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2013-02-15-MapReduce.html
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2013-02-15-MapReduce.html
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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>2013-02-15-MapReduce</title>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8"/>
<meta name="title" content="2013-02-15-MapReduce"/>
<meta name="generator" content="Org-mode"/>
<meta name="generated" content="2013-02-15 08:29:27 PST"/>
<meta name="author" content="Jim Blomo"/>
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<link href='http://fonts.googleapis.com/css?family=Lobster+Two:700|Yanone+Kaffeesatz:700|Open+Sans' rel='stylesheet' type='text/css'>
</head>
<body>
<div id="preamble">
</div>
<div id="content">
<h1 class="title">2013-02-15-MapReduce</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#sec-1">1 MapReduce</a>
<ul>
<li><a href="#sec-1-1">1.1 Spoilers</a></li>
</ul>
</li>
<li><a href="#sec-2">2 Yelp has a problem</a>
<ul>
<li><a href="#sec-2-1">2.1 Too long</a></li>
</ul>
</li>
<li><a href="#sec-3">3 Solution?</a>
<ul>
<li><a href="#sec-3-1">3.1 New Challenges</a></li>
</ul>
</li>
<li><a href="#sec-4">4 Do It Yourself</a>
<ul>
<li><a href="#sec-4-1">4.1 Dependencies</a></li>
</ul>
</li>
<li><a href="#sec-5">5 Big Idea</a>
<ul>
<li><a href="#sec-5-1">5.1 Really Big Idea</a></li>
</ul>
</li>
<li><a href="#sec-6">6 MapReduce</a>
<ul>
<li><a href="#sec-6-1">6.1 Reading</a></li>
<li><a href="#sec-6-2">6.2 Example</a>
<ul>
<li><a href="#sec-6-2-1">6.2.1 Details</a></li>
</ul>
</li>
<li><a href="#sec-6-3">6.3 Logs</a>
<ul>
<li><a href="#sec-6-3-1">6.3.1 Logs</a></li>
</ul>
</li>
<li><a href="#sec-6-4">6.4 Map</a></li>
<li><a href="#sec-6-5">6.5 Map Example</a>
<ul>
<li><a href="#sec-6-5-1">6.5.1 Counts</a></li>
</ul>
</li>
<li><a href="#sec-6-6">6.6 Actions?</a>
<ul>
<li><a href="#sec-6-6-1">6.6.1 Middle Step</a></li>
</ul>
</li>
<li><a href="#sec-6-7">6.7 Reduce</a>
<ul>
<li><a href="#sec-6-7-1">6.7.1 Values</a></li>
</ul>
</li>
<li><a href="#sec-6-8">6.8 Reduce Example</a>
<ul>
<li><a href="#sec-6-8-1">6.8.1 Details</a></li>
</ul>
</li>
<li><a href="#sec-6-9">6.9 Example Output</a></li>
</ul>
</li>
<li><a href="#sec-7">7 Point?</a>
<ul>
<li><a href="#sec-7-1">7.1 Details</a></li>
<li><a href="#sec-7-2">7.2 Implementation</a></li>
<li><a href="#sec-7-3">7.3 Intermediate</a></li>
<li><a href="#sec-7-4">7.4 "Shuffle"</a></li>
<li><a href="#sec-7-5">7.5 Distribute</a></li>
<li><a href="#sec-7-6">7.6 Inputs</a>
<ul>
<li><a href="#sec-7-6-1">7.6.1 Splitting Files</a></li>
</ul></li>
</ul>
</li>
<li><a href="#sec-8">8 Word Count</a>
<ul>
<li><a href="#sec-8-1">8.1 Classic</a></li>
</ul>
</li>
<li><a href="#sec-9">9 Steps</a>
<ul>
<li><a href="#sec-9-1">9.1 Hints</a></li>
<li><a href="#sec-9-2">9.2 Examples</a></li>
</ul>
</li>
<li><a href="#sec-10">10 Multi-Step</a>
<ul>
<li><a href="#sec-10-1">10.1 Output as Input</a></li>
<li><a href="#sec-10-2">10.2 Examples</a></li>
<li><a href="#sec-10-3">10.3 PageRank</a></li>
</ul>
</li>
<li><a href="#sec-11">11 Unique Review</a>
<ul>
<li><a href="#sec-11-1">11.1 Questions</a></li>
<li><a href="#sec-11-2">11.2 Step 2: Count Unique Words</a></li>
<li><a href="#sec-11-3">11.3 Questions</a></li>
<li><a href="#sec-11-4">11.4 Step 3: Max</a></li>
<li><a href="#sec-11-5">11.5 Questions</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="outline-container-1" class="outline-2">
<h2 id="sec-1"><span class="section-number-2">1</span> MapReduce <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-1">
</div>
<div id="outline-container-1-1" class="outline-3">
<h3 id="sec-1-1"><span class="section-number-3">1.1</span> Spoilers <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-1-1">
<ul>
<li>Don't look ahead in the slides
</li>
<li>If you know MapReduce, try to let others answer and genuinely think about
how <b>you</b> would solve the problem.
