Add Schema Coordinates RFC

rfcs/SchemaCoordinates.md

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+# RFC: Schema Coordinates
+
+**Proposed by:** [Mark Larah](https://twitter.com/mark_larah) - Yelp
+
+This RFC proposes formalizing "Schema Coordinates" - a human readable syntax to
+uniquely identify a type, field, or field argument defined in a GraphQL Schema.
+
+This should be listed as a non-normative note in the GraphQL specification to
+serve as an official reference for use by third party tooling.
+
+## 📜 Problem Statement
+
+Third party GraphQL tooling and libraries may wish to refer to a field, or set of
+fields in a schema. Use cases include documentation, metrics and logging
+libraries.
+
+![](https://i.fluffy.cc/5Cz9cpwLVsH1FsSF9VPVLwXvwrGpNh7q.png)
+
+_(Example shown from GraphiQL's documentation search tab)_
+
+There already exists a convention used by some third party libraries for writing
+out fields in a unique way for such purposes. However, there is no formal
+specification or name for this convention.
+
+### Use cases
+
+1. A GraphQL server wants to **log how often each field in the schema is
+   requested**. This may be implemented by incrementing a counter by the name of
+   the schema coordinate for each field executed in a request.
+
+   _Existing implementations: Yelp (internal), Facebook (internal)_
+
+1. GraphiQL and other playgrounds / documentation sites want to show a list of
+   **search results** when a user searches for a type or field name. We can
+   display a list of schema coordinates that match the search term. A schema
+   coordinate can also be used in the hyperlink to form a permalink for
+   documentation for a particular field.
+
+   _Existing implementations: GraphiQL, Apollo Studio (see "Prior Art")_
+
+1. A developer may want to perform **analytics** on all known
+   [persisted queries][apq] - e.g. what are the most commonly used fields across
+   all documents. Schema coordinates may be used as the index/lookup keys when
+   storing this information in the database.
+
+   _Existing implementations: Yelp (internal)_
+
+   [apq]: https://www.apollographql.com/docs/apollo-server/performance/apq/
+
+1. A **GitHub bot** may want to warn developers in a Pull Request comment
+   whenever the schema diff contains a breaking change. Schema selectors can be
+   used to provide a list of which fields were broken.
+
+   _Existing implementations: GraphQL Inspector (see "Prior Art")_
+
+1. **GraphQL IDEs** (e.g. GraphiQL, GraphQL Playground, Apollo Studio) may wish
+   to display the schema definition type of a node in a query when hovering over
+   it.
+
+   <details>
+   <summary>Example</summary>
+   ![](https://i.fluffy.cc/g78sJCjCJ0MsbNPhvgPXP46Kh9knBCKF.png)
+   </details>
+
+   Schema coordinates can be used to form the left hand side of this popover.
+
+_Existing implementations: Apollo Studio (see "Prior Art")_
+
+## 🚫 What this RFC does _not_ propose
+
+- This does not cover "selectors" or "wildcard" syntax - e.g. `User.*`. _(See
+  alternatives considered.)_
+- There are **no proposed GraphQL language/syntax changes**
+- There are **no proposed GraphQL runtime changes**
+- [Schema coordinate non-goals](#-syntax-non-goals)
+
+## ✨ Worked Examples
+
+For example, consider the following schema:
+
+```graphql
+type Person {
+  name: String
+}
+
+type Business {
+  name: String
+  owner: Person
+}
+
+type Query {
+  searchBusinesses(name: String): [Business]
+}
+```
+
+We can write the following list of Schema Coordinates:
+
+- `Person` uniquely identifies the the "Person" type
+- `Business` uniquely identifies the the "Business" type
+- `Person.name` uniquely identifies the "name" field on the "Person" type
+- `Business.name` uniquely identifies the "name" field on the "Business"
+  type
+- `Business.owner` uniquely identifies the "owner" field on the "Business" type
+- `Query.searchBusinesses` uniquely identifies the "searchBusinesses" field on
+  the "Query" type
+- `Query.searchBusinesses(name)` uniquely identifies the "name" argument on the
+  "searchBusinesses" field on the "Query" type
+
+This RFC standardizes how we write coordinates GraphQL Schema members as above.
+
+## 🎨 Prior art
+
+- The name "schema coordinates" is inspired from [GraphQL Java](https://github.com/graphql-java/graphql-java)
+  (4.3k stars), where "field coordinates" are already used in a similar way as
+  described in this RFC.
