Type: New Feature
Affects Version/s: None
Fix Version/s: None
Some features like TTLs or access control lists have use cases that call for per-value configurability.
Currently in HBase TTLs are set per column family. This leads to potentially awkward "bucketing" of values into column families set up to accommodate the common desired TTLs for all values within – an unnecessarily wide schema, with resulting unnecessary reduction in I/O locality in access patterns, more store files than otherwise, and so on.
HBASE-1697 we're considering setting ACLs on column families. However, we are aware of other BT-like systems which support per-value ACLs. This allows for multitenancy in a single table as opposed to really requiring tables for each customer (or, at least column families). The scale out properties for a single table are better than alternatives. I think supporting per-row ACLs would be generally sufficient: customer ID could be part of the row key. We can still plan to maintain column-family level ACLs. We would therefore not have to bloat the store with per-row ACLs for the normal case – but it would be highly useful to support overrides for particular rows. So how to do that?
I propose to introduce metacolumns.
A metacolumn would be a column family intrinsic to every table, created by the system at table create time. It would be accessible like any other column family, but we expect a default ACL that only allows access by the system and operator principals, and would function like any other, except administrative actions such as renaming or deletion would not be allowed. Into the metacolumn would be stored per-row overrides for such things as ACLs and TTLs. The metacolumn therefore would be as sparse as possible; no storage would required for any overrides if a value is committed with defaults. A reasonably sparse metacolumn for a region may fit entirely within blockcache. It may be possible for all metacolumns on a RS to fit within blockcache without undue pressure on other users. We can aim design effort at this target.
The scope of changes required to support this is:
- Introduce metacolumn concept in the code and into the security model (default ACL): A flag in HCD, a default ACL, and a few additional checks for rejecting disallowed administrative actions.
- Automatically create metacolumns at table create time.
- Consult metacolumn as part of processing reads or mutations, perhaps using a bloom filter to shortcut lookups for rows with no metaentries, and apply configuration or security policy overrides if found.