What causes "Cannot watch or send commands on different slots" in redis/redis-py?

Use hash tags to force keys into the same slot: 'user:{1}' and 'account:{1}'

redis/redis-py · error · CrossSlotTransactionError

Cannot watch or send commands on different slots

Error message

Cannot watch or send commands on different slots

What it means

CrossSlotTransactionError raised in TransactionStrategy._execute_command (redis/asyncio/cluster.py:3184) during the WATCH/immediate path. While watching, all commands must target the same slot as the already-registered pipeline slots; attempting to watch or run a command whose slot differs from the existing one breaks the single-node transaction guarantee. Redis Cluster transactions are only atomic within one hash slot.

Solutions

Use hash tags to force keys into the same slot: 'user:{1}' and 'account:{1}'
Split the work into separate single-slot transactions
Switch to the non-atomic pipeline strategy if cross-slot operation is acceptable

Example fix

// before
await pipe.watch('user:1')
await pipe.watch('account:2')
// after - same slot via hash tag
await pipe.watch('user:{1}')
await pipe.watch('account:{1}')

Defensive patterns

Strategy: validation

Validate before calling

from redis.cluster import key_slot
slots = {key_slot(k.encode()) for k in watch_keys}
if len(slots) > 1:
    raise ValueError('watch keys map to different slots; use hash tags')

Try / catch

try:
    await pipe.watch(*keys)
except CrossSlotTransactionError:
    # rebucket keys with hash tags or split transactions

Prevention

Use hash tags to colocate transaction keys in one slot
Validate slots up front when building cluster transactions

When it happens

Trigger: `pipe.watch('user:1')` then `pipe.watch('user:2')` where the two keys hash to different slots; mixing a watched key with an immediate command on a different slot before MULTI.

Common situations: Forgetting to use hash tags ({tag}) when transacting over logically related keys; assuming cluster transactions behave like standalone transactions.

Related errors

AI-assisted analysis of redis/redis-py@6a6b581b48 (2026-08-10). Data as JSON: /api/errors/38bdd069a0735e8e. Report an issue: GitHub.

Appendix: source

Thrown at redis/asyncio/cluster.py:3184

    async def _execute_command(
        self, *args: Union[KeyT, EncodableT], **kwargs: Any
    ) -> Any:
        if self._pipe.cluster_client._initialize:
            await self._pipe.cluster_client.initialize()

        slot_number: Optional[int] = None
        if args[0] not in self.NO_SLOTS_COMMANDS:
            slot_number = await self._resolve_transaction_slot(*args)

        if (
            self._watching or args[0] in self.IMMEDIATE_EXECUTE_COMMANDS
        ) and not self._explicit_transaction:
            if args[0] == "WATCH":
                self._validate_watch()

            if slot_number is not None:
                if self._pipeline_slots and slot_number not in self._pipeline_slots:
                    raise CrossSlotTransactionError(
                        "Cannot watch or send commands on different slots"
                    )

                self._pipeline_slots.add(slot_number)
            elif args[0] not in self.NO_SLOTS_COMMANDS:
                raise RedisClusterException(
                    f"Cannot identify slot number for command: {args[0]},"
                    "it cannot be triggered in a transaction"
                )

            return self._immediate_execute_command(*args, **kwargs)
        else:
            if slot_number is not None:
                self._pipeline_slots.add(slot_number)

            return super().execute_command(*args, **kwargs)

    def _validate_watch(self):

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