Comment by fc417fc802

> With perhaps a storage limit of say 1k undelivered messages.

If you want this to scale you'd need a scheme to deal with limited cache per device. Something like having each device assign a random priority to each message it has in transit. That way everyone culls a different set when things fill up.

> would need a ttl, say 24 hours

Probably better off scaling priority by age. That way you deliver if at all possible, until it eventually falls out of cache. Some people will be able to dedicate much more storage than others.