HLD FUNDAMENTALS

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FOUNDATIONS5 concepts
001
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📐
Scalability
Design systems that grow with demand without redesigning from scratch.
growthtrafficdemand
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002
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🟢
Availability
Keep systems operational even when components fail.
uptimeSLA99.9%
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003
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🔒
Reliability
Build systems that behave correctly under load and partial failure.
correctnessdurabilityfaults
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004
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⏱
Latency vs Throughput vs Bandwidth
Understand the three core performance axes and how they trade off.
latencythroughputbandwidth
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005
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🖥
Client-Server Architecture
The fundamental request-response model everything else builds on.
HTTPclientserver
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NETWORKING & PROTOCOLS13 concepts
006
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HTTP vs HTTPS
How web requests travel, and why encryption matters.
HTTPTLSencryption
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007
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TCP vs UDP
Connection-oriented vs best-effort delivery — pick the right transport.
TCPUDPtransport
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008
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OSI Model
Seven layers that model how data moves from application to wire.
layersnetwork stackprotocols
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009
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TLS/SSL
How the internet encrypts data in transit — the mechanism behind HTTPS.
encryptioncertificateshandshake
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010
PREVIEW
DNS
How domain names resolve to IP addresses — the internet's phone book.
DNSresolutionnameserver
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011
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DNS Load Balancing
Distribute traffic across servers using DNS TTL and round-robin records.
DNSTTLround-robin
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012
PRO
Anycast Routing
Route requests to the geographically nearest server using a single IP.
anycastBGPCDN
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013
PRO
Proxy vs Reverse Proxy
Forward proxies protect clients; reverse proxies protect servers.
proxynginxgateway
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014
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WebSockets
Persistent bidirectional connections for real-time features.
WebSocketreal-timepush
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015
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WebRTC
Peer-to-peer media streaming directly between browsers.
WebRTCP2Pvideo
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016
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Long Polling
Simulate push by holding open HTTP requests until data is ready.
pollingpushHTTP
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017
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Server-Sent Events
One-way push from server to browser over a persistent HTTP stream.
SSEpushstreaming
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018
PRO
Webhooks
Event-driven HTTP callbacks triggered by changes in external systems.
webhookcallbackevents
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APIS & COMMUNICATION8 concepts
019
PREVIEW
API Design
Principles for building APIs that are intuitive, stable, and easy to consume.
APIRESTcontracts
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020
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REST APIs
Stateless HTTP APIs with resources, verbs, and status codes.
RESTHTTPJSON
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021
PRO
gRPC / Protocol Buffers
High-performance binary RPC using strongly typed contracts.
gRPCprotobufstreaming
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022
PRO
GraphQL
Flexible query language that lets clients request exactly the data they need.
GraphQLqueryschema
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023
PREVIEW
API Gateways
Centralized entry point for routing, auth, rate limiting, and observability.
gatewayroutingmiddleware
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024
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↔️
Synchronous vs Asynchronous
Request-response vs fire-and-forget — when to use each.
syncasynccoupling
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025
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Rate Limiting
Throttle requests to prevent abuse, protect downstream systems, and enforce quotas.
throttletoken bucketquota
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026
PRO
API Versioning
Evolve APIs without breaking existing clients.
versioningbackwards compatsemver
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DATABASES13 concepts
027
PREVIEW
Databases
Relational, document, key-value, graph — understand the landscape.
SQLNoSQLstorage
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028
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SQL vs NoSQL
Choose the right database model for your access patterns and consistency needs.
SQLNoSQLschema
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029
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ACID vs BASE
Strong consistency guarantees vs eventual consistency — the fundamental tradeoff.
ACIDBASEconsistency
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030
PRO
Indexing
Speed up queries by trading write overhead for read performance.
indexB-treequery
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031
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Denormalization
Duplicate data strategically to eliminate expensive joins at read time.
denormalizationjoinsperformance
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032
PREVIEW
Read Replicas
Offload read traffic to synchronized copies of the primary database.
replicaRDSread scaling
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033
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Data Replication
Copy data across nodes to improve availability and read throughput.
replicationsynclag
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034
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Connection Pooling
Reuse database connections to avoid per-request handshake overhead.
poolconnectionslatency
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035
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Query Optimization
Write and structure queries so the database engine can execute them efficiently.
queryexplainindex
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036
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Materialized Views
Pre-compute and store expensive query results for instant reads.
materialized viewcacheprecompute
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037
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Full-Text Search Engine
Index and rank text documents for relevance-based search.
