System Design Availability and Reliability🌱 Beginner

Availability is the percentage of time a system is up and responding; reliability is doing the *right* thing correctly. A system can be available (responding) but unreliable (returning wrong results).

The “nines”

Each extra nine is dramatically harder and more expensive. availability = uptime / (uptime + downtime).

SLA, SLO, SLI

Achieving high availability

• Remove single points of failure — redundancy at every layer (servers, databases, zones).
• Replication & failover — a standby takes over automatically when a component dies.
• Multiple availability zones / regions — survive a whole datacenter outage.
• Health checks + load balancers — route away from unhealthy instances.

Redundancy math

Components in series multiply failure risk; redundant components in parallel multiply availability. Two independent 99% components in parallel give ~99.99% — which is why redundancy is the foundation of HA.

Implementing a Circuit Breaker for High Availability

A circuit breaker prevents cascading failures by stopping requests to a struggling dependency:

class CircuitBreaker {
  private failures = 0;
  private state: "CLOSED" | "OPEN" | "HALF_OPEN" = "CLOSED";
  private nextAttempt = 0;

  constructor(
    private failureThreshold = 5,
    private resetTimeoutMs = 10000
  ) {}

  async execute<T>(fn: () => Promise<T>): Promise<T> {
    if (this.state === "OPEN") {
      if (Date.now() > this.nextAttempt) {
        this.state = "HALF_OPEN"; // Test if service recovered
      } else {
        throw new Error("Circuit is OPEN: Request blocked to prevent cascade failure");
      }
    }

    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (err) {
      this.onFailure();
      throw err;
    }
  }

  private onSuccess() {
    this.failures = 0;
    this.state = "CLOSED";
  }

  private onFailure() {
    this.failures++;
    if (this.failures >= this.failureThreshold) {
      this.state = "OPEN";
      this.nextAttempt = Date.now() + this.resetTimeoutMs;
    }
  }
}