GraphQL

GraphQL is a query language and execution engine tied to any backend servic...

GraphQL

graphql/graphql-spec spec.graphql.org graphql-relay

README

GraphQL

The GraphQL specification is edited in the markdown files found in

[/spec](./spec) the latest release of which is published at

https://graphql.github.io/graphql-spec/.

The latest draft specification can be found at

https://graphql.github.io/graphql-spec/draft/ which tracks the latest commit to

the main branch in this repository.

Previous releases of the GraphQL specification can be found at permalinks that

match their release tag. For

example, https://graphql.github.io/graphql-spec/October2016/. If you are linking

directly to the GraphQL specification, it's best to link to a tagged permalink

for the particular referenced version.

Overview

This is a Working Draft of the Specification for GraphQL, a query language for

APIs created by Facebook.

The target audience for this specification is not the client developer, but

those who have, or are actively interested in, building their own GraphQL

implementations and tools.

In order to be broadly adopted, GraphQL will have to target a wide variety of

backend environments, frameworks, and languages, which will necessitate a

collaborative effort across projects and organizations. This specification

serves as a point of coordination for this effort.

Looking for help? Find resources

from the community.

Getting Started

GraphQL consists of a type system, query language and execution semantics,

static validation, and type introspection, each outlined below. To guide you

through each of these components, we've written an example designed to

illustrate the various pieces of GraphQL.

This example is not comprehensive, but it is designed to quickly introduce the

core concepts of GraphQL, to provide some context before diving into the more

detailed specification or the

GraphQL.js reference implementation.

The premise of the example is that we want to use GraphQL to query for

information about characters and locations in the original Star Wars trilogy.

Type System

At the heart of any GraphQL implementation is a description of what types of

objects it can return, described in a GraphQL type system and returned in the

GraphQL Schema.

For our Star Wars example, the

starWarsSchema.ts

file in GraphQL.js defines this type system.

The most basic type in the system will be Human, representing characters like

Luke, Leia, and Han. All humans in our type system will have a name, so we

define the Human type to have a field called "name". This returns a String,

and we know that it is not null (since all Humans have a name), so we will

define the "name" field to be a non-nullable String. Using a shorthand notation

that we will use throughout the spec and documentation, we would describe the

human type as:

```graphql
type Human {
name: String
}
```

This shorthand is convenient for describing the basic shape of a type system;

the JavaScript implementation is more full-featured, and allows types and fields

to be documented. It also sets up the mapping between the type system and the

underlying data; for a test case in GraphQL.js, the underlying data is a

set of JavaScript objects,

but in most cases the backing data will be accessed through some service, and

this type system layer will be responsible for mapping from types and fields to

that service.

A common pattern in many APIs, and indeed in GraphQL is to give objects an ID

that can be used to refetch the object. So let's add that to our Human type.

We'll also add a string for their home planet.

```graphql
type Human {
id: String
name: String
homePlanet: String
}
```

Since we're talking about the Star Wars trilogy, it would be useful to describe

the episodes in which each character appears. To do so, we'll first define an

enum, which lists the three episodes in the trilogy:

```graphql
enum Episode {
NEWHOPE
EMPIRE
JEDI
}
```

Now we want to add a field to Human describing what episodes they were in.

This will return a list of Episodes:

```graphql
type Human {
id: String
name: String
appearsIn: [Episode]
homePlanet: String
}
```

Now, let's introduce another type, Droid:

```graphql
type Droid {
id: String
name: String
appearsIn: [Episode]
primaryFunction: String
}
```

Now we have two types! Let's add a way of going between them: humans and droids

both have friends. But humans can be friends with both humans and droids. How do

we refer to either a human or a droid?

If we look, we note that there's common functionality between humans and droids;

they both have IDs, names, and episodes in which they appear. So we'll add an

interface, Character, and make both Human and Droid implement it. Once we

have that, we can add the friends field, that returns a list of Characters.

Our type system so far is:

```graphql
enum Episode {
NEWHOPE
EMPIRE
JEDI
}
interface Character {
id: String
name: String
friends: [Character]
appearsIn: [Episode]
}
type Human implements Character {
id: String
name: String
friends: [Character]
appearsIn: [Episode]
homePlanet: String
}
type Droid implements Character {
id: String
name: String
friends: [Character]
appearsIn: [Episode]
primaryFunction: String
}
```

One question we might ask, though, is whether any of those fields can return

null. By default, null is a permitted value for any type in GraphQL, since

fetching data to fulfill a GraphQL query often requires talking to different

services that may or may not be available. However, if the type system can

guarantee that a type is never null, then we can mark it as Non Null in the type

system. We indicate that in our shorthand by adding an "!" after the type. We

can update our type system to note that the id is never null.

