1# Cosmos DB Partition Key Selection
2 
3## Good Partition Keys
4 
5A good partition key should have:
6 
7- **High cardinality** - Many distinct values
8- **Even data distribution** - No hot partitions
9- **Even request distribution** - Balanced workload
10- **Used in most queries** - Enables efficient routing
11 
12## Examples by Scenario
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14| Scenario | Partition Key | Reason |
15|----------|---------------|--------|
16| User-centric data | `/userId` | Queries typically filter by user |
17| Multi-tenant apps | `/tenantId` | Isolates tenant data |
18| E-commerce orders | `/customerId` | Orders queried by customer |
19| IoT telemetry | `/deviceId` | High cardinality, even distribution |
20 
21## Hierarchical Partition Keys
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23For complex scenarios, use hierarchical keys:
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25```bicep
26partitionKey: {
27  paths: ['/tenantId', '/userId']
28  kind: 'MultiHash'
29}
30```
31 
32## Anti-Patterns
33 
34Avoid these partition key choices:
35 
36| Bad Choice | Problem |
37|------------|---------|
38| Timestamp | Creates hot partitions |
39| Boolean values | Only 2 partitions |
40| Low cardinality enums | Uneven distribution |
41| Random GUID | Can't query efficiently |
42