How to fix ERR EXEC without MULTI in Redis

Ensure your code calls MULTI before any transaction commands, and only calls EXEC after queueing the desired commands.

Correct pattern:

MULTI          # Starts transaction
SET key1 val1  # Gets queued
SET key2 val2  # Gets queued
EXEC           # Executes all queued commands

Incorrect pattern:

SET key1 val1  # Executed immediately
EXEC           # Error: ERR EXEC without MULTI

DISCARD aborts a transaction and resets the connection state. If you call DISCARD, the transaction is ended and you cannot call EXEC.

Incorrect:

MULTI
SET key1 val1
DISCARD        # Transaction ended
EXEC           # Error: ERR EXEC without MULTI

If you need to cancel a transaction, only call DISCARD and then start a new MULTI/EXEC block if needed.

If using Spring Data Redis or similar libraries, ensure all MULTI/EXEC commands use the same Redis connection. Use SessionCallback or similar transaction support to bind operations to a single connection.

redisTemplate.execute(new SessionCallback<List<Object>>() {
    @Override
    public <K, V> List<Object> execute(RedisOperations<K, V> operations)
            throws DataAccessException {
        operations.multi();
        operations.opsForValue().set("key1", "value1");
        operations.opsForValue().set("key2", "value2");
        return operations.exec();
    }
});

This ensures all commands are executed on the same connection instance.

For clients like vert-x3, Lettuce, or Jedis, verify that:

- The same connection is used for MULTI, all queued commands, and EXEC
- Connection pooling doesn't multiplex commands from different transactions onto the same connection
- Enable debugging to trace which connection handles each command

If using load testing, reduce concurrency initially to verify the basic pattern works. Under high load (30+ rps), connection switching becomes more likely.