Note that while in our current implementation, we can guarantee that more fields

are non-null (since our current implementation has hard-coded data), we didn't

mark them as non-null. One can imagine we would eventually replace our hardcoded

data with a backend service, which might not be perfectly reliable; by leaving

these fields as nullable, we allow ourselves the flexibility to eventually

return null to indicate a backend error, while also telling the client that the

error occurred.

```graphql
enum Episode {
NEWHOPE
EMPIRE
JEDI
}
interface Character {
id: String!
name: String
friends: [Character]
appearsIn: [Episode]
}
type Human implements Character {
id: String!
name: String
friends: [Character]
appearsIn: [Episode]
homePlanet: String
}
type Droid implements Character {
id: String!
name: String
friends: [Character]
appearsIn: [Episode]
primaryFunction: String
}
```

We're missing one last piece: an entry point into the type system.

When we define a schema, we define an object type that is the basis for all

query operations. The name of this type is Query by convention, and it

describes our public, top-level API. Our Query type for this example will look

like this:

```graphql
type Query {
hero(episode: Episode): Character
human(id: String!): Human
droid(id: String!): Droid
}
```

In this example, there are three top-level operations that can be done on our

schema:

- hero returns the Character who is the hero of the Star Wars trilogy; it

takes an optional argument that allows us to fetch the hero of a specific

episode instead.

- human accepts a non-null string as a query argument, a human's ID, and

returns the human with that ID.

- droid does the same for droids.

These fields demonstrate another feature of the type system, the ability for a

field to specify arguments that configure their behavior.

When we package the whole type system together, defining the Query type above

as our entry point for queries, this creates a GraphQL Schema.

This example just scratched the surface of the type system. The specification

goes into more detail about this topic in the "Type System" section, and the

type directory in

GraphQL.js contains code implementing a specification-compliant GraphQL type

system.

Query Syntax

GraphQL queries declaratively describe what data the issuer wishes to fetch from

whoever is fulfilling the GraphQL query.

For our Star Wars example, the

starWarsQueryTests.js

file in the GraphQL.js repository contains a number of queries and responses.

That file is a test file that uses the schema discussed above and a set of

sample data, located in

starWarsData.js.

This test file can be run to exercise the reference implementation.

An example query on the above schema would be:

```graphql
query HeroNameQuery {
hero {
name
}
}
```

The initial line, query HeroNameQuery, defines a query with the operation name

HeroNameQuery that starts with the schema's root query type; in this case,

Query. As defined above, Query has a hero field that returns a

Character, so we'll query for that. Character then has a name field that

returns a String, so we query for that, completing our query. The result of

this query would then be:

``` json
{
"hero": {
"name": "R2-D2"
}
}
```

Specifying the query keyword and an operation name is only required when a

GraphQL document defines multiple operations. We therefore could have written

the previous query with the query shorthand:

```graphql
{
hero {
name
}
}
```

Assuming that the backing data for the GraphQL server identified R2-D2 as the

hero. The response continues to vary based on the request; if we asked for

R2-D2's ID and friends with this query:

```graphql
query HeroNameAndFriendsQuery {
hero {
id
name
friends {
id
name
}
}
}
```

then we'll get back a response like this:

``` json
{
"hero": {
"id": "2001",
"name": "R2-D2",
"friends": [
{
"id": "1000",
"name": "Luke Skywalker"
},
{
"id": "1002",
"name": "Han Solo"
},
{
"id": "1003",
"name": "Leia Organa"
}
]
}
}
```

One of the key aspects of GraphQL is its ability to nest queries. In the above

query, we asked for R2-D2's friends, but we can ask for more information about

each of those objects. So let's construct a query that asks for R2-D2's friends,

gets their name and episode appearances, then asks for each of _their_ friends.

```graphql
query NestedQuery {
hero {
name
friends {
name
appearsIn
friends {
name
}
}
}
}
```

which will give us the nested response

``` json
{
"hero": {
"name": "R2-D2",
"friends": [
{
"name": "Luke Skywalker",
"appearsIn": ["NEWHOPE", "EMPIRE", "JEDI"],
"friends": [
{ "name": "Han Solo" },
{ "name": "Leia Organa" },
{ "name": "C-3PO" },
{ "name": "R2-D2" }
]
},
{
"name": "Han Solo",
"appearsIn": ["NEWHOPE", "EMPIRE", "JEDI"],
"friends": [
{ "name": "Luke Skywalker" },
{ "name": "Leia Organa" },
{ "name": "R2-D2" }
]
},
{
"name": "Leia Organa",
"appearsIn": ["NEWHOPE", "EMPIRE", "JEDI"],
"friends": [
{ "name": "Luke Skywalker" },
{ "name": "Han Solo" },
{ "name": "C-3PO" },
{ "name": "R2-D2" }
]
}
]
}
}
```