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-2" class="outline-2">
<h2 id="sec-2"><span class="section-number-2">2</span> Yelp has a problem <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-2">
<ul>
<li>250+ GB of logs per day
</li>
<li>Each GB takes 10 minutes to process
</li>
<li>How long to handle a day's logs?
</li>
</ul>
<p><img src="img/yelp-growth.png" alt="img/yelp-growth.png" />
</p>
</div>
<div id="outline-container-2-1" class="outline-3">
<h3 id="sec-2-1"><span class="section-number-3">2.1</span> Too long <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-2-1">
<ul>
<li>On a single machine 40+ hours!
</li>
<li>If we really had only a single machine, we wouldn't be able to keep up!
</li>
<li>Mistake can't be fixed in a day (billing especially important)
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-3" class="outline-2">
<h2 id="sec-3"><span class="section-number-2">3</span> Solution? <span class="tag"><span class="slide">slide</span> <span class="animate">animate</span></span></h2>
<div class="outline-text-2" id="text-3">
<ul>
<li>Don't use one machine!
</li>
<li>What are the new challenges?
</li>
<li>Distributing data
</li>
<li>Calculating overall statistics
</li>
<li>Failures
</li>
</ul>
</div>
<div id="outline-container-3-1" class="outline-3">
<h3 id="sec-3-1"><span class="section-number-3">3.1</span> New Challenges <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-3-1">
<ul>
<li>With many machines, how to they get access to the 100 GB of logs?
</li>
<li>How do they coordinate who gets which section of logs?
</li>
<li>How do we calculate the average?
</li>
<li>What happens when one of the boxes dies?
<ul>
<li>Detecting failure (timeout waiting for data? Out of band?)
</li>
<li>Decide who takes over the data
</li>
</ul>
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-4" class="outline-2">
<h2 id="sec-4"><span class="section-number-2">4</span> Do It Yourself <span class="tag"><span class="slide">slide</span> <span class="two_col">two_col</span></span></h2>
<div class="outline-text-2" id="text-4">
<ul>
<li>There are many ways to deal with these challenges
</li>
<li>Often, people would "roll" their own solutions depending on the problem
</li>
<li>Google implemented a generic solution, shared idea
</li>
</ul>
<p><img src="img/mapreduce-paper.png" alt="img/mapreduce-paper.png" />
</p>
</div>
<div id="outline-container-4-1" class="outline-3">
<h3 id="sec-4-1"><span class="section-number-3">4.1</span> Dependencies <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-4-1">
<ul>
<li>Did you have a super-computer?
</li>
<li>What programming language were you using?
