CircuitBreaker

    The circuit breaker protects your system from stacking requests to unhealthy services (resulting in cascading failures).

    When your system is healthy, the circuit is close (normal operations). When your system becomes unhealthy, the circuit becomes open and the requests are no longer forwarded (but handled by a fallback mechanism).

    To assess if your system is healthy, the circuit breaker constantly monitors the services.

    • The CircuitBreaker only analyses what happens after it is positioned in the middleware chain. What happens before has no impact on its state.
    • The CircuitBreaker only affects the routers that use it. Routers that don't use the CircuitBreaker won't be affected by its state.

    Important

    Each router will eventually gets its own instance of a given circuit breaker.

    If two different routers refer to the same circuit breaker definition, they will get one instance each. It means that one circuit breaker can be open while the other stays closed: their state is not shared.

    This is the expected behavior, we want you to be able to define what makes a service healthy without having to declare a circuit breaker for each route.

    2. apiVersion: traefik.containo.us/v1alpha1
    3. kind: Middleware
    4. metadata:
    5.   name: latency-check
    6. spec:
    7.   circuitBreaker:
    8.     expression: LatencyAtQuantileMS(50.0) > 100
    1. "labels": {
    2.   "traefik.http.middlewares.latency-check.circuitbreaker.expression": "LatencyAtQuantileMS(50.0) > 100"
    3. }
    1. # Latency Check
    2. [http.middlewares]
    3.     expression = "LatencyAtQuantileMS(50.0) > 100"

    There are three possible states for your circuit breaker:

    • Close (your service operates normally)
    • Open (the fallback mechanism takes over your service)
    • Recovering (the circuit breaker tries to resume normal operations by progressively sending requests to your service)

    While close, the circuit breaker only collects metrics to analyze the behavior of the requests.

    Open

    While open, the fallback mechanism takes over the normal service calls for a duration of FallbackDuration. After this duration, it will enter the recovering state.

    While recovering, the circuit breaker will progressively send requests to your service again (in a linear way, for ). If your service fails during recovery, the circuit breaker becomes open again. If the service operates normally during the whole recovering duration, then the circuit breaker returns to close.

    Configuring the Trigger

    You can specify an expression that, once matched, will trigger the circuit breaker (and apply the fallback mechanism instead of calling your services).

    The expression can check three different metrics:

    • The network error ratio (NetworkErrorRatio)
    • The status code ratio (ResponseCodeRatio)
    • The latency at quantile, in milliseconds (LatencyAtQuantileMS)

    NetworkErrorRatio

    If you want the circuit breaker to trigger at a 30% ratio of network errors, the expression will be NetworkErrorRatio() > 0.30

    ResponseCodeRatio

    You can trigger the circuit breaker based on the ratio of a given range of status codes.

    The ResponseCodeRatio accepts four parameters, from, to, dividedByFrom, dividedByTo.

    The operation that will be computed is sum(to -> from) / sum (dividedByFrom -> dividedByTo).

    If sum (dividedByFrom -> dividedByTo) equals 0, then returns 0.

    fromis inclusive, to is exclusive.

    LatencyAtQuantileMS

    You can trigger the circuit breaker when a given proportion of your requests become too slow.

    For example, the expression LatencyAtQuantileMS(50.0) > 100 will trigger the circuit breaker when the median latency (quantile 50) reaches 100MS.

    You must provide a float number (with the trailing .0) for the quantile value

    Using multiple metrics

    You can combine multiple metrics using operators in your expression.

    Supported operators are:

    • AND (&&)
    • OR (||)

    For example, ResponseCodeRatio(500, 600, 0, 600) > 0.30 || NetworkErrorRatio() > 0.10 triggers the circuit breaker when 30% of the requests return a 5XX status code, or when the ratio of network errors reaches 10%.

    Operators

    Here is the list of supported operators:

    • Greater than (>)
    • Greater or equal than (>=)
    • Lesser than (<)
    • Lesser or equal than (<=)
    • Equal (==)
    • Not Equal (!=)

    The fallback mechanism returns a HTTP 503 Service Unavailable to the client (instead of calling the target service). This behavior cannot be configured.

    CheckPeriod

    The interval used to evaluate expression and decide if the state of the circuit breaker must change. By default, CheckPeriod is 100ms. This value cannot be configured.

    By default, FallbackDuration is 10 seconds. This value cannot be configured.

    RecoveringDuration

    By default, is 10 seconds. This value cannot be configured.