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<body class="article toc2 toc-left">
<div id="header">
<h1>Erlang GraphQL Tutorial</h1>
<div class="details">
<span id="author" class="author">Jesper Louis Andersen</span><br>
<span id="email" class="email"><a href="https://github.com/jlouis">@jlouis</a></span><br>
<span id="author2" class="author">Martin Gausby</span><br>
<span id="email2" class="email"><a href="https://github.com/gausby">@gausby</a></span><br>
<span id="author3" class="author">ShopGun</span><br>
<span id="email3" class="email"><a href="https://github.com/shopgun">@shopgun</a></span><br>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#introduction">Introduction</a>
<ul class="sectlevel2">
<li><a href="#_on_this_tutorial">On this tutorial</a></li>
<li><a href="#_prerequisites">Prerequisites</a></li>
<li><a href="#_supported_platforms">Supported Platforms</a></li>
<li><a href="#_comments_contact">Comments & Contact</a></li>
<li><a href="#_license">License</a></li>
<li><a href="#_acknowledgments">Acknowledgments</a></li>
</ul>
</li>
<li><a href="#_why_graphql">Why GraphQL</a></li>
<li><a href="#system-tour">System Tour</a>
<ul class="sectlevel2">
<li><a href="#_first_query">First query</a></li>
<li><a href="#_more_advanced_queries">More advanced queries</a></li>
<li><a href="#_simple_mutations">Simple Mutations</a></li>
<li><a href="#_more_complex_mutations">More complex mutations</a></li>
<li><a href="#_this_tutorial">This tutorial</a></li>
</ul>
</li>
<li><a href="#getting_started">Getting Started</a>
<ul class="sectlevel2">
<li><a href="#_what_we_do_not_cover">What we do not cover</a></li>
<li><a href="#_overview">Overview</a></li>
<li><a href="#_plan">Plan</a></li>
</ul>
</li>
<li><a href="#_mnesia">Mnesia</a>
<ul class="sectlevel2">
<li><a href="#_setting_up_an_initial_mnesia_schema">Setting up an initial Mnesia schema</a></li>
<li><a href="#_populating_the_database">Populating the database</a></li>
<li><a href="#_creating_a_fallback_for_the_database">Creating a FALLBACK for the database</a></li>
</ul>
</li>
<li><a href="#schema">GraphQL Schema</a>
<ul class="sectlevel2">
<li><a href="#identity-encoding">Identity encoding</a></li>
<li><a href="#_the_node_interface">The Node Interface</a></li>
<li><a href="#_object_types">Object types</a></li>
<li><a href="#queries-and-mutations">Queries & Mutations</a></li>
<li><a href="#_input_objects">Input objects</a></li>
<li><a href="#interfaces-and-unions">Interfaces & Unions</a></li>
<li><a href="#schema-default-values">Schema default values</a></li>
</ul>
</li>
<li><a href="#_loading_the_schema">Loading the Schema</a>
<ul class="sectlevel2">
<li><a href="#_root_setup">Root setup</a></li>
<li><a href="#_mapping_rules">Mapping rules</a></li>
</ul>
</li>
<li><a href="#scalar-resolution">Scalar Resolution</a>
<ul class="sectlevel2">
<li><a href="#_example">Example</a></li>
</ul>
</li>
<li><a href="#type-resolution">Type Resolution</a></li>
<li><a href="#object-resolution">Object Resolution</a>
<ul class="sectlevel2">
<li><a href="#_execution">Execution</a></li>
<li><a href="#input-argument-rules">Input argument rules</a></li>
<li><a href="#_handling_planets">Handling Planets</a></li>
<li><a href="#_handling_starships">Handling Starships</a></li>
<li><a href="#_loading_data">Loading Data</a></li>
<li><a href="#walking-in-the-graph">Walking in the Graph</a></li>
<li><a href="#_default_mapping">Default Mapping</a></li>
<li><a href="#resolving-lists">Resolving lists</a></li>
<li><a href="#resolution-of-mutations">Mutations</a></li>
<li><a href="#_mutation_examples">Mutation Examples</a></li>
<li><a href="#_anatomy_of_a_query">Anatomy of a query</a></li>
</ul>
</li>
<li><a href="#_transports">Transports</a>
<ul class="sectlevel2">
<li><a href="#cowboy-handler">Cowboy Handler</a></li>
</ul>
</li>
<li><a href="#graphiql">GraphiQL</a></li>
<li><a href="#error-handling">Error Handling</a></li>
<li><a href="#_relay_modern">Relay Modern</a>
<ul class="sectlevel2">
<li><a href="#_node_interface">Node Interface</a></li>
<li><a href="#inputs-and-payloads">Inputs & Payloads</a></li>
<li><a href="#pagination">Pagination</a></li>
</ul>
</li>
<li><a href="#_security">Security</a>
<ul class="sectlevel2">
