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Posted to commits@zookeeper.apache.org by ma...@apache.org on 2010/02/19 08:02:07 UTC
svn commit: r911716 [1/3] - in /hadoop/zookeeper/trunk: ./ docs/
docs/images/ src/docs/src/documentation/content/xdocs/
src/docs/src/documentation/resources/images/
Author: mahadev
Date: Fri Feb 19 07:02:06 2010
New Revision: 911716
URL: http://svn.apache.org/viewvc?rev=911716&view=rev
Log:
ZOOKEEPER-607. improve bookkeeper overview (flavio via mahadev)
Added:
hadoop/zookeeper/trunk/docs/images/bk-overview.jpg (with props)
hadoop/zookeeper/trunk/src/docs/src/documentation/resources/images/bk-overview.jpg (with props)
Modified:
hadoop/zookeeper/trunk/CHANGES.txt
hadoop/zookeeper/trunk/docs/bookkeeperOverview.html
hadoop/zookeeper/trunk/docs/bookkeeperOverview.pdf
hadoop/zookeeper/trunk/src/docs/src/documentation/content/xdocs/bookkeeperOverview.xml
Modified: hadoop/zookeeper/trunk/CHANGES.txt
URL: http://svn.apache.org/viewvc/hadoop/zookeeper/trunk/CHANGES.txt?rev=911716&r1=911715&r2=911716&view=diff
==============================================================================
--- hadoop/zookeeper/trunk/CHANGES.txt (original)
+++ hadoop/zookeeper/trunk/CHANGES.txt Fri Feb 19 07:02:06 2010
@@ -283,6 +283,8 @@
ZOOKEEPER-664. BookKeeper API documentation (flavio via mahadev)
+ ZOOKEEPER-607. improve bookkeeper overview (flavio via mahadev)
+
NEW FEATURES:
ZOOKEEPER-539. generate eclipse project via ant target. (phunt via mahadev)
Modified: hadoop/zookeeper/trunk/docs/bookkeeperOverview.html
URL: http://svn.apache.org/viewvc/hadoop/zookeeper/trunk/docs/bookkeeperOverview.html?rev=911716&r1=911715&r2=911716&view=diff
==============================================================================
--- hadoop/zookeeper/trunk/docs/bookkeeperOverview.html (original)
+++ hadoop/zookeeper/trunk/docs/bookkeeperOverview.html Fri Feb 19 07:02:06 2010
@@ -209,11 +209,23 @@
<a href="#bk_Overview">BookKeeper overview</a>
<ul class="minitoc">
<li>
-<a href="#bk_basicComponents">Basic elements</a>
+<a href="#bk_Intro">BookKeeper introduction</a>
</li>
<li>
<a href="#bk_moreDetail">In slightly more detail...</a>
</li>
+<li>
+<a href="#bk_basicComponents">Bookkeeper elements and concepts</a>
+</li>
+<li>
+<a href="#bk_initialDesign">Bookkeeper initial design</a>
+</li>
+<li>
+<a href="#bk_metadata">Bookkeeper metadata management</a>
+</li>
+<li>
+<a href="#bk_closingOut">Closing out ledgers</a>
+</li>
</ul>
</li>
</ul>
@@ -225,11 +237,79 @@
<a name="N10009"></a><a name="bk_Overview"></a>
<h2 class="h3">BookKeeper overview</h2>
<div class="section">
-<p>This document explains basic concepts of BookKeeper. We start by discussing
- the basic elements of BookKeeper, and next we discuss how they work together.
+<a name="N1000F"></a><a name="bk_Intro"></a>
+<h3 class="h4">BookKeeper introduction</h3>
+<p>
+ BookKeeper is a replicated service to reliably log streams of records. In BookKeeper,
+ servers are "bookies", log streams are "ledgers", and each unit of a log (aka record) is a
+ "ledger entry". BookKeeper is designed to be reliable; bookies, the servers that store
+ ledgers, can crash, corrupt data, discard data, but as long as there are enough bookies
+ behaving correctly the service as a whole behaves correctly.
+ </p>
+<p>
+ The initial motivation for BookKeeper comes from the namenode of HDFS. Namenodes have to
+ log operations in a reliable fashion so that recovery is possible in the case of crashes.
