GraphQL

It is a data query and manipulation language that allows specifying what data is to be retrieved or modified. This is a control/signalling plane technology developed by Facebook in 2012 and open sourced in 2015, and is now maintained by the GraphQL foundation, hosted by the non-profit Linux foundation.

GraphQL is a query language for APIs that lets clients request exactly the data they need. Unlike REST, it avoids over-fetching/under-fetching and exposes a single endpoint.

Core concepts:

1. Operations - Query, Mutation, Subscription
2. Schemas - Define types, queries, mutations, and relationships. Acts as contract between server and client
3. Resolvers - Functions for fetching data for a field
4. Introspection - Ability to query the schema itself via __schema and __type
5. Fragments
6. Error handling
7. Variables
8. Directives

Implementation concepts:

1. Authentication
2. Batching and Batching utilities(DataLoader)
3. Federation(see Apollo Federation)
4. Subscriptions usage(WebSockets)
5. Query complexity limits using cost
6. Union vs Interface
7. Caching
8. Persisted Queries
9. Securing
10. File Uploads(see Apollo Upload Server)

Operations

1. Query for reading
2. Mutation for modifying(create, delete, update)
3. Subscription for real time updates