The Query type above defined a way to fetch a human given their ID. We can use

it by hard-coding the ID in the query:

```graphql
query FetchLukeQuery {
human(id: "1000") {
name
}
}
```

to get

``` json
{
"human": {
"name": "Luke Skywalker"
}
}
```

Alternately, we could have defined the query to have a query parameter:

```graphql
query FetchSomeIDQuery($someId: String!) {
human(id: $someId) {
name
}
}
```

This query is now parameterized by $someId; to run it, we must provide that

ID. If we ran it with $someId set to "1000", we would get Luke; set to "1002",

we would get Han. If we passed an invalid ID here, we would get null back for

the human, indicating that no such object exists.

Notice that the key in the response is the name of the field, by default. It is

sometimes useful to change this key, for clarity or to avoid key collisions when

fetching the same field with different arguments.

We can do that with field aliases, as demonstrated in this query:

```graphql
query FetchLukeAliased {
luke: human(id: "1000") {
name
}
}
```

We aliased the result of the human field to the key luke. Now the response

is:

``` json
{
"luke": {
"name": "Luke Skywalker"
}
}
```

Notice the key is "luke" and not "human", as it was in our previous example

where we did not use the alias.

This is particularly useful if we want to use the same field twice with

different arguments, as in the following query:

```graphql
query FetchLukeAndLeiaAliased {
luke: human(id: "1000") {
name
}
leia: human(id: "1003") {
name
}
}
```

We aliased the result of the first human field to the key luke, and the

second to leia. So the result will be:

``` json
{
"luke": {
"name": "Luke Skywalker"
},
"leia": {
"name": "Leia Organa"
}
}
```

Now imagine we wanted to ask for Luke and Leia's home planets. We could do so

with this query:

```graphql
query DuplicateFields {
luke: human(id: "1000") {
name
homePlanet
}
leia: human(id: "1003") {
name
homePlanet
}
}
```

but we can already see that this could get unwieldy, since we have to add new

fields to both parts of the query. Instead, we can extract out the common fields

into a fragment, and include the fragment in the query, like this:

```graphql
query UseFragment {
luke: human(id: "1000") {
...HumanFragment
}
leia: human(id: "1003") {
...HumanFragment
}
}
fragment HumanFragment on Human {
name
homePlanet
}
```

Both of those queries give this result:

``` json
{
"luke": {
"name": "Luke Skywalker",
"homePlanet": "Tatooine"
},
"leia": {
"name": "Leia Organa",
"homePlanet": "Alderaan"
}
}
```

The UseFragment and DuplicateFields queries will both get the same result,

but UseFragment is less verbose; if we wanted to add more fields, we could add

it to the common fragment rather than copying it into multiple places.

We defined the type system above, so we know the type of each object in the

output; the query can ask for that type using the special field __typename,

defined on every object.

```graphql
query CheckTypeOfR2 {
hero {
__typename
name
}
}
```

Since R2-D2 is a droid, this will return

``` json
{
"hero": {
"__typename": "Droid",
"name": "R2-D2"
}
}
```

This was particularly useful because hero was defined to return a Character,

which is an interface; we might want to know what concrete type was actually

returned. If we instead asked for the hero of Episode V:

```graphql
query CheckTypeOfLuke {
hero(episode: EMPIRE) {
__typename
name
}
}
```

We would find that it was Luke, who is a Human:

``` json
{
"hero": {
"__typename": "Human",
"name": "Luke Skywalker"
}
}
```

As with the type system, this example just scratched the surface of the query

language. The specification goes into more detail about this topic in the

"Language" section, and the

language

directory in GraphQL.js contains code implementing a specification-compliant

GraphQL query language parser and lexer.

Validation

By using the type system, it can be predetermined whether a GraphQL query is

valid or not. This allows servers and clients to effectively inform developers

when an invalid query has been created, without having to rely on runtime

checks.

For our Star Wars example, the file

starWarsValidationTests.js

contains a number of demonstrations of invalid operations, and is a test file

that can be run to exercise the reference implementation's validator.