+ We have found the applications for BookKeeper extend far beyond HDFS, however. Essentially,
+ any application that requires an append storage can replace their implementations with
+ BookKeeper. BookKeeper has the advantage of scaling throughput with the number of servers.
+ </p>
+<p>
+ At a high level, a bookkeeper client receives entries from a client application and stores it to
+ sets of bookies, and there are a few advantages in having such a service:
+ </p>
+<ul>
+
+<li>
+
+<p>
+ We can use hardware that is optimized for such a service. We currently believe that such a
+ system has to be optimized only for disk I/O;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ We can have a pool of servers implementing such a log system, and shared among a number of servers;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ We can have a higher degree of replication with such a pool, which makes sense if the hardware necessary for it is cheaper compared to the one the application uses.
+ </p>
+
+</li>
+
+</ul>
+<a name="N10034"></a><a name="bk_moreDetail"></a>
+<h3 class="h4">In slightly more detail...</h3>
+<p> BookKeeper implements highly available logs, and it has been designed with write-ahead logging in mind. Besides high availability
+ due to the replicated nature of the service, it provides high throughput due to striping. As we write entries in a subset of bookies of an
+ ensemble and rotate writes across available quorums, we are able to increase throughput with the number of servers for both reads and writes.
+ Scalability is a property that is possible to achieve in this case due to the use of quorums. Other replication techniques, such as
+ state-machine replication, do not enable such a property.
</p>
-<a name="N10012"></a><a name="bk_basicComponents"></a>
-<h3 class="h4">Basic elements</h3>
+<p> An application first creates a ledger before writing to bookies through a local BookKeeper client instance.
+ Upon creating a ledger, a BookKeeper client writes metadata about the ledger to ZooKeeper. Each ledger currently
+ has a single writer. This writer has to execute a close ledger operation before any other client can read from it.
+ If the writer of a ledger does not close a ledger properly because, for example, it has crashed before having the
+ opportunity of closing the ledger, then the next client that tries to open a ledger executes a procedure to recover
+ it. As closing a ledger consists essentially of writing the last entry written to a ledger to ZooKeeper, the recovery
+ procedure simply finds the last entry written correctly and writes it to ZooKeeper.
+ </p>
+<p>
+ Note that currently this recovery procedure is executed automatically upon trying to open a ledger and no explicit action is necessary.
+ Although two clients may try to recover a ledger concurrently, only one will succeed, the first one that is able to create the close znode
+ for the ledger.
+ </p>
+<a name="N10044"></a><a name="bk_basicComponents"></a>
+<h3 class="h4">Bookkeeper elements and concepts</h3>
<p>
BookKeeper uses four basic elements:
</p>
@@ -281,37 +361,305 @@
</li>
</ul>
-<a name="N10046"></a><a name="bk_moreDetail"></a>
-<h3 class="h4">In slightly more detail...</h3>
-<p> BookKeeper implements highly available logs, and it has been designed with write-ahead logging in mind. Besides high availability
- due to the replicated nature of the service, it provides high throughput due to striping. As we write entries in a subset of bookies of an
- ensemble and rotate writes across available quorums, we are able to increase throughput with the number of servers for both reads and writes.
- Scalability is a property that is possible to achieve in this case due to the use of quorums. Other replication techniques, such as
- state-machine replication, do not enable such a property.
+<a name="N10078"></a><a name="bk_initialDesign"></a>
+<h3 class="h4">Bookkeeper initial design</h3>
+<p>
+ A set of bookies implements BookKeeper, and we use a quorum-based protocol to replicate data across the bookies.
+ There are basically two operations to an existing ledger: read and append. Here is the complete API list
+ (mode detail <a href="bookkeeperProgrammer.html">
+ here</a>):
+ </p>
+<ul>
+
+<li>
+
+<p>
+ Create ledger: creates a new empty ledger;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ Open ledger: opens an existing ledger for reading;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ Add entry: adds a record to a ledger either synchronously or asynchronously;
</p>
-<p> An application first creates a ledger before writing to bookies through a local BookKeeper client instance. To
- create a ledger, an application has to specify which kind of ledger it wants to use: self-verifying or generic. Self-verifying
- includes a digest on every entry, which enables a reduction on the degree of replication. Generic ledgers do not store a digest
- along with entries at the cost of using more bookies.
