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Juniper Bot f42054f407 Updated book for master ***NO_CI***
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<h1 class="menu-title">Juniper - GraphQL Server for Rust</h1>
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<a class="header" href="#avoiding-the-n1-problem-with-dataloaders" id="avoiding-the-n1-problem-with-dataloaders"><h1>Avoiding the N+1 Problem With Dataloaders</h1></a>
<p>A common issue with graphql servers is how the resolvers query their datasource.
This issue results in a large number of unneccessary database queries or http requests.
Say you were wanting to list a bunch of cults people were in</p>
<pre><code class="language-graphql">query {
persons {
id
name
cult {
id
name
}
}
}
</code></pre>
<p>What would be executed by a SQL database would be:</p>
<pre><code class="language-sql">SELECT id, name, cult_id FROM persons;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 1;
SELECT id, name FROM cults WHERE id = 2;
SELECT id, name FROM cults WHERE id = 2;
SELECT id, name FROM cults WHERE id = 2;
# ...
</code></pre>
<p>Once the list of users has been returned, a separate query is run to find the cult of each user.
You can see how this could quickly become a problem.</p>
<p>A common solution to this is to introduce a <strong>dataloader</strong>.
This can be done with Juniper using the crate <a href="https://github.com/cksac/dataloader-rs">cksac/dataloader-rs</a>, which has two types of dataloaders; cached and non-cached.</p>
<a class="header" href="#cached-loader" id="cached-loader"><h4>Cached Loader</h4></a>
<p>DataLoader provides a memoization cache, after .load() is called once with a given key, the resulting value is cached to eliminate redundant loads.</p>
<p>DataLoader caching does not replace Redis, Memcache, or any other shared application-level cache. DataLoader is first and foremost a data loading mechanism, and its cache only serves the purpose of not repeatedly loading the same data in the context of a single request to your Application. <a href="https://github.com/graphql/dataloader#caching">(read more)</a></p>
<a class="header" href="#what-does-it-look-like" id="what-does-it-look-like"><h3>What does it look like?</h3></a>
<p>!FILENAME Cargo.toml</p>
<pre><code class="language-toml">[dependencies]
actix-identity = &quot;0.2&quot;
actix-rt = &quot;1.0&quot;
actix-web = {version = &quot;2.0&quot;, features = []}
juniper = { git = &quot;https://github.com/graphql-rust/juniper&quot; }
futures = &quot;0.3&quot;
postgres = &quot;0.15.2&quot;
dataloader = &quot;0.12.0&quot;
async-trait = &quot;0.1.30&quot;
</code></pre>
<pre><code class="language-rust ignore">// use dataloader::cached::Loader;
use dataloader::non_cached::Loader;
use dataloader::BatchFn;
use std::collections::HashMap;
use postgres::{Connection, TlsMode};
use std::env;
pub fn get_db_conn() -&gt; Connection {
let pg_connection_string = env::var(&quot;DATABASE_URI&quot;).expect(&quot;need a db uri&quot;);
println!(&quot;Connecting to {}&quot;, pg_connection_string);
let conn = Connection::connect(&amp;pg_connection_string[..], TlsMode::None).unwrap();
println!(&quot;Connection is fine&quot;);
conn
}
#[derive(Debug, Clone)]
pub struct Cult {
pub id: i32,
pub name: String,
}
pub fn get_cult_by_ids(hashmap: &amp;mut HashMap&lt;i32, Cult&gt;, ids: Vec&lt;i32&gt;) {
let conn = get_db_conn();
for row in &amp;conn
.query(&quot;SELECT id, name FROM cults WHERE id = ANY($1)&quot;, &amp;[&amp;ids])
.unwrap()
{
let cult = Cult {
id: row.get(0),
name: row.get(1),
};
hashmap.insert(cult.id, cult);
}
}
pub struct CultBatcher;
#[async_trait]
impl BatchFn&lt;i32, Cult&gt; for CultBatcher {
// A hashmap is used, as we need to return an array which maps each original key to a Cult.
async fn load(&amp;self, keys: &amp;[i32]) -&gt; HashMap&lt;i32, Cult&gt; {
println!(&quot;load cult batch {:?}&quot;, keys);
let mut cult_hashmap = HashMap::new();
get_cult_by_ids(&amp;mut cult_hashmap, keys.to_vec());
cult_hashmap
}
}
pub type CultLoader = Loader&lt;i32, Cult, CultBatcher&gt;;
// To create a new loader
pub fn get_loader() -&gt; CultLoader {
Loader::new(CultBatcher)
// Usually a DataLoader will coalesce all individual loads which occur
// within a single frame of execution before calling your batch function with all requested keys.
// However sometimes this behavior is not desirable or optimal.
// Perhaps you expect requests to be spread out over a few subsequent ticks
// See: https://github.com/cksac/dataloader-rs/issues/12
// More info: https://github.com/graphql/dataloader#batch-scheduling
// A larger yield count will allow more requests to append to batch but will wait longer before actual load.
.with_yield_count(100)
}
#[juniper::graphql_object(Context = Context)]
impl Cult {
// your resolvers
// To call the dataloader
pub async fn cult_by_id(ctx: &amp;Context, id: i32) -&gt; Cult {
ctx.cult_loader.load(id).await
}
}
</code></pre>
<a class="header" href="#how-do-i-call-them" id="how-do-i-call-them"><h3>How do I call them?</h3></a>
<p>Once created, a dataloader has the async functions <code>.load()</code> and <code>.load_many()</code>.
In the above example <code>cult_loader.load(id: i32).await</code> returns <code>Cult</code>. If we had used <code>cult_loader.load_many(Vec&lt;i32&gt;).await</code> it would have returned <code>Vec&lt;Cult&gt;</code>.</p>
<a class="header" href="#where-do-i-create-my-dataloaders" id="where-do-i-create-my-dataloaders"><h3>Where do I create my dataloaders?</h3></a>
<p><strong>Dataloaders</strong> should be created per-request to avoid risk of bugs where one user is able to load cached/batched data from another user/ outside of its authenticated scope.
Creating dataloaders within individual resolvers will prevent batching from occurring and will nullify the benefits of the dataloader.</p>
<p>For example:</p>
<p><em>When you declare your context</em></p>
<pre><code class="language-rust ignore">use juniper;
#[derive(Clone)]
pub struct Context {
pub cult_loader: CultLoader,
}
impl juniper::Context for Context {}
impl Context {
pub fn new(cult_loader: CultLoader) -&gt; Self {
Self {
cult_loader
}
}
}
</code></pre>
<p><em>Your handler for GraphQL (Note: instantiating context here keeps it per-request)</em></p>
<pre><code class="language-rust ignore">pub async fn graphql(
st: web::Data&lt;Arc&lt;Schema&gt;&gt;,
data: web::Json&lt;GraphQLRequest&gt;,
) -&gt; Result&lt;HttpResponse, Error&gt; {
// Context setup
let cult_loader = get_loader();
let ctx = Context::new(cult_loader);
// Execute
let res = data.execute(&amp;st, &amp;ctx).await;
let json = serde_json::to_string(&amp;res).map_err(error::ErrorInternalServerError)?;
Ok(HttpResponse::Ok()
.content_type(&quot;application/json&quot;)
.body(json))
}
</code></pre>
<a class="header" href="#further-example" id="further-example"><h3>Further Example:</h3></a>
<p>For a full example using Dataloaders and Context check out <a href="https://github.com/jayy-lmao/rust-graphql-docker">jayy-lmao/rust-graphql-docker</a>.</p>
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