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
3.8k
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
330
Why Service is the worst API in Kubernetes, and what we can do about it
thockin
2
870
Kubernetes Pod Probes
thockin
6
4.3k
Go Workspaces for Kubernetes
thockin
2
1k
Code Review in Kubernetes
thockin
2
1.7k
Multi-cluster: past, present, future
thockin
0
490
Kubernetes Network Models (why is this so dang hard?)
thockin
9
1.9k
KubeCon EU 2020: SIG-Network Intro and Deep-Dive
thockin
8
1.3k
A Non-Technical Kubernetes Talk (KubeCon EU 2020)
thockin
3
590
Other Decks in Technology
See All in Technology
生成AIのユースケースをとにかく集めてまるっと学ぶ!/ all about generative ai usecases
gakumura
2
240
テストって楽しい!開発を加速させるテストの魅力 / Testing is Fun! The Fascinating of Testing to Accelerate Development
aiandrox
0
110
Aspire をカスタマイズしよう & Aspire 9.2
nenonaninu
0
180
JPOUG Tech Talk #12 UNDO Tablespace Reintroduction
nori_shinoda
2
160
2025-04-24 "Manga AI Understanding & Localization" Furukawa Arata (CyberAgent, Inc)
ornew
2
280
Making a MIDI controller device with PicoRuby/R2P2 (RubyKaigi 2025 LT)
risgk
1
320
AWSで作るセキュアな認証基盤with OAuth mTLS / Secure Authentication Infrastructure with OAuth mTLS on AWS
kaminashi
0
190
Winning at PHP in Production in 2025
beberlei
1
190
Bazel for Ruby (RubyKaigi 2025)
p0deje
0
120
新卒エンジニアがCICDをモダナイズしてみた話
akashi_sn
2
260
MCPを活用した検索システムの作り方/How to implement search systems with MCP #catalks
quiver
13
7.1k
React ABC Questions
hirotomoyamada
0
550
Featured
See All Featured
Understanding Cognitive Biases in Performance Measurement
bluesmoon
29
1.6k
Visualization
eitanlees
146
16k
It's Worth the Effort
3n
184
28k
KATA
mclloyd
29
14k
Why You Should Never Use an ORM
jnunemaker
PRO
55
9.3k
Put a Button on it: Removing Barriers to Going Fast.
kastner
60
3.8k
For a Future-Friendly Web
brad_frost
177
9.7k
Code Review Best Practice
trishagee
67
18k
Designing Dashboards & Data Visualisations in Web Apps
destraynor
231
53k
ピンチをチャンスに:未来をつくるプロダクトロードマップ #pmconf2020
aki_iinuma
119
51k
Keith and Marios Guide to Fast Websites
keithpitt
411
22k
Code Reviewing Like a Champion
maltzj
523
40k
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.