Redis Sentinel配置中的法定人數是什麼？

Quorum在Redis Sentinel配置中是指觸發故障轉移前必須達成共識的Sentinel節點最小數量。例如，若設置5個Sentinel且quorum為3，則至少需3個Sentinel確認主節點不可達才會發起故障轉移。 1. Quorum決定標記主節點為主觀下線（SDOWN）和客觀下線（ODOWN）所需的最低共識數；2. 設置過高可能導致無法及時故障轉移，設置過低可能引發誤判；3. 推薦使用奇數個Sentinel並將quorum設為略超過總數的一半；4. 需結合部署規模、容錯能力與網絡環境綜合考慮；5. Quorum影響主節點下線判斷及副本提升決策，是保障可靠性與可用性的關鍵參數。

A quorum in Redis Sentinel configuration refers to the minimum number of Sentinels that must agree on a condition (like a master being down) before any action is taken, such as promoting a new master or reconfiguring replicas.

This number doesn't have to match the total number of Sentinels, but setting it wisely ensures both reliability and availability. If set too high, you risk not reaching agreement when you need to fail over. If too low, you might trigger unnecessary failovers.

What Does Quorum Actually Do?

At its core, the quorum value determines how many Sentinels must independently confirm that a master is unreachable before marking it as subjectively down (SDOWN) and then objectively down (ODOWN).

• When a Sentinel detects that a master isn't responding, it marks it as SDOWN.
• It then checks with other Sentinels to see if they also think the master is unreachable.
• If the number of Sentinels agreeing exceeds the quorum value, the master is marked ODOWN.
• Only after ODOWN is declared will a failover be initiated.

So, the quorum is essentially your threshold for consensus among Sentinels.

For example, if you have 5 Sentinels and set quorum to 3, at least 3 Sentinels must agree that the master is unreachable before any failover starts.

How to Choose the Right Quorum Value

Choosing the right quorum depends on:

• The total number of Sentinels deployed
• Your tolerance for false positives vs. downtime
• The likelihood of network partitions

Here are some practical guidelines:

• Always use an odd number of Sentinels to avoid ties.
• Set quorum to just over half the number of Sentinels for majority-based decisions (eg, 2 out of 3, 3 out of 5).
• Avoid setting quorum equal to the total number of Sentinels unless you want extreme reliability at the cost of availability during failures.

If you're running in a multi-datacenter setup, consider how network partitions may affect communication between Sentinels.

Quorum vs. Other Sentinel Settings

Quorum often gets confused with other Sentinel settings like down-after-milliseconds or parallel-syncs . But they serve different purposes:

• quorum : Dictates how many Sentinels must agree before taking action.
• down-after-milliseconds : How long a Sentinel waits before considering a master down.
• failover-timeout : Controls how long a Sentinel waits before retrying a failed failover.

They all work together, but quorum has the biggest impact on decision-making reliability.

Also, keep in mind that quorum affects both detection and promotion:

• At least quorum Sentinels must agree to mark a master down.
• A Sentinel needs permission from at least quorum Sentinels to start promoting a replica.

Final Notes

The quorum setting is powerful but simple once you understand what it controls. It's not about speed—it's about agreement. Misconfigure it, and you either risk false failovers or lose automatic recovery when you need it most.

You don't need complex math here—just make sure you're aligning quorum with your deployment size and fault tolerance goals.

基本上就這些。

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