TIL (Today I Learned)

A collection of concise, practical notes on things I learn daily across DevOps, cloud infrastructure, system design, and software engineering. Each entry is a deep-dive reference designed for quick review and interview preparation.

Categories

System Design (25 entries)

Core concepts for system design interviews — distributed systems, databases, networking, and architecture patterns.

Topic	Key Concepts
CAP Theorem	CP vs AP, PACELC, real-world databases
Consistent Hashing	Virtual nodes, replication, implementation
Rate Limiting	Token bucket, sliding window, distributed with Redis
Database Sharding	Strategies, shard key selection, resharding
Database Indexing	B-Tree, composite, covering, partial indexes
Caching Strategies	Cache-aside, stampede prevention, multi-level
Load Balancing	L4 vs L7, algorithms, GSLB, health checks
Event-Driven Architecture	Outbox pattern, DLQ, choreography vs orchestration
Microservices Communication	REST vs gRPC, circuit breaker, bulkhead
Distributed Consensus	Raft, Paxos, quorum, practical usage
Saga Pattern	Orchestration vs choreography, compensation
Message Queues vs Streams	SQS vs Kafka, when to use which
API Design	REST, pagination, versioning, error handling
Back-of-Envelope Estimation	Essential numbers, estimation framework
URL Shortener Design	Capacity planning, architecture, caching
Circuit Breaker	States, implementation, when to use
CQRS	Command/query separation
Event Sourcing	Event log, state reconstruction
Bloom Filters	Probabilistic data structure
Distributed Locking	Redis, ZooKeeper, fencing tokens
Leader Election	Bully, Raft-based, ZooKeeper
CDN Architecture	Edge caching, cache invalidation
SQL vs NoSQL	Decision guide
Back Pressure	Flow control in distributed systems
Twelve-Factor App	Cloud-native application methodology

Kubernetes (8 entries)

Production Kubernetes operations, networking, and troubleshooting.

Topic	Key Concepts
Networking & Services	CNI, NetworkPolicy, Ingress, DNS
Autoscaling (HPA/VPA/KEDA)	All 4 autoscalers, Karpenter
Resource Requests & Limits	QoS classes, right-sizing, LimitRange
Troubleshooting Guide	Decision trees, debugging commands
Helm Charts	Package management for K8s
RBAC	Roles, ClusterRoles, ServiceAccounts
Pod Disruption Budgets	Safe eviction policies
Resource Management	Quotas, limits, scheduling

AWS (12 entries)

Topic	Key Concepts
ECS vs EKS vs Lambda	Decision framework, cost comparison
DynamoDB Design Patterns	Single-table, GSI, write sharding, streams
IAM & Least Privilege	Policy evaluation, ABAC, IRSA
VPC & Networking	VPC design, Transit Gateway
Lambda Cold Starts	Optimization strategies
S3 Storage Classes	Lifecycle policies, cost optimization
Aurora vs RDS	When to choose which
EventBridge	Event routing, rules, targets
Step Functions	Workflow orchestration
Secrets Manager	Secret rotation
CloudFront	CDN, edge functions
Organizations	Multi-account strategy

Azure (15 entries)

Topic	Key Concepts
Cosmos DB Consistency	5 levels, RU costs, multi-region
App Service vs Functions vs AKS	Compute decision guide
Networking Overview	VNet, NSG, Azure Firewall
Identity & Access	Entra ID, RBAC
Managed Identity	Passwordless authentication
DevOps Pipelines	CI/CD best practices
Key Vault	Secret management
Monitor & App Insights	Observability
Container Apps	Serverless containers
Service Bus	Messaging patterns
Front Door	Global load balancing
Durable Functions	Workflow patterns
Bicep	IaC for Azure
Policy	Governance
Storage Options	Blob, File, Queue, Table

GCP (15 entries)

Topic	Key Concepts
Compute Options	GCE, GKE, Cloud Run, Functions
Networking VPC	Shared VPC, firewall rules
IAM & Service Accounts	Workload Identity
Cloud Spanner	Globally distributed SQL
Pub/Sub	Messaging patterns
BigQuery	Data warehouse tips
Cloud Build	CI/CD
Cloud Functions v2	Serverless
Cloud SQL vs AlloyDB vs Spanner	Database decision
Firestore vs Bigtable	NoSQL options
Load Balancing	Global, regional
Secret Manager	Secrets management
Artifact Registry	Container registry
Workload Identity	GKE IAM integration
Pub/Sub Patterns	Advanced patterns

DevOps & SRE (12 entries)

Topic	Key Concepts
SRE Golden Signals	USE/RED, SLI/SLO/SLA, alerting
Deployment Strategies	Canary, blue-green, Argo Rollouts
Observability	Metrics, logs, traces, OpenTelemetry
Incident Management	Response framework, post-mortem
GitOps	ArgoCD, Flux, principles
CI/CD Best Practices	IaC comparison
Chaos Engineering	Principles and tools
Feature Flags	Strategies and implementation
Container Security	Best practices
Zero Trust Security	Architecture
SLI/SLO/SLA	Error budgets
Infrastructure Testing	Testing pyramid

Networking (2 entries)

Topic	Key Concepts
TCP/IP, HTTP, TLS	Protocols, handshakes, HTTP/2 vs 3
DNS Resolution	Record types, resolution flow

Terraform (2 entries)

Topic	Key Concepts
State Management	Remote backend, locking, workspaces
Modules Best Practices	Reusable module patterns

Docker (1 entry)

Topic	Key Concepts
Multi-Stage Builds & Security	Layer caching, minimal images, security

Linux (2 entries)

Topic	Key Concepts
Performance Troubleshooting	USE method, essential commands
Process Signals	SIGTERM, SIGKILL, signal handling

Python (2 entries)

Topic	Key Concepts
Asyncio Basics	Event loop, async/await
Context Managers	with statement, resource management

Git (1 entry)

Topic	Key Concepts
Rebase vs Merge	When to use which

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Total: 105+ entries across 12 categories. Updated regularly.

AI & Machine Learning (15 entries)

Deep dives into modern AI — LLMs, agents, RAG, and production AI systems.

Topic	Key Concepts
Transformer Architecture	Self-attention, multi-head, encoder vs decoder
Prompt Engineering	Few-shot, CoT, ReAct, temperature tuning
RAG	Chunking, embeddings, vector search, hybrid retrieval
AI Agents & Tool Use	ReAct loop, function calling, multi-agent
LangChain & LangGraph	LCEL, stateful workflows, human-in-the-loop
Vector Databases	HNSW, IVF, Pinecone vs Qdrant, similarity search
Fine-Tuning vs RAG	Decision framework, LoRA, QLoRA
MCP Protocol	Tools, resources, prompts, server implementation
LLM Evaluation	RAGAS, LLM-as-judge, CI testing
AI Safety & Guardrails	Prompt injection, output filtering, NeMo
Multi-Agent Systems	CrewAI, orchestration patterns, collaboration
Tokenization & Context	BPE, context limits, cost management
LLMOps	Latency, caching, model routing, monitoring
Structured Output	JSON mode, Pydantic, Instructor, parallel calls
Open Source LLMs	vLLM, Ollama, quantization, K8s deployment