Managing FastAPI Projects with Poetry: A Step-by-Step Guide

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This guide will walk you through how to use Poetry to manage dependencies and structure your project in FastAPI . It includes detailed explanations of Poetry's core concepts and commands to help prevent issues that can arise during team collaboration and deployment. 1. What is Poetry? Poetry is a dependency management and packaging tool for Python. It goes beyond simply installing libraries by allowing you to clearly declare the dependencies your project needs and ensuring that all developers on the project have the same library versions. Clarity in Dependency Management : Explicitly manage your project's basic information and required libraries through the pyproject.toml file. Reproducible Builds : By locking all dependency versions in the poetry.lock file, it fundamentally prevents "it works on my machine" problems. Integrated Development Environment : It automatically creates and manages isolated virtual environments for each project and handles mo...
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System Design] Reliability vs. Availability

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Software systems must provide users with reliable and continuous service. Two key concepts at the heart of this goal are Reliability and Availability.
Many people confuse these terms or use them interchangeably. However, in system architecture, it is crucial to understand the difference and consider them separately.

1. Reliability

How long does the system operate without failure?

Definition

Reliability refers to a system’s ability to operate continuously without errors.
In other words, once the system starts, how long can it run without interruption or failure?

Key Concepts

  • MTBF (Mean Time Between Failures): The average time between two consecutive failures.
  • Fault Tolerance: The system should continue to function even if one component fails.

Example

  • A server operates flawlessly for 180 out of 365 days, then experiences a critical failure and remains down for 2 days. → High reliability (long stable period), but low availability (long downtime when it fails)
  • Typical technologies to improve reliability include RAID configurations, dual power supplies, and ECC RAM.

2. Availability

How often is the system accessible to users?

Definition

Availability is the proportion of time a system is able to respond to user requests — essentially, how often the system is in a usable state.

Formula

Availability = MTBF / (MTBF + MTTR)
  • MTBF: Mean Time Between Failures
  • MTTR (Mean Time To Repair): Average time to restore the system after a failure

Example Calculation

  • MTBF = 100 hours, MTTR = 1 hour → Availability = 100 / (100 + 1) ≈ 99.01%

Availability Tiers

Level Allowed Downtime per Year
99.9% (Three Nines) ~8.76 hours
99.99% (Four Nines) ~52.6 minutes

Example

  • A server fails once per day, but automatically recovers in 1 second. → Low reliability (frequent failures), but high availability (almost always up)
  • Cloud service providers typically guarantee 99.99% availability in their SLAs (Service Level Agreements).

3. Technical Approaches: Reliability vs. Availability

Goal Core Strategy Example Technologies
Reliability Prevent failures from occurring - Static analysis, unit tests, CI/CD- Error tracking, logging
- Hardware redundancy (RAID, UPS)
- Memory error protection (ECC RAM)
- Consumer Contract Testing for microservices
Availability Recover quickly from failures - Kubernetes self-healing- Auto Scaling Groups (ASG)
- Active-Passive / Active-Active redundancy
- Load Balancer for traffic distribution
- Circuit Breakers to prevent cascading failures
- Zero-downtime deployment (Blue-Green, Canary)
Common to Both Monitoring & incident response - Monitoring (Prometheus, Grafana)- Health checks
- Alerting (PagerDuty, Opsgenie)
- Logging & tracing (ELK, Jaeger)

4. How to Apply These in Real-World Design

Reliability and Availability are both essential — but they serve different purposes.
A highly reliable system doesn’t necessarily guarantee high availability, and vice versa. Design priorities should be driven by business goals and user expectations.

When Designing for High Reliability

  • Focus on preventing failures through careful design
  • Use robust hardware, extensive testing, and fail-safe patterns
  • Example: spacecraft control systems, medical devices

When Designing for High Availability

  • Ensure fast recovery even if failures occur
  • Use redundancy, automated failover, and traffic rerouting
  • Example: e-commerce websites, video streaming platforms

5. Domain-Based Prioritization

System Type Primary Focus Reason
Hospital System Reliability Lives depend on uninterrupted operation
E-commerce Site Availability Must always be accessible to maintain sales
Financial System Both Requires real-time responsiveness and trustworthiness
Location: Calgary, AB, Canada

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