<li><a href="#_limiting_clients_stored_procedures">Limiting Clients—​Stored Procedures</a></li>
<li><a href="#authentication">Authentication</a></li>
<li><a href="#authorization">Authorization</a></li>
</ul>
</li>
<li><a href="#annotations">Annotations</a></li>
<li><a href="#tricks">Tricks</a>
<ul class="sectlevel2">
<li><a href="#object-representation">Object Representation</a></li>
<li><a href="#non-isomorphism">Avoid Isomorphic representations</a></li>
<li><a href="#middleware-stacks">Middleware stacks</a></li>
<li><a href="#_data_loader">Data Loader</a></li>
<li><a href="#_fragments">Fragments</a></li>
</ul>
</li>
<li><a href="#_terminology">Appendix A: Terminology</a>
<ul class="sectlevel2">
<li><a href="#null-propagation">Null Propagation</a></li>
<li><a href="#HATEOAS">Hypertext as the engine of application state</a></li>
<li><a href="#context">Context</a></li>
<li><a href="#cqrs">CQRS</a></li>
<li><a href="#cursor">Cursor</a></li>
</ul>
</li>
<li><a href="#_code_overview">Appendix B: Code Overview</a>
<ul class="sectlevel2">
<li><a href="#_root">Root</a></li>
<li><a href="#_application_code_sw_core_code">Application <code>sw_core</code></a></li>
<li><a href="#_application_code_sw_web_code">Application <code>sw_web</code></a></li>
</ul>
</li>
<li><a href="#changelog">Appendix C: Changelog</a></li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>Erlang GraphQL Tutorial</p>
</div>
<div class="paragraph">
<p>The guide here is a running example of an API implemented in Erlang
through the ShopGun GraphQL engine. The API is a frontend to a
database, containing information about the Star Wars films by George
Lucas. The intent is to provide readers with enough information they
can go build their own GraphQL servers in Erlang.</p>
</div>
<div class="paragraph">
<p>We use the GraphQL system at <a href="https://shopgun.com" class="bare">https://shopgun.com</a> as a data backend. We
sponsor this tutorial as part of our Open Source efforts. We developed
this GraphQL system to meet our demands as our system evolves. The
world of tracking businesses and offers is a highly heterogeneous
dataset, which requires the flexibility of something like GraphQL.</p>
</div>
<div class="paragraph">
<p>Because GraphQL provides a lot of great tooling, we decided to move
forward and implement a server backend for Erlang, which didn’t exist
at the time.</p>
</div>
<div class="paragraph">
<p>At the same time, we recognize other people may be interested in the
system and its development. Hence the decision was made to open source
the GraphQL parts of the system.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="introduction">Introduction</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The Erlang GraphQL system allows you to implement GraphQL servers in
Erlang. It works as a library which you can use on top of existing web
servers such as Cowboy, Webmachine, Yaws and so on.</p>
</div>
<div class="paragraph">
<p>As a developer, you work by providing a <em>schema</em> which defines the
query structure which your server provides. Next, you map your schema
unto Erlang modules which then defines a binding of the two worlds.</p>
</div>
<div class="paragraph">
<p>Clients execute <em>queries</em> to the server according to the structure of
the schema. The GraphQL system then figures out a <em>query plan</em> for the
query and executes the query. This in turn calls your bound modules
and this allows you to process the query, load data, and so on.</p>
</div>
<div class="paragraph">
<div class="title">Changes</div>
<p>For a complete list of changes over time to this document, take a look
at the <a href="#changelog">Changelog</a> appendix.</p>
</div>
<div class="sect2">
<h3 id="_on_this_tutorial">On this tutorial</h3>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<i class="fa icon-caution" title="Caution"></i>
</td>
<td class="content">
We are currently building the document and are still making
changes to it. Things can still move around and change. If you see a
“TBD” marker it means that section is “To Be Done” and will be
written at a later point. In the same vein, the code base is being
built up as well, so it may not be that everything is fully described
yet.