</li>
<li>Type of problem being solved (working on graphs, or web logs, …)
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-5" class="outline-2">
<h2 id="sec-5"><span class="section-number-2">5</span> Big Idea <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-5">
<ul>
<li>Simplify, limit solution expression
</li>
<li>Enable sophisticated implementation
</li>
</ul>
<ul>
<li>Interface: Map() Reduce()
</li>
<li>Implementation: Reliably run over 1000s of machines
</li>
</ul>
</div>
<div id="outline-container-5-1" class="outline-3">
<h3 id="sec-5-1"><span class="section-number-3">5.1</span> Really Big Idea <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-5-1">
<ul>
<li>Limiting yourself to what can be expressed may seem like a loss
</li>
<li>But it enables the implementation to handle the problems we talked about
</li>
<li>And then can be used as understandable building blocks
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6" class="outline-2">
<h2 id="sec-6"><span class="section-number-2">6</span> MapReduce <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-6">
<dl>
<dt>Map</dt><dd>Extract a property to summarize over
</dd>
<dt>Reduce</dt><dd>Summarize all items with a particular property
</dd>
</dl>
<ul>
<li>Simple: Each operation stateless
</li>
</ul>
</div>
<div id="outline-container-6-1" class="outline-3">
<h3 id="sec-6-1"><span class="section-number-3">6.1</span> Reading <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-6-1">
<ul>
<li>Reading this week includes a video explaining MapReduce much more generally
</li>
<li>This lecture will focus on it from a practical standpoint for homework
</li>
<li>MapReduce's main benefits are for running over many machines, fault
tolerance
</li>
<li>But we'll just practice on one machine
</li>
</ul>
</div>
</div>
<div id="outline-container-6-2" class="outline-3">
<h3 id="sec-6-2"><span class="section-number-3">6.2</span> Example <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-2">
<ul>
<li>Web application logs
</li>
<li>How many actions have we seen?
<ul>
<li>Business views
</li>
<li>User profile views
</li>
<li>Searches
</li>
</ul>
</li>
</ul>
</div>
<div id="outline-container-6-2-1" class="outline-4">
<h4 id="sec-6-2-1"><span class="section-number-4">6.2.1</span> Details <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-2-1">
<dl>
<dt>Business Views</dt><dd>Triple Rock, Bear Raman
</dd>
<dt>User profile</dt><dd>jimblomo.yelp.com
</dd>
<dt>Searches</dt><dd>query, location
</dd>
</dl>
</div>
</div>
</div>
<div id="outline-container-6-3" class="outline-3">
<h3 id="sec-6-3"><span class="section-number-3">6.3</span> Logs <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-3">
<pre class="src src-json">{'page_type': 'search',
'user': 'jim', 'query': ...}
{'page_type': 'biz_view',
'user': 'shreyas', 'biz_id': 55}
{'page_type': 'user_profile',
'user': null, 'profile_id: 123}
...
</pre>
</div>
<div id="outline-container-6-3-1" class="outline-4">
<h4 id="sec-6-3-1"><span class="section-number-4">6.3.1</span> Logs <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-3-1">
<ul>
<li>JSON logs, various types of information
</li>
<li>entire record on one line (wrapped for slides)
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6-4" class="outline-3">
<h3 id="sec-6-4"><span class="section-number-3">6.4</span> Map <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-4">
<dl>
<dt>Input</dt><dd>Key, Value
</dd>
<dt>Output</dt><dd>Keys, Values
</dd>
</dl>
</div>
</div>
<div id="outline-container-6-5" class="outline-3">
<h3 id="sec-6-5"><span class="section-number-3">6.5</span> Map Example <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-5">
<dl>
<dt>Input Key</dt><dd>Log line number
</dd>
<dt>Input Value</dt><dd>Log line text
</dd>
<dt>Output Key</dt><dd>Action
</dd>
<dt>Output Value</dt><dd>times this action has occurred <b>on this line</b>
</dd>
</dl>
</div>
<div id="outline-container-6-5-1" class="outline-4">
<h4 id="sec-6-5-1"><span class="section-number-4">6.5.1</span> Counts <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-5-1">
<ul>
<li>Log line number is not helpful in our specific case
</li>
<li>Log line text: we hope it is machine readable so we can accurately extract
the action
</li>
<li>It has datetime, cookie, action, etc.
</li>
<li>How many times has this action occurred? 1
</li>
<li>Tunnel vision: all we care about is this line
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6-6" class="outline-3">
<h3 id="sec-6-6"><span class="section-number-3">6.6</span> Actions? <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-6">
<pre class="src src-text">search 1
biz_view 1
user_profile 1
search 1
biz_view 1
search 1
biz_view 1
user_profile 1
search 1
</pre>
</div>
<div id="outline-container-6-6-1" class="outline-4">
<h4 id="sec-6-6-1"><span class="section-number-4">6.6.1</span> Middle Step <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-6-1">
<ul>
<li>From log lines, we've extracted the information out that we care about
</li>
<li>The counts and the actions
</li>
<li>Next step summarize
</li>
<li>Next step after Map?
