评估Hazelcast时关于同步复制机制的技术疑问咨询

Hey there, let's unpack this confusion around Hazelcast's replication mechanisms—it's totally understandable given how the docs cover different scenarios based on configuration and use cases.

First, let's confirm the core default behavior you found in the Backing Up Maps docs:

Backup operations are synchronous, so when a map.put(key, value) returns, it is guaranteed that the map entry is replicated to one other member

This is 100% accurate for Hazelcast's default synchronous backup setup. By default, Hazelcast uses a backup-count of 1 (configurable), which means every write operation like map.put() blocks until the entry is successfully replicated to at least one backup node. This gives you strong consistency guarantees for the in-memory state of your map.

Now, the reason the other doc page might be causing confusion is likely because it's covering one of these edge cases or configurable alternatives:

• Async Backups: If you configure async-backup-count to a value greater than 0, those backups are replicated asynchronously in the background. The map.put() call will return immediately without waiting for these async backups to complete—so there's no guarantee the entry is on the backup node when the method returns. This is a tradeoff for higher write throughput, but with weaker immediate consistency for those backup copies.
• Consistency Models for Distributed Scenarios: Hazelcast operates in an AP (Availability/Partition tolerance) mode by default, but offers CP (Consistency/Partition tolerance) subsystems for specific use cases. If the other doc is discussing CP data structures (like the CP Map), the replication rules might differ slightly, focusing on linearizability rather than just synchronous backups.
• MapStore/Loader Integration: If you're using Hazelcast's MapStore to persist entries to an external database, the default behavior is to sync the MapStore write with the put() call—but you can configure this to be async too. The doc might be clarifying that the MapStore write isn't guaranteed to complete before put() returns if you've enabled async persistence.

To resolve your confusion, I'd recommend:

1. Double-check your Hazelcast map configuration to see if you're using synchronous backups (default) or have enabled async backups.
2. Look at the full context of the conflicting doc page—note if it's talking about async backups, CP subsystems, or external persistence scenarios.
3. Align your configuration with your business needs: if you need strong immediate consistency for backups, stick with the default synchronous setup; if throughput is a higher priority, consider async backups (but be aware of the consistency tradeoff).

内容的提问来源于stack exchange，提问作者vamsi