Elasticsearchinverted indexsearch
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038
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Database Transactions
Group multiple operations into an atomic, all-or-nothing unit.
transactioncommitrollback
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130
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🔒
Two-Phase Locking
Database locking protocol for serializable transactions — distinct from 2PC.
2PLlocksserializable
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SPECIALIZED DATABASES2 concepts
039
PREVIEW
Time Series Database
Purpose-built storage for timestamped metrics and events at high write rates.
InfluxDBTimescaleDBmetrics
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040
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Vector Database
Store and search high-dimensional embeddings for AI and similarity search.
embeddingssimilarityAI
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STORAGE5 concepts
041
PRO
Distributed File Systems
Stripe large files across many nodes for fault-tolerant storage at scale.
HDFSGFSdistributed
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042
PRO
Block vs File vs Object Storage
Three storage abstractions: raw blocks, hierarchical files, and flat objects.
S3EBSEFS
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043
PREVIEW
Object Storage
Infinitely scalable flat storage for blobs, images, and backups.
S3objectbucket
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044
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Data Compression
Reduce storage cost and network bandwidth by encoding data more compactly.
gzipsnappycompression
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045
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Erasure Coding
Reconstruct lost data from parity fragments without full replication.
parityRAIDdurability
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CACHING7 concepts
047
PREVIEW
Caching
Store frequently accessed data closer to where it's needed to reduce latency.
cacheRedishit rate
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048
PRO
Cache Invalidation
Keep cached data consistent with the source of truth when it changes.
TTLinvalidationstale
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049
PRO
Cache Eviction Policies
LRU, LFU, FIFO — how caches decide what to remove when full.
LRULFUeviction
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050
PRO
Distributed Cache
Share a cache layer across many app servers for consistency and scale.
RedisMemcachedcluster
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051
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Cache Stampede
Thundering herd on cache expiry — and how to prevent it.
stampededog-pilemutex
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052
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Cache Warming
Pre-populate caches at startup to avoid cold-start latency spikes.
warm-uppre-loadcold start
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053
PREVIEW
🌍
CDN
Distribute static and dynamic content from edge locations close to users.
CDNCloudFrontedge
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SCALABILITY PATTERNS6 concepts
054
PRO
Vertical Scaling
Upgrade a single server to handle more load. Understand the ceiling — and why it's not enough.
EC2CPUinstance types
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055
PREVIEW
Horizontal Scaling
Add more servers instead of bigger ones. Distribute load across a fleet.
EC2 ×NstatelessALB
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056
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Load Balancing
Spread requests evenly across a server fleet. No single node gets overwhelmed.
ALBround-robinhealth check
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057
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Sharding
Split a large dataset into smaller, independently hosted partitions.
shardingpartition keyhorizontal
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058
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Data Partitioning
Divide data across nodes based on a partitioning strategy for scale.
partitionrangehash
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059
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Consistent Hashing
Distribute keys across nodes in a way that minimizes reshuffling when nodes change.
ringconsistent hashdistributed
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AVAILABILITY & RESILIENCE9 concepts
060
PREVIEW
Single Point of Failure
Identify and eliminate components whose failure takes down the entire system.
SPOFredundancyHA
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061
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High Availability vs Fault Tolerance
The difference between recovering quickly and never going down.
HAFTuptime
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062
PREVIEW
Circuit Breaker Pattern
Stop calling a failing service to give it time to recover.
circuit breakerfallbackresilience
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063
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Bulkhead Pattern
Isolate failures so one overloaded subsystem can't take down the rest.
bulkheadisolationthreadpool
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064
PRO
✂️
Network Partitions
What happens when nodes can't communicate — and how to design for it.
partitionsplitCAP
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065
PRO
Stateful vs Stateless Design
Stateless services scale horizontally; stateful services require sticky routing.
statelesssessionscalability
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066
PRO
Health Checks
Detect unhealthy instances so load balancers and orchestrators can reroute.
health checklivenessreadiness
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067
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Idempotency
Design operations so retrying them produces the same result as calling them once.
idempotencyretrysafe
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128
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🐃
Thundering Herd Problem
When many clients hammer a recovering resource, and how jitter and backoff smooth the surge.
thundering herdjitterretry
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DISTRIBUTED SYSTEMS FUNDAMENTALS9 concepts
068
PREVIEW
CAP Theorem
In a partition, you must choose between consistency and availability.
