Book 03 · Cheat sheet

Backend & APIs

Server-side services, API design and the ways software systems talk to each other.

01.NET: .NET is Microsoft's free, open-source developer platform for building web, desktop, mobile, cloud and game applications, mainly in C#.
.NET is Microsoft's open-source platform for many kinds of apps, mainly in C#.
.NET Core in 2016 made it cross-platform; .NET 5 in 2020 unified the line.
Code runs on the CLR, which JIT-compiles it and manages memory.
02APIApplication Programming Interface: An API is a set of rules that lets one piece of software request data or actions from another in a predictable, documented way.
An API defines how two pieces of software communicate.
Callers don't need to know the internal implementation.
Web APIs usually work over HTTP and return JSON.
03API Gateway: An API gateway is a server that sits in front of a group of backend services and acts as the single entry point that receives, checks, and routes API requests.
An API gateway is the single entry point for clients calling many backend services.
It routes each request to the right service based on its path, host, or headers.
It centralizes authentication, rate limiting, logging, and TLS.
04API Versioning: API versioning is the practice of labeling and managing changes to an API so existing clients keep working while new versions add or change features.
Versioning lets an API make breaking changes without breaking existing clients.
Common approaches put the version in the URL path, a header, or a query parameter.
Adding optional fields is backward compatible; removing or renaming fields is a breaking change.
05Backend: The backend is the server side of an application: the code, databases and services that store data, apply business rules and answer the frontend's requests.
The backend is the server side: data, business rules and security.
It answers requests from the frontend and other programs through an API.
It usually combines application code, databases, caches and queues.
06Background Job: A background job is a task that a server runs outside the normal request-response cycle, so slow work like sending emails doesn't make users wait.
Background jobs move slow work out of the request so users get fast responses.
The app enqueues a job; separate worker processes run it later.
Failed jobs are retried, often with exponential backoff.
07Backpressure: Backpressure is a mechanism that lets a slow consumer signal a fast producer to slow down, so data doesn't pile up faster than it can be processed.
Backpressure lets a slow consumer tell a fast producer to slow down or pause.
Without it, unbounded buffers grow until latency spikes or memory runs out.
Common strategies are pull-based consumption, bounded buffers that block, and load shedding.
08Cache: A cache is a fast, temporary storage layer that keeps copies of frequently used data so later requests can be served quickly without repeating slow work.
A cache keeps copies of data in fast storage to avoid repeating slow work.
A cache hit returns stored data; a cache miss falls back to the original source.
Caches exist in CPUs, browsers, CDNs, and applications, for example Redis.
09Cron Job: A cron job is a command or script that runs automatically on a repeating schedule, such as every night at 2 a.m., defined by a five-field cron expression.
A cron job runs a command automatically on a time-based schedule.
Schedules have five fields: minute, hour, day of month, month, and day of week.
0 2 * * * means every day at 2:00; */15 * * * * means every 15 minutes.
10CRUDCreate, Read, Update, Delete: CRUD stands for create, read, update and delete, the four basic operations for working with stored data in databases, APIs and most business applications.
CRUD stands for create, read, update, and delete.
In SQL, the operations are INSERT, SELECT, UPDATE, and DELETE.
In REST APIs, they usually map to POST, GET, PUT or PATCH, and DELETE.
11Django: Django is an open-source Python web framework that comes with an ORM, an admin panel, authentication and security protections, so teams can build quickly.
Django is an open-source, "batteries included" Python web framework.
Models, views and templates form its MTV structure.
Its ORM maps Python classes to database tables.
12Endpoint: An endpoint is a specific URL, combined with an HTTP method, where an API receives requests and returns responses for one particular resource or action.
An endpoint is a URL where an API accepts requests.
The same URL with different HTTP methods can act as different endpoints.
Endpoint URLs combine a base URL, a resource path, and optional query parameters.
13Event Loop: The event loop is a mechanism that lets a single thread handle many tasks by running callbacks one at a time as events and I/O results become ready.
The event loop runs callbacks one at a time on a single thread.
Slow I/O happens in the background, so the thread never sits idle waiting.
Microtasks, like promise callbacks, run before the next task, like a setTimeout callback.
14Event Streaming: Event streaming is the practice of recording events as a continuous, ordered and durable log that many applications can read, replay and process in real time.
Events are recorded in order in a durable, append-only log.
Many consumers read the same stream independently, each tracking its own offset.
Streams can be replayed from any point within the retention period.
15Exponential Backoff: Exponential backoff is a retry strategy that waits longer after each failed attempt, such as 1, 2, 4 and 8 seconds, so a struggling service can recover.
Each retry waits longer than the last, usually doubling the delay.
Random jitter keeps many clients from retrying in lockstep.
Cap both the maximum delay and the number of attempts.
16ExpressExpress.js: Express is the most widely used Node.js web framework: a minimal layer that handles routing, requests and responses through a chain of middleware functions.
Express is a minimal web framework for Node.js, first released in 2010.
Routes map HTTP methods and paths to handler functions.
Middleware functions process each request in order and call next().
