eolivelli commented on a change in pull request #1138: BP-31 BookKeeper 
Durability Anchor
URL: https://github.com/apache/bookkeeper/pull/1138#discussion_r168313262

 File path: site/bps/BP-31-durability
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+title: "BP-31: BookKeeper Durability(Anchor)"
+issue: https://github.com/apache/bookkeeper/BP-31-durability
+state: 'Anchor BP'
+release: "x.y.z"
+## Motivation
+Apache BookKeeper is transitioning into a full fledged distributed storage 
that can keep the data for long term. Durability is paramount to achieve the 
status of trusted store. This Anchor BP discusses many gaps and areas of 
improvement.  Each issue listed here will have another issue and this BP is 
expected to be updated when that issue is created.
+## Durability Contract
+1. **Maintain WQ copies all the time**. If any ledger falls into under 
replicated state, there needs to be an SLA on how quickly the replication 
levels can be brought back to normal levels.
+2. **Enforce Placement Policy** strictly during write and replication.
+3. **Protect the data** against corruption on the wire or at rest.
+## Work Grouping (In the order of priority)
+### Detect Durability Validation
+First step is to understand the areas of durability breaches. Design metrics 
that record durability contract violations. 
+* At the Creation: Validate durability contract the extent is being created
+* At the Deletion: Validate durability contract when extent is deleted
+* During lifetime: Validate durability contract during the lifetime of the 
extent.(periodic validator)
+* During Read: IO or Checksum errors in the read path
+### Delete Discipline
+* Build a single delete choke point with stringent validations
+* Archival bit in the metadata to assist Two phase Deletes
+* Stateful/Explicit Deletes
+### Metadata Recovery
+* Metadata recovery tool to reconstruct the metadata if the metadata server 
gets wiped out. This tool need to make sure that the data is readable even if 
we can't get all the metadata (ex: ctime) back.
 Review comment:
   I am citing custom metadata because they cannot be recreated from data on 
bookies actually and they are very useful at application layer (can be used to 
tell about the tenant which owns data for instance).
   Okay for leaving the phrase as it is now actually

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