CAPconsistencyavailability
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069
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PACELC Theorem
CAP extended: even without partitions, latency and consistency trade off.
PACELClatencyconsistency
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070
PREVIEW
Consistency Models
Strong, eventual, causal — the spectrum of what 'consistent' means.
strongeventuallinearizable
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071
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🧠
Split-Brain Problem
When a network partition causes two nodes to each believe they are the leader.
split-brainfencingpartition
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072
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Heartbeats
Periodic signals that prove a node is alive — the basis of failure detection.
heartbeatgossiptimeout
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073
PRO
Leader Election
Pick one node to coordinate work — and handle the case where it dies.
leaderelectionRaft
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074
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Consensus Algorithms
Get a distributed set of nodes to agree on a single value.
consensusPaxosRaft
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075
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Quorum
Require a majority to agree before committing — the simplest consensus primitive.
quorummajorityvotes
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076
PRO
🕐
Clock Synchronization Problem
Distributed clocks drift — and the ordering problems that causes.
NTPdriftordering
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DISTRIBUTED ALGORITHMS5 concepts
077
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Paxos Algorithm
The original consensus algorithm — complex but provably correct.
Paxosprepareaccept
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078
PREVIEW
Raft Algorithm
A consensus algorithm designed to be understandable — used in etcd and CockroachDB.
Raftlogleader
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079
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Gossip Protocol
Spread information through a cluster like a rumor — decentralized and fault-tolerant.
gossipepidemicmembership
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080
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Lamport Timestamps
Assign logical timestamps to events to establish a causal ordering.
Lamporthappens-beforeordering
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081
PRO
Vector Clocks
Track causal history per-node to detect conflicting concurrent writes.
vector clockcausalityDynamo
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DISTRIBUTED TRANSACTIONS5 concepts
082
PRO
Distributed Transactions
Atomically commit changes across multiple services or databases.
distributedatomic2PC
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083
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Two-Phase Commit
The classic distributed commit protocol — safe but blocking.
2PCpreparecommit
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084
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Three-Phase Commit
Add a pre-commit phase to 2PC to avoid blocking on coordinator failure.
3PCnon-blockingcoordinator
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085
PREVIEW
SAGA Pattern
Long-running distributed transactions using compensating actions instead of locks.
SAGAchoreographyorchestration
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086
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Outbox Pattern
Guarantee message delivery by writing events to an outbox table in the same transaction.
outboxCDCat-least-once
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MESSAGING & EVENTS6 concepts
089
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Message Queues
Decouple producers from consumers using an async buffer.
SQSqueueasync
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090
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Pub/Sub
Broadcast events to many subscribers without the publisher knowing who they are.
pub/subSNStopic
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091
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Event-Driven Architecture
Build systems that react to events rather than calling each other directly.
eventsdecoupledasync
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087
PRO
📬
Delivery Semantics
At-most-once, at-least-once, exactly-once — and the cost of each.
deliveryidempotencyexactly-once
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088
PRO
Change Data Capture
Stream database changes as events to downstream consumers.
CDCDebeziumstreaming
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Backpressure
Signal to producers to slow down when consumers can't keep up.
backpressureflow controlqueue
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ARCHITECTURE PATTERNS13 concepts
092
PRO
Monolithic Architecture
All functionality in a single deployable — simple to start, hard to scale.
monolithsingle deploycoupling
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093
PREVIEW
Microservices Architecture
Split functionality into independently deployable services around business domains.
microservicesbounded contextAPI
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094
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Serverless Architecture
Deploy functions without managing servers — pay per invocation.
LambdaFaaSserverless
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095
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CQRS
Separate the models for reading and writing data to optimize each independently.
CQRSread modelwrite model
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096
PRO
Event Sourcing
Store state as an append-only log of events instead of current values.
event sourcinglogreplay
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097
PRO
Backend for Frontend
A dedicated API layer tailored to each client type (mobile, web, TV).
