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<main>
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<h1 id="alternative-backends"><a class="header" href="#alternative-backends">Alternative backends</a></h1>
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<p>A “backend” is simply a program which <code>mdbook</code> will invoke during the book
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rendering process. This program is passed a JSON representation of the book and
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configuration information via <code>stdin</code>. Once the backend receives this
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information it is free to do whatever it wants.</p>
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<p>See <a href="../format/configuration/renderers.html">Configuring Renderers</a> for more information about using backends.</p>
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<p>The community has developed several backends.
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See the <a href="https://github.com/rust-lang/mdBook/wiki/Third-party-plugins">Third Party Plugins</a> wiki page for a list of available backends.</p>
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<h2 id="setting-up"><a class="header" href="#setting-up">Setting up</a></h2>
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<p>This page will step you through creating your own alternative backend in the form
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of a simple word counting program. Although it will be written in Rust, there’s
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no reason why it couldn’t be accomplished using something like Python or Ruby.</p>
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<p>First you’ll want to create a new binary program and add <code>mdbook-renderer</code> as a
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||
dependency.</p>
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<pre><code class="language-shell">$ cargo new --bin mdbook-wordcount
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$ cd mdbook-wordcount
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$ cargo add mdbook-renderer
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</code></pre>
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<p>When our <code>mdbook-wordcount</code> plugin is invoked, <code>mdbook</code> will send it a JSON
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version of <a href="https://docs.rs/mdbook-renderer/latest/mdbook_renderer/struct.RenderContext.html"><code>RenderContext</code></a> via our plugin’s <code>stdin</code>. For convenience, there’s
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a <a href="https://docs.rs/mdbook-renderer/latest/mdbook_renderer/struct.RenderContext.html#method.from_json"><code>RenderContext::from_json()</code></a> constructor which will load a <code>RenderContext</code>.</p>
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<p>This is all the boilerplate necessary for our backend to load the book.</p>
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<pre class="playground"><code class="language-rust edition2018">// src/main.rs
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use std::io;
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use mdbook_renderer::RenderContext;
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fn main() {
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let mut stdin = io::stdin();
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let ctx = RenderContext::from_json(&mut stdin).unwrap();
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}</code></pre>
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<blockquote>
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<p><strong>Note:</strong> The <code>RenderContext</code> contains a <code>version</code> field. This lets backends
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figure out whether they are compatible with the version of <code>mdbook</code> it’s being
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called by. This <code>version</code> comes directly from the corresponding field in
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<code>mdbook</code>’s <code>Cargo.toml</code>.</p>
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<p>It is recommended that backends use the <a href="https://crates.io/crates/semver"><code>semver</code></a> crate to inspect this field
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and emit a warning if there may be a compatibility issue.</p>
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</blockquote>
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<h2 id="inspecting-the-book"><a class="header" href="#inspecting-the-book">Inspecting the book</a></h2>
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<p>Now our backend has a copy of the book, lets count how many words are in each
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chapter!</p>
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<p>Because the <code>RenderContext</code> contains a <a href="https://docs.rs/mdbook-renderer/latest/mdbook_renderer/book/struct.Book.html"><code>Book</code></a> field (<code>book</code>), and a <code>Book</code> has
|
||
the <a href="https://docs.rs/mdbook-renderer/latest/mdbook_renderer/book/struct.Book.html#method.iter"><code>Book::iter()</code></a> method for iterating over all items in a <code>Book</code>, this step
|
||
turns out to be just as easy as the first.</p>
|
||
<pre class="playground"><code class="language-rust edition2018">
|
||
fn main() {
|
||
let mut stdin = io::stdin();
|
||
let ctx = RenderContext::from_json(&mut stdin).unwrap();
|
||
|
||
for item in ctx.book.iter() {
|
||
if let BookItem::Chapter(ref ch) = *item {
|
||
let num_words = count_words(ch);
|
||
println!("{}: {}", ch.name, num_words);
|
||
}
|
||
}
|
||
}
|
||
|
||
fn count_words(ch: &Chapter) -> usize {
|
||
ch.content.split_whitespace().count()
|
||
}</code></pre>
|
||
<h2 id="enabling-the-backend"><a class="header" href="#enabling-the-backend">Enabling the backend</a></h2>
|
||
<p>Now we’ve got the basics running, we want to actually use it. First, install the
|
||
program.</p>
|
||
<pre><code class="language-shell">$ cargo install --path .
|
||
</code></pre>
|
||
<p>Then <code>cd</code> to the particular book you’d like to count the words of and update its
|
||
<code>book.toml</code> file.</p>
|
||
<pre><code class="language-diff"> [book]
|
||
title = "mdBook Documentation"
|
||
description = "Create book from markdown files. Like Gitbook but implemented in Rust"
|
||
authors = ["Mathieu David", "Michael-F-Bryan"]
|
||
|
||
+ [output.html]
|
||
|
||
+ [output.wordcount]
|
||
</code></pre>
|
||
<p>When it loads a book into memory, <code>mdbook</code> will inspect your <code>book.toml</code> file to
|
||
try and figure out which backends to use by looking for all <code>output.*</code> tables.
