HTTP clients

A client behind its circuit

A circuit with http_client decorates that service with CircuitBreakingHttpClient, as phil_circuit_breaker.http_client.<key>:

framework:
    http_client:
        scoped_clients:
            payments.client:
                base_uri: 'https://api.payments.example'

phil_circuit_breaker:
    circuits:
        http.payments:
            failure_rate_percent: 50
            minimum_calls: 10
            window_seconds: 60
            open_seconds: 30
            http_client: payments.client

Every service that receives payments.client now receives it behind its circuit. One circuit per client, or per destination: a client that reaches several partners is better split into scoped clients, each behind its own circuit, so that one failing partner never trips the others.

What is refused, what is counted

• While the circuit is open, or forced open by an override, a request is refused before it leaves with CircuitOpen, a TransportExceptionInterface: code that already handles an unreachable host handles it too.
• An answer of 5xx or a transport error is a failure.
• Any other answer, 4xx included, is a success: a 4xx is the caller's mistake, not the third party's.

The outcome is recorded on the first chunk of the response, without waiting for its body, so the decorator keeps the client asynchronous. Each request is measured up to that chunk, or up to its transport error: the duration is the transport's own total_time when it gives a positive one, the monotonic time since the request was made otherwise. It goes with the outcome, so that a circuit with slow_call_milliseconds counts the requests that last longer, and opens when their share reaches slow_call_rate_percent.

use Phil\CircuitBreakerBundle\Infrastructure\HttpClient\Exception\CircuitOpen;

try {
    $charge = $this->paymentsClient->request('POST', '/charges', ['json' => $payload])->toArray();
} catch (CircuitOpen) {
    return Charge::deferred($payload);        // the fallback
}

Around any other call

A breaker can guard any other call: get it from CircuitBreakers, ask isAvailable(), then report with record(), its duration included, or with recordSuccess() or recordFailure(). phil/circuit-breaker describes it.