1.5 Redis as a message brokering system

Starting with Redis 5, redis supports streams. A stream is a list of messages. Streams can be used as a reliable alternative to the older Redis PUB/SUB (Publish Subscribe) mechanism that has no memory, i.e., if a node is down when a message arrives the message is missed. In addition, they can be used to have each message processed by a consumer that belongs to a consumer group. Both facilities are supported by library(redis_streams) (section 3)

Redis streams provide all the low-level primitives to realise message brokering. Putting it all together is non-trivial though. Notably:

• We must take care of messages that have been sent to some consumer but the consumer fails to process the message and (thus) ACK it is processed. This is handled by xlisten_group/5 using several options. Good defaults for these options are hard to give as it depends on the required processing time for a message, how common failures are and an acceptable delay time in case of a failure, what to do in case of a persistent failure, etc.
• Streams are independent from consumer groups and acknowledged messages remain in the stream. xstream_set/3 can be used to limit the length of the stream, discarding the oldest messages. However, it is hard to give a sensible default. The required queue length depends on the the size of the messages, whether messages come in more or less randomly or in bursts (that cause the stream to grow for a while), available memory, how bad it is if some messages get lost, etc.

The directory doc/packages/examples/redis in the installation provides an example using streams and consumer groups to realise one or more clients connected to one or more compute nodes.