jvrao commented on a change in pull request #1117: BP-29: Metadata API module
URL: https://github.com/apache/bookkeeper/pull/1117#discussion_r165885490
 
 

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 File path: site/bps/BP-29-metadata-store-api-module.md
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+---
+title: "BP-29: Metadata API module"
+issue: https://github.com/apache/bookkeeper/<issue-number>
+state: 'Under Discussion'
+release: "x.y.z"
+---
+
+### Motivation
+
+We have already abstracted all the metadata operations into interfaces. And 
all the bookkeeper implementations only reply on metadata interfaces,
+rather than depending on zookeeper. This proposal is to organize the metadata 
interfaces and its implementations in a separate module and make
+bookkeeper implementation only depends on metadata interfaces, not depends on 
zookeeper. This would a few benefits:
+
+- It allows supporting different metadata storages, without bringing in 
dependencies of metadata store implementation directly into
+  bookkeeper-server module. The development of different metadata storage can 
be done without interleaving with each other.
+- It would define a clean module dependency between bookkeeper implementation 
and metadata api, and how bookkeeper load a different metadata
+  implementation.
+
+### Public Interfaces
+
+A more generic setting `metadataServiceUri` is introduced for replacing 
implementation specific settings `zkServers` and `zkLedgersRootPath`.
+
+A metadata service uri defines the location of a metadata storage. In 
zookeeper based implementation, the metadata service url will be
+`zk://<zkServers>/<zkLedgersRootPath>`.
+
+This new setting in bookie configuration will be like as below:
+
+```
+metadataServiceUri=zk://127.0.0.1/ledgers
+```
+
+If we eventually support Etcd as one of the metadata storages. Then the 
setting in bookie configuration to use Etcd will be:
+
+```
+metadataServiceUri=etcd://<etcd_servers>/<etcd_key_prefix>
+```
+
+### Proposed Changes
+
+#### Configuration
+
+This BP proposes introducing a more generic metadata setting 
`metadataServiceUri` to replace implementation specific settings
+`zkServers` and `zkLedgersRootPath`. All implementation specific settings 
should be considered moving to implementation itself.
+
+The `metadataServiceUri` can also be used for replacing the need of 
configuring `ledgerManagerFactoryClass`, `registrationClientClass` and
+`registrationManagerClass`. It is unnecessarily complicated to configure 
multiple settings to load a specific metadata implementation.
+We can just use the `scheme` field in `metadataServiceUri` to resolve which 
metadata implementation to use. Using uri to resolve
+different driver or implementation is commonly seen at java world, for 
example, jdbc to support different database drivers. Also, distributedlog
+uses this pattern to load different metadata driver.
+
+So in zookeeper based metadata implementation, the metadata service uri can be:
+
+- `zk+flat://127.0.0.1/ledgers`: the scheme is "zk+flat". it means a zookeeper 
base metadata implementation and it uses flat ledger manager.
+- `zk+hierarchical://127.0.0.1/ledgers`: the scheme is "zk+flat". it means a 
zookeeper base metadata implementation and it
+  uses hierarchical ledger manager.
+- `zk+longhierarchical://127.0.0.1/ledgers`: the scheme is "zk+flat". it means 
a zookeeper base metadata implementation and it
 
 Review comment:
   Nit: zk+longhierarchical not zk+flat. 

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