- </p>
-<p> Upon creating a ledger, a BookKeeper clients writes metadata about the ledger to ZooKeeper. A given client first creates
- a znode named "L" as a child of "/ledger" with the SEQUENCE flag. ZooKeeper consequently assigns a unique sequence number to the
- node, naming the node "/Lx", where x is the sequence number assigned. We use this sequence number as the identifier of the ledger.
- This identifier is necessary when opening a ledger. We also store the ensemble composition so that readers know which set of bookies
- of access for a given ledger.
- </p>
-<p>
- Each ledger currently has a single writer. This writer has to execute a close ledger operation before any other client can read
- from it. If the writer of a ledger does not close a ledger properly because, for example, it has crashed before having the
- opportunity of closing the ledger, then the next client that tries to open a ledger executes an procedure to recover it. As closing a ledger
- consists essentially of writing the last entry written to a ledger to ZooKeeper, the recovery procedure simply finds the last entry
- written correctly and writes it to ZooKeeper in the form of a close znode as a child of "/Lx", where x is the identifier of the ledger.
+
+</li>
+
+
+<li>
+
+<p>
+ Read entries: reads a sequence of entries from a ledger either synchronously or asynchronously
</p>
+
+</li>
+
+</ul>
<p>
- Note that currently this recovery procedure is executed automatically upon trying to open a ledger and no explicit action is necessary.
- Although two clients may try to recover a ledger concurrently, only one will succeed, the first one that is able to create the close znode
- for the ledger.
+ There is only a single client that can write to a ledger. Once that ledger is closed or the client fails,
+ no more entries can be added. (We take advantage of this behavior to provide our strong guarantees.)
+ There will not be gaps in the ledger. Fingers get broken, people get roughed up or end up in prison when
+ books are manipulated, so there is no deleting or changing of entries.
+ </p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<tr>
+<td>BookKeeper Overview</td>
+</tr>
+<tr>
+<td>
+
+ <img alt="" src="images/bk-overview.jpg">
+
+ </td>
+</tr>
+</table>
+<p>
+ A simple use of BooKeeper is to implement a write-ahead transaction log. A server maintains an in-memory data structure
+ (with periodic snapshots for example) and logs changes to that structure before it applies the change. The application
+ server creates a ledger at startup and store the ledger id and password in a well known place (ZooKeeper maybe). When
+ it needs to make a change, the server adds an entry with the change information to a ledger and apply the change when
+ BookKeeper adds the entry successfully. The server can even use asyncAddEntry to queue up many changes for high change
+ throughput. BooKeeper meticulously logs the changes in order and call the completion functions in order.
</p>
+<p>
+ When the application server dies, a backup server will come online, get the last snapshot and then it will open the
+ ledger of the old server and read all the entries from the time the snapshot was taken. (Since it doesn't know the
+ last entry number it will use MAX_INTEGER). Once all the entries have been processed, it will close the ledger and
+ start a new one for its use.
+ </p>
+<p>
+ A client library takes care of communicating with bookies and managing entry numbers. An entry has the following fields:
+ </p>
+<table class="ForrestTable" cellspacing="1" cellpadding="4">
+<caption>Entry fields</caption>
+<title>Entry fields</title>
+
+
+<tr>
+
+<th>Field</th>
+ <th>Type</th>
+ <th>Description</th>
+
+</tr>
+
+
+<tr>
+
+<td>Ledger number</td>
+ <td>long</td>
+ <td>The id of the ledger of this entry</td>
+
+</tr>
+
+<tr>
+
+<td>Entry number</td>
+ <td>long</td>
+ <td>The id of this entry</td>
+
+</tr>
+
+
+<tr>
+
+<td>last confirmed (<em>LC</em>)</td>
+ <td>long</td>
+ <td>id of the last recorded entry</td>
+
+</tr>
+
+<tr>
+
+<td>data</td>
+ <td>byte[]</td>
+ <td>the entry data (supplied by application)</td>
+
+</tr>
+
+<tr>
+
+<td>authentication code</td>
+ <td>byte[]</td>
+ <td>Message authentication code that includes all other fields of the entry</td>
+
+</tr>
+
+
+
+</table>
+<p>
+ The client library generates a ledger entry. None of the fields are modified by the bookies and only the first three
+ fields are interpreted by the bookies.
