Choosing Aerospike Commit Levels: Balancing Write Latency and Data Durability

The Durability vs. Latency Trade-off

When configuring Aerospike, the most critical decision regarding write performance is the commit level. This setting determines when the server tells the client a write was successful: immediately upon receiving the data, or only after the data has been safely replicated to other nodes in the cluster.

Choosing the wrong level leads to either unacceptable application latency (if too strict) or catastrophic data loss during a node failure (if too permissive). The goal is to align the commit level with your specific Recovery Point Objective (RPO).

Comparing Commit Level Options

Aerospike allows you to define how many replicas must acknowledge a write before the operation is considered complete. This is typically configured at the namespace level or overridden via client-side policies.

Option	Acknowledgment Trigger	Latency	Durability Risk	Best Use Case
Async (One)	Primary node receives write	Lowest	High (Loss if primary fails before replication)	Real-time telemetry, session caches
Sync (All)	All replicas acknowledge write	Highest	Low (Data survives multiple node failures)	Financial ledgers, user account balances

Engineering Trade-offs

Network Topology Constraints

The impact of a SYNC commit level is magnified by your network architecture. In a single-rack deployment, the round-trip time (RTT) for replication is negligible. However, if your cluster spans multiple availability zones or uses Cross-Datacenter Replication (XDR), SYNC writes can cause application-level timeouts and severe throughput degradation because the primary node must wait for a response across the WAN.

Storage Engine Influence

The storage engine interacts with the commit level to define the total risk profile:

• RAM-only: Data is volatile. Even a SYNC write is lost if the entire cluster loses power.
• Hybrid Memory: Data is written to an index in RAM and a record in Flash (NVMe). A SYNC write here ensures the data is persisted to non-volatile media on multiple nodes, providing the highest durability.

Implementation and Validation

Commit levels are primarily managed via the namespace configuration in aerospike.conf. To ensure strong consistency, you must set the write-block-timeout to allow enough time for replicas to respond.

Configuration Example

To configure a namespace for high durability (Sync), modify your server configuration file (typically /etc/aerospike/aerospike.conf) with the following parameters:

namespace test {
    storage-engine device
    replication-factor 2
    # Ensure the write is committed to all replicas
    # This is often handled via the client policy, 
    # but the server must support the replication factor.
    write-block-timeout 100ms
}

Client-Side Control (Java Example)

While the server manages the data, the client defines the WritePolicy to specify the required acknowledgment. Run this within your application logic using the Aerospike Java Client:

WritePolicy wp = new WritePolicy();
// Set to ALL to ensure all replicas acknowledge the write
wp.commitLevel = CommitLevel.ALL;
// Set a timeout to prevent the application from hanging on network lag
wp.timeout = 500; 

client.put(wp, key, bins);

Verification and Diagnostics

To verify the current state of your cluster and ensure the namespace is operating under the expected storage and replication settings, run the asinfo tool on any cluster node (requires root or aerospike user permissions):

# Check replication factor and namespace settings
asinfo -v "namespace-config"

Expected Result: The output should list the replication-factor matching your design. If you are using CommitLevel.ALL, ensure the replication-factor is greater than 1; otherwise, the setting has no effect.

Limitations and Risks

• Write Amplification: Using SYNC increases the internal network traffic and CPU load on replica nodes.
• Availability Risk: If you require ALL replicas to acknowledge a write and one replica node is down or lagging, the write operation will fail, potentially reducing your system's write availability.
• Timeout Cascades: If write-block-timeout is set too low for a SYNC configuration, you will see an increase in TIMEOUT errors in your client logs, even if the data eventually reaches the server.

Rollback Procedure

If SYNC writes are causing application timeouts, revert to ASYNC by updating the client WritePolicy to CommitLevel.ONE. This change takes effect immediately upon application restart and does not require a server reboot.