To start, let's take a complex valid query. This is the NestedQuery example

from the above section, but with the duplicated fields factored out into a

fragment:

```graphql
query NestedQueryWithFragment {
hero {
...NameAndAppearances
friends {
...NameAndAppearances
friends {
...NameAndAppearances
}
}
}
}
fragment NameAndAppearances on Character {
name
appearsIn
}
```

And this query is valid. Let's take a look at some invalid queries!

When we query for fields, we have to query for a field that exists on the given

type. So as hero returns a Character, we have to query for a field on

Character. That type does not have a favoriteSpaceship field, so this query:

```graphql
# INVALID: favoriteSpaceship does not exist on Character
query HeroSpaceshipQuery {
hero {
favoriteSpaceship
}
}
```

is invalid.

Whenever we query for a field and it returns something other than a scalar or an

enum, we need to specify what data we want to get back from the field. Hero

returns a Character, and we've been requesting fields like name and

appearsIn on it; if we omit that, the query will not be valid:

```graphql
# INVALID: hero is not a scalar, so fields are needed
query HeroNoFieldsQuery {
hero
}
```

Similarly, if a field is a scalar, it doesn't make sense to query for additional

fields on it, and doing so will make the query invalid:

```graphql
# INVALID: name is a scalar, so fields are not permitted
query HeroFieldsOnScalarQuery {
hero {
name {
firstCharacterOfName
}
}
}
```

Earlier, it was noted that a query can only query for fields on the type in

question; when we query for hero which returns a Character, we can only

query for fields that exist on Character. What happens if we want to query for

R2-D2s primary function, though?

```graphql
# INVALID: primaryFunction does not exist on Character
query DroidFieldOnCharacter {
hero {
name
primaryFunction
}
}
```

That query is invalid, because primaryFunction is not a field on Character.

We want some way of indicating that we wish to fetch primaryFunction if the

Character is a Droid, and to ignore that field otherwise. We can use the

fragments we introduced earlier to do this. By setting up a fragment defined on

Droid and including it, we ensure that we only query for primaryFunction

where it is defined.

```graphql
query DroidFieldInFragment {
hero {
name
...DroidFields
}
}
fragment DroidFields on Droid {
primaryFunction
}
```

This query is valid, but it's a bit verbose; named fragments were valuable above

when we used them multiple times, but we're only using this one once. Instead of

using a named fragment, we can use an inline fragment; this still allows us to

indicate the type we are querying on, but without naming a separate fragment:

```graphql
query DroidFieldInInlineFragment {
hero {
name
... on Droid {
primaryFunction
}
}
}
```

This has just scratched the surface of the validation system; there are a number

of validation rules in place to ensure that a GraphQL query is semantically

meaningful. The specification goes into more detail about this topic in the

"Validation" section, and the

validation

directory in GraphQL.js contains code implementing a specification-compliant

GraphQL validator.

Introspection

It's often useful to ask a GraphQL schema for information about what queries it

supports. GraphQL allows us to do so using the introspection system!

For our Star Wars example, the file

starWarsIntrospectionTests.js

contains a number of queries demonstrating the introspection system, and is a

test file that can be run to exercise the reference implementation's

introspection system.

We designed the type system, so we know what types are available, but if we

didn't, we can ask GraphQL, by querying the __schema field, always available

on the root type of a Query. Let's do so now, and ask what types are available.

```graphql
query IntrospectionTypeQuery {
__schema {
types {
name
}
}
}
```

and we get back:

``` json
{
"__schema": {
"types": [
{
"name": "Query"
},
{
"name": "Character"
},
{
"name": "Human"
},
{
"name": "String"
},
{
"name": "Episode"
},
{
"name": "Droid"
},
{
"name": "__Schema"
},
{
"name": "__Type"
},
{
"name": "__TypeKind"
},
{
"name": "Boolean"
},
{
"name": "__Field"
},
{
"name": "__InputValue"
},
{
"name": "__EnumValue"
},
{
"name": "__Directive"
}
]
}
}
```

Wow, that's a lot of types! What are they? Let's group them:

- Query, Character, Human, Episode, Droid - These are the ones that we

defined in our type system.

- String, Boolean - These are built-in scalars that the type system

provided.

- __Schema, __Type, __TypeKind, __Field, __InputValue,

__EnumValue, __Directive - These all are preceded with a double

underscore, indicating that they are part of the introspection system.