</td>
</tr>
</table>
</div>
<div class="admonitionblock caution">
<table>
<tr>
<td class="icon">
<i class="fa icon-caution" title="Caution"></i>
</td>
<td class="content">
The current version of Erlang GraphQL returns some errors which
are hard to parse and understand. It is our intention to make the
error handling better and more clean in a later version.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>The tutorial you are now reading isn’t really a tutorial per se where
you type in stuff and see the output. There is a bit too much code for
that kind of exposition. Rather, the tutorial describes a specific
project implemented by means of the GraphQL system. You can use the
ideas herein to build your own.</p>
</div>
<div class="paragraph">
<p>There are examples of how things are built however, so you may be able
to follow along and check out the construction of the system as a
whole. Apart from being a small self-contained functional GraphQL
project, it is also a small self-contained functional rebar3 project.
So there’s that.</p>
</div>
</div>
<div class="sect2">
<h3 id="_prerequisites">Prerequisites</h3>
<div class="paragraph">
<p>Some Erlang knowledge is expected for reading this guide. General
Erlang concept will not be explained, but assumed to be known. Some
Mnesia knowledge will also help a bit in understanding what is going
on, though if you know anything about databases in general, that is
probably enough. Furthermore, some knowledge of the web in general is
assumed. We don’t cover the intricacies of HTTP 1.1 or HTTP/2 for
instance.</p>
</div>
<div class="paragraph">
<p>This tutorial uses a couple of dependencies:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Rebar3 is used to build the software</p>
</li>
<li>
<p>Cowboy 1.x is used as a web server for the project</p>
</li>
<li>
<p>GraphiQL is used as a web interface to the Graph System</p>
</li>
<li>
<p>Erlang/OTP version 19.3.3 was used in the creation of this tutorial</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_supported_platforms">Supported Platforms</h3>
<div class="paragraph">
<p>The GraphQL system should run on any system which can run Erlang. The
library does not use any special tooling, nor does it make any
assumptions about the environment. If Erlang runs on your platform,
chances are that GraphQL will too.</p>
</div>
</div>
<div class="sect2">
<h3 id="_comments_contact">Comments & Contact</h3>
<div class="paragraph">
<p>The official repository location is</p>
</div>
<div class="paragraph">
<p><a href="https://github.com/shopgun/graphql-erlang-tutorial" class="bare">https://github.com/shopgun/graphql-erlang-tutorial</a></p>
</div>
<div class="paragraph">
<p>If you have comments on the document or corrections, please open an
<a href="https://github.com/shopgun/graphql-erlang-tutorial/issues">Issue</a> in
the above repository on the thing that is missing. Also, feel free to
provide pull requests against the code itself.</p>
</div>
<div class="paragraph">
<p>Things we are particularly interested in:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>Parts you don’t understand. These often means something isn’t
described well enough and needs improvement.</p>
</li>
<li>
<p>Code sequences that doesn’t work for you. There is often some
prerequisite the document should mention but doesn’t.</p>
</li>
<li>
<p>Bad wording. Things should be clear and precise. If a particular
sentence doesn’t convey information clearly, we’d rather rewrite it
then confuse the next reader.</p>
</li>
<li>
<p>Bugs in the code base.</p>
</li>
<li>
<p>Bad code structure. A problem with a tutorial repository is that it
can “infect” code in the future. People copy from this repository,
so if it contains bad style, then that bad style is copied into
other repositories, infecting them with the same mistakes.</p>
</li>
<li>
<p>Stale documentation. Parts of the documentation which were relevant
in the past but isn’t anymore. For instance ID entries which doesn’t
work anymore.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_license">License</h3>
<div class="paragraph">
<p>Copyright © 2017 ShopGun.</p>
</div>
<div class="paragraph">
<p>Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at</p>
</div>
<div class="literalblock">
<div class="content">
<pre>http://www.apache.org/licenses/LICENSE-2.0</pre>
</div>
</div>
<div class="paragraph">
