Custom filter development

You can extend Botkube functionality by writing additional filters. The FilterEngine runs these filters on the Event struct before forwarding it as a notification to a channel. These filters can check resource specs, validate some checks and add messages to the Event struct.

We have already defined a filter to add suggestions in the notifications if container image in pod specs is using latest tag.

Let's see, how we can write a filter like this.

A. Writing a filter

Prerequisites:

• As of now, you can write filters only using Go language. So you need to be familiar with it.
• Understanding of Kubernetes Objects needed (https://kubernetes.io/docs/concepts/overview/working-with-objects/kubernetes-objects/)

1. Create a new .go file

Create a new file (e.g object_annotation_checker.go) in the pkg/filterengine/filters/ directory.

Set package name as "filters" and import required packages:

package filters

import (
    "context"
    "fmt"

    "github.com/sirupsen/logrus"
    "k8s.io/apimachinery/pkg/api/meta"
    metaV1 "k8s.io/apimachinery/pkg/apis/meta/v1"
    "k8s.io/client-go/dynamic"

    "github.com/kubeshop/botkube/pkg/events"
    "github.com/kubeshop/botkube/pkg/utils"
)

2. Create a structure and implement "Run() and Describe()" methods for the struct.

FilterEngine has an interface Filter defined for the filters:

type Filter interface {
    Run(context.Context, *events.Event)
    Name() string
    Describe() string
}

Create a struct which implements the Filter interface. Use logger instance taken as an argument from the constructor:

// ObjectAnnotationChecker forwards events to specific channels based on a special annotation if it is set on a given K8s resource.
type ObjectAnnotationChecker struct {
    log        logrus.FieldLogger
    dynamicCli dynamic.Interface
    mapper     meta.RESTMapper
}

// NewObjectAnnotationChecker creates a new ObjectAnnotationChecker instance.
func NewObjectAnnotationChecker(log logrus.FieldLogger, dynamicCli dynamic.Interface, mapper meta.RESTMapper) *ObjectAnnotationChecker {
    return &ObjectAnnotationChecker{log: log, dynamicCli: dynamicCli, mapper: mapper}
}

// Run filer and modifies event struct
func (f *NamespaceChecker) Run(ctx context.Context, event *events.Event) {

    // your logic goes here

}

// Name returns the filter's name.
func (f *ObjectAnnotationChecker) Name() string {
    return "ObjectAnnotationChecker"
}

// Describe describes the filter.
func (f *ObjectAnnotationChecker) Describe() string {
    return "Filters or reroutes events based on botkube.io/* Kubernetes resource annotations."
}

3. Add your logic in the Run() function

Now, put your logic in the Run() function to parse resource object, run validation and modify Event struct. The fields in the Event struct can be found here.

// Run filters and modifies event struct.
func (f *ObjectAnnotationChecker) Run(ctx context.Context, event *events.Event) error {
    // get objects metadata
    obj, err := utils.GetObjectMetaData(ctx, f.dynamicCli, f.mapper, event.Object)
    if err != nil {
        return fmt.Errorf("while getting object metadata: %w", err)
    }

    // Check annotations in object
    if f.isObjectNotifDisabled(obj) {
        event.Skip = true
        f.log.Debug("Object Notification Disable through annotations")
    }

    if channel, ok := f.reconfigureChannel(obj); ok {
        event.Channel = channel
        f.log.Debugf("Redirecting Event Notifications to channel: %s", channel)
    }

    f.log.Debug("Object annotations filter successful!")
    return nil
}

4. Register your filter in the Filter Engine

Open pkg/filterengine/with_all_filters.go file and call the constructor of your new filter in the WithAllFilters method:

// WithAllFilters returns new DefaultFilterEngine instance with all filters registered.
func WithAllFilters(logger *logrus.Logger, dynamicCli dynamic.Interface, mapper meta.RESTMapper, cfg config.Filters) *DefaultFilterEngine {
    filterEngine := New(logger.WithField(componentLogFieldKey, "Filter Engine"))
    filterEngine.Register([]RegisteredFilter{
        {
            Filter:  filters.NewObjectAnnotationChecker(logger.WithField(filterLogFieldKey, "Object Annotation Checker"), dynamicCli, mapper),
            Enabled: cfg.Kubernetes.ObjectAnnotationChecker,
        },

        // Your filter goes here:
        {
            Filter:  filters.NewNodeEventsChecker(
                logger.WithField(filterLogFieldKey, "Node Events Checker") // make sure to use `logger.WithField`
            ),
            Enabled: cfg.Kubernetes.NodeEventsChecker, // you can use a configuration field or set a fixed `true` or `false` default value to enable/disable the filter during the initial app startup
        },
    }...)

    return filterEngine
}

B. Rebuild and deploy the Botkube backend

• Build the Botkube backend docker image with make container-image.
• Push the image to Dockerhub registry.
• Install/Upgrade your Botkube deployment (Steps are provided here).

The implementation of built-in filters can be found at: https://github.com/kubeshop/botkube/tree/main/pkg/filterengine/filters