Return-Path: Delivered-To: apmail-zookeeper-commits-archive@www.apache.org Received: (qmail 26475 invoked from network); 28 Feb 2011 23:22:25 -0000 Received: from hermes.apache.org (HELO mail.apache.org) (140.211.11.3) by minotaur.apache.org with SMTP; 28 Feb 2011 23:22:25 -0000 Received: (qmail 16431 invoked by uid 500); 28 Feb 2011 23:22:25 -0000 Delivered-To: apmail-zookeeper-commits-archive@zookeeper.apache.org Received: (qmail 16405 invoked by uid 500); 28 Feb 2011 23:22:24 -0000 Mailing-List: contact commits-help@zookeeper.apache.org; run by ezmlm Precedence: bulk List-Help: List-Unsubscribe: List-Post: List-Id: Reply-To: dev@ Delivered-To: mailing list commits@zookeeper.apache.org Received: (qmail 16397 invoked by uid 99); 28 Feb 2011 23:22:24 -0000 Received: from athena.apache.org (HELO athena.apache.org) (140.211.11.136) by apache.org (qpsmtpd/0.29) with ESMTP; Mon, 28 Feb 2011 23:22:24 +0000 X-ASF-Spam-Status: No, hits=-2000.0 required=5.0 tests=ALL_TRUSTED X-Spam-Check-By: apache.org Received: from [140.211.11.4] (HELO eris.apache.org) (140.211.11.4) by apache.org (qpsmtpd/0.29) with ESMTP; Mon, 28 Feb 2011 23:22:22 +0000 Received: by eris.apache.org (Postfix, from userid 65534) id 0DC1F2388A40; Mon, 28 Feb 2011 23:22:02 +0000 (UTC) Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Subject: svn commit: r786254 [1/16] - in /websites/staging/zookeeper/trunk/content: ./ doc/r3.3.3/ doc/r3.3.3/images/ doc/r3.3.3/skin/ doc/r3.3.3/skin/images/ Date: Mon, 28 Feb 2011 23:22:00 -0000 To: commits@zookeeper.apache.org From: buildbot@apache.org X-Mailer: svnmailer-1.0.8 Message-Id: <20110228232202.0DC1F2388A40@eris.apache.org> Author: buildbot Date: Mon Feb 28 23:21:57 2011 New Revision: 786254 Log: Staging update by buildbot Added: websites/staging/zookeeper/trunk/content/doc/r3.3.3/ websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperConfig.html 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websites/staging/zookeeper/trunk/content/lists.html websites/staging/zookeeper/trunk/content/privacy.html websites/staging/zookeeper/trunk/content/releases.html websites/staging/zookeeper/trunk/content/svn.html Modified: websites/staging/zookeeper/trunk/content/bylaws.html ============================================================================== --- websites/staging/zookeeper/trunk/content/bylaws.html (original) +++ websites/staging/zookeeper/trunk/content/bylaws.html Mon Feb 28 23:21:57 2011 @@ -296,13 +296,10 @@ actions will also be referred to the Documentation Modified: websites/staging/zookeeper/trunk/content/credits.html ============================================================================== --- websites/staging/zookeeper/trunk/content/credits.html (original) +++ websites/staging/zookeeper/trunk/content/credits.html Mon Feb 28 23:21:57 2011 @@ -101,13 +101,10 @@

Documentation

Added: websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperConfig.html ============================================================================== --- websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperConfig.html (added) +++ websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperConfig.html Mon Feb 28 23:21:57 2011 @@ -0,0 +1,372 @@ + + + + + + + +BookKeeper Administrator's Guide + + + + + + + + + +
+ +
+Apache > Hadoop > ZooKeeper +
+ +
+ + + + + + + +
+
+  + +
+
+ + + + +
+
+
+
+ +
+ + +
+ +
+ +   +
+ + + + + +
+
+PDF -icon
+ PDF +
+

BookKeeper Administrator's Guide

+

Setup Guide

+ + + + + + + + + +

Deployment

+
+

This section contains information about deploying BookKeeper and + covers these topics:

+ +

The first section tells you how many machines you need. The second explains how to bootstrap bookies + (BookKeeper storage servers). The third section explains how we use ZooKeeper and our requirements with + respect to ZooKeeper. +

+ +

System requirements

+

A typical BookKeeper installation comprises a set of bookies and a set of ZooKeeper replicas. The exact number of bookies + depends on the quorum mode, desired throughput, and number of clients using this installation simultaneously. The minimum number of + bookies is three for self-verifying (stores a message authentication code along with each entry) and four for generic (does not + store a message authentication codewith each entry), and there is no upper limit on the number of bookies. Increasing the number of + bookies, in fact, enables higher throughput. +

+

For performance, we require each server to have at least two disks. It is possible to run a bookie with a single disk, but + performance will be significantly lower in this case. Of course, it works with one disk, but performance is significantly lower. +

+

For ZooKeeper, there is no constraint with respect to the number of replicas. Having a single machine running ZooKeeper + in standalone mode is sufficient for BookKeeper. For resilience purposes, it might be a good idea to run ZooKeeper in quorum + mode with multiple servers. Please refer to the ZooKeeper documentation for detail on how to configure ZooKeeper with multiple + replicas +

+ +

Running bookies

+

+ To run a bookie, we execute the following command: +

+

+ + java -cp .:./zookeeper-<version>-bookkeeper.jar:./zookeeper-<version>.jar:../log4j/apache-log4j-1.2.15/log4j-1.2.15.jar\ + -Dlog4j.configuration=log4j.properties org.apache.bookkeeper.proto.BookieServer 3181 /path_to_log_device/\ + /path_to_ledger_device/ + +