+ </p>
+<p>
+ To add to a ledger, the client generates the entry above using the ledger number. The entry number will be one more
+ than the last entry generated. The <em>LC</em> field contains the last entry that has been successfully recorded by BookKeeper.
+ If the client writes entries one at a time, <em>LC</em> is the last entry id. But, if the client is using asyncAddEntry, there
+ may be many entries in flight. An entry is considered recorded when both of the following conditions are met:
+ </p>
+<ul>
+
+<li>
+
+<p>
+ the entry has been accepted by a quorum of bookies
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ all entries with a lower entry id have been accepted by a quorum of bookies
+ </p>
+
+</li>
+
+</ul>
+<p>
+
+<em>LC</em> seems mysterious right now, but it is too early to explain how we use it; just smile and move on.
+ </p>
+<p>
+ Once all the other fields have been field in, the client generates an authentication code with all of the previous fields.
+ The entry is then sent to a quorum of bookies to be recorded. Any failures will result in the entry being sent to a new
+ quorum of bookies.
+ </p>
+<p>
+ To read, the client library initially contacts a bookie and starts requesting entries. If an entry is missing or
+ invalid (a bad MAC for example), the client will make a request to a different bookie. By using quorum writes,
+ as long as enough bookies are up we are guaranteed to eventually be able to read an entry.
+ </p>
+<a name="N10132"></a><a name="bk_metadata"></a>
+<h3 class="h4">Bookkeeper metadata management</h3>
+<p>
+ There are some meta data that needs to be made available to BookKeeper clients:
+ </p>
+<ul>
+
+<li>
+
+<p>
+ The available bookies;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ The list of ledgers;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ The list of bookies that have been used for a given ledger;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ The last entry of a ledger;
+ </p>
+
+</li>
+
+</ul>
+<p>
+ We maintain this information in ZooKeeper. Bookies use ephemeral nodes to indicate their availability. Clients
+ use znodes to track ledger creation and deletion and also to know the end of the ledger and the bookies that
+ were used to store the ledger. Bookies also watch the ledger list so that they can cleanup ledgers that get deleted.
+ </p>
+<a name="N1015A"></a><a name="bk_closingOut"></a>
+<h3 class="h4">Closing out ledgers</h3>
+<p>
+ The process of closing out the ledger and finding the last ledger is difficult due to the durability guarantees of BookKeeper:
+ </p>
+<ul>
+
+<li>
+
+<p>
+ If an entry has been successfully recorded, it must be readable.
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ If an entry is read once, it must always be available to be read.
+ </p>
+
+</li>
+
+</ul>
+<p>
+ If the ledger was closed gracefully, ZooKeeper will have the last entry and everything will work well. But, if the
+ BookKeeper client that was writing the ledger dies, there is some recovery that needs to take place.
+ </p>
+<p>
+ The problematic entries are the ones at the end of the ledger. There can be entries in flight when a BookKeeper client
+ dies. If the entry only gets to one bookie, the entry should not be readable since the entry will disappear if that bookie
+ fails. If the entry is only on one bookie, that doesn't mean that the entry has not been recorded successfully; the other
+ bookies that recorded the entry might have failed.
+ </p>
+<p>
+ The trick to making everything work is to have a correct idea of a last entry. We do it in roughly three steps:
+ </p>
+<ol>
+
+<li>
+
+<p>
+ Find the entry with the highest last recorded entry, <em>LC</em>;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ Find the highest consecutively recorded entry, <em>LR</em>;
+ </p>
+
+</li>
+
+
+<li>
+
+<p>
+ Make sure that all entries between <em>LC</em> and <em>LR</em> are on a quorum of bookies;
+ </p>
+
+</li>
+
+
+</ol>
</div>
<p align="right">