Kubernetes operators in rust

Writing light weight cloud services without go

When interacting with kubernetes it’s generally been standard practice to use either client-go via go, or kubectl via shell.

While these are good, non-controversial choices, the advancement of client libraries, and smarter openapi bindings, combined with the generics and procedural macros of rust-lang, it’s now quite possible to write fully fledged kube operators, using slim rust kube clients.

..couldn’t we have done this for ages?

Yes, but you’d have to set up all the watch machinery yourself and decide on a strategy for tracking the state.

The most painstaking part of this is dealing with watch events and its resourceVersion properties directly (metadata returned by kube so you can keep calling watch without getting duplicate events). The events themselves are also returned as newline delimited json, each a wrapped struct of either what you want, or a wrapped error type.

In client-go, a lot of this logic is wrapped up in something they call a reflector; a local cache responsible for updating itself and making sure its internal state reflects the state of etcd for the resource it’s watching.

The land of kube clients

First off, there are several good crates availble for kubernetes usage already. We have k8s-openapi; the increasingly clever set of rust bindings from the openapi spec. There’s kubernetes; a is a very sensible convenience wrapper around reqwest + k8s-openapi, but it lacks quite a bit of error handling.

In a perfect world, we should all use k8s-openapi because it operates on the sans-io principle where you can just plug in any client. However, it’s awkward when dealing with multiple values, and it doesn’t really support CRDs well yet and this is the main thing we wanted to use a kube client for.

So to tackle the CRD use case, we started out with an older version of the kubernetes crate, before k8s-openapi was added as a dependency, and added error handling.

Stealing Reflectors

So we’ve got an unnecessary fork of a kube client. Let’s do something interesting with it. A Reflector<T> is a Sync + Send cache of some T with everything it needs to keep itself up to date:

#[derive(Clone)]
pub struct Reflector<T> where
  T: Debug + Clone + Named + DeserializeOwned
{
    data: Arc<RwLock<Cache<T>>>,
    client: APIClient,
    resource: ApiResource,
}

Here, T is meant to be the deserializable struct you own that represents the .spec portion of a CRD. This is wrapped up in several containers, first Arc + RwLock for thread safety (this data needs to be readable across workers in actix at the very least). The Reflector implements methods for the write() part of RwLock while consumers can read() continuously.

Cache<T> is the state + bookkeeping markers. Ultimately, you are able to extract a BTreeMap<String, T> out of it (aliased to ResourceMap<T>). The key is Named how you like (probably using the .name key of the CRD).

By calling the provided .poll() methods as frequently as you’d like, you’ll be able to read the up-to-date ResourceMap from .read() and use the Reflector<T> as a cache.

New crate: kube

Once the Reflector was added and working in our fork, we decided to release it and see how useable it would be directly. The kube crate is available at version 0.2.0 at the moment:

[dependencies]
kube = "0.2.0"

It should behave like the kubernetes crate, but with more error handling, and various generic structs including reflectors.

Whether this fork will stick around depends. We were hoping this would serve as, if not a source of inspiration, then a source of ridicule.

How would you actually use this?

Easy. You’ll have a struct that defines your CRD:

#[derive(Debug, Deserialize, Serialize, Clone)]
pub struct FooResource {
  name: String,
  info: String,
}

Make it nameable for quick cache access:

impl Named for FooResource {
    // we want Foo identified by self.name in the cache
    fn name(&self) -> String {
        self.name.clone()
    }
}

create your state container with an instance of Reflector<FooResource>:

#[derive(Clone)]
pub struct State {
    foos: Reflector<FooResource>,
}

This is useable, but it won’t update without you driving it. Let’s make a nice constructor with some methods on it, since it’ll be the interface you’ll use from your application (and maybe you’ll need more resources):

impl State {
    fn new(client: APIClient) -> Result<Self> {
        let namespace = env::var("NAMESPACE").unwrap_or("kube-system".into());
        let fooresource = ApiResource {
            group: "clux.dev".into(),
            resource: "foos".into(),
            namespace: namespace,
        };
        let foos = Reflector::new(client, fooresource)?;
        Ok(State { foos })
    }
    /// Internal poll for internal thread
    fn poll(&self) -> Result<()> {
        self.foos.poll()
    }
    /// Exposed refresh button for use by app
    pub fn refresh(&self) -> Result<()> {
        self.foos.refresh()
    }
    /// Exposed getter for read access to state for app
    pub fn foos(&self) -> Result<ResourceMap<FooResource>> {
        self.foos.read()
    }
}

with that set up, we can set up a simple system that runs the reflector, by making sure Reflector::poll is called continuously. Here we illustrate it by using a simple thread, that polls every 10 seconds (the same as kube’s internal timeout for watch calls):

pub fn init(cfg: Configuration) -> Result<State> {
    let state = State::new(APIClient::new(cfg))?; // for app to read
    let state_clone = state.clone(); // clone for internal thread
    std::thread::spawn(move || {
        loop {
            std::thread::sleep(Duration::from_secs(10));
            match state_clone.poll() {
                Ok(_) => trace!("State refreshed"), // normal case
                Err(e) => {
                    // Can't recover: boot as much as kubernetes' backoff allows
                    error!("Failed to refesh cache '{}' - rebooting", e);
                    std::process::exit(1); // boot might fix it if network is failing
                }
            }
        }
    });
    Ok(state)
}

The .poll() call watch for events since the last internal resourceVersion and modify the Cache according to the WatchEvent returned by kubernetes. If for some reason we’ve desynced and the resourceVersion is too old (happens occasionally) then the Reflector will attempt to refresh the full state internally.

