Redis Sentinel Config Generator

Generate sentinel.conf for a monitored primary. Two things decide whether failover works, and only one of them is the quorum: performing a failover also needs a majority of all sentinels to be reachable, which is why two sentinels can never fail over.

The primary

Primary name Clients connect by this name rather than by host, so changing it is a client change.

Primary host A real address, not a loopback one: sentinels hand clients the address they were given, so 127.0.0.1 here is what every client receives.

Primary port

The quorum

Quorum decides how many sentinels must AGREE the primary is down. A majority of sentinels must also be reachable to authorise the failover, which is a separate condition and the one people conflate with quorum.

Number of sentinels Three is the minimum that tolerates one failure. Two cannot form a majority once one is gone, so it fails over never rather than always.

Quorum Leave this at a majority of the sentinel count unless you have a specific reason.

down-after-milliseconds Too low fails over on a garbage collection pause or a network blip. Too high extends every real outage by exactly this long.

sentinel.conf

updates as you type

Examples

Worked setups you can load into the form above. Each one is a decision the generator makes differently, and the reason it makes it.

Two sentinels

Losing one leaves no majority, so failover can never be authorised

master-name: mymaster
sentinels: 2

An even number

Four buys nothing over three and makes a split vote possible

master-name: mymaster
sentinels: 4

Common mistakes

These are the ones that fail silently. The config is accepted, nothing raises an error, and the consequence arrives later.

Running two sentinels
A majority of two is two, so losing either one makes failover impossible. Two sentinels detect an outage and can do nothing about it.
Instead:Run three, on three separate hosts.
Lowering the quorum to make failover easier
The quorum only controls agreement that the primary is down. The majority requirement for performing the failover is separate and cannot be lowered.
Instead:Add sentinels rather than lowering the quorum.
Pointing clients at the primary's address
After a promotion the client keeps writing to the old primary, which now rejects writes with READONLY.
Instead:Use the client library's Sentinel support.
Deploying sentinel.conf from configuration management on every restart
Sentinel rewrites the file to record the current primary and its peers. Overwriting it discards that state.
Instead:Template it once, then exclude it.

The quorum is not the whole story

Sentinel has two separate thresholds and configuring only the visible one is the usual reason a failover does not happen when it should.

Quorum decides agreement, majority decides authority

The quorum is how many sentinels must agree the primary is down. Actually performing the failover additionally requires a majority of ALL configured sentinels to be reachable. Setting a quorum of 1 across two sentinels does not make failover easier: the majority is still 2.

Two sentinels can never fail over

Losing one leaves a single sentinel, which is not a majority of two, so no failover can be authorised. Three is the minimum, on three separate hosts, and an even number above that buys nothing over the odd number below it.

Clients must talk to Sentinel, not to the primary

Sentinel does not proxy traffic; it tells clients where the primary is. A client configured with the primary's host and port keeps pointing at the old primary after a promotion, and the symptom is writes failing with READONLY.

down-after-milliseconds is a trade, not a tuning knob

Too low and an ordinary garbage collection pause triggers a failover nobody wanted. Too high and a real outage lasts longer than it needs to. The right value depends on how long your primary can legitimately stall.

Sentinel rewrites its own config file

It records the current primary and the other sentinels it has discovered. Deploying the same file to every host from configuration management will overwrite that state, so exclude it after the first start.

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