<p>Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.</p>
</div>
</div>
<div class="sect2">
<h3 id="_acknowledgments">Acknowledgments</h3>
<div class="ulist">
<ul>
<li>
<p>Everyone involved in the Star Wars API. We use that data extensively.</p>
</li>
<li>
<p>The GraphQL people who did an excellent job at answering questions
and provided us with a well-written specification.</p>
</li>
<li>
<p>Josh Price. The parser was derived from his initial work though it
has been changed a lot since the initial commit.</p>
</li>
</ul>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_why_graphql">Why GraphQL</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A worthy question to ask is “Why GraphQL?”</p>
</div>
<div class="paragraph">
<p>GraphQL is a natural extension of what we are already doing on the
Web. As our systems grow, we start realizing our systems become
gradually more heterogeneous in the sense that data becomes more
complex and data gets more variance.</p>
</div>
<div class="paragraph">
<p>In addition—​since we usually have a single API serving multiple
different clients, written in different languages for different
platforms—​we need to be flexible in query support. Clients are likely
to evolve dynamically, non-linearly, and at different paces. Thus, the
backend must support evolution while retaining backwards
compatibility. Also, we must have a contract or protocol between the
clients and the server that is standardized. Otherwise, we end up
inventing our own system again and again, and this is a strenuous
affair which has little to no reuse between systems.</p>
</div>
<div class="paragraph">
<p>The defining characteristic of GraphQL is that the system is
client-focused and client-centric. Data is driven by the client of the
system, and not by the server. The consequence is that the
delivery-time for features tend to be shorter. As soon as the product
knows what change to make, it can often be handled with less
server-side interaction than normally. Especially for the case where
you are recombining existing data into a new view.</p>
</div>
<div class="paragraph">
<p>RESTful APIs have served us well for a long time. And they are likely
to continue serving as well in a large number of situations. However,
if you have a system requiring more complex interaction, chances are
you are better off by taking the plunge and switching your system to
GraphQL.</p>
</div>
<div class="paragraph">
<div class="title">Efficiency</div>
<p>RESTful APIs recently got a powerful improvement in HTTP/2 which
allows RESTful APIs to pipeline far better than what they did earlier.
However, you still pay the round trip time between data dependencies
in an HTTP/2 setting: You need the listing of keys before you can
start requesting the data objects on those keys. In contrast, GraphQL
queries tend to be a single round-trip only. A full declarative query
is formulated and executed, without the need of any intermediate
query. This means faster response times. Even in the case where a
single query becomes slower since there is no need for followup
queries.</p>
</div>
<div class="paragraph">
<p>A major (subtle) insight is that in a GraphQL server, you don’t have
to hand-code the looping constructs which tend to be present in a lot
of RESTful APIs. To avoid the round-trip describes in the preceding
paragraph, you often resolve to a solution where a specialized
optimized query is constructed and added to the system. This
specialized endpoint is then looping over the data in one go so you
avoid having to do multiple round-trips.</p>
</div>
<div class="paragraph">
<p>In a GraphQL system, that looping is handled once-and-for-all by the
GraphQL engine. You are only implementing callbacks that run as part
of the loop. A lot of tedious code is then handled by GraphQL and we
avoid having to code this again and again for each RESTful web service
we write.</p>
</div>
<div class="paragraph">
<div class="title">Porting</div>
<p>You can often move your system onto GraphQL a bit at a time. You don’t
have to port every endpoint in the beginning. Often, people add some
kind of field, <code>previousId</code> say, which is used as an identifier in the
old system. Then you can gradually take over data from an old system
and port it on top of GraphQL. Once the ball is rolling, it is likely
that more and more clients want to use it, as it is a easier interface