Exposing it from actix

To wrap this up and use it in an actix-web application, create your kube::config, pass it to init to start and initialize your State. The state can be embedded straight onto App::data:

let kubecfg = match env::var("HOME").expect("have HOME dir").as_ref() {
    "/root" => kube::config::incluster_config(),
    _ => kube::config::load_kube_config(),
}.expect("Failed to load kube config");

let state = init(kubecfg).expect("Failed to initialize reflectors");
HttpServer::new(move || {
    App::new()
        .data(state.clone())
    })
    .bind("0.0.0.0:8080").expect("Can not bind to 0.0.0.0:8080")
    .start();

and from there, it’s more or less following actix examples to read shared state in an http handler. The following will do:

fn get_foos(state: Data<State>, req: HttpRequest) -> HttpResponse {
    let foos = state.foos().unwrap();
    HttpResponse::Ok().json(foos)
}

Full example

To see how it’s all put together, you can browse the source for operator-rs; a full example that you can deploy directly onto kube with its 7MB docker image using only the necessary access

Unresolved problems

This is an early stage happy path. It works for custom resources very well, but any other resources can benefit from k8s-openapi as a side-dependency at the moment.

When can we use this for Pods/Deployments/X?

The chosen abstraction in the kube client is one targetting an ApiResource, which maps onto a url of the form /apis/{group}/v1/namespaces/{namespace}/{resource}. A lot of the kube apis use that format (pods has /api/v1/namespaces/{namespace}/pods/), so it might be somewhat be reuseable between all native structs once we get some optionals in this:

pub struct ApiResource {
    /// API Resource name
    pub resource: String,
    /// API Group
    pub group: String,
    /// Namespace the resources reside
    pub namespace: String,
}

While this is arguably an optimistic use of kube’s rest api, it might be the simpler approach than having a bunch of massive function wrappers doing more or less the same thing.

Similarly, our WatchEvent enum might also be reuseable:

#[serde(tag = "type", content = "object", rename_all = "UPPERCASE")]
pub enum WatchEvent<T> where
  T: Debug + Clone
{
    Added(T),
    Modified(T),
    Deleted(T),
    Error(ApiError),
}

This type of watch events seem to come out of many watch calls, but it’s equally unclear if it can be used correctly in a generic setting.

The same can be said about our Metadata, Resource<T>, and ResourceList<T>. These are defined in resource.rs, and are currently unexported implementation details of kube. If they are useful, it’s possible that these will be exposed in future versions of kube.

There’s an interesting discussion about similar generic design in k8s-openapi#20.

The plan is to test it out a bit further on data types beyond CRDs and see how well it generalizes, because it’d be a nice api to just use the resource names and groups we already know how to use from kube rbac rules.

A thread for polling? Why not actors?

Why not indeed. It might be better to expose actors? We have not tried yet. It might be a better fit long term, especially if we try to go down this route futher with Informer<T>. It’s a more complicated lifecycle model for the type of complexity we’re showcasing here though.

Library Divergence?

What about the fact that there’s now like 3 kube clients in rust land, all of which have the same config parsing and x509 gunk?

Yeah, that’s not great.

Maybe there’s a need for an actual crate that deals with Configuration alone so that effort isn’t duplicated.

It might also be good if we could factor this subjective view of what a reflector should do out of a kube library, but that style of sans-io based setup would require some restructuring.

But I was too tired to go on

That’s the state of things as of 30/04/19. There’s likely dragons around the corner, as well as missing features not ported from existing clients. Suggestions and ideas are welcome.

Obligatory shout-out to one of the best libraries of rust: serde for generating generic serialization/deserialization code on top of generic structs. There was a bit of a learning curve to implement bounds, but thankfully, the most complicated stuff that ended up being in kube was this one-line annotation:

#[derive(Deserialize)]
pub struct ResourceList<T> where
  T: Debug + Clone
{
    pub apiVersion: String,
    pub kind: String,
    pub metadata: Metadata,
    #[serde(bound(deserialize = "Vec<T>: Deserialize<'de>"))]
    pub items: Vec<T>,
}

Next steps

Would be to port raftcat (a small operator in babylon’s cloud) to use this client. Hopefully, after some battle testing in a slightly more advanced setting, this stuff can be less kubernetes alpha client mode.

See also