Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Kubernetes Controllers - are they loops or events?
Search
Tim Hockin
February 20, 2021
Technology
11
3.4k
Kubernetes Controllers - are they loops or events?
Tim Hockin
February 20, 2021
Tweet
Share
More Decks by Tim Hockin
See All by Tim Hockin
Kubernetes in the 2nd Decade
thockin
0
140
Why Service is the worst API in Kubernetes, and what we can do about it
thockin
1
550
Kubernetes Pod Probes
thockin
6
3.2k
Go Workspaces for Kubernetes
thockin
2
890
Code Review in Kubernetes
thockin
2
1.5k
Multi-cluster: past, present, future
thockin
0
370
Kubernetes Network Models (why is this so dang hard?)
thockin
9
1.7k
KubeCon EU 2020: SIG-Network Intro and Deep-Dive
thockin
8
1.2k
A Non-Technical Kubernetes Talk (KubeCon EU 2020)
thockin
3
550
Other Decks in Technology
See All in Technology
一生覚えておきたい「システム開発=コミュニケーション」〜初めての実務案件振り返りLT〜
maimyyym
2
330
開発パフォーマンスを最大化するための開発体制
ham0215
7
1.1k
いいたいことちゃんという
tkengo
0
240
web-application-security
matsuihidetoshi
1
190
【NW X Security JAWS#3】L3-4:AWS環境のIPv6移行に向けて知っておきたいこと
shotashiratori
1
670
How to do well in consulting–Balkan Ruby 2024
irinanazarova
0
150
MapLibreとAmazon Location Service
dayjournal
1
190
VSCodeの拡張機能を作っている話
ebarakazuhiro
1
810
One engineer company with Ruby on Rails
rstankov
2
440
Babylon.jsと色々なものを組み合わせる:ブラウザのAPIやガジェットや2D描画ライブラリなど / Babylon.js 勉強会 vol.3
you
PRO
0
170
require(ESM)とECMAScript仕様
uhyo
4
990
アクセス制御にまつわる改善 / Improving access control
itkq
0
590
Featured
See All Featured
Practical Orchestrator
shlominoach
183
9.7k
RailsConf 2023
tenderlove
9
560
Keith and Marios Guide to Fast Websites
keithpitt
408
22k
The Mythical Team-Month
searls
216
42k
I Don’t Have Time: Getting Over the Fear to Launch Your Podcast
jcasabona
22
1.6k
Embracing the Ebb and Flow
colly
80
4.2k
Navigating Team Friction
lara
179
13k
[RailsConf 2023 Opening Keynote] The Magic of Rails
eileencodes
14
8.3k
Producing Creativity
orderedlist
PRO
338
39k
Sharpening the Axe: The Primacy of Toolmaking
bcantrill
22
1.4k
Build your cross-platform service in a week with App Engine
jlugia
226
17k
Exploring the Power of Turbo Streams & Action Cable | RailsConf2023
kevinliebholz
8
3.4k
Transcript
Kubernetes Controllers Are they loops or events? Tim Hockin @thockin
v1
Background on “reconciliation”: https://speakerdeck.com/thockin/kubernetes-what-is-reconciliation
Background on “edge vs. level”: https://speakerdeck.com/thockin/edge-vs-level-triggered-logic
Usually when we talk about controllers we refer to them
as a “loop”
Imagine a controller for Pods (aka kubelet). It has 2
jobs: 1) Actuate the pod API 2) Report status on pods
What you’d expect looks something like:
Node Kubernetes API a kubelet b c Get all pods
Node Kubernetes API a kubelet b c { name: a,
... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet b c for each pod
p { if p is running { verify p config } else { start p } gather status }
Node Kubernetes API a kubelet b c Set status c
a b
...then repeat (aka “a poll loop”)
Here’s where it matters
Node Kubernetes API a kubelet b c c a b
kubectl delete pod b
Node Kubernetes API a kubelet c c a b kubectl
delete pod b
Node Kubernetes API a kubelet c Get all pods c
a b
Node Kubernetes API a kubelet c { name: a, ...
} { name: c, ... } c a b
Node Kubernetes API a kubelet c I have “b” but
API doesn’t - delete it! c a b
Node Kubernetes API a kubelet c Set status c a
This is correct level-triggered reconciliation Read desired state, make it
so
Some controllers are implemented this way, but it’s inefficient at
scale
Imagine thousands of controllers (kubelet, kube-proxy, dns, ingress, storage...) polling
continuously
We need to achieve the same behavior more efficiently
We could poll less often, but then it takes a
long (and variable) time to react - not a great UX
Enter the “list-watch” model
Node Kubernetes API a kubelet b c Get all pods
Node Kubernetes API a kubelet b c { name: a,
... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet b c Cache: { name:
a, ... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet b c Watch all pods
Cache: { name: a, ... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet b c Cache: { name:
a, ... } { name: b, ... } { name: c, ... } for each pod p { if p is running { verify p config } else { start p } gather status }
Node Kubernetes API a kubelet b c Set status c
a b Cache: { name: a, ... } { name: b, ... } { name: c, ... }
We trade memory (the cache) for other resources (API server
CPU in particular)
There’s no point in polling my own cache, so what
happens next?
Remember that watch we did earlier? That’s an open stream
for events.
Node Kubernetes API a kubelet b c c a b
kubectl delete pod b Cache: { name: a, ... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet c c a b kubectl
delete pod b Cache: { name: a, ... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet c Delete: { name: b,
... } c a b Cache: { name: a, ... } { name: b, ... } { name: c, ... }
Node Kubernetes API a kubelet c Delete: { name: b,
... } c a b Cache: { name: a, ... } { name: c, ... }
Node Kubernetes API a kubelet c Cache: { name: a,
... } { name: c, ... } c a b API said to delete pod “b”.
Node Kubernetes API a kubelet c Cache: { name: a,
... } { name: c, ... } c a API said to delete pod “b”.
“But you said edge-triggered is bad!”
It is! But this isn’t edge-triggered.
The cache is updated by events (edges) but we are
still reconciling state
“???”
The controller can be restarted at any time and the
cache will be reconstructed - we can’t “miss an edge*” * modulo bugs, read on
Even if you miss an event, you can still recover
the state
Ultimately it’s all just software, and software has bugs. Controllers
should re-list periodically to get full state...
...but we’ve put a lot of energy into making sure
that our list-watch is reliable.