What's the size limitation of a message when PUB/SUB in Redis channel

Updated Answer

Subsequent to my original answer below, I came across the actual answer. There is a configuration parameter in redis.conf called:

client-output-buffer-limit

You can check it with:

config get client-output-buffer limit
"normal 0 0 0 slave 268435456 67108864 60 pubsub 33554432 8388608 60"

The 33554432 after pub/sub is the maximum buffer size for pub/sub clients, and the 8388608 is a soft limit that may not be exceeded for more than 60 seconds.

So, my answer below changes if you raise the limit using a command like the following:

config set client-output-buffer-limit "normal 0 0 0 slave 268435456 67108864 60 pubsub 53554432 8388608 60"

I have not done any tests as to whether this is advisable, or safe or even sensible but think it is materially relevant to the answer.

Original Answer

I don't know where the limit arises, but just FYI, I did some simple tests with the redis-cli client, subscribing like this:

redis-cli SUBSCRIBE myStream

And I sent data, in multiples of 1MB like this:

dd if=/dev/zero bs=$((1024*1024)) count=20  | redis-cli -x PUBLISH myStream

So, the command above worked, at 20MB but the following command for 21MB failed:

dd if=/dev/zero bs=$((1024*1024)) count=21  | redis-cli -x PUBLISH myStream

and the subscriber was disconnected with the message:

Error: Server closed the connection

I'm calling this an "empirical" answer.

Just for your information, I experimented myself, the approximate ceiling size is around 21MB to 22MB on my machine (Debian 5.4.8-1) (it may vary depends on the OS and redis version), it will cause a redis ConnectionError: Connection closed by server.