We use Druid in a multi-tenant environment. In general multi-tenancy is good for query speed because excess capacity is shared. Right NOW all the limitations on cluster usage are done at a level above Druid.
Multi-tenancy on the real-time indexing side is not as good right now because the druid indexer doesn’t have great support for resource-usage awareness per tenant. So if all your tenants use approximately the same resources for real time indexing, then its fine… but if you have a large disparity between tenants, then corner cases where one or the other are performing sub-optimally, or where one or the other on rare occasions have a perfect storm of resource collisions… can happen.
If you’re just talking about QUERY multi-tenancy, then yes, druid does fine in our experience.
And to your second point, our cost accounting is related to data size, so if tenants want to send us more data that is good and we will happily grow to whatever size is needed to fit their requirements.
Note that you CAN set up multiple historical tiers to help control cost at the expense of more relaxed speed expectations. Populating data in different tiers is enforced on two criteria: data source, and age of data. For us different tiers revolve around the amount of cpu and memory is available per on-disk byte of queryable data. Where the more performant tiers have more memory and cpu available over less data. So you can say “All data from the last 5 weeks must be really fast, and data up to 13 months must be fast, and data up to 3 years must be queryable” See http://www.slideshare.net/CharlesAllen9/programmatic-bidding-data-streams-druid#34 for a screenshot of our metrics cluster (our cluster for monitoring our cluster).
There are resources currently in play to have better access rights in place: https://github.com/druid-io/druid/issues/2355 but I haven’t seen the ask for more limits natively supported by druid. Would you mind filing some github issues at https://github.com/druid-io/druid/issues , preferably one issue per “thing you would like druid to do”.