17FastAPI: FastAPI is a modern Python API framework that uses standard type hints to validate requests, convert data and generate OpenAPI documentation automatically.
FastAPI builds APIs in Python using standard type hints.
Types drive request validation through Pydantic models.
Interactive OpenAPI docs are generated automatically at /docs.
18Flask: Flask is a lightweight Python web framework offering routing, request handling and templates, leaving the database and project structure to the developer.
Flask is a lightweight Python web framework released in 2010.
It is built on Werkzeug for HTTP and Jinja for templates.
Routes are functions marked with decorators such as @app.route.
19GraphQL: GraphQL is a query language and runtime for APIs that lets clients request exactly the data they need, often from a single endpoint in a single request.
Clients specify exactly which fields they want in the response.
A typed schema describes all available data and operations.
Queries read data, mutations change it, and subscriptions stream updates.
20gRPC: gRPC is an open-source framework for calling functions on a remote server as if they were local, using Protocol Buffers and HTTP/2 for fast, typed messages.
gRPC lets clients call methods on a remote server like local functions.
Services and messages are defined in .proto files using Protocol Buffers.
Client and server code is generated automatically for many languages.
21Health Check: A health check is a small automated test, usually an HTTP endpoint, that reports whether a service is up and able to handle requests, so failures show fast.
A health check is a quick endpoint or command that reports whether a service is healthy.
Load balancers and orchestrators call it regularly and reroute traffic or restart automatically.
Liveness checks trigger restarts; readiness checks control whether traffic is sent.
22HTTP Caching: HTTP caching is the reuse of stored HTTP responses by browsers, CDNs and proxies, controlled by headers like Cache-Control and ETag, to avoid repeat downloads.
Cache-Control tells browsers and CDNs whether, and for how long, they may reuse a response.
ETag and Last-Modified let clients revalidate with a cheap 304 Not Modified response.
no-cache means revalidate before every use; no-store means never store.
23HTTP Method: An HTTP method is the verb in an HTTP request, such as GET, POST, PUT, PATCH or DELETE, that tells the server what action to perform on the requested resource.
The method is the verb of an HTTP request, such as GET, POST, PUT, PATCH, or DELETE.
GET reads, POST creates or triggers actions, PUT replaces, PATCH partially updates, and DELETE removes.
Safe methods don't change server state; idempotent methods can be repeated safely.
24Idempotency: Idempotency is the property of an operation that produces the same result whether it runs once or many times, so accidentally repeating a request is safe.
An idempotent operation has the same effect no matter how many times it runs.
It makes retries safe after timeouts, crashes, and duplicate messages.
GET, PUT, and DELETE are idempotent in HTTP; POST is not.
25JSONJavaScript Object Notation: JSON is a lightweight, text-based format for storing and exchanging structured data as key-value pairs and lists, readable by both humans and machines.
JSON stands for JavaScript Object Notation but works with almost every language.
Data is written as objects ({}), arrays ([]), strings, numbers, booleans, and null.
Keys and strings must use double quotes.
26KafkaApache Kafka: Apache Kafka is a distributed event streaming platform that stores events in durable, ordered logs for many services to publish, read in real time or replay.
Kafka stores events in durable, ordered, append-only logs called topics.
Topics are split into partitions spread across a cluster of brokers.
Consumers track their own offset, so they can resume or replay history.
27Laravel: Laravel is a PHP framework for full web applications, with an elegant syntax and built-in routing, the Eloquent ORM, authentication, queues and migrations.
Laravel is the most popular PHP framework, first released in 2011.
It uses MVC with routes, controllers, Eloquent models and Blade views.
Eloquent is an expressive active record ORM; migrations version the schema.
28Long Polling: Long polling is a technique where a client sends a request that the server holds open until new data is ready, imitating real-time push over plain HTTP.
The server holds each request open until new data is available or a timeout expires.
The client sends a new request immediately after each response.
It delivers updates faster, and with fewer empty responses, than regular polling.
29Message Queue: A message queue is a component that stores messages from one service until another is ready to process them, so parts of a system can work asynchronously.
Producers send messages; consumers process them later, at their own pace.
The queue decouples services and absorbs traffic spikes.
Consumers acknowledge messages after processing, so work isn't lost if they crash.
30Middleware: Middleware is software that sits between two layers of a system, most often code that runs between an incoming request and the final response in a web server.
Middleware runs between receiving a request and sending the response.
Multiple middleware functions form a chain that runs in order.
Each one can modify the request, end it early, or pass it on.
31Nginx: Nginx is a fast, open-source web server that is also widely used as a reverse proxy, load balancer and HTTP cache in front of application servers.
Nginx is an open-source web server, reverse proxy, load balancer and cache.
An event-driven design lets it handle many thousands of connections cheaply.
It serves static files and forwards dynamic requests to app servers.
32Node.js: Node.js is an open-source JavaScript runtime that runs JavaScript outside the browser, most often to build web servers, APIs, and command-line tools.
Node.js runs JavaScript outside the browser, on servers and developer machines.
It is a runtime built on Chrome's V8 engine, not a language or a framework.
A non-blocking event loop lets one process handle many concurrent connections.
