OAuth 2.0 Overview

Ideas you will meet

OAuth

Delegated authorization flows for APIs and apps.

API Gateway

A single entry point for routing, auth and throttling.

What we already know about this

Our own lessons on these ideas. Read them here before you start, so it makes sense.

• Core Concepts: How Things Actually Work · 9 minSSO, OAuth 2.0 and OpenID ConnectHow 'Sign in with Google' works, and the difference between authentication and authorisation.Read hereClose

  Two questions, two words

  • Authentication: who are you?
  • Authorisation: what are you allowed to do?

  OAuth 2.0: delegated access

  OAuth 2.0 lets a user give an app limited access to their data at another service, without sharing their password. Roles:

  • Resource owner: the user.
  • Client: the app asking for access.
  • Authorization server: issues tokens (for example Google, GitHub).
  • Resource server: the API holding the data.

  The user grants specific scopes (for example "read my calendar"), and the client receives an access token that is limited to those scopes and expires.

  The authorization code flow

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  Figure SDA-CORE-07-1 · Authorization code flow · © forcsguy.com

  Important details:

  • state: a random value you check on return, which stops attackers forging the callback (CSRF).
  • PKCE: a one-time proof that the party exchanging the code is the one that started the flow. Use it for all clients, essential for mobile and single-page apps.
  • Register exact redirect URIs and compare them strictly.

  OpenID Connect: adding identity

  OAuth alone says "this token may call the API", not "this is Jane". OpenID Connect (OIDC) adds an ID token (a signed JWT) with claims about the user: subject id, email, name. To sign someone in, validate the ID token's signature, issuer, audience and expiry.

  SSO

  With single sign-on, many applications trust one identity provider. The user signs in once; the provider vouches for them to each app. Enterprises use SAML or OIDC for this. Benefits: fewer passwords, central policies like MFA, and one place to revoke access when someone leaves.

  Single sign-on means the second application never sees your password. It trusts the sign-in service you already used:

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  Figure SDA-CORE-07-2 · One sign-in, two applications · © forcsguy.com

  Common mistakes

  • Using the access token as proof of identity instead of validating an ID token.
  • Skipping state or accepting any redirect URI.
  • Storing tokens somewhere scripts can read them.
  • Requesting more scopes than the feature needs.

  More on this site

  Related reading

  • RFC 6749: The OAuth 2.0 Authorization FrameworkIETFDocumentation
  • OAuth 2.0 OverviewOAuth Community · oauth.netDocumentation
  • Introduction to JSON Web TokensAuth0 · jwt.ioDocumentation
  • System Design 101ByteByteGo · ByteByteGoHqGitHub

  Show 6 moreShow fewer

  • OAuth Audit Toolbox at IndeedIndeed engineers · IndeedCase Studies
  • The Architecture of Open Source ApplicationsVarious authors · aosabook.orgBooks
  • Secret Detector: Preventing Secrets in Source Code at YelpYelp engineers · YelpCase Studies
  • LevelDBGoogle · GitHubGitHub
  • Garbage Collection in Go Application at TwitchTwitch engineers · TwitchCase Studies
  • DevOps resource center - Azure DevOpsMicrosoft engineers · MicrosoftBlogs

  Open this lesson with its quiz and exercise

• Advanced Distributed Systems · 12 minLoad balancing: layer 4, layer 7 and gatewaysWhat a load balancer can see and do at each layer, the algorithms behind it, and how API gateways add routing, aggregation and policy.Read hereClose

  What a load balancer sees

  A load balancer spreads incoming work across servers, removes failed servers and makes scaling invisible to clients. What it can do depends on which layer it works at.

  	Layer 4 (transport)	Layer 7 (application)
  Sees	IP addresses, ports, TCP/UDP connections	HTTP method, URL, headers, cookies, body
  Decides by	Connection details	Request content
  TLS	Passes through, or terminates in some products	Usually terminates TLS
  Speed and cost	Very fast, low overhead	More work per request
  Typical features	Spread connections, health checks	Path routing, header rules, sticky sessions, rewrites, caching, WAF rules
  Examples	Network load balancers, LVS, HAProxy in TCP mode	NGINX, Envoy, HAProxy HTTP mode, cloud application balancers

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  Figure SDA-ADS-05-1 · Layers of balancing in front of an app · © forcsguy.com

  Many designs use both: an L4 tier for raw scale and an L7 tier for smart routing.

  Before: layer 4: ports and addresses

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  After: layer 7: reads the request

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  What changed

  • Layer 7 can route by path, header or cookie
  • Layer 7 can terminate TLS and retry safely
  • Layer 4 is simpler and faster, with less to configure

  Choosing a target: algorithms

  • Round robin / weighted round robin: simple rotation; weights favour bigger machines.
  • Least connections (and weighted): send to the server with fewest active requests; good when requests vary in length.
  • Least response time: combine active load with measured latency.
  • IP hash / consistent hashing: the same client or key goes to the same server, useful for caches and sticky state (see consistent hashing).
  • Resource-based: servers report load and the balancer follows.

  Health checks (active probes and passive error tracking) remove unhealthy servers; connection draining lets in-flight requests finish before a server is removed during a deploy.

  Global and DNS balancing

  Across regions, DNS-based or global server load balancing directs users to the nearest or healthiest region (see the DNS and CDN lessons). DNS answers are cached, so failover takes as long as the TTL; anycast and global balancers react faster.

  API gateway: policy at the edge

  An API gateway is an L7 component specialised for APIs. Beyond routing it commonly provides:

  • Authentication and authorisation (validate tokens, apply scopes),
  • Rate limiting and quotas (see the rate-limiter lesson),
  • Request and response transformation, versioning, and protocol translation,
  • Aggregation: one client call fans out to several services and merges results, saving round trips for mobile clients,
  • Caching, logging, metrics and tracing in one place.

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  Figure SDA-ADS-05-2 · Gateway routing and aggregation · © forcsguy.com

  Risks: the gateway is a single entry point (run several instances, keep it stateless) and can become a bottleneck or a place where business logic piles up. Keep it to cross-cutting concerns. A variation is a backend for frontend: one gateway per client type (web, mobile) tailored to its needs.

  Inside the cluster

  Between services, balancing is done by client-side libraries or sidecar proxies that know the healthy instances from service discovery, so no central balancer is needed for east-west traffic. See the microservices lessons for service mesh ideas.

  Failure habits

  • Run at least two of every balancer and gateway instance across zones.
  • Keep timeouts and retry budgets (see the resilience lesson) so the balancer does not amplify an outage.
  • Use slow start for new instances so they warm up before taking a full share.

  Real-world reading

  Related reading

  • Maglev: A Fast and Reliable Software Network Load Balancer (paper)Eisenbud et al. · GooglePapers
  • NGINX HTTP Load BalancingNGINX · F5 / NGINXDocumentation
  • Scaling Your API with Rate LimitersPaul Tarjan · StripeBlogs
  • Active-Active for Multi-Regional ResiliencyNetflix Engineering · NetflixCase Studies

  Show 6 moreShow fewer

  • ByteByteGo: System Design Video SeriesAlex Xu · ByteByteGoYouTube
  • Rate Limiting with NGINXNGINX · F5 / NGINXBlogs
  • Implementing health checksAmazon Builders' Library · Amazon Web ServicesBlogs
  • Google SRE Book: Load Balancing in the DatacenterGoogle SRE team · GoogleBlogs
  • Traffic Steering using Rum DNS at LinkedInLinkedIn engineers · LinkedInCase Studies
  • Google SRE BookBetsy Beyer et al. · GoogleCourses

  Open this lesson with its quiz and exercise