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Exercise

Build a multi-provider failover chain

Implement a ProviderFailoverChain that:

Takes a list of provider callables (async functions with signature async (messages: list[dict]) -> str)
complete(messages: list[dict]) -> tuple[str, int] — tries providers in order, returns (response, provider_index) for the first one that succeeds
Skips providers whose circuit breaker is open (use a simple in-memory circuit breaker: open after 3 consecutive failures, auto-reset after 60 seconds)
Raises RuntimeError("all providers exhausted") if every provider fails or has an open circuit
Tracks which provider was last used and how many times each provider has been called

Demonstrate with a test where provider 0 always raises an exception, provider 1 succeeds.

import asyncio
import time

async def provider_always_fails(messages):
    raise ConnectionError("Provider 0 is down")

async def provider_always_succeeds(messages):
    return "Hello from provider 1"

Deployment / medium / Step 5 of 6

Practice stage

General drill

Hint

Keep the solution explicit and reviewable.

Next drill

Implement a cost-aware model router

Success criteria

Make the solution explicit, debuggable, and easy to explain.

Review checklist

Review where the boundary is, what gets validated, and what would be hard to debug later.

import asyncio
import time
from dataclasses import dataclass, field

@dataclass
class SimpleCircuitBreaker:
    failure_threshold: int = 3
    reset_after_seconds: float = 60.0
    _failures: int = field(default=0, init=False)
    _opened_at: float | None = field(default=None, init=False)

@property
    def is_open(self) -> bool:
        # TODO: return True if open and cooldown has not elapsed
        raise NotImplementedError

def record_failure(self) -> None:
        # TODO: increment failure count and open if threshold reached
        raise NotImplementedError

def record_success(self) -> None:
        # TODO: reset state on success
        raise NotImplementedError

class ProviderFailoverChain:
    def __init__(self, providers: list):
        # TODO: initialize breakers and call counts
        raise NotImplementedError

async def complete(self, messages: list[dict]) -> tuple[str, int]:
        # TODO: try providers in order, respecting circuit breakers
        raise NotImplementedError

Practice

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