Krator: My God, it's Full of States!

Announcing Krator

Last September the Krustlet project refactored our API to employ a state machine pattern for describing the lifecycle of Pods. The design of this API was covered in depth in my previous blog post. Since then we have made a number of refinements to this interface and extended our use of the pattern to individual containers within each Pod. These changes also served to simplify and make more generic our API, and were done in part as a stepping stone to the crate we are announcing today.

Since introducing this API, we have experienced positive feedback from the community, including a number of requests to generalize this state machine architecture so that it can be used to build Kubernetes Operators. It turned out to be fairly straightforward to extract the “operator” portion of Krustlet to a new crate, and today we are pleased to announce Krator (pronounced crater), which stands for [K]ubernetes [R]ust st[at]e machine Operat[or]. Krator was released alongside the latest release of Krustlet, 0.6.0, and Krustlet now uses Krator internally to watch Pods and manage their state machines.

Implementing an Operator involves a number of moving parts, so I have put together a fairly minimal example, which I will walk through in the rest of this post. If you just want to browse the code, it currently lives in the Krustlet GitHub repository here, and documentation can be found on (once Krustlet 0.6.0 is released).

Writing an Operator

As an example, we will create an operator to manage Moose custom resources. Using Krator, we will write a state machine that will describe the possible states of a Moose, and how it should transition between these states. First, we must write a custom resource definition, which is very easy using the CustomResource derive macro from the kube crate!

use kube_derive::CustomResource;
use serde::{Deserialize, Serialize};

#[derive(CustomResource, Debug, Serialize, Deserialize, Clone, Default)]
    group = "",
    version = "v1",
    kind = "Moose",
    derive = "Default",
    status = "MooseStatus",
struct MooseSpec {
    /// Shoulder height in meters.
    height: f64,
    /// Body weight in kilograms.
    weight: f64,
    antlers: bool,

Each Moose has a spec which includes height, weight, and whether it has antlers. CustomResource lets us also specify that a resource should support the Kubernetes status API, which is required for a Krator Operator. We specified that our status type was MooseStatus which we will define as follows:

#[derive(Debug, Serialize, Deserialize, Clone)]
enum MoosePhase {

#[derive(Debug, Serialize, Deserialize, Clone)]
struct MooseStatus {
    phase: Option<MoosePhase>,
    /// ~ Moose noises ~
    message: Option<String>,

As our state machine transitions to new states, Krator will automatically update the object’s status with the Kubernetes API. To do this, Krator requires that this status type implement a trait, ObjectStatus:

impl ObjectStatus for MooseStatus {
    /// Use to generate an error status if an unhandled error occurs.
    fn failed(e: &str) -> MooseStatus {
        MooseStatus {
            message: Some(format!("Error tracking moose: {}.", e)),
            phase: None,

    /// Converts the arbitrary status type to a JSON patch to send to
    /// Kubernetes.
    fn json_patch(&self) -> serde_json::Value {
        // Generate a map containing only set fields.
        let mut status = serde_json::Map::new();

        if let Some(phase) = self.phase.clone() {
            status.insert("phase".to_string(), serde_json::json!(phase));

        if let Some(message) = self.message.clone() {
            status.insert("message".to_string(), serde_json::Value::String(message));

        // Create status patch with map.
        serde_json::json!({ "status": serde_json::Value::Object(status) })

Next we need to define data types for:

  • Data that is shared across state handlers for a single object (moose in this case)
  • Data that is shared across state machines (shared between objects)

We must also implement a trait, ObjectState which lets Krator know how all of these types relate to the state machine.

struct MooseState {
    /// All moose names start with 'M'
    name: String,
    /// Food mass in kilograms.
    food: f64,

struct SharedMooseState {
    /// Map moose name to set of moose friend's names (moose friendships are directed).
    friends: HashMap<String, HashSet<String>>,

impl ObjectState for MooseState {
    type Manifest = Moose;
    type Status = MooseStatus;
    type SharedState = SharedMooseState;

    /// Clean up data related to a specific object.
    async fn async_drop(self, shared: &mut Self::SharedState) {

We are now ready to implement our state machine. Below is a diagram of the states we will represent. When a Moose object is first created, Krator will initialize the state machine in the Tagged state. This will populate our shared state and then transition to the Roam state. The moose will roam the wilderness and with some probability encounter other moose and become friends. Eventually, the moose will get hungry and transition to the Eat state, which will replenish its food level but also make it tired. It will then transition to the Sleep state for 60 seconds before returning to the Roam state. Note that the final state, Released, is only reached when the Moose object is marked for deletion in the Kubernetes API and Krator interrupts the state machine and transitions to Released. In some operators, end states are also reachable under certain conditions from within the state machine graph, such as a Pod transitioning to Completed.