Now, let's try and figure out a good place to start exploring what queries are

available. When we designed our type system, we specified what type all queries

would start at; let's ask the introspection system about that!

```graphql
query IntrospectionQueryTypeQuery {
__schema {
queryType {
name
}
}
}
```

and we get back:

``` json
{
"__schema": {
"queryType": {
"name": "Query"
}
}
}
```

And that matches what we said in the type system section, that the Query type

is where we will start! Note that the naming here was just by convention; we

could have named our Query type anything else, and it still would have been

returned here if we had specified it as the starting type for queries. Naming it

Query, though, is a useful convention.

It is often useful to examine one specific type. Let's take a look at the

Droid type:

```graphql
query IntrospectionDroidTypeQuery {
__type(name: "Droid") {
name
}
}
```

and we get back:

``` json
{
"__type": {
"name": "Droid"
}
}
```

What if we want to know more about Droid, though? For example, is it an

interface or an object?

```graphql
query IntrospectionDroidKindQuery {
__type(name: "Droid") {
name
kind
}
}
```

and we get back:

``` json
{
"__type": {
"name": "Droid",
"kind": "OBJECT"
}
}
```

kind returns a __TypeKind enum, one of whose values is OBJECT. If we asked

about Character instead:

```graphql
query IntrospectionCharacterKindQuery {
__type(name: "Character") {
name
kind
}
}
```

and we get back:

``` json
{
"__type": {
"name": "Character",
"kind": "INTERFACE"
}
}
```

We'd find that it is an interface.

It's useful for an object to know what fields are available, so let's ask the

introspection system about Droid:

```graphql
query IntrospectionDroidFieldsQuery {
__type(name: "Droid") {
name
fields {
name
type {
name
kind
}
}
}
}
```

and we get back:

``` json
{
"__type": {
"name": "Droid",
"fields": [
{
"name": "id",
"type": {
"name": null,
"kind": "NON_NULL"
}
},
{
"name": "name",
"type": {
"name": "String",
"kind": "SCALAR"
}
},
{
"name": "friends",
"type": {
"name": null,
"kind": "LIST"
}
},
{
"name": "appearsIn",
"type": {
"name": null,
"kind": "LIST"
}
},
{
"name": "primaryFunction",
"type": {
"name": "String",
"kind": "SCALAR"
}
}
]
}
}
```

Those are our fields that we defined on Droid!

id looks a bit weird there, it has no name for the type. That's because it's a

"wrapper" type of kind NON_NULL. If we queried for ofType on that field's

type, we would find the String type there, telling us that this is a non-null

String.

Similarly, both friends and appearsIn have no name, since they are the

LIST wrapper type. We can query for ofType on those types, which will tell

us what these are lists of.

```graphql
query IntrospectionDroidWrappedFieldsQuery {
__type(name: "Droid") {
name
fields {
name
type {
name
kind
ofType {
name
kind
}
}
}
}
}
```

and we get back:

``` json
{
"__type": {
"name": "Droid",
"fields": [
{
"name": "id",
"type": {
"name": null,
"kind": "NON_NULL",
"ofType": {
"name": "String",
"kind": "SCALAR"
}
}
},
{
"name": "name",
"type": {
"name": "String",
"kind": "SCALAR",
"ofType": null
}
},
{
"name": "friends",
"type": {
"name": null,
"kind": "LIST",
"ofType": {
"name": "Character",
"kind": "INTERFACE"
}
}
},
{
"name": "appearsIn",
"type": {
"name": null,
"kind": "LIST",
"ofType": {
"name": "Episode",
"kind": "ENUM"
}
}
},
{
"name": "primaryFunction",
"type": {
"name": "String",
"kind": "SCALAR",
"ofType": null
}
}
]
}
}
```

Let's end with a feature of the introspection system particularly useful for

tooling; let's ask the system for documentation!

```graphql
query IntrospectionDroidDescriptionQuery {
__type(name: "Droid") {
name
description
}
}
```

yields

``` json
{
"__type": {
"name": "Droid",
"description": "A mechanical creature in the Star Wars universe."
}
}
```

So we can access the documentation about the type system using introspection,

and create documentation browsers, or rich IDE experiences.

This has just scratched the surface of the introspection system; we can query

for enum values, what interfaces a type implements, and more. We can even

introspect on the introspection system itself. The specification goes into more

detail about this topic in the "Introspection" section, and the

introspection

file in GraphQL.js contains code implementing a specification-compliant GraphQL

query introspection system.

Additional Content

This README walked through the GraphQL.js reference implementation's type

system, query execution, validation, and introspection systems. There's more in

both GraphQL.js and specification,

including a description and implementation for executing queries, how to format

a response, explaining how a type system maps to an underlying implementation,

and how to format a GraphQL response, as well as the grammar for GraphQL.

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