jiangphcn opened a new pull request #2707: Set DbPrefix with value allocated by erlfdb_hca URL: https://github.com/apache/couchdb/pull/2707 <!-- Thank you for your contribution! Please file this form by replacing the Markdown comments with your text. If a section needs no action - remove it. Also remember, that CouchDB uses the Review-Then-Commit (RTC) model of code collaboration. Positive feedback is represented +1 from committers and negative is a -1. The -1 also means veto, and needs to be addressed to proceed. Once there are no objections, the PR can be merged by a CouchDB committer. See: http://couchdb.apache.org/bylaws.html#decisions for more info. --> ## Overview <!-- Please give a short brief for the pull request, what problem it solves or how it makes things better. --> In CouchDB 4.0, directories and indirection access in FoundationDB are already used to better build data model. One key/value pair is used to build reference from Dbkey to DbPrefix. All other key/value pairs are based on DbPrefix instead of DbKey. This decouples the direct relationship between DBName and data in this database. The current implementation for `DBKey -> DBPrefix` is `{?ALL_DBS, DbName} -> {?DBS, DbName}`. So you can see below information in FoundationDB using fdbcli, etc. ``` {?ALL_DBS, DbName} -> {?DBS, DbName} {?DBS, DbName, other part of key} -> <value> ``` To support soft-deletion, especially allowing one database to be deleted/re-created multiple time, we need to use different DbPrefix for the same DbKey/DBName. The proposed change is to use a unique value allocated via High Contention Allocator(HCA) algorithm. ``` DbPrefixAllocator = erlfdb_hca:create(?ERLFDB_EXTEND(DbId, <<"hca">>)),, DbPrefix = erlfdb_hca:allocate(DBPrefixAllocator, Tx), erlfdb:set(Tx, DbKey, DbPrefix), ``` The data in FoundationDB looks like: ``` {?ALL_DBS, DbName} -> <unique key allocated by hca> {<unique key allocated by hca>, other part of key} -> <value> ``` Using HCA algorithm, it can acquire one unique key quickly while avoiding conflicting. The more important, it is shorter enough to save space because `DBPrefix` exists in almost every key/value pair for database. ## Testing recommendations <!-- Describe how we can test your changes. Does it provides any behaviour that the end users could notice? --> All existing CURD test case for database should pass. ## Related Issues or Pull Requests <!-- If your changes affects multiple components in different repositories please put links to those issues or pull requests here. --> https://github.com/apache/couchdb/pull/2666 ## Checklist - [X] Code is written and works correctly - [X] Changes are covered by tests - [ ] Any new configurable parameters are documented in `rel/overlay/etc/default.ini` - [ ] A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation
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