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6-7" class="outline-3">
<h3 id="sec-6-7"><span class="section-number-3">6.7</span> Reduce <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-7">
<dl>
<dt>Input</dt><dd>Key, Values
</dd>
<dt>Output</dt><dd>Keys, Values
</dd>
</dl>
</div>
<div id="outline-container-6-7-1" class="outline-4">
<h4 id="sec-6-7-1"><span class="section-number-4">6.7.1</span> Values <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-7-1">
<ul>
<li>Note: The input is values! Plural
</li>
<li>Because we get a key and all of its associated values
</li>
<li>Remind me: what are we trying to get out of this computation?
</li>
<li>So what do you think the output keys are?
</li>
<li>Values?
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6-8" class="outline-3">
<h3 id="sec-6-8"><span class="section-number-3">6.8</span> Reduce Example <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-8">
<dl>
<dt>Input Key</dt><dd>Action
</dd>
<dt>Input Values</dt><dd>Counts: <code>[1,1,1,1]</code>
</dd>
<dt>Output Key</dt><dd>Action
</dd>
<dt>Output Value</dt><dd>Total Count
</dd>
</dl>
</div>
<div id="outline-container-6-8-1" class="outline-4">
<h4 id="sec-6-8-1"><span class="section-number-4">6.8.1</span> Details <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-6-8-1">
<ul>
<li>Action is <b>one of</b> search biz<sub>view</sub> profile<sub>view</sub>
</li>
<li>To get total count, sum all of the counts
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-6-9" class="outline-3">
<h3 id="sec-6-9"><span class="section-number-3">6.9</span> Example Output <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-6-9">
<dl>
<dt>Output Key</dt><dd>Action
</dd>
<dt>Output Value</dt><dd>Total Count
</dd>
</dl>
<pre class="src src-html">"search" 4
"user_profile" 2
"biz_view" 3
</pre>
</div>
</div>
</div>
<div id="outline-container-7" class="outline-2">
<h2 id="sec-7"><span class="section-number-2">7</span> Point? <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-7">
<ul>
<li>A lot of work for counting!
</li>
<li>More complex calculations can be done this way, eg. PageRank
</li>
<li>Stateless constraint means it can be used across thousands of computers
</li>
</ul>
</div>
<div id="outline-container-7-1" class="outline-3">
<h3 id="sec-7-1"><span class="section-number-3">7.1</span> Details <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-7-1">
<ul>
<li>By only looking at keys and values, can optimize a lot of back-end work
</li>
<li>Where to send the results?
</li>
<li>What to do when a computer fails? (Just restart failed part)
</li>
</ul>
</div>
</div>
<div id="outline-container-7-2" class="outline-3">
<h3 id="sec-7-2"><span class="section-number-3">7.2</span> Implementation <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-7-2">
<pre class="src src-text">biz_view 1
user_profile 1
search 1
search 1
biz_view 1
search 1
biz_view 1
user_profile 1
search 1
</pre>
</div>
</div>
<div id="outline-container-7-3" class="outline-3">
<h3 id="sec-7-3"><span class="section-number-3">7.3</span> Intermediate <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-7-3">
<ul>
<li>This was the situation after map
</li>
<li>Keys all jumbled
</li>
<li>What Hadoop does is sort them and distribute them to computers
</li>
</ul>
</div>
</div>
<div id="outline-container-7-4" class="outline-3">
<h3 id="sec-7-4"><span class="section-number-3">7.4</span> "Shuffle" <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-7-4">
<pre class="src src-text">biz_view 1
biz_view 1
biz_view 1
search 1
search 1
search 1
search 1
user_profile 1
user_profile 1
</pre>
</div>
</div>
<div id="outline-container-7-5" class="outline-3">
<h3 id="sec-7-5"><span class="section-number-3">7.5</span> Distribute <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-7-5">
<ul>
<li>Now it is easy to distribute, and can handle all the <code>biz_view</code> at once
</li>
</ul>
</div>
</div>
<div id="outline-container-7-6" class="outline-3">
<h3 id="sec-7-6"><span class="section-number-3">7.6</span> Inputs <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-7-6">
<ul>
<li>MapReduce distributes computing power by distributing input
