Smith

Smith is a Kubernetes workflow engine / resource manager.

It's functional and under active development.

News

13.02.2019: A lot of changes went in and we are using it in production. Time for v2.
01.01.2018: Milestone v1.0 is complete and v1.0.0 released!

The idea

What if we build a service that allows us to manage Kubernetes' built-in resources and other Custom Resources (CRs) in a generic way? Similar to how AWS CloudFormation (or Google Deployment Manager) allows us to manage any AWS/GCE and custom resource. Then we could expose all the resources we need to integrate as Custom Resources and manage them declaratively. This is an open architecture with Kubernetes as its core. Other controllers can create/update/watch CRs to co-ordinate their work/lifecycle.

Implementation

A group of resources is defined using a Bundle (just like a Stack for AWS CloudFormation). The Bundle itself is also a Kubernetes CR. Smith watches for new instances of a Bundle (and events to existing ones), picks them up and processes them.

Processing involves parsing the bundle, building a dependency graph (which is implicitly defined in the bundle), walking the graph, and creating/updating necessary resources. Each created/referenced resource gets a controller owner reference pointing at the origin Bundle.

Example bundle

CR definitions:

For Bundle see 0-crd.yaml.

apiVersion: apiextensions.k8s.io/v1beta1
kind: CustomResourceDefinition
metadata:
  name: postgresql-resources.smith.atlassian.com
spec:
  group: smith.atlassian.com
  names:
    kind: PostgresqlResource
    plural: postgresqlresources
    singular: postgresqlresource
  versions:
  - name: v1
    served: true
    storage: true

Bundle:

apiVersion: smith.atlassian.com/v1
kind: Bundle
metadata:
  name: bundle1
spec:
  resources:
  - name: db1
    spec:
      object:
        apiVersion: smith.atlassian.com/v1
        kind: PostgresqlResource
        metadata:
          name: db1
        spec:
          disk: 100GiB
  - name: app1
    references:
    - resource: db1
    spec:
      object:
        apiVersion: apps/v1
        kind: Deployment
        metadata:
          name: app1
        spec:
          replicas: 1
          bundle:
            metadata:
              labels:
                app: app1
            spec:
              containers:
              - name: app1
                image: quay.io/some/app1

Outputs

Some resource types can have Outputs:

Values placed into Status field of the object
New objects likes Secrets and/or ConfigMaps
Service Catalog objects

Resources can reference outputs of other resources within the same bundle. See what is supported.

Dependencies

Resources may depend on each other explicitly via object references. Resources are created in the reverse dependency order.

States

READY is the state of a Resource when it can be considered created. E.g. if it is a DB then it means it was provisioned and set up as requested. State is often part of Status but it depends on kind of resource.

Event-driven and stateless

Smith does not block while waiting for a resource to reach the READY state. Instead, when walking the dependency graph, if a resource is not in the READY state (still being created) it skips processing of that resource. Resources that don't have their dependencies READY are not processed. Resources that can be created concurrently are created concurrently. Full bundle re-processing is triggered by events about the watched resources. Smith is watching all supported resource kinds and reacts to events to determine which bundle should be re-processed. This scales better than watching individual resources and much better than polling individual resources. Smith controller is built according to recommendations and following the same behaviour, semantics and code "style" as native Kubernetes controllers as closely as possible.

Features

Supported object kinds: Deployment, Service, ConfigMap, Secret, Ingress, ServiceAccount, HorizontalPodAutoscaler, PodDisruptionBudget;
Service Catalog support: objects with kind ServiceInstance and ServiceBinding. See an example and recording of the presentation to Service Catalog SIG;
Dynamic Custom Resources support via special annotations;
References between objects in the graph to pull parts of objects/fields from dependencies;
Smith will delete objects which were removed from a Bundle when Bundle reconciliation is performed (e.g. on a Bundle update);
Plugins framework for injecting custom behavior when walking the dependency graph;

Notes

Presentations

Smith has been presented to:

SIG Service Catalog - see information, screencast and recording here.
SIG Apps - see recoding of the meeting.

On App Controller

Mirantis App Controller (discussed here kubernetes/kubernetes#29453) is a very similar workflow engine with a few differences.

Graph of dependencies is defined explicitly.
It uses polling and blocks while waiting for the resource to become READY.
The goal of Smith is to manage Custom Resources and Service Catalog objects. App Controller cannot manage them as of this writing (?).
Smith has very advanced support for Service Catalog objects.

On Helm

Helm is a package manager for Kubernetes. Smith operates on a lower level, even though it can be used by a human, that is not the main use case. Smith is built to be used as a foundation component with human-friendly tooling built on top of it. E.g. Helm could probably use Smith under the covers to manipulate Kubernetes API objects. Another use case is a PaaS that delegates (some) object manipulations to Smith.

Requirements

Kubernetes 1.11+ is required - we use /status subresource and OpenAPI schema features that became available in this version;
List of project dependencies and their versions can be found in go.mod and go.sum files.

Building, testing and running

Go modules are used for package management. You need Go v1.12 or newer.
Bazel is used as the build tool. Please install it.
To install dependencies run

make setup

Integration tests can be run against any Kubernetes context that is configured locally. To see which contexts are available run:

kubectl config get-contexts

By default a context named minikube is used. If you use minikube and want to run tests against that context then you don't need to do anything extra. If you want to run against some other context you may do so by setting the KUBE_CONTEXT environment variable which is honored by the makefile.

E.g. to run against Kubernetes-for-Docker use KUBE_CONTEXT=docker-for-desktop.

To run integration tests run

make integration-test

To run integration tests for Service Catalog support run

make integration-test-sc

This command assumes Service Catalog and UPS Broker are installed in the cluster. To install them follow the Service Catalog walkthrough.

To run Smith locally against the configured context run

make run
# or to run with Service Catalog support enabled
make run-sc

To build the Docker image run

make docker

This command only builds the image, which is not very useful. If you want to import it into your Docker run

make docker-export

Documentation

Sample deployment YAMLs
Design documents for implemented and potential future features

Contributing

Pull requests, issues and comments welcome. For pull requests:

Add tests for new features and bug fixes
Follow the existing style
Separate unrelated changes into multiple pull requests

See the existing issues for things to start contributing.

For bigger changes, make sure you start a discussion first by creating an issue and explaining the intended change.

Atlassian requires contributors to sign a Contributor License Agreement, known as a CLA. This serves as a record stating that the contributor is entitled to contribute the code/documentation/translation to the project and is willing to have it used in distributions and derivative works (or is willing to transfer ownership).

Prior to accepting your contributions we ask that you please follow the appropriate link below to digitally sign the CLA. The Corporate CLA is for those who are contributing as a member of an organization and the individual CLA is for those contributing as an individual.

atlassian/smith