BFFAPIclient-specific
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098
PRO
Strangler Fig Pattern
Incrementally replace a monolith by routing traffic to new services one piece at a time.
strangler figmigrationincremental
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099
PRO
Sidecar Pattern
Attach a helper container to each service for shared concerns like proxying and logging.
sidecarcontainerproxy
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100
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Service Mesh
Offload service-to-service networking — mTLS, retries, tracing — to a mesh layer.
IstioEnvoymTLS
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101
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Service Discovery
Let services find each other dynamically without hardcoded addresses.
Consulservice registryDNS
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102
PRO
🌏
Multi-Region Architecture
Deploy across multiple geographic regions for latency and disaster recovery.
multi-regiongeoDR
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111
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Blue/Green & Canary Deployments
Release changes safely by splitting traffic between old and new versions.
blue-greencanaryrollout
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112
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Feature Flags
Toggle features on/off at runtime without deploying new code.
feature flagLaunchDarklyrollout
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SECURITY & IDENTITY8 concepts
103
PREVIEW
Authentication vs Authorization
Who you are vs what you're allowed to do — two distinct problems.
authnauthzidentity
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Session vs Token Authentication
Server-side sessions vs stateless signed tokens — tradeoffs for scale.
sessiontokenstateless
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105
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OAuth / OpenID Connect
Delegate authentication to a trusted identity provider without sharing passwords.
OAuth2OIDCidentity provider
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106
PREVIEW
JWT
Self-contained signed tokens that carry claims — no database lookup required.
JWTclaimssignature
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107
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Role-Based Access Control
Assign permissions to roles, then assign roles to users.
RBACrolespermissions
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108
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Single Sign-On
Authenticate once and access multiple services without re-logging in.
SSOSAMLfederation
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109
PRO
Secrets Management
Store and rotate API keys, credentials, and certs without hardcoding them.
Vaultsecretsrotation
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110
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Mutual TLS (mTLS)
Require both sides to authenticate with certificates — essential in microservices.
mTLScertificateszero-trust
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OBSERVABILITY6 concepts
113
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Observability
Logs, metrics, and traces — the three pillars of understanding distributed systems.
observabilitypillarstelemetry
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Logging
Record discrete events from your system for debugging and audit.
logsstructuredELK
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115
PREVIEW
Metrics
Measure system behavior over time with counters, gauges, and histograms.
metricsPrometheusGrafana
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116
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Distributed Tracing
Follow a request through multiple services to find where latency lives.
tracingJaegerspans
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117
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Correlation IDs
Attach a unique ID to each request so logs across services can be joined.
correlation IDrequest IDtrace
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129
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📋
SLA / SLO / SLI
Define and measure service reliability commitments formally.
SLASLOSLI
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DATA ENGINEERING4 concepts
118
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Batch vs Stream Processing
Process data in large periodic batches vs continuously as it arrives.
batchstreamKafka
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119
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ETL Pipelines
Extract, transform, and load data from sources to destinations.
ETLpipelinedata warehouse
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MapReduce
Distribute computation across a cluster by mapping then reducing.
MapReduceHadoopdistributed
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121
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🏔
Data Lake vs Data Warehouse
Raw storage for everything vs structured storage optimized for analytics.
data lakewarehouseSnowflake
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ALGORITHMS & DATA STRUCTURES6 concepts
046
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B-trees and B+ Trees
The data structure powering most database indexes and file systems.
B-treeindexdisk
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123
PREVIEW
Bloom Filter
Probabilistic set membership — fast 'definitely not / maybe' with zero false negatives.
Bloom filterprobabilisticset
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LSM Tree
Write-optimized tree structure used in Cassandra, RocksDB, and LevelDB.
LSMCassandrawrite-ahead
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125
PREVIEW
Merkle Tree
Efficiently verify data integrity across distributed nodes using hash trees.
Merklehashintegrity
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126
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HyperLogLog
Count unique items in massive datasets using constant memory.
HyperLogLogcardinalityapproximate
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127
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✅
Checksums
Detect data corruption in transit or at rest using hash-based fingerprints.
checksumCRCintegrity
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