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commit 2741ee0c2843c1c29b1b9bd761ee167ccff9b24a
Author: jenkins <[email protected]>
AuthorDate: Mon Feb 26 08:59:04 2018 +0000
Updated site at revision c1988c1
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.../bps/BP-28-etcd-as-metadata-store/index.html | 396 +++++++++++++++++++++
content/community/bookkeeper_proposals/index.html | 6 +-
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+ <h1 class="title">BP-28: use etcd as metadata store</h1>
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+
+ <hr />
+
+ <div class="content is-medium">
+ <section class="bk-community-content">
+ <h3 id="motivation">Motivation</h3>
+
+<p>Currently bookkeeper uses zookeeper as the metadata store. However there is
a couple of issues with current approach, especially using zookeeper.</p>
+
+<p>These issues includes:</p>
+
+<ol>
+ <li>You need to allocate special nodes for zookeeper. These nodes need to be
treated specially, and have their own monitoring.
+Ops need to understand both bookies and zookeeper.</li>
+ <li>ZooKeeper is the scalability bottleneck. ZooKeeper doesn’t scale writes
as you add nodes. This means that if your bookkeeper
+cluster reaches the maximum write throughput that ZK can sustain, you’ve
reached the maximum capacity of your cluster, and there’s nothing you
+can do (except buy bigger hardware for your special nodes).</li>
+ <li>ZooKeeper enforces you into its programming model. In general, its
programming model is not too bad. However it becomes problematic when
+the scale goes up (e.g. the number of clients and watcher increase). The
issues usually comes from <em>session expires</em> and <em>watcher</em>.
+ <ul>
+ <li><em>Session Expires</em>: For simplicity, ZooKeeper ties session
state directly with connection state. So when a connection is broken, a session
is usually expired (unless it reconnects before session expires), and when a
session is expired, the underlying connection can not be used anymore, the
application has to close the connection and re-establish a new client (a new
connection). It is understandable that it makes zookeeper development easy.
However in reality, it means if [...]
+ <li><em>Watchers</em>: The zookeeper watcher is one time watcher,
applications can’t reliably use it to get updates. In order to set a watcher,
you have to read a znode or get children. Imagine such a use case, clients are
watching a list of znodes (e.g. list of bookies), when those clients expire,
they have to get the list of znodes in order to rewatch the list, even the list
is never changed.</li>
+ <li>The combination of session expires and watchers is often the root
cause of critical zookeeper outages.</li>
+ </ul>
+ </li>
+</ol>
+
+<p>This proposal is to explore other existing systems such as etcd as the
metadata store. Using Etcd doesn’t address concerns #1, however it might
potentially
+address concern #2 and #3 to some extend. And if you are running bookkeeper in
k8s, there is already an Etcd instance available. It can become easier to run
+bookkeeper on k8s if we can use Etcd as the metadata store.</p>
+
+<p>NOTE: this proposal has some overlaps on goals/vision with the distributed
k/v store work (a separate BP will be sent out soon). However they are not
conflicting with each other.
+Both proposals are exploring a better metadata storage solution for
bookkeeper.</p>
+
+<h3 id="public-interfaces">Public Interfaces</h3>
+
+<p>A new metadata store module <code
class="highlighter-rouge">metadata-store-etcd</code> will be added to
bookkeeper. This module will be implementing all the required metadata
interfaces:</p>
+
+<p>These interfaces include:</p>
+
+<ul>
+ <li>RegistrationClient</li>
+ <li>RegistrationManager</li>
+ <li>LayoutManager</li>
+ <li>LedgerIdGenerator</li>
+ <li>LedgerManager</li>
+ <li>LedgerUnderreplicatedManager</li>
+ <li>LedgerManagerFactory</li>
+</ul>
+
+<h3 id="proposed-changes">Proposed Changes</h3>
+
+<p>Since Etcd provides a key/value model rather than a tree-like structure. So
the metadata will be organized in a key/value way as below:</p>
+
+<ul>
+ <li><code class="highlighter-rouge">scope</code>: the prefix used for
prefixing all the keys used for storing the metadata for a given cluster. The
<code class="highlighter-rouge">scope</code> is effectively same as <code
class="highlighter-rouge">zkLedgerRootPath</code>.</li>
+ <li><code class="highlighter-rouge">scope</code>/LAYOUT: key for storing
layout data</li>
+ <li><code class="highlighter-rouge">scope</code>/INSTANCEID: key for storing
instance id</li>
+ <li><code class="highlighter-rouge">scope</code>/IDGEN/<code
class="highlighter-rouge"><bucket></code>: key for id generation. <code
class="highlighter-rouge"><bucket></code> is to allow concurrent id
generation.</li>
+ <li><code class="highlighter-rouge">scope</code>/cookies/<code
class="highlighter-rouge"><bookieid></code>: key for storing a bookie’s
cookie</li>
+ <li><code class="highlighter-rouge">scope</code>/available/readwrite/<code
