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
320
Why Service is the worst API in Kubernetes, and what we can do about it
thockin
2
840
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
470
Kubernetes Network Models (why is this so dang hard?)
thockin
9
1.8k
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
AWS のポリシー言語 Cedar を活用した高速かつスケーラブルな認可技術の探求 #phperkaigi / PHPerKaigi 2025
ytaka23
7
1.4k
我々に残された仕事はあるのか?
taishiyade
0
200
Go製のマイグレーションツールの git-schemalex の紹介と運用方法
shinnosuke_kishida
1
350
空が堕ち、大地が割れ、海が涸れた日~もしも愛用しているフレームワークが開発停止したら?~ #phperkaigi 2025
77web
2
980
Redefine_Possible
upsider_tech
0
190
BCMathを高速化した一部始終をC言語でガチ目に解説する / BCMath performance improvement explanation
sakitakamachi
2
1.2k
AIエージェント完全に理解した
segavvy
4
230
caching_sha2_passwordのはなし
boro1234
0
170
Amazon Q Developer 他⽣成AIと⽐較してみた
takano0131
1
110
Cloud Native PG 使ってみて気づいたことと最新機能の紹介 - 第52回PostgreSQLアンカンファレンス
seinoyu
0
160
ドメインイベントを活用したPHPコードのリファクタリング
kajitack
2
1.1k
職種に名前が付く、ということ/The fact that a job title has a name
bitkey
1
220
Featured
See All Featured
Practical Tips for Bootstrapping Information Extraction Pipelines
honnibal
PRO
14
1.1k
KATA
mclloyd
29
14k
Speed Design
sergeychernyshev
28
850
10 Git Anti Patterns You Should be Aware of
lemiorhan
PRO
656
60k
Raft: Consensus for Rubyists
vanstee
137
6.8k
Documentation Writing (for coders)
carmenintech
69
4.7k
実際に使うSQLの書き方 徹底解説 / pgcon21j-tutorial
soudai
176
52k
Done Done
chrislema
183
16k
Reflections from 52 weeks, 52 projects
jeffersonlam
349
20k
How to Ace a Technical Interview
jacobian
276
23k
GitHub's CSS Performance
jonrohan
1030
460k
Build your cross-platform service in a week with App Engine
jlugia
229
18k
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.