Adding a custom filter

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 image_tag_checker.go) in botkube/pkg/filterengine/filters/ directory

Set package name as “filters” and import required packages

package filters

import (
        "strings"

        "github.com/infracloudio/botkube/pkg/events"
        "github.com/infracloudio/botkube/pkg/filterengine"
        log "github.com/infracloudio/botkube/pkg/logging"

        apiV1 "k8s.io/api/core/v1"
)

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

FilterEngine has an interface Filter defined for the filters

// Filter has method to run filter
type Filter interface {
        Run(interface{}, *events.Event)
        Describe() string
}

To implement the Filter interface, your struct should have Run(interface{}, *events.Event) and Describe() string methods

// ImageTagChecker add recommendations to the event object if latest image tag is used in pod containers
type ImageTagChecker struct {
}

// Run filer and modifies event struct
func (f *ImageTagChecker) Run(object interface{}, event *events.Event) {

    // your logic goes here

}

// Describe filter
func (f ImageTagChecker) Describe() string {
        return "Checks and adds recommendation if 'latest' image tag is used for container image."
}

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 filers and modifies event struct
func (f *ImageTagChecker) Run(object interface{}, event *events.Event) {

    // Run checks only for Pod resource kind and when event type is CreateEvent
    if event.Kind != "Pod" || event.Type != config.CreateEvent {
        return
    }

    // Convert Unstructured object into K8s typed object
    // https://godoc.org/k8s.io/api/core/v1#Pod
    var podObj coreV1.Pod
    err := utils.TransformIntoTypedObject(object.(*unstructured.Unstructured), &podObj)
    if err != nil {
        log.Errorf("Unable to tranform object type: %v, into type: %v", reflect.TypeOf(object), reflect.TypeOf(podObj))
    }

    // Check image tag in initContainers
    for _, ic := range podObj.Spec.InitContainers {
        images := strings.Split(ic.Image, ":")
        if len(images) == 1 || images[1] == "latest" {

            // Add messages in the Recommedations list
            event.Recommendations = append(event.Recommendations, ":latest tag used in image '"+ic.Image+"' of initContainer '"+ic.Name+"' should be avoided.\n")
        }
    }

    // Check image tag in Containers
    for _, c := range podObj.Spec.Containers {
        images := strings.Split(c.Image, ":")
        if len(images) == 1 || images[1] == "latest" {

            // Add messages in the Recommedations list
            event.Recommendations = append(event.Recommendations, ":latest tag used in image '"+c.Image+"' of Container '"+c.Name+"' should be avoided.\n")
        }
    }
    log.Logger.Info("Image tag filter successful!")
}

4. Register your filter to filterengine

You can call Register() method implemented by filterengine to register the filter at startup

// Register the filter
func init() {
        filterengine.DefaultFilterEngine.Register(ImageTagChecker{})
}

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/infracloudio/botkube/tree/develop/pkg/filterengine/filters