Cassandra Consistency Levels Reference
This page provides quick-reference tables for Cassandra consistency levels. For detailed explanations of how consistency works, see Consistency.
Write Consistency Levels
Section titled “Write Consistency Levels”| Level | Replicas Required | Description | Availability |
|---|---|---|---|
ANY | 1 (including hints) | Write succeeds if any node (including coordinator) acknowledges. May store as hint only. | Highest |
ONE | 1 | Single replica must acknowledge | High |
TWO | 2 | Two replicas must acknowledge | Medium |
THREE | 3 | Three replicas must acknowledge | Medium |
QUORUM | RF/2 + 1 (global) | Majority of all replicas across all datacenters | Medium |
LOCAL_ONE | 1 (local DC) | Single replica in coordinator's datacenter | High |
LOCAL_QUORUM | local_RF/2 + 1 | Majority in coordinator's datacenter only | High |
EACH_QUORUM | local_RF/2 + 1 per DC | Majority in every datacenter (each DC's RF used independently) | Low |
ALL | RF (all replicas) | Every replica must acknowledge | Lowest |
Read Consistency Levels
Section titled “Read Consistency Levels”| Level | Replicas Contacted | Description | Availability |
|---|---|---|---|
ONE | 1 | Single replica responds | Highest |
TWO | 2 | Two replicas respond; newest returned | High |
THREE | 3 | Three replicas respond; newest returned | Medium |
QUORUM | RF/2 + 1 (global) | Majority of all replicas; newest returned | Medium |
LOCAL_ONE | 1 (local DC) | Single replica in coordinator's datacenter | Highest |
LOCAL_QUORUM | local_RF/2 + 1 | Majority in coordinator's datacenter; newest returned | High |
ALL | RF (all replicas) | All replicas respond; newest returned | Lowest |
EACH_QUORUM
EACH_QUORUM is write-only. It is not supported for read operations.
Serial Consistency Levels (LWT)
Section titled “Serial Consistency Levels (LWT)”Lightweight transactions use separate consistency levels for the Paxos consensus phase.
| Level | Scope | Description |
|---|---|---|
SERIAL | All datacenters | Paxos consensus across all replicas globally |
LOCAL_SERIAL | Local datacenter | Paxos consensus within coordinator's datacenter only |
See Lightweight Transactions for CQL syntax and Paxos for architecture details.
Quorum Calculation
Section titled “Quorum Calculation”Formula: QUORUM = floor(RF / 2) + 1
| Replication Factor | QUORUM | Can Tolerate Failures |
|---|---|---|
| 1 | 1 | 0 |
| 2 | 2 | 0 |
| 3 | 2 | 1 |
| 4 | 3 | 1 |
| 5 | 3 | 2 |
| 6 | 4 | 2 |
| 7 | 4 | 3 |
Multi-Datacenter Behavior
Section titled “Multi-Datacenter Behavior”For a cluster with RF=3 per datacenter (total RF=6 across 2 DCs):
| Level | Replicas Required | Cross-DC Wait | DC Failure Tolerance |
|---|---|---|---|
ONE | 1 (any DC) | No | Yes |
LOCAL_ONE | 1 (local DC) | No | Yes |
QUORUM | 4 (any DCs) | Yes | No |
LOCAL_QUORUM | 2 (local DC) | No | Yes |
EACH_QUORUM | 2 per DC | Yes | No |
ALL | 6 (all DCs) | Yes | No |
Recommended for Multi-DC
LOCAL_QUORUM is recommended for most multi-datacenter deployments. It provides strong consistency within each datacenter. Datacenter failure tolerance depends on client placement; clients must be able to reach another DC to fail over.
Strong Consistency Formula
Section titled “Strong Consistency Formula”Heuristic: R + W > RF generally provides strong consistency (reads see latest writes) under normal operation. This assumes no concurrent failures affecting the same replica set and does not account for edge cases like ANY writes or transient divergence.
| Write CL | Read CL | RF=3 | Strong? | Notes |
|---|---|---|---|---|
QUORUM | QUORUM | 2+2=4 > 3 | Yes | Standard strong consistency |
ONE | ALL | 1+3=4 > 3 | Yes | Write-heavy workloads |
ALL | ONE | 3+1=4 > 3 | Yes | Read-heavy workloads |
LOCAL_QUORUM | LOCAL_QUORUM | 2+2=4 > 3 | Yes | Per-datacenter strong consistency |
ONE | ONE | 1+1=2 < 3 | No | Eventual consistency |
ONE | QUORUM | 1+2=3 = 3 | No | Not strictly greater |
Failure Tolerance
Section titled “Failure Tolerance”Single Datacenter (RF=3)
Section titled “Single Datacenter (RF=3)”| Nodes Down | ONE | QUORUM | ALL |
|---|---|---|---|
| 0 | ✅ | ✅ | ✅ |
| 1 | ✅ | ✅ | ❌ |
| 2 | ✅ | ❌ | ❌ |
Multi-Datacenter (RF=3 per DC)
Section titled “Multi-Datacenter (RF=3 per DC)”| Failure Scenario | LOCAL_QUORUM | QUORUM | EACH_QUORUM |
|---|---|---|---|
| 1 node in local DC | ✅ | ✅ | ✅ |
| 2 nodes in local DC | ❌ | ✅* | ❌ |
| Entire remote DC down | ✅ | ❌ | ❌ |
*QUORUM may succeed if remote DC nodes compensate.
Latency Comparison
Section titled “Latency Comparison”| Level | Single DC | Multi-DC | Notes |
|---|---|---|---|
ONE | ~1-2ms | ~1-2ms | Fastest |
LOCAL_ONE | Low | Low | Same as ONE for local |
QUORUM | Low | Higher (cross-DC) | Cross-DC round trip in multi-DC |
LOCAL_QUORUM | Low | Low | No cross-DC wait |
EACH_QUORUM | N/A | Higher (cross-DC) | Waits for slowest DC |
ALL | Higher | Higher (cross-DC) | Waits for slowest replica |
Actual latencies are workload-, topology-, and hardware-dependent.
Quick Decision Guide
Section titled “Quick Decision Guide”| Requirement | Recommended Write CL | Recommended Read CL |
|---|---|---|
| Strong consistency, single DC | QUORUM | QUORUM |
| Strong consistency, multi-DC | LOCAL_QUORUM | LOCAL_QUORUM |
| Global strong consistency | QUORUM | QUORUM |
| Maximum throughput | ONE | ONE |
| Write-heavy, strong reads | ONE | ALL |
| Read-heavy, strong writes | ALL | ONE |
| Time-series / metrics | LOCAL_ONE | LOCAL_ONE |
| Compare-and-set (LWT) | N/A (use IF clause) | N/A |
Related Documentation
Section titled “Related Documentation”- Consistency (Concepts) - How consistency works in detail
- Replication - How replicas are placed
- Lightweight Transactions - CQL syntax for LWT
- Paxos - Consensus algorithm for LWT