[LUCENE-2592] Add static readSnapshotsInfo to PersistentSnapshotDeletionPolicy

• Type: Improvement
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 3.1, 4.0-ALPHA
• Component/s: core/index
• Lucene Fields: New, Patch Available
• Assignee: Shai Erera
• Reporter: Shai Erera
• Created: 10/Aug/10 10:32
• Updated: 28/Aug/22 12:30
• Resolved: 21/Aug/10 19:19

Description

PSDP persists the snapshots information in a Directory. When you open PSDP, it obtains a write lock on the snapshots dir (by keeping an open IndexWriter), and updates the directory when snapshots are created/released.

This causes problem in the following scenario – you have two processes, one updates the 'content' index and keeps PSDP open (because it also takes snapshots). Another process wants to read the existing snapshots information and open a read-only IndexReader on the 'content' index. The other process cannot read the existing snapshots information, because p1 keeps a write lock on the snapshots directory.

There are two possible solutions:

1. Have PSDP open the IndexWriter over the directory for each snapshot/release. A bit expensive, and unnecessary.
2. Introduce a static readSnapshotsInfo on PSDP which accepts a Directory and returns the snapshots information. IMO it's cleaner, and won't have the performance overhead of opening/closing the IW as before.

I'll post a patch (implementing the 2nd approach) shortly. I'd appreciate any comments.