Stages Configuration #

This document walks you through how to configure the Stages of Lifecycle.

What is a Stage? #

The Stage is a kwok Configuration that allows users to define and simulate different stages in the lifecycle of Kubernetes resources, such as nodes and pods. Each Stage resource specifies a resourceRef field that identifies the type of resource that the stage applies to, and a selector field that determines when the stage should be executed.

A Stage resource has the following fields:

kind: Stage
  name: <string>
    apiGroup: <string>
    kind: <string>
      <string>: <string>
      <string>: <string>
    - key: <expressions-string>
      operator: <string>
      - <string>
  weight: <int>
    durationMilliseconds: <int>
      expressionFrom: <expressions-string>
    jitterDurationMilliseconds: <int>
      expressionFrom: <expressions-string>
    statusTemplate: <string>
      type: <string>
      reason: <string>
      message: <string>
      - value: <string>
      - value: <string>
      empty: <bool>
    delete: <bool>
  immediateNextStage: <bool>

By setting the selector and next fields in the spec section of a Stage resource, users can specify the conditions that need to be met for the stage to be applied, and the changes that will be made to the resource when the stage is applied. The next field allows users to define the new status of the resource using the statusTemplate field, and even delete the resource.

statusTemplate and delete are the two fundamental fields in next that respectively represent the two basic phases of resource lifecycle simulation: status update and resource deletion. statusTemplate provides a way to define resource status based on go template rendering. Please see go template in kwok for more detailed instructions. delete: true has higher priority than a non-empty statusTemplate, which means kwok will delete the resource rather than update its status if both are set.

Apart from the two fundamental fields, there are two other fields in next that allow users to do some additional actions on a simulated resource before status update or deletion. event allows users to emit an associated Kubernetes event if there is something to record. finalizers allows users to add or remove finalizers. Please note that both fields can exist on their own without specifying delete or statusTemplate field. In this case, kwok will only send an event or modify finalizers and will not change the status or delete a resource when applying a Stage.

It is worth noting that there is no dedicated field for arranging the execution order if multiple stages of a resource type are provided. The execution order of stages can be controlled by utilizing selector.matchExpressions and next field together. Specifically, users can chain the stages by ensuring that selector.matchExpressions of a stage match the status content specified in the next field of a previous stage. Please refer to Default Pod Stages for a detailed example. If multiple stages of a resource type share the same selector setting, kwok will randomly choose a stage to apply for a specific resource. Users can also customize the probability of a stage being selected via the weight field. This is useful when you want the resources under a certain type to enter different stages according to a certain probability distribution. Please note that weight only takes effect among stages with same resourceRef and selector settings.

Additionally, the delay field in a Stage resource allows users to specify a delay before the stage is applied, and introduce jitter to the delay to specify the latest delay time to make the simulation more realistic. This can be useful for simulating real-world scenarios where events do not always happen at the same time. Please refer to How Delay is Calculated for more details.

By configuring the delay, selector, and next fields in a Stage, you can control when and how the stage is applied, providing a flexible and scalable way to simulate real-world scenarios in your Kubernetes cluster. This allows you to create complex and realistic simulations for testing, validation, and experimentation, and gain insights into the behavior and performance of your applications and infrastructure.

Expressions string #

The <expressions-string> is provided by the Go Implementation of JQ Expressions

How it works #

Stages can be generally divided into two categories based on different settings of the next field. A Stage that has a non-empty statusTemplate is a “Change Stage”, which will be used by kwok for updating resource status. A Stage with delete being true represents a “Delete Stage”, which means to kwok to delete the resource.

It is the Resource Lifecycle Simulation Controller in kwok that applies the Stages. The controller watches resource events from the apiserver and applies a Stage on a resource when it receives an associated event. Let’s take a particular resource as an example. Starting from receiving an Added event of the resource, kwok checks whether the associated object matches a Stage. kwok then updates the resource status if a “Change Stage” is matched. The update action itself consequently causes a new Modified resource to be generated, which will be caught by the controller later and trigger the next check and apply round. kwok deletes the resource until a “Delete Stage” is matched.


However, this event-driven approach to applying Stages has a limitation: kwok won’t apply a Stage until a new event associated with that resource is received. To address the limitation, users can utilize the immediateNextStage field to make the controller apply Stages immediately rather than waiting for an event pushed from the apiserver.

How Delay is Calculated #

The delay time of applying a Stage is obtained by adding a constant time period and a randomized interval, which will be denoted by duration and jitter respectively in the following.

  • durationMilliseconds: provides duration.
  • jitterDurationMilliseconds: calculates jitter by random(jitterDurationMilliseconds-duration). It shall be larger than durationMilliseconds if you want to inject jitter to the delay. Otherwise, it will be used directly as the delay time without any randomization.

kwok also provides other fields to flexibly handle more situations. They are used to extract and parse a RFC3339 timestamp field of a resource to help determine the delay time dynamically.

  • durationFrom: calculates duration by durationFrom-now
  • jitterDurationFrom: calculates jitter by random(jitterDurationFrom-now-duration), where now is the timestamp when the delay starts.
durationFrom and jitterDurationFrom have a higher priority than durationMilliseconds and jitterDurationMilliseconds if both are set.

Let’s explain a little bit about the motivation behind these two advanced fields. But before that, for a better understanding, we briefly describe how kubelet “delete” a pod from a node.

Here are the steps to remove a pod in Kubernetes:

  1. Execute the command kubectl delete pod. The apiserver receives the deletion request but does not immediately remove the corresponding pod resource from etcd.
  2. The apiserver sets the metadata.deletionTimestamp field to the time the request was issued plus a short period, defined by metadata.deletionGracePeriodSeconds (default 30s).
  3. The kubelet detects a non-null metadata.deletionTimestamp for a pod and starts to send a TERM signal to the main process of the container.
  4. If the metadata.deletionTimestamp expires before the process stops by itself, the main process is then terminated using the KILL signal.
  5. After all the containers in the pod have stopped running, the kubelet sends a force deletion request to the apiserver.
  6. The apiserver removes the pod object from etcd.

To simulate this situation, you can set jitterDurationFrom of a “Delete Stage” (next.delete: true) to point to metadata.deletionTimestamp. This will cause the deletion operation to occur at a random moment before metadata.deletionTimestamp expires. You can also let kwok perform the deletion in a deterministic way by pointing durationFrom to metadata.deletionTimple, making the deletion happen exactly at metadata.deletionTimple.

Examples #

Node Stages #

This example shows how to configure the simplest and fastest stages of Node resource, which is also the default Node stages for kwok.

Default Node Stages

Pod Stages #

This example shows how to configure the simplest and fastest stages of Pod resource, which is also the default Pod stages for kwok.

Default Pod Stages


Pod Stages that simulate real behavior as closely as possible #

General Pod Stages