Purposes of index:
- Tell garbage collector what is eligible for deletion (and some partial order of preference)
- Tell replication enforcer which blocks should be stored how many × (and in which [types of] backing store)
- Tell rebalancer which blocks should be moved to redistribute free space and reduce probe time
- Tell managers how much disk space is being conserved due to CAS
- Tell managers how much disk space is occupied in a given backing store service
- Tell managers how disk usage would be affected by modifying storage policy
- Tell users how much disk space is represented by a given set of collections
- Tell users how much disk space can be made available by garbage collection
- Tell users how soon they should expect their cached data to disappear
- Tell users performance statistics (how fast should I expect my job to read data?)
- Tell ops where each block was most recently read/written, in case data recovery is needed
- Tell ops how unbalanced the backing stores are across the cluster
- Tell ops activity level and performance statistics
- Tell ops activity level vs. amount of space (how much of the data is being accessed by users?)
- Tell ops disk performance/error/status trends to help identify bad hardware
Basic kinds of data in the index:
- Which blocks are used by which collections (and which collections are valued by which users/groups)
- Which blocks are stored on which disks
- Which disks are attached to which nodes
- Read events
- Write events
- Exceptions (checksum mismatch, IO error)
- REST service
- API server may cache/proxy some queries
- API server may redirect some queries
- Support +A tokens like Keep server when accepting collection/blob uuids in request?
- Require admin api_token for some queries, site-configurable?
- Easy to run multiple keep index services.
- Most features do not need synchronous operation / real time data.
- Features that move or delete data should be tied to a single "primary" indexing service (failover event likely requires resetting some state).
- Substantial disagreement between multiple index services should be easy to flag on admin dashboard.