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From maha...@apache.org
Subject svn commit: r911716 [1/3] - in /hadoop/zookeeper/trunk: ./ docs/ docs/images/ src/docs/src/documentation/content/xdocs/ src/docs/src/documentation/resources/images/
Date Fri, 19 Feb 2010 07:02:07 GMT
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">



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