What two patterns do you need to call an unreliable external API safely?

Category: System Design

Answer

Tenacity (retry with exponential backoff + jitter) handles transient errors. Circuit breaker (closed -> open -> half-open) handles sustained outages by short-circuiting before they pile up. Use both: tenacity in the inner loop, breaker in the outer. The breaker decides whether to retry at all.

Concrete examples from the fca project context

Example 1

Tenacity: retry 3x, wait_exponential_jitter (1, 2, 4 sec with ±30%).

Example 2

Breaker: trip after 5 failures in 60 sec; cool-down 30 sec; half-open trial.

Example 3

Pattern: if breaker.closed: try with tenacity; else: return cached or 503.

Data flow / flow chart

request -> breaker (closed/open/half-open)
  if closed: tenacity.retry(call) -> success/fail
  if open:   503 fast

Takeaway

Tenacity is “try harder”; circuit breaker is “stop trying”. You need both.