Lacinia Manual | Lacinia Tutorial | API Documentation
This library is a full implementation of Facebook's GraphQL specification.
Lacinia should be viewed as roughly analogous to the official reference JavaScript implementation. In other words, it is a backend-agnostic GraphQL query execution engine. Lacinia is not an Object Relational Mapper ... it's simply the implementation of a contract sitting between the GraphQL client and your data.
Lacinia features:
-
An EDN-based schema language.
-
High performance parser for GraphQL queries, built on Antlr4.
-
Efficient and asynchronous query execution.
-
Full support for GraphQL types, interfaces, unions, enums, input objects, and custom scalars.
-
Full support for GraphQL subscriptions.
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Full support of inline and named query fragments.
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Full support for GraphQL Schema Introspection.
Lacinia has been developed with a set of core philosophies:
-
Prefer data over macros and other tricks. Compose your schema in whatever mix of data and code works for you.
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Embrace Clojure: Use EDN data, keywords, functions, and persistent data structures.
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Keep it simple: You provide the schema and a handful of functions to resolve data, and Lacinia does the rest.
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Do the right thing: apply reasonable defaults without a lot of "magic".
This library can be plugged into any Clojure HTTP pipeline. The companion library lacinia-pedestal provides full HTTP support, including GraphQL subscriptions, for Pedestal.
For more detailed documentation, read the manual.
GraphQL starts with a schema definition of exposed types.
A schema starts as an EDN file; the example below demonstrates a small subset of the available options:
{:enums
{:episode
{:description "The episodes of the original Star Wars trilogy."
:values [:NEWHOPE :EMPIRE :JEDI]}}
:objects
{:droid
{:fields {:primary_functions {:type (list String)}
:id {:type Int}
:name {:type String}
:appears_in {:type (list :episode)}}}
:human
{:fields {:id {:type Int}
:name {:type String}
:home_planet {:type String}
:appears_in {:type (list :episode)}}}}
:queries
{:hero {:type (non-null :human)
:args {:episode {:type :episode}}
:resolve :get-hero}
:droid {:type :droid
:args {:id {:type String :default-value "2001"}}
:resolve :get-droid}}}
A schema alone describes what data is available to clients, but doesn't identify where the data comes from; that's the job of a field resolver, provided by the :resolve key inside fields such as the :hero and :droid query.
The values here, :get-hero and :get-droid, are placeholders; the startup code
of the application will use
com.walmartlabs.lacinia.util/attach-resolvers
to attach the actual
field resolver function.
A field resolver is just a function which is passed the application context, a map of arguments values, and a resolved value from a parent field. The field resolver returns a value. If it's a scalar type, it should return a value that conforms to the defined type in the schema. If not, it's a type error.
The field resolver is totally responsible for obtaining the data from whatever external store you use: whether it is a database, a web service, or something else.
It's important to understand that every field has a field resolver, even if you don't define it explicitly. If you don't supply a field resolver, Lacinia provides a default field resolver, customized to the field.
Here's what the get-hero
field resolver might look like:
(defn get-hero [context arguments value]
(let [{:keys [episode]} arguments]
(if (= episode :NEWHOPE)
{:id 1000
:name "Luke"
:home_planet "Tatooine"
:appears_in ["NEWHOPE" "EMPIRE" "JEDI"]}
{:id 2000
:name "Lando Calrissian"
:home_planet "Socorro"
:appears_in ["EMPIRE" "JEDI"]})))
In this greatly simplified example, the field resolver can simply return the resolved value. Field resolvers that return multiple values return a list, vector, or set of values.
In real applications, a field resolver might execute a query against a database, or send a request to another web service.
After attaching resolvers, it is necessary to compile the schema; this step performs validations, provide defaults, and organizes the schema for efficient execution of queries.
This needs only be done once, in application startup code:
(require '[clojure.edn :as edn]
'[com.walmartlabs.lacinia.util :refer [attach-resolvers]]
'[com.walmartlabs.lacinia.schema :as schema])
(def star-wars-schema
(-> "schema.edn"
slurp
edn/read-string
(attach-resolvers {:get-hero get-hero
:get-droid (constantly {})})
schema/compile))
With the compiled application available, it can be used to execute requests; this typically occurs inside a Ring handler function:
(require '[com.walmartlabs.lacinia :refer [execute]]
'[clojure.data.json :as json])
(defn handler [request]
{:status 200
:headers {"Content-Type" "application/json"}
:body (let [query (get-in request [:query-params :query])
result (execute star-wars-schema query nil nil)]
(json/write-str result)})
Lacinia doesn't know about the web tier at all, it just knows about parsing and executing queries against a compiled schema. A companion library, lacinia-pedestal, is one way to expose your schema on the web.
User queries are provided as the body of a request with the content type application/graphql
.
The GraphQL query language is designed to look familiar to someone who is versant in JSON.
{
hero {
id
name
}
}
The execute
function returns EDN data that can be easily converted to JSON.
The :data key contains the value requested for the hero
query in the request.
{:data
{:hero {:id 2000
:name "Lando Calrissian"}}}
This example request has no errors, and contained only a single query. GraphQL supports multiple queries in a single request. There may be errors executing the query, Lacinia will process as much as it can, and will report errors in the :errors key.
One of the benefits of GraphQL is that the client has the power to rename fields in the response:
{
hero(episode: NEWHOPE) {
movies: appears_in
}
}
{:data {:hero {:movies [:NEWHOPE :EMPIRE :JEDI]}}}
This is just an overview, far more detail is available in the manual.
Although this library is used in production at Walmart, it is still considered alpha software - subject to change. We expect to stabilize it in the near future.
To use this library with Clojure 1.9, specify an exclusion like:
[com.walmartlabs/lacinia "x.x.x" :exclusions [clojure-future-spec]]
.
More details are in the manual.
Copyright © 2017 WalmartLabs
Distributed under the Apache License, Version 2.0.