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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 [3/3] - in /hadoop/zookeeper/trunk: ./ docs/
docs/images/ src/docs/src/documentation/content/xdocs/
src/docs/src/documentation/resources/images/
Modified: hadoop/zookeeper/trunk/src/docs/src/documentation/content/xdocs/bookkeeperOverview.xml
URL: http://svn.apache.org/viewvc/hadoop/zookeeper/trunk/src/docs/src/documentation/content/xdocs/bookkeeperOverview.xml?rev=911716&r1=911715&r2=911716&view=diff
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--- hadoop/zookeeper/trunk/src/docs/src/documentation/content/xdocs/bookkeeperOverview.xml (original)
+++ hadoop/zookeeper/trunk/src/docs/src/documentation/content/xdocs/bookkeeperOverview.xml Fri Feb 19 07:02:06 2010
@@ -42,14 +42,82 @@
</abstract>
</articleinfo>
<section id="bk_Overview">
- <title>BookKeeper overview</title>
+ <title>BookKeeper overview</title>
+
+ <section id="bk_Intro">
+ <title>BookKeeper introduction</title>
+ <para>
+ 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.
+ </para>
- <para>This document explains basic concepts of BookKeeper. We start by discussing
- the basic elements of BookKeeper, and next we discuss how they work together.
+ <para>
+ 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.
</para>
+
+ <para>
+ 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:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ 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;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ We can have a pool of servers implementing such a log system, and shared among a number of servers;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ 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.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ </section>
+
+ <section id="bk_moreDetail">
+ <title>In slightly more detail...</title>
+ <para> 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.
+ </para>
+
+ <para> 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.
+ </para>
+
+ <para>
+ 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.
+ </para>
+ </section>
+
<section id="bk_basicComponents">
- <title>Basic elements</title>
+ <title>Bookkeeper elements and concepts</title>
<para>
BookKeeper uses four basic elements:
</para>
@@ -87,42 +155,265 @@
</itemizedlist>
</section>
- <section id="bk_moreDetail">
- <title>In slightly more detail...</title>
+ <section id="bk_initialDesign">
+ <title>Bookkeeper initial design</title>
+ <para>
+ 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 <ulink url="bookkeeperProgrammer.html">
+ here</ulink>):
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Create ledger: creates a new empty ledger;
+ </para>
+ </listitem>
- <para> 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.
- </para>
+ <listitem>
+ <para>
+ Open ledger: opens an existing ledger for reading;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Add entry: adds a record to a ledger either synchronously or asynchronously;
+ </para>
+ </listitem>
- <para> 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.
+ <listitem>
+ <para>
+ Read entries: reads a sequence of entries from a ledger either synchronously or asynchronously
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ 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.
</para>
+
+ <figure>
+ <title>BookKeeper Overview</title>
- <para> 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.
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="images/bk-overview.jpg" width="3in" depth="3in" contentwidth="3in" contentdepth="3in" scalefit="0"/>
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>
+ 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.
+ </para>
+
+ <para>
+ 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.
</para>
<para>
- 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.
+ A client library takes care of communicating with bookies and managing entry numbers. An entry has the following fields:
</para>
+
+ <table frame='all'><title>Entry fields</title>
+ <tgroup cols='3' align='left' colsep='1' rowsep='1'>
+ <colspec colname='Field'/>
+ <colspec colname='Type'/>
+ <colspec colname='Description'/>
+ <colspec colnum='5' colname='c5'/>
+ <thead>
+ <row>
+ <entry>Field</entry>
+ <entry>Type</entry>
+ <entry>Description</entry>
+ </row>
+ </thead>
+ <tfoot>
+ <row>
+ <entry>Ledger number</entry>
+ <entry>long</entry>
+ <entry>The id of the ledger of this entry</entry>
+ </row>
+ <row>
+ <entry>Entry number</entry>
+ <entry>long</entry>
+ <entry>The id of this entry</entry>
+ </row>
+ </tfoot>
+ <tbody>
+ <row>
+ <entry>last confirmed (<emphasis>LC</emphasis>)</entry>
+ <entry>long</entry>
+ <entry>id of the last recorded entry</entry>
+ </row>
+ <row>
+ <entry>data</entry>
+ <entry>byte[]</entry>
+ <entry>the entry data (supplied by application)</entry>
+ </row>
+ <row>
+ <entry>authentication code</entry>
+ <entry>byte[]</entry>
+ <entry>Message authentication code that includes all other fields of the entry</entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
<para>
- 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.
- </para>
- </section>
+ 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.
+ </para>
+
+ <para>
+ 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 <emphasis>LC</emphasis> field contains the last entry that has been successfully recorded by BookKeeper.
+ If the client writes entries one at a time, <emphasis>LC</emphasis> 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:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ the entry has been accepted by a quorum of bookies
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ all entries with a lower entry id have been accepted by a quorum of bookies
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ <emphasis>LC</emphasis> seems mysterious right now, but it is too early to explain how we use it; just smile and move on.
+ </para>
+
+ <para>
+ 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.
+ </para>
+
+ <para>
+ 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.
+ </para>
+
+ </section>
+
+ <section id="bk_metadata">
+ <title>Bookkeeper metadata management</title>
+
+ <para>
+ There are some meta data that needs to be made available to BookKeeper clients:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ The available bookies;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The list of ledgers;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The list of bookies that have been used for a given ledger;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ The last entry of a ledger;
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ 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.
+ </para>
+
+ </section>
+
+ <section id="bk_closingOut">
+ <title>Closing out ledgers</title>
+
+ <para>
+ The process of closing out the ledger and finding the last ledger is difficult due to the durability guarantees of BookKeeper:
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ If an entry has been successfully recorded, it must be readable.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ If an entry is read once, it must always be available to be read.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ <para>
+ 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.
+ </para>
+
+ <para>
+ 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.
+ </para>
+
+ <para>
+ The trick to making everything work is to have a correct idea of a last entry. We do it in roughly three steps:
+ </para>
+ <orderedlist>
+ <listitem>
+ <para>
+ Find the entry with the highest last recorded entry, <emphasis>LC</emphasis>;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Find the highest consecutively recorded entry, <emphasis>LR</emphasis>;
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Make sure that all entries between <emphasis>LC</emphasis> and <emphasis>LR</emphasis> are on a quorum of bookies;
+ </para>
+ </listitem>
+
+ </orderedlist>
+ </section>
</section>
</article>
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