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Document updated on Dec 5, 2022

Circuit Breaker Pattern

The Circuit Breaker is a straightforward state machine in the middle of the request and response that monitors all your backend failures. When they reach a configured threshold, the circuit breaker will prevent sending more traffic to a failing backend alleviating its pressure under challenging conditions.

When KrakenD demands more throughput than your actual API stack can deliver properly, the Circuit Breaker mechanism will detect the failures and prevent stressing your servers by not sending requests that are likely to fail. It is also helpful for dealing with network and other communication problems by preventing too many requests from dying due to timeouts, etc.

It is important to remark that the number of maximum errors are consecutive errors, and not the total of errors in the period. This approach works better when your traffic is variable, as it’s based on a probabilistic pattern and it’s not affected by the volume you might have.

A must have configuration
The Circuit Breaker is an automatic protection measure for your API stack and avoids cascade failures, keeping your API responsive and resilient. It has a small consumption of resources. Try to implement it always.

Circuit breaker configuration

The Circuit Breaker is available in the namespace qos/circuit-breaker inside the extra_config key. The following configuration is an example of how to add circuit breaker capabilities to a backend:

{
    "endpoints": [
    {
        "endpoint": "/myendpoint",
        "method": "GET",
        "backend": [
        {
            "host": [
                "http://127.0.0.1:8080"
            ],
            "url_pattern": "/mybackend-endpoint",
            "extra_config": {
                "qos/circuit-breaker": {
                    "interval": 60,
                    "timeout": 10,
                    "max_errors": 1,
                    "name": "cb-myendpoint-1",
                    "log_status_change": true
                }
            }
        }
        ]
    }
    ]
}

The attributes available for the configuration are:

Fields of Circuit Breaker
* required fields

interval * integer
Time window where the errors count, in seconds.
log_status_change boolean
Whether to log the changes of state of this circuit breaker or not.
Defaults to false
max_errors * integer
The CONSECUTIVE (not total) number of errors within the interval window to consider the backend unhealthy. All HTTP status codes different than 20x are considered an error, except for the no-op encoding that does not evaluate status codes and is limited to connectivity/networking, security and component errors. See the definition of error below.
name string
A friendly name to follow this circuit breaker’s activity in the logs.
Example: "cb-backend-1"
timeout * integer
For how many seconds the circuit breaker will wait before testing again if the backend is healthy.

How the Circuit Breaker works

It’s easy to picture the state of the circuit breaker as an electrical component, where an open circuit means no flow of electricity between the ends, and a closed one normal flow:

Krakend logo

The Circuit Breaker starts with the CLOSED state, meaning the electricty can flow to the backends as they are considered healthy (innocent until proven guilty).

Then the component watches the state of the connections with your backend(s), with a tolerance to consecutive failures (max_errors) during a time interval (interval). it stops all the interaction with the backend for the next N seconds (the timeout). We call this state OPEN.

After waiting for this time window, the state changes to HALF-OPEN and allows a single connection to pass and test the system again. At this stage:

  • If the test connection fails, the state returns to “open” and the circuit breaker will wait N seconds again to test it again.
  • If it succeeds, it will return to the “closed “state, and the system is considered healthy.

This is the way the states change:

Circuit Breaker transitions
Krakend logo
  • CLOSED: In the initial state, the system is healthy and sending connections to the backend.
  • OPEN: When a consecutive number of supported errors from the backend (max_errors) is exceeded, the system changes to OPEN, and no further connections are sent to the backend. The system will stay in OPEN state for N seconds ( the timeout).
  • HALF-OPEN: After the timeout, it changes to this state and allows one connection to pass. If the connection succeeds, the state changes to CLOSED, and the backend is considered to be healthy again. But if it fails, it switches back to OPEN for another timeout.

Definition of error

When the circuit breaker counts the number of consecutive max_errors, an error could be anything that prevents having a successful connection with the service and completing the work.

no-op endpoints do not check HTTP status codes
Because a no-op does not evaluate the response status code, the circuit breaker does not see the reponse status code of the backend and the errors are limited to the following list below.

An error could be any of the following:

  • Network or connectivity problems
  • Security policies
  • Timeouts
  • Components in the list returning errors or having issues:
    • Proxy rate limit (qos/ratelimit/proxy)
    • Lua backend scripts (modifier/lua-backend)
    • CEL in the backend (validation/cel)
    • Lambda (backend/lambda)
    • AMQP or PubSub issues

For endpoints that DO NOT use no-op

In addition, only when you work with json, or any other encoding different than no-op, the gateway also takes into account the HTTP responses back from the backend and marks as errors:

  • Status codes different than 200 or 201 (including client credentials)
  • Decoding issues
  • Martian issues
Scarf

Unresolved issues?

The documentation is only a piece of the help you can get! Whether you are looking for Open Source or Enterprise support, see more support channels that can help you.

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