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1. Incremental i.e. only the changed data is saved instead of a full copy. This allows users to save on storage costs as well as significantly speed up the backup process if only small changes have happened since the last backup.
2. Cloud friendly i.e. supports one or more blob storage systems available in major public clouds such as Amazon S3, Google Cloud Storage, Azure Blob Storage etc.
3. Safe against index corruption i.e. backups should succeed only if the backed up index is not corrupt
4. Restorable to existing collections i.e. it should be possible to restore to the source collection (or any existing collection, assuming it is compatible with the source)

Observant readers might recognize (1) and (3) from code that Cao Manh Dat proposed in SOLR-13608.  In that sense this SIP is a superset of that ticket, created to cover a broader swath of functionality and generate more discussion/review of the design.  Both Dat's ticket and this SIP are informed by code written by Dat, Shalin Mangar, and others, which is available in rough form here

Public Interfaces

The proposed changes involve changes to several different levels of public interfaces.

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