+
+  - [GitHub comment](https://github.com/graphql/graphql-spec/issues/735#issuecomment-646979049)
+  - [Implementation](https://github.com/graphql-java/graphql-java/blob/2acb557474ca73/src/main/java/graphql/schema/FieldCoordinates.java)
+
+- GraphiQL displays schema coordinates in its documentation search tab:
+
+  ![](https://i.fluffy.cc/5Cz9cpwLVsH1FsSF9VPVLwXvwrGpNh7q.png)
+
+- [GraphQL Inspector](https://github.com/kamilkisiela/graphql-inspector) (840
+  stars) shows schema coordinates in its output:
+
+  ![](https://i.imgur.com/HAf18rz.png)
+
+- [Apollo Studio](https://www.apollographql.com/docs/studio/) shows schema
+  coordinates when hovering over fields in a query:
+
+  ![](https://i.fluffy.cc/g78sJCjCJ0MsbNPhvgPXP46Kh9knBCKF.png)
+
+## 🥣 Document -> Schema Coordinate serialization
+
+Use cases 3 and 5 above imply that a mapping from GraphQL query nodes to schema
+coordinates is performed.
+
+For example, consider the following schema:
+
+```graphql
+type Person {
+  name: String
+}
+
+type Business {
+  name: String
+  owner: Person
+}
+
+type Query {
+  searchBusiness(name: String): [Business]
+}
+```
+
+And the following query:
+
+```graphql
+query {
+  searchBusinesses(name: "El Greco Deli") {
+    name
+    owner {
+      name
+    }
+  }
+}
+```
+
+From the query above, we may calculate the following list of schema coordinates:
+
+- `Query.searchBusinesses`
+- `Business.name`
+- `Business.owner`
+- `Person.name`
+
+_`Query.searchBusinesses(name)` is also a valid member of the output set. The
+serialization algorithm may optionally choose to output all permutations of field
+arguments used, should this be specified._
+
+A library has been written to demonstrate this mapping: https://github.com/sharkcore/extract-schema-coordinates.
+
+## 🗳️ Alternatives considered
+
+### Naming
+
+- **"Schema Selectors"**
+
+  "Selectors" is a term used in [HTML](https://www.w3.org/TR/selectors-api/) and
+  [CSS](https://drafts.csswg.org/selectors-4/) to _select_ parts of an HTML
+  document.
+
+  This would be a compelling, familiar choice - however, we've decided to not
+  support wildcard expansion in this spec. See the section
+  [Syntax Non-goals](#-syntax-non-goals).
+
+- **"type/field pairs"**
+
+  This was the original working name. However, there already exists more
+  established terminology for this concept, and we also wish to describe more
+  than just types on fields.
+
+- **"Field Coordinates"**
+
+  "Field Coordinates" is already understood and used by the popular
+  [GraphQL Java](https://github.com/graphql-java/graphql-java) project.
+
+  [Feedback in the August GraphQL Working Group meeting](https://youtu.be/FYF15RA9H3k?t=3786)
+  hinted that since we're targeting also describing arguments, _field_
+  coordinates might not be the right name. Hence "Schema Coordinates" is chosen
+  instead, as a more generalized form of this.
+
+- **"GraphQL Coordinates"**
+
+  Similar to Field Coordinates/Schema Coordinates - however, "GraphQL
+  Coordinates" is potentially ambiguous as to if it describes _schema_ members,
+  _query/document_ members or response object members.
+
+- **"Field path" / "GraphQL path"**
+
+  [`path` exists as an attribute on `GraphQLResolveInfo`](https://github.com/graphql/graphql-js/blob/8f3d09b54260565/src/type/definition.js#L951).
+
+  Given the following query:
+
+  ```graphql
+  query {
+    searchBusinesses(name: "El Greco Deli") {
+      name
+      owner {
+        name
+      }
+    }
+  }
+  ```
+
+  `Person.name` in the response may be written as the following "field path":
+
+  ```json
+  ["query", "searchBusinesses", 1, "owner", "name"]
+  ```
+
+  Note that here, the "path" is a serialized _response_ tree traversal, instead
+  of describing the location of the field in the _schema_.
+
+  Since "path" is already used in GraphQL nomenclature to describe the location
+  of a field in a response, we'll avoid overloading this term.
