The document discusses consistent hashing, which is a technique for distributing data across multiple servers. It works by assigning each server and data item a unique hash value and storing each data item on the first server whose hash value comes after the data's hash value. This allows redistributing only a fraction of data when servers are added or removed. The key aspects are using a hash function to assign all items unique values and treating the hash ring as a circular space to determine data placement.

1. The Easiest
Consistent Hashing
charsyam@naver.com

2. Consistent Hashing?

3. WHY?

4. There was data
In the beginning.

5. 1
2
3 4
5
6
7
89

6. How do you distribute
data pairly into 3 servers?
Considering the future.

7. 1
2
3
4
5
6
7
8
9
Sequence

8. 1 2 3
4 5 6
7 8 9
Modular

9. If you add one server, or
remove one server
What happened?

10. 1
2
3
4
5
6
Add one server for Sequence
7
8
9

11. Add one server for Modular
1 2 3
4 5 6
7 8 9

12. How do you redistribute
these data?

13. Redistribute by Modular
1 2 3 4
5 6 7 8
9

14. Most of data is
redistributed!

15. Redistribution is
Burden.

16. What is a good way
to reduce redistribution?

17. Consistent Hashing
Can Do!!!

18. Consistent Hashing
redistribute
only N/K data
N = data size K = servers

19. Key Concept is
Hash

20. What is main
concept of hash

21. If you use same hash
function?
The result is always the same.

22. hash(“abc”) = 1
hash(“abc1”) = 2
hash(“abc”) = 1

23. You have 3 servers.
10.0.1.1
10.0.1.2
10.0.1.3

24. There is a hash function.
y = hash(x)

25. You hash 3 servers
hash(“10.0.1.1”) = 100
hash(“10.0.1.2”) = 400
hash(“10.0.1.3”) = 700
Just Giving server address as key

26. Just define a rule.
We will store a key in
hash(key) is higher and
the nearest one.

27. hash(key) Hash value
hash(“10.0.1.1”) 100
hash(“10.0.1.2”) 400
hash(“10.0.1.3”) 700

28. There is key “redis”
hash(“redis”) = 200
Where we store it?

29. hash(key) Hash value
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
hash(“10.0.1.3”) 700

30. There is key “charsyam”
hash(“charsyam”) = 450
Where we store it?

31. hash(key) Hash value
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
hash(“charsyam”) 450 in hash(“10.0.1.3”)
hash(“10.0.1.3”) 700

32. There is key “udemy”
hash(“udemy”) = 50
Where we store it?

33. hash(key) Hash value
hash(“udemy”) 50 in hash(“10.0.1.1”)
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
hash(“charsyam”) 450 in hash(“10.0.1.3”)
hash(“10.0.1.3”) 700

34. There is key “web”
hash(“web”) = 1000
Where we store it?

35. There is no server has
higher hash value 1000.
Where we can store it?

36. Think it is Circle

37. hash(key) Hash value
hash(“udemy”) 50 in hash(“10.0.1.1”)
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
hash(“charsyam”) 450 in hash(“10.0.1.3”)
hash(“10.0.1.3”) 700
hash(“web”) 1000 in hash(“10.0.1.1”)

38. Key “web” is stored in
First Server.

39. If we add new server
It is “10.0.1.4”.
And hash(“10.0.1.4”) = 500

40. hash(key) Hash value
hash(“udemy”) 50 in hash(“10.0.1.1”)
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
DELETED hash(“charsyam”) 450
hash(“10.0.1.4”) 500
hash(“10.0.1.3”) 700
hash(“web”) 1000 in hash(“10.0.1.1”)

41. After adding “10.0.1.4”
Key “charsyam” is
missing

42. But other keys are
never changed.

43. You can still find key
“udemy” in “10.0.1.1”

44. There is key “charsyam”
hash(“charsyam”) = 450
Where we store it?

45. hash(key) Hash value
hash(“udemy”) 50 in hash(“10.0.1.1”)
hash(“10.0.1.1”) 100
hash(“redis”) 200 in hash(“10.0.1.2”)
hash(“10.0.1.2”) 400
hash(“charsyam”) 450 in hash(“10.0.1.4”)
hash(“10.0.1.4”) 500
hash(“10.0.1.3”) 700
hash(“web”) 1000 in hash(“10.0.1.1”)

46. A
Add A Server

47. A
B
Add B Server

48. A
BC
Add C Server

49. A
BC
Add Key 1
1

50. A
BC
Add Key 2
1
2

51. A
BC
Add Key 3
1
2
3

52. A
BC
Add Key 4
1
2
3
4

53. A
BC
Add Key 5
1
2
3
4
5

54. A
C
Fail B Server
2
3
4
5

55. A
C
Add Key 1
2
3
4
5
1

56. In Next Lecture
● We will discuss belows topics
○ How to use Consistent Hashing in Real World.