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How Does Reloader Work?

Reloader watches for ConfigMap and Secret and detects if there are changes in data of these objects. After change detection Reloader performs rolling upgrade on relevant Pods via associated Deployment, Daemonset and Statefulset:

flowchart LR
    subgraph Reloader
    controller("Controller watches in a loop") -- "Detects a change" --> upgrade_handler("Upgrade handler checks if the change is a valid data change by comparing the change hash")
    upgrade_handler -- "Update resource" --> update_resource("Updates the resource with computed hash of change")
    end
    Reloader -- "Watches" --> secret_configmaps("Secrets/ConfigMaps")
    Reloader -- "Updates resources with Reloader environment variable" --> resources("Deployments/DaemonSets/StatefulSets resources with Reloader annotation")
    resources -- "Restart pods based on StrategyType" --> Pods
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How Does Change Detection Work?

Reloader watches changes in configmaps and secrets data. As soon as it detects a change in these. It forwards these objects to an update handler which decides if and how to perform the rolling upgrade.

Requirements for Rolling Upgrade

To perform rolling upgrade a deployment, daemonset or statefulset must have

  • support for rolling upgrade strategy
  • specific annotation for configmaps or secrets

The annotation value is comma separated list of configmaps or secrets. If a change is detected in data of these configmaps or secrets, Reloader will perform rolling upgrades on their associated deployments, daemonsets or statefulsets.

Annotation for Configmap

For a Deployment called foo have a ConfigMap called foo. Then add this annotation* to your Deployment, where the default annotation can be changed with the --configmap-annotation flag:

metadata:
  annotations:
    configmap.reloader.stakater.com/reload: "foo"

Annotation for Secret

For a Deployment called foo have a Secret called foo. Then add this annotation to your Deployment, where the default annotation can be changed with the --secret-annotation flag:

metadata:
  annotations:
    secret.reloader.stakater.com/reload: "foo"

Above mentioned annotation are also work for Daemonsets Statefulsets and Rollouts

How Does Rolling Upgrade Work?

When Reloader detects changes in configmap. It gets two objects of configmap. First object is an old configmap object which has a state before the latest change. Second object is new configmap object which contains latest changes. Reloader compares both objects and see whether any change in data occurred or not. If Reloader finds any change in new configmap object, only then, it moves forward with rolling upgrade.

After that, Reloader gets the list of all deployments, daemonsets and statefulset and looks for above mentioned annotation for configmap. If the annotation value contains the configmap name, it then looks for an environment variable which can contain the configmap or secret data change hash.

Environment Variable for ConfigMap

If configmap name is foo then

STAKATER_FOO_CONFIGMAP

Environment Variable for Secret

If Secret name is foo then

STAKATER_FOO_SECRET

If the environment variable is found then it gets its value and compares it with new configmap hash value. If old value in environment variable is different from new hash value then Reloader updates the environment variable. If the environment variable does not exist then it creates a new environment variable with latest hash value from configmap and updates the relevant deployment, daemonset or statefulset

Note: Rolling upgrade also works in the same way for secrets.

Hash Value Computation

Reloader uses SHA1 to compute hash value. SHA1 is used because it is efficient and less prone to collision.

Monitor All Namespaces

By default Reloader deploys in default namespace and monitors changes in all namespaces. To monitor changes in a specific namespace deploy the Reloader in that namespace and set the watchGlobally flag to false in values file located under deployments/kubernetes/chart/reloader and render manifest file using helm command:

helm --namespace {replace this with namespace name} template . > reloader.yaml

The output file can then be used to deploy Reloader in specific namespace.

Compatibility With Helm Install and Upgrade

Reloader has no impact on helm deployment cycle. Reloader only injects an environment variable in deployment, daemonset or statefulset. The environment variable contains the SHA1 value of configmap's or secret's data. So if a deployment is created using Helm and Reloader updates the deployment, then next time you upgrade the helm release, Reloader will do nothing except changing that environment variable value in deployment , daemonset or statefulset.