redis/redis-py · critical · MasterNotFoundError

No master found for {service_name!r}{error_info}

Error message

No master found for {service_name!r}{error_info}

What it means

Raised as MasterNotFoundError from Sentinel.discover_master (redis/asyncio/sentinel.py:344) after iterating every configured Sentinel: none returned a master state for service_name that passes check_master_state (is_master True, not SDOWN/ODOWN, and enough peer sentinels). The error_info suffix lists per-sentinel connection errors if any sentinels were unreachable.

Source

Thrown at redis/asyncio/sentinel.py:344

            state = masters.get(service_name)
            if state and self.check_master_state(state, service_name):
                # Put this sentinel at the top of the list
                self.sentinels[0], self.sentinels[sentinel_no] = (
                    sentinel,
                    self.sentinels[0],
                )

                ip = (
                    self._force_master_ip
                    if self._force_master_ip is not None
                    else state["ip"]
                )
                return ip, state["port"]

        error_info = ""
        if len(collected_errors) > 0:
            error_info = f" : {', '.join(collected_errors)}"
        raise MasterNotFoundError(f"No master found for {service_name!r}{error_info}")

    def filter_slaves(
        self, slaves: Iterable[Mapping]
    ) -> Sequence[Tuple[EncodableT, EncodableT]]:
        """Remove slaves that are in an ODOWN or SDOWN state"""
        slaves_alive = []
        for slave in slaves:
            if slave["is_odown"] or slave["is_sdown"]:
                continue
            slaves_alive.append((slave["ip"], slave["port"]))
        return slaves_alive

    def filter_replicas(
        self, replicas: Iterable[Mapping]
    ) -> Sequence[Tuple[EncodableT, EncodableT]]:
        """Remove replicas that are in an ODOWN or SDOWN state.

        This is an alias for :py:meth:`filter_slaves`,

View on GitHub (pinned to da03cdc7e8)

Solutions

1. Verify service_name matches the Sentinel-monitored master name exactly.
2. Check Sentinel connectivity — the error_info suffix names which sentinels failed and why.
3. Lower min_other_sentinels if quorum is over-strict, or wait out an in-progress failover and retry.
4. Confirm at least one Sentinel reports the master as is_master with is_sdown/is_odown False.

Example fix

# before
master = sentinel.master_for('mymaster-typo')
await master.set('k','v')  # MasterNotFoundError
# after
master = sentinel.master_for('mymaster')  # correct service name

states = await sentinel.sentinel_masters()
if service_name not in states:
    raise ConfigError(f'service {service_name!r} not known to sentinel')

from redis.sentinel import MasterNotFoundError
try:
    addr = await sentinel.discover_master(service_name)
except MasterNotFoundError as e:
    logger.error('master discovery failed: %s', e)
    raise

When it happens

Trigger: First command on a master_for(...) client, or connection reset mid-life, when no Sentinel knows a healthy master; service_name typo; all Sentinels unreachable (collected_errors populated); failover in progress with no acknowledged master yet.

Common situations: Wrong service_name; Sentinel nodes misconfigured/unreachable; failover mid-flight; min_other_sentinels set too high so otherwise-valid responses are rejected.

Related errors

• The previous master is now a slave
• No slave found for {self.service_name!r}
• The previous master is now a slave
• No slave found for {self.service_name!r}
• No master found for {service_name!r}{error_info}

AI-assisted analysis of redis/redis-py@da03cdc7e8 (2026-08-04). Data as JSON: /data/errors/65e382ad22dfdf73.json. Report an issue: GitHub.