Add TCP Middlewares support
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# CircuitBreaker
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Don't Waste Time Calling Unhealthy Services
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{: .subtitle }
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The circuit breaker protects your system from stacking requests to unhealthy services, resulting in cascading failures.
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When your system is healthy, the circuit is closed (normal operations).
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When your system becomes unhealthy, the circuit opens, and the requests are no longer forwarded, but instead are handled by a fallback mechanism.
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To assess if your system is healthy, the circuit breaker constantly monitors the services.
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!!! note ""
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The CircuitBreaker only analyzes what happens _after_ its position within the middleware chain. What happens _before_ has no impact on its state.
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!!! important
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Each router gets its own instance of a given circuit breaker.
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One circuit breaker instance can be open while the other remains closed: their state is not shared.
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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.
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## Configuration Examples
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```yaml tab="Docker"
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# Latency Check
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labels:
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- "traefik.http.middlewares.latency-check.circuitbreaker.expression=LatencyAtQuantileMS(50.0) > 100"
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```
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```yaml tab="Kubernetes"
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# Latency Check
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apiVersion: traefik.containo.us/v1alpha1
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kind: Middleware
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metadata:
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name: latency-check
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spec:
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circuitBreaker:
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expression: LatencyAtQuantileMS(50.0) > 100
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```
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```yaml tab="Consul Catalog"
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# Latency Check
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- "traefik.http.middlewares.latency-check.circuitbreaker.expression=LatencyAtQuantileMS(50.0) > 100"
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```
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```json tab="Marathon"
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"labels": {
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"traefik.http.middlewares.latency-check.circuitbreaker.expression": "LatencyAtQuantileMS(50.0) > 100"
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}
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```
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```yaml tab="Rancher"
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# Latency Check
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labels:
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- "traefik.http.middlewares.latency-check.circuitbreaker.expression=LatencyAtQuantileMS(50.0) > 100"
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```
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```toml tab="File (TOML)"
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# Latency Check
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[http.middlewares]
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[http.middlewares.latency-check.circuitBreaker]
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expression = "LatencyAtQuantileMS(50.0) > 100"
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```
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```yaml tab="File (YAML)"
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# Latency Check
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http:
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middlewares:
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latency-check:
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circuitBreaker:
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expression: "LatencyAtQuantileMS(50.0) > 100"
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```
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## Possible States
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There are three possible states for your circuit breaker:
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- Closed (your service operates normally)
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- Open (the fallback mechanism takes over your service)
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- Recovering (the circuit breaker tries to resume normal operations by progressively sending requests to your service)
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### Closed
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While the circuit is closed, the circuit breaker only collects metrics to analyze the behavior of the requests.
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At specified intervals (`checkPeriod`), the circuit breaker evaluates `expression` to decide if its state must change.
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### Open
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While open, the fallback mechanism takes over the normal service calls for a duration of `FallbackDuration`.
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After this duration, it enters the recovering state.
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### Recovering
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While recovering, the circuit breaker sends linearly increasing amounts of requests to your service (for `RecoveryDuration`).
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If your service fails during recovery, the circuit breaker opens again.
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If the service operates normally during the entire recovery duration, then the circuit breaker closes.
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## Configuration Options
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### Configuring the Trigger
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You can specify an `expression` that, once matched, opens the circuit breaker and applies the fallback mechanism instead of calling your services.
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The `expression` option can check three different metrics:
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- The network error ratio (`NetworkErrorRatio`)
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- The status code ratio (`ResponseCodeRatio`)
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- The latency at a quantile in milliseconds (`LatencyAtQuantileMS`)
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#### `NetworkErrorRatio`
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If you want the circuit breaker to open at a 30% ratio of network errors, the `expression` is `NetworkErrorRatio() > 0.30`
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#### `ResponseCodeRatio`
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You can configure the circuit breaker to open based on the ratio of a given range of status codes.
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The `ResponseCodeRatio` accepts four parameters, `from`, `to`, `dividedByFrom`, `dividedByTo`.
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The operation that will be computed is sum(`to` -> `from`) / sum (`dividedByFrom` -> `dividedByTo`).
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!!! note ""
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If sum (`dividedByFrom` -> `dividedByTo`) equals 0, then `ResponseCodeRatio` returns 0.
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`from`is inclusive, `to` is exclusive.
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For example, the expression `ResponseCodeRatio(500, 600, 0, 600) > 0.25` will trigger the circuit breaker if 25% of the requests returned a 5XX status (amongst the request that returned a status code from 0 to 5XX).
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#### `LatencyAtQuantileMS`
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You can configure the circuit breaker to open when a given proportion of your requests become too slow.
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For example, the expression `LatencyAtQuantileMS(50.0) > 100` opens the circuit breaker when the median latency (quantile 50) reaches 100ms.
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!!! note ""
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You must provide a floating point number (with the trailing .0) for the quantile value
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#### Using Multiple Metrics
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You can combine multiple metrics using operators in your `expression`.
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Supported operators are:
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- AND (`&&`)
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- OR (`||`)
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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%.
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#### Operators
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Here is the list of supported operators:
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- Greater than (`>`)
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- Greater or equal than (`>=`)
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- Lesser than (`<`)
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- Lesser or equal than (`<=`)
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- Equal (`==`)
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- Not Equal (`!=`)
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### Fallback mechanism
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The fallback mechanism returns a `HTTP 503 Service Unavailable` to the client instead of calling the target service.
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This behavior cannot be configured.
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### `CheckPeriod`
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The interval used to evaluate `expression` and decide if the state of the circuit breaker must change.
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By default, `CheckPeriod` is 100ms. This value cannot be configured.
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### `FallbackDuration`
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By default, `FallbackDuration` is 10 seconds. This value cannot be configured.
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### `RecoveringDuration`
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The duration of the recovering mode (recovering state).
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By default, `RecoveringDuration` is 10 seconds. This value cannot be configured.
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