Conflict Free Replicated Data Types

Collaboration with
Con
fl
ict Free Replicated Datatypes
(CRDTs)

What are CRDTs?
Datatypes
Arrays
Maps
Text
Counters
Registers
...

Shared Datatypes?
CRDTs
Computer A
CRDTs
Computer B
Changes

Shared Datatypes?
Project X Document
Lorem ipsum dolor sit amet,
consectetuer adipiscing elit.
Aenean commodo ligula eget
dolor
Donec quam felis, ultricies nec,
pellentesque eu, pretium quis,
sem. Nulla consequat massa quis
enim
Donec pede justo, fringilla vel,
aliquet nec, vulputate eget, arcu
Computer A
Project X Document
dolor
enim
Computer B

Shared Datatypes?
Project X Document
dolor elementum
enim
Computer A
Project X Document
dolor
enim
Computer B
elementum

Shared Datatypes?
Project X Document
dolor elementum
enim
Computer A
Project X Document
dolor
enim ullamcorper
Computer B
elementum
ullamcorper

Project X Document
dolor elementum
enim
Computer A
Shared Datatypes?
Project X Document
dolor
enim ullamcorper
Computer B
elementum
ullamcorper

More than just
collaborative text editing...
About that example

It's about collaboration
in general
About that example

Working together
on a shared task
Collaboration?

Project X Document
dolor elementum
enim
Computer A
Project X Document
dolor
enim ullamcorper
Computer B
Asynchronously

Project X Document
dolor elementum
enim
Computer A
Project X Document
dolor
enim ullamcorper
Computer B
Concurrently

What is collaborative software?

"collaborative software"
What examples come to mind?

Most software isn't
really collaborative

Most software isn't really collaborative
Why?

Project X Document
dolor elementum
enim
Computer A
Project X Document
dolor
enim ullamcorper
Computer B
If you had to build this...

We need better tools...
And that's what
CRDTs offer

How do they work?
De
fi
ne the shape
of your data
model

De
fi
ne the shape of your model...
CRDTs
Arrays
Maps
Sets
Registers
Counters
...
Arrays
Maps
Sets
Registers
Application
data model

How is data replicated
CRDTs
Application
data model
Application
data model
CRDTs

How is data replicated
CRDTs
Application
data model
Application
data model
CRDTs
Server

CRDTs
Created following rules...
info to
merge changes
w/o con
fl
ict

To fully understand CRDTs...
Order theory
join semi-lattices

You probably don't need to know...
Order theory
unless you're a
CRDT library author

Still curious...
How CRDTs work?

First look at CRDTs
CRDT Counter

Counting things on a conveyor...
≤
Count: 2
1
0

Counting people entering a concert...

Counting people entering a concert...
Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
1 1 1
2 2 2
3 3 3

≤
Count: 2 Total Count
+ -
3 Total Count
+ -
3

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
1 1 1
Can we use primitives?
// model
count = ;
// model
count = 0;
// model
count = 0;
0 0 0
1
1
1

Total Count
+ -
Total Count
+ -
Total Count
+ -
1 1 1
2 2 2
3 3 3
// model
count = ;
// model
count = 0;
// model
count = 0;
0 1 1
1
1
1 2
2
2 2
2
2

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
// model
count = 0;
// model
count = 0;
// model
count = 0;

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
CRDT Counter
// model
counter =
new CRDTCounter();
// model
counter =
new CRDTCounter();
// model
counter =
new CRDTCounter();

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
// model
counter =
new CRDTCounter();
// increment
counter.increment(1);
// model
counter =
new CRDTCounter();
// increment
// model
counter =
new CRDTCounter();
// increment
CRDT Counter

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
// model
id = 100;
count = {}
// model
id = 200;
count = {}
// model
id = 300;
count = {}
CRDT Counter

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
// model
id = 100;
count = {}
// model
id = 200;
count = {}
// model
id = 300;
count = {}
100
CRDT Counter

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0 0
// model
id = 100;
count = {
:
}
// model
id = 200;
count = {}
// model
id = 300;
count = {}
100 1
1
CRDT Counter
{
100: 1
}
{
100: 1
}

Total Count
+ -
Total Count
+ -
Total Count
+ -
0 0
// model
id = 100;
count = {
:
}
// model
id = 200;
count = {
}
// model
id = 300;
count = {
}
100 1
1
CRDT Counter
{
100: 1
}
{
100: 1
}
1 1

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
:
}
// model
id = 200;
count = {
}
// model
id = 300;
count = {
}
100 1
1
CRDT Counter
{
100: 1
}
{
100: 1
}
1 1

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
:
}
// model
id = 200;
count = {
}
// model
id = 300;
count = {
}
100 1
1
CRDT Counter
{
100: 1
}
{
100: 1
}
1 1
200 300

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
:
}
// model
id = 200;
count =
// model
id = 300;
count =
100 1
1
CRDT Counter
1 1
{
100: 1,
:
}
{
100: 1,
:
}
200 300
1 1
2 2 300: 1
300: 1
200: 1
200: 1

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
: ,
}
// model
id = 200;
count =
// model
id = 300;
count =
100 1
1
CRDT Counter
{
100: 1,
:
}
{
100: 1,
: ,
}
200 300
1 1
2 2 300: 1
300: 1
200: 1
200: 1
3

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
: ,
}
// model
id = 200;
count =
// model
id = 300;
count =
100 1
1
CRDT Counter
{
100: 1,
:
}
{
100: 1,
: ,
}
200 300
1 1
2 300: 1
300: 1
200: 1
200: 1
3
2

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
: ,
,
}
// model
id = 200;
count =
// model
id = 300;
count =
100 1
CRDT Counter
{
100: 1,
: ,
}
{
100: 1,
: ,
}
200 300
1 1
2 300: 1
300: 1 200: 1
200: 1
3
2
3 3

Total Count
+ -
Total Count
+ -
Total Count
+ -
// model
id = 100;
count = {
: ,
,
}
// model
id = 200;
count =
// model
id = 300;
count =
100 1
CRDT Counter
{
100: 1,
: ,
}
{
100: 1,
: ,
}
200 300
1 1
300: 1
300: 1 200: 1
200: 1
3
3 3

CRDT Tradeoffs
Extra Memory / Metadata
Strong Eventual Consistancy

Networking Protocols
Websockets
WebRTC
...

Local First
No spinners
Multi device
Network optional
Data ownership

Libraries to look at
Yjs (JavaScript)
Automerge (JavaScript)
Y-crdt (Rust)
diamond-types (Rust)

Conflict Free Replicated Data Types

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