This document discusses how Redis implements conflict-free replicated data types (CRDTs) to allow for concurrent updates and automatic conflict resolution across multiple replicas. It provides examples of how common Redis data structures like strings, sets, lists, hashes and sorted sets are replicated across nodes using CRDT techniques. The examples show how writes are performed concurrently on different replicas and then synced together, with the last write taking precedence in some cases and replicas converging on a consistent state in all cases.

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4. • Geo Distributed Trading/Bidding
- Auctions, Bids/Asks
Lists/Sorted Sets tracking Bids and Asks
• Dashboards & Scoreboards
- Tracking Geo Distributed Scoreboards
Sorted Sets tracking ordered scores
• Real-time session management and
Metering Apps
- HA for session management
- Tracking Usage/Consumption
- A/B testing
Hashes/Sets/Lists/Expiry Tracking Consumption Events
• Fraud Detection
- Geo Distributed Event Tracking
Sets Gathering Geo Distributed Events
• Social Engagement Apps
- Encoding Social Engagement
Distributed Counters for “Likes”, “Shares”, “Retweets”
• Collaboration Apps
- Constructing Smart Timelines
Merged Lists Ordering Posts
- Instant Messaging & Conversation Tracking
Merged Lists Ordering Conversations

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10. compressed and secured replication
compressed and secured
replication
compressed and secured replication
compressedand
securedreplication

12. conflict-free replicated data type ( data structure which
can be replicated across multiple computers in a network, each
holding local replication of the value (replica) where:
1. Local writes = Replicas can be updated independently and
concurrently WITHOUT coordination between the replicas
2. After replication, Inconsistency resolved = It is always
mathematically possible to resolve inconsistencies which might
occur

14. A
B
C
{A=1,B=0,C=0}
{A=1,B=1,C=0}
{A=0}
{B=0}
{C=0}
{A=0,B=0,C=1} {A=0,B=0,C=2}
{A=1,B=2,C=0}
{A=0,B=0,C=3}
{A=1,B=3,C=3}
{A=2,B=3,C=3}

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15

16. Bi-directional secured
compressed
Bi-directional secured
compressed
Bi-directional
secured
compressed
CRDT
Module
CRDT
Module
CRDT
Module

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<add image of the
items view>

20. Replica A Replica B
Two replicas, working over single data sets
Each step is done concurrently and then a synchronization takes
place
In reality, un-synced situations are limited to a few millisec
(depends on the network)
Examples are displayed from the client side point of view and not
the redis internals

22. Replica A Replica B
> INCRBY mykey 10
(integer) 10
> GET mykey
(nil)
Syncing
Synced
> GET mykey
“10”
> GET mykey
“10”
> GET mykey
“10”

23. Replica A Replica B
> INCRBY mykey 5
(integer) 15
> INCRBY mykey 8
(integer) 18
Syncing
Synced
> GET mykey
“23”
> GET mykey
“23”

24. Replica A Replica B
> DEL mykey
(integer) 1
> DECRBY mykey 3
(integer) 20
Syncing
Synced
> GET mykey
(nil)
> GET mykey
“-3”
> GET mykey
“-3”

25. Replica A Replica B
> DEL mykey
(integer)
> GET mykey
“-3”
Syncing
Synced
> GET mykey
(nil)
> GET mykey
(nil)
> GET mykey
(nil)

27. Replica A Replica B
> SADD myset X
(integer) 1
Syncing
Synced
> SMEMBERS myset
1) “X”
> SMEMBERS myset
1) “X”
> SMEMBERS myset
1) “X”
> SADD myset X
(integer) 1
> SREM myset X
(integer) 1
> SMEMBERS myset
(empty list or set)

28. Replica A Replica B
Syncing
Synced
> SMEMBERS myset
1) “Y”
2) “X”
> SMEMBERS myset
1) “Y”
> SMEMBERS myset
1) “Y”
> DEL myset
(integer) 0
> SMEMBERS myset
(empty list or set)
> SADD myset Y
(integer) 1

30. Replica A Replica B
> SET mykey “A”
OK
Syncing
Synced
> GET mykey
“A”
> GET mykey
1) “B”
> GET mykey
1) “B”
> GET mykey
“B”
> SET mykey “B”
OK
Replica B write time is later the replica A

31. Replica A Replica B
> DEL mykey
(integer)
> SET mykey “C”
OK
Syncing
Synced
> GET mykey
(nil)
> GET mykey
“C”
> GET mykey
“C”
> GET mykey
“C”

32. Strings
⇒ Observed Remove with Last write win
Counter
⇒ Observed Remove with local replica changes,
Sets
⇒ Observed Remove Set
Hashes
⇒ Based on the implementation of sets, with fields of registers and counters
Lists
⇒ Directed acyclic graph
Sorted Sets
⇒ Based on combining sets and Counters