What exactly is a "Null reply" in Redis?

Question

I use Redis with Scala (rediscala library). I need to create a transaction like this:

redisTransaction.watch(key)
redisTransaction.zremrangebyscore(key, limit, limit) *>
redisTransaction.zadd(key, (someValue, entry)) *>
redisTransaction.set[ByteString](s"$lastSeqNumKey", sequenceNum) *>
redisTransaction.exec().void

The last expression should return a Future[Unit] which I expect to fail in case of failed transaction. However, Redis documentation states that:

When using WATCH, EXEC can return a Null reply if the execution was aborted.

What exactly is a "null reply"? Is it possible that my Future[Unit] will succeed with a null value, or empty String or something similar? I'd like to write a test and I need some hints about it.

Answer 1

One correction, there is no void method on redisTransaction.exec().

Short answer:

exec() call will return Future(MultiBulk(None)) if the transaction fails or the execution is aborted.

Long answer:

As the documentation says,

When using WATCH, EXEC can return a Null reply if the execution was aborted.

and links to NULL reply 's explanation

RESP Bulk Strings can also be used in order to signal non-existence of a value using a special format that is used to represent a Null value. In this special format the length is -1, and there is no data, so a Null is represented as:

"$-1\r\n" This is called a Null Bulk String.

The client library API should not return an empty string, but a nil object, when the server replies with a Null Bulk String. For example a Ruby library should return 'nil' while a C library should return NULL (or set a special flag in the reply object), and so forth

This points to the fact that server will reply with a NULL response and the client has to deal with it. It can't be an empty string and in this library it is dealt to decode & map the response from server as MultiBulk(None)

There is a test case for the decoding of NULL responses from redis.