Alice's exports correspond to Bob's imports, and vice versa. Each entry in the export table has a signed integer ID, which is used to reference it. You can think of these IDs like file descriptors in a POSIX system. Unlike file descriptors, though, IDs can be negative, and an ID is never reused over the lifetime of a connection.

  At the start of the connection, Alice and Bob each populate their export tables with a single entry, numbered zero, representing their "main" interfaces.

  Typically, when one side is acting as the "server", they will export their main public RPC interface as ID zero, whereas the "client" will export an empty interface. However, this is up to the application: either side can export whatever they want.