ipcz

Overview

ipcz is a fully cross-platform library for interprocess communication (IPC) intended to address two generic problems: routing and data transfer.

Routing

With ipcz, applications create pairs of entangled portals to facilitate bidirectional communication. These are grouped into collections called nodes, which typically correspond 1:1 to OS processes.

Nodes may be explicitly connected by an application to establish portal pairs which span the node boundary.

                   ┌───────┐
       Connect     │       │     Connect
       ┌──────────>O   A   O<───────────┐
       │           │       │            │
       │           └───────┘            │
       │                                │
       v                                v
   ┌───O───┐                        ┌───O───┐
   │       │                        │       │
   │   B   │                        │   C   │
   │       │                        │       │
   └───────┘                        └───────┘

Here nodes A and B are explicitly connected by the application, as are nodes A and C. B can put stuff into its portal, and that stuff will come out of the linked portal on A.

But portals may also be sent over other portals. For example, B may create a new pair of portals...

                   ┌───────┐
       Connect     │       │     Connect
       ┌──────────>O   A   O<───────────┐
       │           │       │            │
       │           └───────┘            │
       │                                │
       v                                v
   ┌───O───┐                        ┌───O───┐
   │       │                        │       │
   │   B   │                        │   C   │
   │ O───O │                        │       │
   └───────┘                        └───────┘

...and send one over B's existing portal to A:

                   ┌───────┐
       Connect     │       │     Connect
       ┌──────────>O   A   O<───────────┐
       │ ┌────────>O       │            │
       │ │         └───────┘            │
       │ │                              │
       v v                              v
   ┌───O─O─┐                        ┌───O───┐
   │       │                        │       │
   │   B   │                        │   C   │
   │       │                        │       │
   └───────┘                        └───────┘

Node A may then forward this new portal along its own existing portal to node C:

                   ┌───────┐
       Connect     │       │     Connect
       ┌──────────>O   A   O<───────────┐
       │           │       │            │
       │           └───────┘            │
       │                                │
       v                                v
   ┌───O───┐                        ┌───O───┐
   │       │                        │       │
   │   B   O────────────────────────O   C   │
   │       │                        │       │
   └───────┘                        └───────┘

As a result, the application ends up with a portal on node B linked directly to a portal on node C, despite no explicit effort by the application to connect these two nodes directly.

ipcz enables this seamless creation and transferrence of routes with minimal end-to-end latency and amortized overhead.

Data Transfer

ipcz supports an arbitrarily large number of interconnected portals across the system, with potentially hundreds of thousands of individual portal pairs spanning any two nodes. Apart from managing how these portal pairs route their communications (i.e. which nodes they must traverse from end-to-end), ipcz is also concerned with how each communication is physically conveyed from one node to another.

In a traditional IPC system, each transmission typically corresponds to a system I/O call of some kind (e.g. POSIX writev(), or WriteFile() on Windows). These calls may require extra copies of transmitted data, incur additional context switches, and potentially elicit other forms of overhead (e.g. redundant idle CPU wakes) under various conditions.

ipcz tries to avoid such I/O operations in favor of pure userspace memory transactions, falling back onto system I/O only for signaling and less frequent edge cases. To facilitate this behavior, every pair of interconnected nodes has a private shared memory pool managed by ipcz.

Usage

Applications (specifically Mojo Core) must provide each node with an implementation of the IpczDriver interface to perform a variety of straightforward, platform- and environment-specific tasks such as establishing a basic I/O transport, generating random numbers, and allocating shared memory regions. See reference drivers for examples.

This directory in the Chromium tree is the source of truth for ipcz. It is integrated directly into Mojo Core (//mojo/core) and is not intended to be used as a standalone or embeddable library outside of Chromium.

Consequently, ipcz is implemented using Chromium's //base library.

In the past ipcz allowed other applications to link against it as a library with stable C interfaces, which was called 'standalone' mode. In order to support standalone mode, //base was not allowed as a dependency. The standalone mode is no longer supported by ipcz. In particular, currently using //base is allowed, but a few of its polyfills are still referenced in code (like safe_math.h) until they are factored out.

Design

Some extensive coverage of ipcz design details can be found here.