for them to use.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="system-tour">System Tour</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Since a system as large as GraphQL can seem incomprehensible when you
first use it, we will begin by providing a system tour explaining by
example how the system works. In order to start the system for the
first time, we must construct a <em>release</em>.</p>
</div>
<div class="sidebarblock">
<div class="content">
<div class="title">Releases</div>
<div class="paragraph">
<p>Erlang uses a concept called <em>releases</em> for real applications. A
release consists of:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>An erlang emulator for the virtual machine, BEAM as a binary in the
operating system.</p>
</li>
<li>
<p>The set of applications which is configured to be inside the
release.</p>
</li>
<li>
<p>Every dependent application needed by the release-configuration.</p>
</li>
<li>
<p>Support libraries for executing the emulator, binary NIF shared
objects (NIF: Native implemented function), and so on.</p>
</li>
<li>
<p>Configuration for the system and runtime</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>A release is a self-contained Erlang system living in its own
directory structure. It can be copied to a target machine and started
without having to install any support libraries, as long as the system
architecture is the same. If you provide an appropriate BEAM emulator
for the given architecture, the code will run with little to no
modification as well since the Erlang byte code is highly portable. It
even works across Erlang/OTP versions (subject to some restrictions).</p>
</div>
</div>
</div>
<div class="paragraph">
<p>To make a release, run the following command:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code>$ make release</code></pre>
</div>
</div>
<div class="paragraph">
<p>This builds a release inside the <code>_build</code> directory and makes it
available. In order to run the release, we can ask to run it with a
console front-end, so we get a shell on the Erlang system:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code>$ _build/default/rel/sw/bin/sw console</code></pre>
</div>
</div>
<div class="paragraph">
<p>The system should boot and start running. A typical invocation looks
like:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code>Erlang/OTP 19 [erts-8.3] [source] [64-bit] [smp:8:8] [async-threads:30] [hipe] [kernel-poll:true] [dtrace]
15:33:05.705 [info] Application lager started on node 'sw@127.0.0.1'
15:33:05.705 [info] Application ranch started on node 'sw@127.0.0.1'
15:33:05.706 [info] Application graphql started on node 'sw@127.0.0.1'
15:33:05.706 [info] Application sw_core started on node 'sw@127.0.0.1'
15:33:05.706 [info] Application cowboy started on node 'sw@127.0.0.1'
15:33:05.706 [info] Starting HTTP listener on port 17290
Eshell V8.3 (abort with ^G)
(sw@127.0.0.1)1></code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
To exit an Erlang node like this, you can either Ctrl-C twice
which stops the system abruptly. Or you can be nice to the system and
ask it to close gracefully one application at a time by entering
<code>q().<RET></code> in the shell.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Once the Erlang emulator is running our <code>sw</code> release, we can point a
browser to <a href="http://localhost:17290/" class="bare">http://localhost:17290/</a> and you should be greeted
with the following screen:</p>
</div>
<div id="img-graphiql" class="imageblock">
<div class="content">
<img src="./images/graphiql.png" alt="GraphiQL">
</div>
<div class="title">Figure 1. The initial greeting screen for the GraphQL system</div>
</div>
<div class="sect2">
<h3 id="_first_query">First query</h3>
<div class="paragraph">
<p>The first query we will run requests a given <strong>Planet</strong> from the system.
In accordance with the rules of Relay Modern GraphQL conventions, we
will load this <strong>Planet</strong> via the <em>Object Identification</em> conventions:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code class="language-graphql" data-lang="graphql">query PlanetQuery {
node(id:"UGxhbmV0OjE=") { <i class="conum" data-value="1"></i><b>(1)</b>
... on Planet { <i class="conum" data-value="2"></i><b>(2)</b>
id <i class="conum" data-value="3"></i><b>(3)</b>
name