Conflict resolution

Learn conflict resolution.

Conflict resolution

A conflict is when the same record is modified both on the device and on the server. For example, while the user is offline they marked a task as 'completed'; the same task was marked 'in progress' on another device. When they come online the two versions conflict.

There are three common strategies for conflict resolution. Server wins: the server's version is always accepted. Device wins: the device's version is accepted. Merge: the fields of both versions are merged. The simplest strategy is 'last write wins'; it decides based on the timestamp. The version with the larger updatedAt field is accepted.

last-write-wins

The last-write-wins strategy compares the updatedAt timestamps of two versions. The newer one (larger timestamp) is accepted; the older one is discarded. This strategy is simple and predictable; but it can overwrite an older change the user made on purpose.

Example: the local task has updatedAt: 1000, the server's has updatedAt: 2000. The server's is newer; so the server's is accepted, the local one is discarded. This decision is written as if (local.updatedAt > server.updatedAt) return local; else return server. On equal timestamps the server is considered the winner; this ensures consistency in distributed systems.

Exercise Resolve the conflict

Complete the resolveConflict function. It takes local and server as parameters. Write if (local.updatedAt > server.updatedAt) return local; else return server.

Show example solution

if (local.updatedAt > server.updatedAt) return local;
return server;

Test yourself

Write the strategy

const strategy = '____';

Show answer

const strategy = 'last-write-wins';

Match each strategy to its purpose

last-write-wins

Timestamp
server wins

Server version
device wins

Device version
merge

Merges fields
What does last-write-wins decide based on?
```
if (local.updatedAt > server.updatedAt) { ... }
```
- Timestamp
- Content size
- Version number
- Name length
Show answer

Timestamp — It decides based on the timestamp; the newer one wins.

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