Could be anything (redis pub/sub with a channel for each shard, sqs with separate queues for each shard, separate partitions in kafka, etc)... haven't committe

Could be anything (redis pub/sub with a channel for each shard, sqs with separate queues for each shard, separate partitions in kafka, etc)... haven't committed to using one particular technology. queues that support retries could be useful in the event that a disconnect happens messages wouldn't be necessarily lost. Something custom built on top of Redis could accomplish the same perhaps.