docs/mojo_and_services.md

# Intro to Mojo & Services

[TOC]

## Overview

This document contains the minimum amount of information needed for a developer
to start using Mojo effectively in Chromium, with example Mojo interface usage,
service definition and hookup, and a brief overview of the Content layer's core
services.

See other [Mojo & Services](/docs/README.md#Mojo-Services) documentation
for introductory guides, API references, and more.

## Mojo Terminology

A **message pipe** is a pair of **endpoints**. Each endpoint has a queue of
incoming messages, and writing a message at one endpoint effectively enqueues
that message on the other (**peer**) endpoint. Message pipes are thus
bidirectional.

A **mojom** file describes **interfaces**, which are strongly-typed collections
of **messages**. Each interface message is roughly analogous to a single proto
message, for developers who are familiar with Google protobufs.

Given a mojom interface and a message pipe, one of the endpoints
can be designated as a **`Remote`** and is used to *send* messages described by
the interface. The other endpoint can be designated as a **`Receiver`** and is used
to *receive* interface messages.

*** aside
NOTE: The above generalization is a bit oversimplified. Remember that the
message pipe is still bidirectional, and it's possible for a mojom message to
expect a reply. Replies are sent from the `Receiver` endpoint and received by the
`Remote` endpoint.
***

The `Receiver` endpoint must be associated with (*i.e.* **bound** to) an