|
||
If none are provided it’ll fall back to using the default HTML renderer.</p>
|
||
<p>Notably, this means if you want to add your own custom backend you’ll also need
|
||
to make sure to add the HTML backend, even if its table just stays empty.</p>
|
||
<p>Now you just need to build your book like normal, and everything should <em>Just
|
||
Work</em>.</p>
|
||
<pre><code class="language-shell">$ mdbook build
|
||
...
|
||
2018-01-16 07:31:15 [INFO] (mdbook::renderer): Invoking the "mdbook-wordcount" renderer
|
||
mdBook: 126
|
||
Command Line Tool: 224
|
||
init: 283
|
||
build: 145
|
||
watch: 146
|
||
serve: 292
|
||
test: 139
|
||
Format: 30
|
||
SUMMARY.md: 259
|
||
Configuration: 784
|
||
Theme: 304
|
||
index.hbs: 447
|
||
Syntax highlighting: 314
|
||
MathJax Support: 153
|
||
Rust code specific features: 148
|
||
For Developers: 788
|
||
Alternative Backends: 710
|
||
Contributors: 85
|
||
</code></pre>
|
||
<p>The reason we didn’t need to specify the full name/path of our <code>wordcount</code>
|
||
backend is because <code>mdbook</code> will try to <em>infer</em> the program’s name via
|
||
convention. The executable for the <code>foo</code> backend is typically called
|
||
<code>mdbook-foo</code>, with an associated <code>[output.foo]</code> entry in the <code>book.toml</code>. To
|
||
explicitly tell <code>mdbook</code> what command to invoke (it may require command-line
|
||
arguments or be an interpreted script), you can use the <code>command</code> field.</p>
|
||
<pre><code class="language-diff"> [book]
|
||
title = "mdBook Documentation"
|
||
description = "Create book from markdown files. Like Gitbook but implemented in Rust"
|
||
authors = ["Mathieu David", "Michael-F-Bryan"]
|
||
|
||
[output.html]
|
||
|
||
[output.wordcount]
|
||
+ command = "python /path/to/wordcount.py"
|
||
</code></pre>
|
||
<h2 id="configuration"><a class="header" href="#configuration">Configuration</a></h2>
|
||
<p>Now imagine you don’t want to count the number of words on a particular chapter
|
||
(it might be generated text/code, etc). The canonical way to do this is via the
|
||
usual <code>book.toml</code> configuration file by adding items to your <code>[output.foo]</code>
|
||
table.</p>
|
||
<p>The <code>Config</code> can be treated roughly as a nested hashmap which lets you call
|
||
methods like <code>get()</code> to access the config’s contents, with a
|
||
<code>get_deserialized()</code> convenience method for retrieving a value and automatically
|
||
deserializing to some arbitrary type <code>T</code>.</p>
|
||
<p>To implement this, we’ll create our own serializable <code>WordcountConfig</code> struct
|
||
which will encapsulate all configuration for this backend.</p>
|
||
<p>First add <code>serde</code> and <code>serde_derive</code> to your <code>Cargo.toml</code>,</p>
|
||
<pre><code>$ cargo add serde serde_derive
|
||
</code></pre>
|
||
<p>And then you can create the config struct,</p>
|
||
<pre class="playground"><code class="language-rust edition2018"><span class="boring">#![allow(unused)]
|
||
</span><span class="boring">fn main() {
|
||
</span>use serde_derive::{Serialize, Deserialize};
|
||
|
||
...
|
||
|
||
#[derive(Debug, Default, Serialize, Deserialize)]
|
||
#[serde(default, rename_all = "kebab-case")]
|
||
pub struct WordcountConfig {
|
||
pub ignores: Vec<String>,
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<p>Now we just need to deserialize the <code>WordcountConfig</code> from our <code>RenderContext</code>
|
||
and then add a check to make sure we skip ignored chapters.</p>
|
||
<pre><code class="language-diff"> fn main() {
|
||
let mut stdin = io::stdin();
|
||
let ctx = RenderContext::from_json(&mut stdin).unwrap();
|
||
+ let cfg: WordcountConfig = ctx.config
|
||
+ .get_deserialized("output.wordcount")
|
||
+ .unwrap_or_default();
|
||
|
||
for item in ctx.book.iter() {
|
||
if let BookItem::Chapter(ref ch) = *item {
|
||
+ if cfg.ignores.contains(&ch.name) {
|
||
+ continue;
|
||
+ }
|
||
+
|
||
let num_words = count_words(ch);
|
||
println!("{}: {}", ch.name, num_words);
|
||
}
|
||
}
|
||
}
|
||
</code></pre>
|
||
<h2 id="output-and-signalling-failure"><a class="header" href="#output-and-signalling-failure">Output and signalling failure</a></h2>
|
||
<p>While it’s nice to print word counts to the terminal when a book is built, it
|
||
might also be a good idea to output them to a file somewhere. <code>mdbook</code> tells a
|
||
backend where it should place any generated output via the <code>destination</code> field
|
||
in <a href="https://docs.rs/mdbook-renderer/latest/mdbook_renderer/struct.RenderContext.html"><code>RenderContext</code></a>.</p>
|
||
<pre><code class="language-diff">+ use std::fs::{self, File};
|
||
+ use std::io::{self, Write};
|
||
- use std::io;
|
||
use mdbook::renderer::RenderContext;
|
||
use mdbook::book::{BookItem, Chapter};
|
||
|
||
fn main() {
|
||
...