33OpenAPI: OpenAPI is an open standard for describing HTTP APIs in a YAML or JSON file, so people and tools can understand every endpoint, parameter, and response.
OpenAPI describes HTTP APIs in a standard YAML or JSON document.
It covers endpoints, parameters, request and response schemas, and authentication.
Tools use it to generate docs, client SDKs, server stubs, mock servers, and tests.
34Pagination: Pagination is the practice of splitting a large set of results into smaller pages, so an API or website returns a manageable number of items per request.
Pagination returns large result sets in smaller pages.
Offset pagination (limit and offset) is simple and allows jumping to any page.
Cursor pagination continues after the last item seen and stays fast on large tables.
35Pub/SubPublish-Subscribe: Pub/sub is a messaging pattern where publishers send messages to topics and every subscriber to a topic gets a copy, without either side knowing the other.
Publishers send messages to topics, and every subscriber to a topic gets a copy.
Publishers and subscribers don't know about each other, which keeps services loosely coupled.
New subscribers can be added without changing the publisher.
36RabbitMQ: RabbitMQ is an open-source message broker that routes producers' messages through exchanges into queues, where consumers take them and confirm when done.
RabbitMQ is an open-source message broker written in Erlang.
Producers send to exchanges, which route messages into queues by bindings.
Direct, topic and fanout exchanges cover one-to-one and one-to-many delivery.
37Rate Limiting: Rate limiting is a technique that caps how many requests a client can make to a server or API within a time window, protecting it from abuse and overload.
Rate limiting caps how many requests a client can make in a period of time.
Clients over the limit usually receive HTTP 429 Too Many Requests.
Limits are typically applied per API key, user, or IP address.
38REST APIRepresentational State Transfer API: A REST API is a web API that exposes data as resources identified by URLs and lets clients read or change them using standard HTTP methods.
REST is an architectural style, not a protocol or a formal standard.
Resources are identified by URLs, such as /users/42.
HTTP methods (GET, POST, PUT, PATCH, DELETE) describe the action.
39RPCRemote Procedure Call: RPC is a communication style in which a program calls a function that runs on another machine as if it were a local function, hiding the network in between.
RPC makes a call to code on another machine look like a local function call.
Client stubs serialize the arguments and send them; the server runs the function and returns the result.
gRPC, JSON-RPC, and Apache Thrift are common RPC implementations.
40Ruby on Rails: Ruby on Rails is a full-stack web framework for Ruby that favors convention over configuration, so database-backed web apps can be built quickly.
Rails is a full-stack Ruby web framework released in 2004.
Convention over configuration means a standard structure and fewer decisions.
Active Record maps models to tables; migrations version the schema.
41Serialization: Serialization is the process of converting in-memory data structures into a format such as JSON or bytes, so they can be stored or sent over a network.
Serialization converts in-memory data into text or bytes for storage or transfer.
Deserialization rebuilds the in-memory data from that format.
JSON, XML, and YAML are text formats; Protocol Buffers and MessagePack are binary.
42Server-Sent Events: Server-Sent Events is a web standard that lets a server push a continuous stream of text updates to the browser over one long-lived HTTP connection.
SSE streams messages from server to browser over one long-lived HTTP response.
The server sends text/event-stream data, and browsers read it with the EventSource API.
Browsers reconnect automatically and resume using the Last-Event-ID header.
43Session: A session is a way for a server to remember a user across many requests, usually by keeping their data on the server and giving the browser a session ID.
Sessions let a server remember a user across stateless HTTP requests.
The server stores session data and gives the client a random session ID.
The session ID usually travels in an HttpOnly, Secure cookie.
44SOAPSimple Object Access Protocol: SOAP is an XML-based messaging protocol for web services that wraps each request and response in a strict envelope, usually defined by a formal WSDL contract.
SOAP messages are XML documents with an envelope, an optional header, and a body.
A WSDL file formally describes the service's operations and data types.
WS-* standards add message-level security and reliable delivery.
45Spring Boot: Spring Boot is a Java framework for quickly building production-ready services on top of Spring, with auto-configuration and an embedded web server.
Spring Boot builds production-ready Java apps on the Spring Framework.
Starters and auto-configuration replace most manual setup.
Apps run as a single JAR with an embedded server such as Tomcat.
46Web Server: A web server is software, or the machine running it, that accepts HTTP requests from browsers and other clients and responds with pages, files, or data.
A web server receives HTTP requests and returns responses such as HTML, files, or JSON.
It usually listens on port 80 for HTTP and port 443 for HTTPS.
Static files are served directly; dynamic requests are passed to application code.
47Webhook: A webhook is an automated HTTP request that one application sends to a URL you provide as soon as a specific event happens, such as a completed payment.
A webhook is an HTTP request triggered by an event.
The sender pushes data to you, so you don't have to poll.
You provide the URL; the other service calls it.
48XMLExtensible Markup Language: XML (Extensible Markup Language) is a text format for structured data that uses nested tags you define yourself, readable by both people and machines.
XML is a text format for structured data with self-defined tags.
It became a W3C standard in 1998.
XSD validates documents, XPath queries them and XSLT transforms them.