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| <h1 class="title">BP-29: Metadata API module</h1> |
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| <h3 id="motivation">Motivation</h3> |
| |
| <p>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:</p> |
| |
| <ul> |
| <li>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.</li> |
| <li>It would define a clean module dependency between bookkeeper implementation and metadata api, and how bookkeeper load a different metadata |
| implementation.</li> |
| </ul> |
| |
| <h3 id="public-interfaces">Public Interfaces</h3> |
| |
| <p>A more generic setting <code class="highlighter-rouge">metadataServiceUri</code> is introduced for replacing implementation specific settings <code class="highlighter-rouge">zkServers</code> and <code class="highlighter-rouge">zkLedgersRootPath</code>.</p> |
| |
| <p>A metadata service uri defines the location of a metadata storage. In zookeeper based implementation, the metadata service url will be |
| <code class="highlighter-rouge">zk://<zkServers>/<zkLedgersRootPath></code>.</p> |
| |
| <p>This new setting in bookie configuration will be like as below:</p> |
| |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>metadataServiceUri=zk://127.0.0.1/ledgers |
| </code></pre></div></div> |
| |
| <p>If we eventually support Etcd as one of the metadata storages. Then the setting in bookie configuration to use Etcd will be:</p> |
| |
| <div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>metadataServiceUri=etcd://<etcd_servers>/<etcd_key_prefix> |
| </code></pre></div></div> |
| |
| <h3 id="proposed-changes">Proposed Changes</h3> |
| |
| <h4 id="configuration">Configuration</h4> |
| |
| <p>This BP proposes introducing a more generic metadata setting <code class="highlighter-rouge">metadataServiceUri</code> to replace implementation specific settings |
| <code class="highlighter-rouge">zkServers</code> and <code class="highlighter-rouge">zkLedgersRootPath</code>. All implementation specific settings should be considered moving to implementation itself.</p> |
| |
| <p>The <code class="highlighter-rouge">metadataServiceUri</code> can also be used for replacing the need of configuring <code class="highlighter-rouge">ledgerManagerFactoryClass</code>, <code class="highlighter-rouge">registrationClientClass</code> and |
| <code class="highlighter-rouge">registrationManagerClass</code>. It is unnecessarily complicated to configure multiple settings to load a specific metadata implementation. |
| We can just use the <code class="highlighter-rouge">scheme</code> field in <code class="highlighter-rouge">metadataServiceUri</code> 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.</p> |
| |
| <p>So in zookeeper based metadata implementation, the metadata service uri can be:</p> |
| |
| <ul> |
| <li><code class="highlighter-rouge">zk+flat://127.0.0.1/ledgers</code>: the scheme is “zk+flat”. it means a zookeeper base metadata implementation and it uses flat ledger manager.</li> |
| <li><code class="highlighter-rouge">zk+hierarchical://127.0.0.1/ledgers</code>: the scheme is “zk+hierarchical”. it means a zookeeper base metadata implementation and it |
| uses hierarchical ledger manager.</li> |
| <li><code class="highlighter-rouge">zk+longhierarchical://127.0.0.1/ledgers</code>: the scheme is “zk+longhierarchical”. it means a zookeeper base metadata implementation and it |
| uses long hierarchical ledger manager.</li> |
| </ul> |
| |
| <h4 id="metadata-stores">Metadata Stores</h4> |
| |
| <p>Introduce a new directory called <code class="highlighter-rouge">metadata-stores</code> for storing all the metadata related modules. Under this directory, it will have following modules:</p> |
| |
| <ul> |
| <li><code class="highlighter-rouge">api</code>: it is the metadata api module <code class="highlighter-rouge">metadata-store-api</code>. It contains all the files defining the metadata interfaces.</li> |
| <li><code class="highlighter-rouge">zookeeper</code>: it is the zookeeper implementation module <code class="highlighter-rouge">metadata-store-zookeeper</code>. It contains all the files that implementing the metadata interfaces |
| using zookeeper.</li> |
| </ul> |
| |
| <p>If a new metadata implementation is added, a new directory will be created under <code class="highlighter-rouge">metadata-stores</code> to contain the implementation. For example, if we |
| are adding <code class="highlighter-rouge">Etcd</code> as the metadata store. A <code class="highlighter-rouge">Etcd</code> directory will be created under <code class="highlighter-rouge">metadata-stores/etcd</code> to store the files that implement metadata |
| interfaces using etcd client. And its module is named as <code class="highlighter-rouge">metadata-store-etcd</code>.</p> |
| |
| <p>We then change bookkeeper-server to depend on <code class="highlighter-rouge">metadata-store-api</code> only.</p> |
| |
| <p>This approach is same as other pluggable modules <code class="highlighter-rouge">stats-api</code> and <code class="highlighter-rouge">http-server</code>.</p> |
| |
| <h3 id="compatibility-deprecation-and-migration-plan">Compatibility, Deprecation, and Migration Plan</h3> |
| |
| <p>No compatibility concern at this moment. New setting is introduced and old settings will still continue to work. |
| No immediate deprecation. |
| No migration is needed.</p> |
| |
| <h3 id="test-plan">Test Plan</h3> |
| |
| <p>This proposal is mostly around refactor. So existing test cases would cover this.</p> |
| |
| <h3 id="rejected-alternatives">Rejected Alternatives</h3> |
| |
| <p>N/A</p> |
| |
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