class="highlighter-rouge"><bookieid></code>: key for registering
readwrite bookie. (lease will be applied to this key)</li>
+ <li><code class="highlighter-rouge">scope</code>/available/readonly/<code
class="highlighter-rouge"><bookieid></code>: key for registering readonly
bookie. (lease will be applied to this key)</li>
+ <li><code class="highlighter-rouge">scope</code>/ledgers/<code
class="highlighter-rouge"><ledgerid></code>: key for storing a ledger
metadata. <code class="highlighter-rouge"><ledgerid></code> is <code
class="highlighter-rouge">String.format("%19d", ledgerId)</code>, pre-padding
with 0s to make sure ledgers are stored in order.</li>
+</ul>
+
+<h4 id="registration-manager">Registration Manager</h4>
+
+<ul>
+ <li>Bookie Register: write to key “<code
class="highlighter-rouge">scope</code>/available/readwrite/<code
class="highlighter-rouge"><bookieid></code>” with a keepalive lease.</li>
+ <li>Bookie Unregister: delete key “<code
class="highlighter-rouge">scope</code>/available/readwrite/<code
class="highlighter-rouge"><bookieid></code>”.</li>
+</ul>
+
+<h4 id="registration-client">Registration Client</h4>
+
+<ul>
+ <li>Get readwrite bookies: range operation to fetch keys between “<code
class="highlighter-rouge">scope</code>/available/readwrite/” and “<code
class="highlighter-rouge">scope</code>/available/readwrite_end/”.</li>
+ <li>Get readonly bookies: range operation to fetch keys between “<code
class="highlighter-rouge">scope</code>/available/readonly/” and “<code
class="highlighter-rouge">scope</code>/available/readonly_end/”.</li>
+ <li>Watch readwrite bookies: watch operation to watch keys between “<code
class="highlighter-rouge">scope</code>/available/readwrite/” and “<code
class="highlighter-rouge">scope</code>/available/readwrite_end/”.</li>
+ <li>Watch readonly bookies: watch operation to watch keys between “<code
class="highlighter-rouge">scope</code>/available/readonly/” and “<code
class="highlighter-rouge">scope</code>/available/readonly_end/”.</li>
+</ul>
+
+<h4 id="layout-manager">Layout Manager</h4>
+
+<ul>
+ <li>Create layout: a txn operation: put layout data to key “<code
class="highlighter-rouge">scope</code>/LAYOUT” when this key doesn’t exist.</li>
+ <li>Delete layout: a delete operation on “<code
class="highlighter-rouge">scope</code>/LAYOUT”</li>
+ <li>Read layout: a get operation on “<code
class="highlighter-rouge">scope</code>/LAYOUT”</li>
+</ul>
+
+<h4 id="ledger-id-generator">Ledger Id Generator</h4>
+
+<ul>
+ <li>Id generation: get key, increment by 1 and then update with a txn
operation.</li>
+</ul>
+
+<h4 id="ledger-manager">Ledger Manager</h4>
+
+<ul>
+ <li>Create Ledger: a txn operation to put ledger metadata to key “<code
class="highlighter-rouge">scope</code>/ledgers/<code
class="highlighter-rouge"><ledgerid></code>” when this key doesn’t
exist</li>
+ <li>Write Ledger: a txn operation to put ledger metadata to key “<code
class="highlighter-rouge">scope</code>/ledgers/<code
class="highlighter-rouge"><ledgerid></code>” when key version matches</li>
+ <li>Delete Ledger: a delete operation on key “<code
class="highlighter-rouge">scope</code>/ledgers/<code
class="highlighter-rouge"><ledgerid></code>”</li>
+ <li>Read Ledger: a get operation on key “<code
class="highlighter-rouge">scope</code>/ledgers/<code
class="highlighter-rouge"><ledgerid></code>”</li>
+ <li>Iteration: a range operation fetch keys between “<code
class="highlighter-rouge">scope</code>/ledgers/” and “<code
class="highlighter-rouge">scope</code>/ledgers_end/”</li>
+</ul>
+
+<h3 id="compatibility-deprecation-and-migration-plan">Compatibility,
Deprecation, and Migration Plan</h3>
+
+<ul>
+ <li>This is a new metadata store. No impacts to existing users.</li>
+ <li>We are not planning to implement any migration tools any time soon. This
only for new users or setting up new clusters.</li>
+</ul>
+
+<h3 id="test-plan">Test Plan</h3>
+
+<ul>
+ <li>Unit tests for all the metadata manager implementation.</li>
+ <li>End-to-end integration tests</li>
+ <li>Jepsen tests</li>
+</ul>
+
+<h3 id="rejected-alternatives">Rejected Alternatives</h3>
+
+<p>N/A</p>
+
+ </section>
+
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diff --git a/content/community/bookkeeper_proposals/index.html
b/content/community/bookkeeper_proposals/index.html
index f793a7f..d727f3a 100644
--- a/content/community/bookkeeper_proposals/index.html
+++ b/content/community/bookkeeper_proposals/index.html
@@ -391,8 +391,12 @@ of the thread is of the format <code
class="highlighter-rouge">[DISCUSS] BP-<
<td style="text-align: left">Draft</td>
</tr>
<tr>
+ <td style="text-align: left"><a
href="../../bps/BP-28-etcd-as-metadata-store">BP-28: use etcd as metadata
store</a></td>
+ <td style="text-align: left">Accepted</td>
+ </tr>
+ <tr>
<td style="text-align: left"><a
href="../../bps/BP-29-metadata-store-api-module">BP-29: Metadata API
module</a></td>
- <td style="text-align: left">Draft</td>
+ <td style="text-align: left">Accepted</td>
</tr>
</tbody>
</table>
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