+

+ The parameters are: +

+
    + +
  • + +

    + Port number that the bookie listens on; +

    + +
    • + + +
  • + +

    + Path for Log Device (stores bookie write-ahead log); +

    + +
    • + + +
  • + +

    + Path for Ledger Device (stores ledger entries); +

    + +
    • + +
+

+ Ideally, /path_to_log_device/ and /path_to_ledger_device/ are each + in a different device. +

+ +

ZooKeeper Metadata

+

+ For BookKeeper, we require a ZooKeeper installation to store metadata, and to pass the list + of ZooKeeper servers as parameter to the constructor of the BookKeeper class ( + org.apache.bookkeeper.client,BookKeeper). + To setup ZooKeeper, please check the + ZooKeeper documentation. +

+
+ +

+ +

+
+ +
 
+
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+0000006903 00000 n +0000007019 00000 n +0000007125 00000 n +trailer +<< +/Size 37 +/Root 2 0 R +/Info 4 0 R +>> +startxref +7842 +%%EOF Added: websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperOverview.html ============================================================================== --- websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperOverview.html (added) +++ websites/staging/zookeeper/trunk/content/doc/r3.3.3/bookkeeperOverview.html Mon Feb 28 23:21:57 2011 @@ -0,0 +1,692 @@ + + + + + + + +BookKeeper overview + + + + + + + + + +
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+

BookKeeper overview

+ + + + + + +

BookKeeper overview

+
+ +

BookKeeper introduction

+

+ 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. +

+

+ 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. +

+

+ 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: +

+
    + +
  • + +

    + 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; +

    + +
    • + + +
  • + +

    + We can have a pool of servers implementing such a log system, and shared among a number of servers; +

    + +
    • + + +
  • + +

    + 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. +

    + +
    • + +
+ +

In slightly more detail...

+

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. +

+

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. +

+

+ 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. +

+ +

Bookkeeper elements and concepts

+

+ BookKeeper uses four basic elements: +

+
    + +
  • + +

    + +Ledger: A ledger is a sequence of entries, and each entry is a sequence of bytes. Entries are + written sequentially to a ledger and at most once. Consequently, ledgers have an append-only semantics; +

    + +
    • + + +
  • + +

    + +BookKeeper client: A client runs along with a BookKeeper application, and it enables applications + to execute operations on ledgers, such as creating a ledger and writing to it; +

    + +
    • + + +
  • + +

    + +Bookie: A bookie is a BookKeeper storage server. Bookies store the content of ledgers. For any given + ledger L, we call an ensemble the group of bookies storing the content of L. For performance, we store on + each bookie of an ensemble only a fragment of a ledger. That is, we stripe when writing entries to a ledger such that + each entry is written to sub-group of bookies of the ensemble. +

    + +
    • + + +
  • + +

    + +Metadata storage service: BookKeeper requires a metadata storage service to store information related + to ledgers and available bookies. We currently use ZooKeeper for such a task. +

    + +
    • + +
+ +

Bookkeeper initial design

+

+ 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 + here): +

+
    + +
  • + +

    + Create ledger: creates a new empty ledger; +

    + +
    • + + +
  • + +

    + Open ledger: opens an existing ledger for reading; +

    + +
    • + + +
  • + +

    + Add entry: adds a record to a ledger either synchronously or asynchronously; +

    + +
    • + + +
  • + +

    + Read entries: reads a sequence of entries from a ledger either synchronously or asynchronously +

    + +
    • + +
+

+ 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. +

+ + + + + + + +
BookKeeper Overview
+ + + +
+

+ 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. +

+

+ 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. +

+

+ A client library takes care of communicating with bookies and managing entry numbers. An entry has the following fields: +

+ + +Entry fields + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
Entry fields
FieldTypeDescription
Ledger numberlongThe id of the ledger of this entry
Entry numberlongThe id of this entry
last confirmed (LC)longid of the last recorded entry
databyte[]the entry data (supplied by application)
authentication codebyte[]Message authentication code that includes all other fields of the entry
+

+ 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. +

+

+ 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 LC field contains the last entry that has been successfully recorded by BookKeeper. + If the client writes entries one at a time, LC 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: +

+
    + +
  • + +

    + the entry has been accepted by a quorum of bookies +

    + +
    • + + +
  • + +

    + all entries with a lower entry id have been accepted by a quorum of bookies +

    + +
    • + +
+

+ +LC seems mysterious right now, but it is too early to explain how we use it; just smile and move on. +

+

+ 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. +

+

+ 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. +

+ +

Bookkeeper metadata management

+

+ There are some meta data that needs to be made available to BookKeeper clients: +

+
    + +
  • + +

    + The available bookies; +

    + +
    • + + +
  • + +

    + The list of ledgers; +

    + +
    • + + +
  • + +

    + The list of bookies that have been used for a given ledger; +

    + +
    • + + +
  • + +

    + The last entry of a ledger; +

    + +
    • + +
+

+ 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. +

+ +

Closing out ledgers

+

+ The process of closing out the ledger and finding the last ledger is difficult due to the durability guarantees of BookKeeper: +

+
    + +
  • + +

    + If an entry has been successfully recorded, it must be readable. +

    + +
    • + + +
  • + +

    + If an entry is read once, it must always be available to be read. +

    + +
    • + +
+

+ 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. +

+

+ 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. +

+

+ The trick to making everything work is to have a correct idea of a last entry. We do it in roughly three steps: +

+
    + +
  1. + +

    + Find the entry with the highest last recorded entry, LC; +

    + +
    2. + + +
  2. + +

    + Find the highest consecutively recorded entry, LR; +

    + +
    3. + + +
  3. + +

    + Make sure that all entries between LC and LR are on a quorum of bookies; +

    + +
    4. + + +
+
+ +

+ +

+
+ +
 
+
+ + +