Lacinia

Lacinia Manual

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.

• Full support of inline and named query fragments.

• 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.

• Embrace Clojure: Use EDN data, keywords, functions, and persistent data structures.

• Keep it simple: You provide the schema and a handful of functions to resolve data, and Lacinia does the rest.

• Do the right thing: apply reasonable defaults without a lot of "magic".

Getting Started

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.

Status

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-alpha17, specify an exclusion like: [com.walmartlabs/lacinia "x.x.x" :exclusions [clojure-future-spec]].

More details are in the manual.

License

Copyright © 2017 WalmartLabs

Distributed under the Apache License, Version 2.0.