Circuit Breakers

Circuit breakers are a powerful technique to improve resilience. By preventing additional connections or requests to an overloaded service, circuit breakers limit the "blast radius" of an overloaded service. By design, Ambassador Edge Stack circuit breakers are distributed, i.e., different Ambassador Edge Stack instances do not coordinate circuit breaker information.

Circuit Breaker Configuration

Circuit breaking configuration can be set for all Ambassador Edge Stack mappings in the ambassador Module or set per Mapping.

The circuit_breakers attribute configures circuit breaking. The following fields are supported:

circuit_breakers:
- priority: <string>
  max_connections: <integer>
  max_pending_requests: <integer>
  max_requests: <integer>
  max_retries: <integer>

priority

(Default: default) Specifies the priority to which the circuit breaker settings apply to; it can be set to either default or high.

max_connections

(Default: 1024) Specifies the maximum number of connections that Ambassador Edge Stack will make to the services. In practice, this is more applicable to HTTP/1.1 than HTTP/2.

max_pending_requests

(Default: 1024) Specifies the maximum number of requests that will be queued while waiting for a connection. In practice, this is more applicable to HTTP/1.1 than HTTP/2.

max_requests

(Default: 1024) Specifies the maximum number of parallel outstanding requests to an upstream service. In practice, this is more applicable to HTTP/2 than HTTP/1.1.

max_retries

(Default: 3) Specifies the maximum number of parallel retries allowed to an upstream service.

Examples

Circuit breakers defined on a single mapping:

---
apiVersion: getambassador.io/v2
kind:  Mapping
metadata:
  name:  quote-backend
spec:
prefix: /backend/
service: quote
circuit_breakers:
- max_connections: 2048
  max_pending_requests: 2048

A global circuit breaker:

apiVersion: getambassador.io/v2
kind:  Module
metadata:
  name:  ambassador
spec:
  config:
    circuit_breakers:
    - max_connections: 2048
      max_pending_requests: 2048
---
apiVersion: getambassador.io/v2
kind:  Mapping
metadata:
  name:  quote-backend
spec:
prefix: /backend/
service: quote

Circuit Breakers and Automatic Retries

Circuit breakers are best used in conjunction with automatic retries. Here are some examples:

• You've configured automatic retries for failed requests to a service. Your service is under heavy load, and starting to time out on servicing requests. In this case, automatic retries can exacerbate your problem, increasing the total request volume by 2x or more. By aggressively circuit breaking, you can mitigate failure in this scenario.
• To circuit break when services are slow, you can combine circuit breakers with retries. Reduce the time out for retries, and then set a circuit breaker that detects many retries. In this setup, if your service doesn't respond quickly, a flood of retries will occur, which can then trip the circuit breaker.

Note that setting circuit breaker thresholds requires careful monitoring and experimentation. We recommend you start with conservative values for circuit breakers and adjust them over time.

More About Circuit Breakers

Responses from a broken circuit contain the x-envoy-overloaded header.

The following are the default values for circuit breaking if nothing is specified:

circuit_breakers:
- priority: default
  max_connections: 1024
  max_pending_requests: 1024
  max_requests: 1024
  max_retries: 3

Circuit breaker metrics are exposed in StatsD. For more information about the specific statistics, see the Envoy documentation.