[jira] [Created] (BOOKKEEPER-590) Another Scan-And-Compare GC Implementation

["Jiannan Wang (JIRA)" <ji...@apache.org>]

Jiannan Wang created BOOKKEEPER-590:
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             Summary: Another Scan-And-Compare GC Implementation
                 Key: BOOKKEEPER-590
                 URL: https://issues.apache.org/jira/browse/BOOKKEEPER-590
             Project: Bookkeeper
          Issue Type: Improvement
          Components: bookkeeper-server
            Reporter: Jiannan Wang
            Assignee: Jiannan Wang


The idea of Scan-And-Compare GC is as below:
   * Assume the ledger id list in local bookie server is *LocalLedgers*
   * At the same time, the ledger id list at metadata storage is *LiveLedgers*
   * Then the ledgers require garbage collection are *LocalLedgers - LiveLedgers*

Under current implementation, an ledger id order guarantee is required when obtain *LiveLedgers* from metadata storage. However, this is unnecessary: we get *LocalLedgers* and we can just remove elements that in *LiveLedgers* one by one in any order.

What's more, without the order requirement when scan all ledger ids, some things become simple:
   * We even don't need radix tree to maintain 64-bits ledger metadata, a hierarchical hash tree is enough (just as what topic metadata management does).
   * Easy to handle 64-bit ledger id backward compatibility for MSLedgerManager:
      * Currently, for MSLedgerManager, we format ledger id to a fixed length (it's 10 now) digit string to make order scan
      * When a 64-bit ledger id is used we need to enlarge the fixed length, then old ledger id backward compatibility turns to be a trouble if we require this order guarantee.

As above reasons, it would better to remove specific order requirement from current Scan-And-Compare GC implementation.

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