|
||
|
||
+ let _ = fs::create_dir_all(&ctx.destination);
|
||
+ let mut f = File::create(ctx.destination.join("wordcounts.txt")).unwrap();
|
||
+
|
||
for item in ctx.book.iter() {
|
||
if let BookItem::Chapter(ref ch) = *item {
|
||
...
|
||
|
||
let num_words = count_words(ch);
|
||
println!("{}: {}", ch.name, num_words);
|
||
+ writeln!(f, "{}: {}", ch.name, num_words).unwrap();
|
||
}
|
||
}
|
||
}
|
||
</code></pre>
|
||
<blockquote>
|
||
<p><strong>Note:</strong> There is no guarantee that the destination directory exists or is
|
||
empty (<code>mdbook</code> may leave the previous contents to let backends do caching),
|
||
so it’s always a good idea to create it with <code>fs::create_dir_all()</code>.</p>
|
||
<p>If the destination directory already exists, don’t assume it will be empty.
|
||
To allow backends to cache the results from previous runs, <code>mdbook</code> may leave
|
||
old content in the directory.</p>
|
||
</blockquote>
|
||
<p>There’s always the possibility that an error will occur while processing a book
|
||
(just look at all the <code>unwrap()</code>’s we’ve written already), so <code>mdbook</code> will
|
||
interpret a non-zero exit code as a rendering failure.</p>
|
||
<p>For example, if we wanted to make sure all chapters have an <em>even</em> number of
|
||
words, erroring out if an odd number is encountered, then you may do something
|
||
like this:</p>
|
||
<pre><code class="language-diff">+ use std::process;
|
||
...
|
||
|
||
fn main() {
|
||
...
|
||
|
||
for item in ctx.book.iter() {
|
||
if let BookItem::Chapter(ref ch) = *item {
|
||
...
|
||
|
||
let num_words = count_words(ch);
|
||
println!("{}: {}", ch.name, num_words);
|
||
writeln!(f, "{}: {}", ch.name, num_words).unwrap();
|
||
|
||
+ if cfg.deny_odds && num_words % 2 == 1 {
|
||
+ eprintln!("{} has an odd number of words!", ch.name);
|
||
+ process::exit(1);
|
||
+ }
|
||
}
|
||
}
|
||
}
|
||
|
||
#[derive(Debug, Default, Serialize, Deserialize)]
|
||
#[serde(default, rename_all = "kebab-case")]
|
||
pub struct WordcountConfig {
|
||
pub ignores: Vec<String>,
|
||
+ pub deny_odds: bool,
|
||
}
|
||
</code></pre>
|
||
<p>Now, if we reinstall the backend and build a book,</p>
|
||
<pre><code class="language-shell">$ cargo install --path . --force
|
||
$ mdbook build /path/to/book
|
||
...
|
||
2018-01-16 21:21:39 [INFO] (mdbook::renderer): Invoking the "wordcount" renderer
|
||
mdBook: 126
|
||
Command Line Tool: 224
|
||
init: 283
|
||
init has an odd number of words!
|
||
2018-01-16 21:21:39 [ERROR] (mdbook::renderer): Renderer exited with non-zero return code.
|
||
2018-01-16 21:21:39 [ERROR] (mdbook::utils): Error: Rendering failed
|
||
2018-01-16 21:21:39 [ERROR] (mdbook::utils): Caused By: The "mdbook-wordcount" renderer failed
|
||
</code></pre>
|
||
<p>As you’ve probably already noticed, output from the plugin’s subprocess is
|
||
immediately passed through to the user. It is encouraged for plugins to follow
|
||
the “rule of silence” and only generate output when necessary (e.g. an error in
|
||
generation or a warning).</p>
|
||
<p>All environment variables are passed through to the backend, allowing you to use
|
||
the usual <code>MDBOOK_LOG</code> to control logging verbosity.</p>
|
||
<h2 id="wrapping-up"><a class="header" href="#wrapping-up">Wrapping up</a></h2>
|
||
<p>Although contrived, hopefully this example was enough to show how you’d create
|
||
an alternative backend for <code>mdbook</code>. If you feel it’s missing something, don’t
|
||
hesitate to create an issue in the <a href="https://github.com/rust-lang/mdBook/issues">issue tracker</a> so we can improve the user
|
||
guide.</p>
|
||
<p>The existing backends mentioned towards the start of this chapter should serve
|
||
as a good example of how it’s done in real life, so feel free to skim through
|
||
the source code or ask questions.</p>
|
||
|
||
</main>
|
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