+
+### Separator
+
+This RFC proposes using "`.`" as the separator character between a type and
+field. The following have also been proposed:
+
+- `Foo::bar`
+- `Foo#bar`
+- `Foo->bar`
+- `Foo~bar`
+- `Foo:bar`
+
+"`.`" is already used in the existing implementations of field coordinates, hence
+the suggested usage in this RFC. However, we may wish to consider one of the
+alternatives above, should this conflict with existing or planned language
+features.
+
+## 🙅 Syntax Non-goals
+
+This syntax consciously does not cover the following use cases:
+
+- **Wildcard selectors**
+
+  Those familiar with `document.querySelector` may be expecting the ability to
+  pass "wildcards" or "star syntax" to be able to select multiple schema
+  elements. This implies multiple ways of _selecting_ a schema node.
+
+  For example, `User.address` and `User.a*` might both resolve to `User.address`.
+  But `User.a*` could also ambiguously refer to `User.age`.
+
+  It's unclear how wildcard expansion would work with respect to field
+  arguments\*, potentially violating the requirement of this schema to _uniquely_
+  identify schema components.
+
+  \* _(e.g. does `Query.getUser` also select all arguments on the `getUser`
+  field? Who knows! A discussion for another time.)_
+
+  A more general purpose schema selector language could be built on top of this
+  spec - however, we'll consider this **out of scope** for now.
+
+- **Directive applications**
+
+  This spec does not support selecting applications of directive.
+
+  For example:
+
+  ```graphql
+  directive @private(scope: String!) on FIELD
+
+  type User {
+      name: String
+      reviewCount: Int
+      friends: [User]
+      email: String @private(scope: 'loggedIn')
+  }
+  ```
+
+  You _can_ select the definition of the `private` directive and its arguments
+  (with `@private` and `@private(scope)` respectively), but you cannot select the
+  application of the `@private` on `User.email`.
+
+  For the stated use cases of this RFC, it is more likely that consumers want to
+  select and track usage and changes to the definition of the custom directive
+  instead.
+
+  If we _did_ want to support this, a syntax such as `User.email@private[0]`
+  could work. (The indexing is necessary since [multiple applications of the same
+  directive is allowed][multiple-directives], and each is considered unique.)
+
+  [multiple-directives]: http://spec.graphql.org/draft/#sec-Directives-Are-Unique-Per-Location
+
+- **Union members**
+
+  This spec does not support selecting members inside a union definition.
+
+  For example:
+
+  ```graphql
+  type Breakfast {
+    eggCount: Int
+  }
+
+  type Lunch {
+    sandwichFilling: String
+  }
+
+  union Meal = Breakfast | Lunch
+  ```
+
+  You may select the `Meal` definition (as "`Meal`"), but you may **not** select
+  members on `Meal` (e.g. `Meal.Breakfast` or `Meal.Lunch`).
+
+  It is unclear what the use case for this would be, so we won't (yet?) support
+  this. In such cases, consumers may select type members directly (e.g. `Lunch`).
+
+## 🤔 Drawbacks / Open questions
+
+- https://github.com/graphql/graphql-spec/issues/735 discusses potential
+  conflicts with the upcoming namespaces proposal - would like to seek clarity on
+  this
+
+- Should we specify an algorithm for doing the query -> set of schema
+  coordinates? Or just hint/imply that this mapping theoretically exists? Is this
+  out of scope?
+
+### Answered questions
+
+- **Is this extensible enough?** The above issue discusses adding arguments as
+  part of this specification - we haven't touched on this here in order to keep
+  this RFC small, but we may wish to consider this in the future (e.g.
+  `Query.searchBusiness:name`).
+
+  _Update:_ As discussed in the [August Working Group Meeting][notes], this RFC
+  now includes the ability to select field arguments
+
+  [notes]: https://github.com/graphql/graphql-wg/blob/master/notes/2020-08-06.md#field-coordinates-rfc-15m-mark
+
+- **Would we want to add a method to graphql-js?** A `fieldCoordinateToFieldNode`
+  method (for example) may take in a field coordinate string and return a field
+  AST node to serve as a helper / reference implementation of the algorithm to
+  look up the field node.
+
+  _Update:_ [This was discussed in the August Working Group Meeting][meeting] -
+  it was suggested to keep any utilities as third party libraries to avoid edge
+  ambiguity problems, and to be able to iterate faster.
+
+  [meeting]: https://youtu.be/FYF15RA9H3k?t=2865