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2.4.Resilience.md

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Resilience

As a Cloud Native traffic orchestrator, Easegress supports build-in resilience features. It is the ability of your system to react to failure and still remain functional. It's not about avoiding failure, but accepting failure and constructing your cloud-native services to respond to it. You want to return to a fully functioning state quickly as possible.[1]

Basic: Load Balance

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin

More Livingness: Resilience of Service

CircuitBreaker

CircuitBreaker leverges a finite state machine to implement the processing logic, the state machine has three states: CLOSED, OPEN, and HALF_OPEN. When the state is CLOSED, requests pass through normally, state transits to OPEN if request failure rate or slow request rate reach a configured threshold and requests will be shor-circuited in this state. After a configured duration, state transits from OPEN to HALF_OPEN, in which a limited number of requests are permitted to pass through while other requests are still short-circuited, and state transit to CLOSED or OPEN based on the results of the permitted requests.

When CLOSED, it uses a sliding window to store and aggregate the result of recent requests, the window can either be COUNT_BASED or TIME_BASED. The COUNT_BASED window aggregates the last N requests and the TIME_BASED window aggregates requests in the last N seconds, where N is the window size.

Below is an example configuration with a COUNT_BASED policy. GET request to paths begin with /books/ uses this policy, which short-circuits requests if more than half of the last 100 requests failed with status code 500, 503, or 504.

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin
    circuitBreakerPolicy: countBased
    failureCodes: [500, 503, 504]

resilience:
- name: countBased
  kind: CircuitBreaker
  slidingWindowType: COUNT_BASED
  failureRateThreshold: 50
  slidingWindowSize: 100

And we can also use a TIME_BASED policy, which short-circuits requests if more than 60% of the requests within the last 200 seconds failed.

resilience:
- name: time-based-policy
  kind: CircuitBreaker
  slidingWindowType: TIME_BASED
  failureRateThreshold: 60
  slidingWindowSize: 200

In addition to failures, we can also short-circuit slow requests. Below configuration regards requests which cost more than 30 seconds as slow requests and short-circuits requests if 60% of recent requests are slow.

resilience:
- name: countBased
  kind: CircuitBreaker
  slowCallRateThreshold: 60
  slowCallDurationThreshold: 30s

For a policy, if the first request fails, the failure rate could be 100% because there's only one request. This is not the desired behavior in most cases, we can avoid it by specifying minimumNumberOfCalls.

resilience:
- name: countBased
  kind: CircuitBreaker
  minimumNumberOfCalls: 10

We can also configure the wait duration in the open state and the max wait duration in the half-open state:

resilience:
- name: countBased
  kind: CircuitBreaker
  waitDurationInOpenState: 2m
  maxWaitDurationInHalfOpenState: 1m

In the half-open state, we can limit the number of permitted requests:

resilience:
- name: countBased
  kind: CircuitBreaker
  permittedNumberOfCallsInHalfOpenState: 10

For the full YAML, see here, and please refer CircuitBreaker Policy for more information.

RateLimiter

NOTE: When there are multiple instances of Easegress, the configuration will be applied for every instance equally. For example, TPS of RateLimiter is configured with 100 in 3-instances cluster, so the total TPS will be 300.

The below configuration limits the request rate for requests to /admin and requests that match regular expression ^/pets/\d+$.

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: rate-limiter
- filter: proxy

filters:
- name: rate-limiter
  kind: RateLimiter
  policies:
  - name: policy-example
    timeoutDuration: 100ms
    limitRefreshPeriod: 10ms
    limitForPeriod: 50
  defaultPolicyRef: policy-example
  urls:
  - methods: [GET, POST, PUT, DELETE]
    url:
      exact: /admin
      regex: ^/pets/\d+$
    policyRef: policy-example
- name: proxy
  kind: Proxy

For the full YAML, see here.

Retry

If we want to retry a failed request, for example, retry on HTTP status codes 500, 503, and 504, we can create a RetryerPolicy with the below configuration, it makes at most 3 attempts on failure.

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin
    retryPolicy: retry3Times
    failureCodes: [500, 503, 504]

resilience:
- name: retry3Times
  kind: Retry
  maxAttempts: 3
  waitDuration: 500ms

By default, the wait duration between two attempts is waitDuration, but this can be changed by specifying backOffPolicy and randomizationFactor.

resilience:
- name: retry3Times
  kind: Retry
  backOffPolicy: Exponential
  randomizationFactor: 0.5

For the full YAML, see here, and please refer Retry Policy for more information.

TimeLimiter

TimeLimiter limits the time of requests, a request is canceled if it cannot get a response in configured duration. As this resilience type only requires config a timeout duration, it is implemented directly on filters like Proxy.

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin
    timeout: 500ms

References

CircuitBreaker

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin
    circuitBreakerPolicy: countBasedPolicy
    failureCodes: [500, 503, 504]

resilience:
- name: countBasedPolicy
  kind: CircuitBreaker
  slidingWindowType: COUNT_BASED
  failureRateThreshold: 50
  slidingWindowSize: 100
  slowCallRateThreshold: 60
  slowCallDurationThreshold: 30s
  minimumNumberOfCalls: 10
  waitDurationInOpenState: 2m
  maxWaitDurationInHalfOpenState: 1m
  permittedNumberOfCallsInHalfOpenState: 10
- name: timeBasedPolicy
  kind: CircuitBreaker
  slidingWindowType: TIME_BASED
  failureRateThreshold: 60
  slidingWindowSize: 200

RateLimiter

name: pipeline-reverse-proxy
kind: Pipeline

flow:
  - filter: rate-limiter
  - filter: proxy

filters:
- name: rate-limiter
  kind: RateLimiter
  policies:
  - name: policy-example
    timeoutDuration: 100ms
    limitRefreshPeriod: 10ms
    limitForPeriod: 50
  defaultPolicyRef: policy-example
  urls:
  - methods: [GET, POST, PUT, DELETE]
    url:
      exact: /admin
      regex: ^/pets/\d+$
    policyRef: policy-example
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin

Retry

name: pipeline-reverse-proxy
kind: Pipeline

flow:
- filter: proxy

filters:
- name: proxy
  kind: Proxy
  pools:
  - servers:
    - url: http://127.0.0.1:9095
    - url: http://127.0.0.1:9096
    - url: http://127.0.0.1:9097
    loadBalance:
      policy: roundRobin
    retryPolicy: retry3Times
    failureCodes: [500, 503, 504]

resilience:
- name: retry3Times
  kind: Retry
  backOffPolicy: Exponential
  randomizationFactor: 0.5
  maxAttempts: 3
  waitDuration: 500ms

Concepts

  1. https://docs.microsoft.com/en-us/dotnet/architecture/cloud-native/resiliency