Moose State Machine

State implementations are currently a little verbose, so I will include a single state implementation here. The rest can be found in the complete example. The important information is within the next and status methods of each trait implementation. The status method is called when each state is entered, so it should describe the state it is associated with. We have considered using macros for implementing State, but have not been satisfied with Rust’s error messages for errors originating from within these macros. If you have any experience with macros and think you can come up with something nice, please let us know!

The next method is the main code block for each state, sometimes referred to as the state handler. This method has a number of important arguments:

  • It takes ownership of self.
  • An Arc<RwLock<_>> to access shared state. For more information on this API, see the Tokio documentation.
  • A mutable reference to the object state.
  • A Manifest<_> which acts as a reflector. This type implements the Stream API for detecting changes to the object manifest within a state handler, or you can simply call latest to obtain a clone of the current object manifest.
#[derive(Debug, Default)]
/// Moose is roaming the wilderness.
struct Roam;

async fn make_friend(
    name: &str,
    shared: &Arc<RwLock<SharedMooseState>>
) -> Option<String> {
    let mut mooses = shared.write().await;
    for (other_moose, friends) in mooses.friends.iter_mut() {
        if name == other_moose {
        if !friends.contains(name) {
            return Some(other_moose.to_string());
    return None;

impl State<MooseState> for Roam {
    async fn next(
        self: Box<Self>,
        shared: Arc<RwLock<SharedMooseState>>,
        state: &mut MooseState,
        _manifest: Manifest<Moose>,
    ) -> Transition<MooseState> {
        loop {
   -= 5.0;
            if <= 50.0 {
                return Transition::next(self, Eat);
            let r: f64 = {
                let mut rng = rand::thread_rng();
            if r < 0.05 {
                if let Some(other_moose) = make_friend(&, &shared).await {
                    info!("{} made friends with {}!",, other_moose);

    async fn status(
        _state: &mut MooseState,
        _manifest: &Moose,
    ) -> anyhow::Result<MooseStatus> {
        Ok(MooseStatus {
            phase: Some(MoosePhase::Roaming),
            message: Some("Gahrooo!".to_string()),

To finish our state machine, we must define valid transitions between states. The compiler will ensure that we do not make invalid transitions, which helps us to avoid undefined behavior.

impl TransitionTo<Roam> for Tagged {}
impl TransitionTo<Eat> for Roam {}
impl TransitionTo<Sleep> for Eat {}

Alternatively, Krator provides a macro for deriving this trait:

#[derive(Debug, Default, TransitionTo)]
/// Moose is sleeping.
struct Sleep;

Finally we must implement the Operator trait to tie this all together. Note that the state machine API itself does not require that the Manifest is a valid Kubernetes API resource. This allows the state machine pattern to be used with a broader set of types (we used it for containers, which are not a Kubernetes resource type). This, however, will require you to reimplement the function which drives the state machine, as we did for containers.

To get the full benefit of Krator, which watches for changes to your API resource, spawns state machines, and handles status updates, you will need to satisfy additional trait bounds which are placed on associated types of the Operator trait. Namely, Manifest must be Metadata<Ty = ObjectMeta>, DeserializeOwned, and Default. Status must be ObjectStatus (as described above).

struct MooseTracker {
    shared: Arc<RwLock<SharedMooseState>>,

impl MooseTracker {
    fn new() -> Self {
        let shared = Arc::new(RwLock::new(SharedMooseState {
            friends: HashMap::new(),
        MooseTracker { shared }

impl Operator for MooseTracker {
    type Manifest = Moose;
    type Status = MooseStatus;
    /// This indicates the starting state of the state machine.
    type InitialState = Tagged;
    /// This is the state that is transitioned to when the object is deleted.
    type DeletedState = Released;
    type ObjectState = MooseState;

    /// Initialize an object state for a new object.
    async fn initialize_object_state(
        manifest: &Self::Manifest,
    ) -> anyhow::Result<Self::ObjectState> {
        let name = manifest.metadata().name.clone().unwrap();
        Ok(MooseState { name, food: 100.0 })

    /// Obtain a reference to the SharedState.
    async fn shared_state(&self) -> Arc<RwLock<SharedMooseState>> {

To run our operator, we initialize an OperatorRuntime and await on its start method which will run forever.

async fn main() -> anyhow::Result<()> {
    let kubeconfig = kube::Config::infer().await?;
    let tracker = MooseTracker::new();

    // Only track mooses in Glacier NP
    let params = ListParams::default().labels("");

    let mut runtime = OperatorRuntime::new(&kubeconfig, tracker, Some(params));

And that is it! While this was a lot to cover in a blog post, we have found that this pattern leads to a very navigable codebase for even complex operators like Kubelets. We believe that this framework eliminates a lot of the error-prone state management involved when writing Operators and can get you off the ground very quickly. If you have questions or feature suggestions, please reach out to us on GitHub or #krustlet on Slack.

Kevin Flansburg