</li>
<li>Input is distributed by splitting on lines (records)
</li>
<li>You cannot depend on lines being "together" in MapReduce
</li>
</ul>
</div>
<div id="outline-container-7-6-1" class="outline-4">
<h4 id="sec-7-6-1"><span class="section-number-4">7.6.1</span> Splitting Files <span class="tag"><span class="notes">notes</span></span></h4>
<div class="outline-text-4" id="text-7-6-1">
<ul>
<li>Image you have a lot of large log files, GB each
</li>
<li>You'd like to let different machines work on the same file
</li>
<li>Split file down the middle, well, at least on a newline
</li>
<li>Enable two separate machines to work on the parts
</li>
<li>You don't know what line came before this one
</li>
<li>You don't know if you will process the next line
</li>
<li>Only view is this line
</li>
<li>Real life slightly more complicated, but mostly hacks around this
</li>
</ul>
</div>
</div>
</div>
</div>
<div id="outline-container-8" class="outline-2">
<h2 id="sec-8"><span class="section-number-2">8</span> Word Count <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-8">
<pre class="src src-json">{<span style="color: #8b2252;">"text"</span>: <span style="color: #8b2252;">"Greatest pizza ever"</span>, <span style="color: #8b2252;">"stars"</span>: 2, <span style="color: #8b2252;">"user"</span>: ...}
{<span style="color: #8b2252;">"text"</span>: <span style="color: #8b2252;">"good pizza selection"</span>, <span style="color: #8b2252;">"stars"</span>: 5, <span style="color: #8b2252;">"user"</span>: ...}
</pre>
<ul>
<li>Total uses of a word in across all reviews
</li>
</ul>
</div>
<div id="outline-container-8-1" class="outline-3">
<h3 id="sec-8-1"><span class="section-number-3">8.1</span> Classic <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-8-1">
<ul>
<li>This is the traditional MapReduce example, so let's solve it
</li>
<li>No skipping ahead
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-9" class="outline-2">
<h2 id="sec-9"><span class="section-number-2">9</span> Steps <span class="tag"><span class="slide">slide</span> <span class="animate">animate</span></span></h2>
<div class="outline-text-2" id="text-9">
<ul>
<li>Map
</li>
<li>Extract <code>text</code>
</li>
<li>Count words in that review
</li>
<li>Key: word , Value: count
</li>
<li>Reduce
</li>
<li>Key: word , Values: all counts
</li>
<li>sum(values)
</li>
</ul>
</div>
<div id="outline-container-9-1" class="outline-3">
<h3 id="sec-9-1"><span class="section-number-3">9.1</span> Hints <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-9-1">
<ul>
<li>What's the first step (of MapReduce)
</li>
<li>What part of the record are we interested in?
</li>
<li>What do we want with those words?
</li>
<li>Mapper: Key Value? What are we grouping by?
</li>
<li>Next step (of MapReduce)
</li>
<li>What are the reducer inputs
</li>
<li>with all of these counts, how do we summarize
</li>
</ul>
</div>
</div>
<div id="outline-container-9-2" class="outline-3">
<h3 id="sec-9-2"><span class="section-number-3">9.2</span> Examples <span class="tag"><span class="slide">slide</span> <span class="animate">animate</span></span></h3>
<div class="outline-text-3" id="text-9-2">
<ul>
<li>"Greatest pizza ever"
</li>
<li>Counts
<ul>
<li>Greatest: 1
</li>
<li>pizza: 1
</li>
<li>ever: 1
</li>
</ul>
</li>
<li>Reducer, Key: pizza
<ul>
<li>Values: [1, 1]
</li>
<li>Output: ["pizza", 2]
</li>
</ul>
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-10" class="outline-2">
<h2 id="sec-10"><span class="section-number-2">10</span> Multi-Step <span class="tag"><span class="slide">slide</span></span></h2>
<div class="outline-text-2" id="text-10">
<ul>
<li>Not all computations can be done in a single MapReduce step
</li>
<li>Map Input: <key, value>
</li>
<li>Reducer Output: <key, value>
</li>
<li>Compose MapReduce steps!
</li>
</ul>
</div>
<div id="outline-container-10-1" class="outline-3">
<h3 id="sec-10-1"><span class="section-number-3">10.1</span> Output as Input <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-10-1">
<ul>
<li>The output of one MapReduce job can be used as the input to another
</li>
</ul>
</div>
</div>
<div id="outline-container-10-2" class="outline-3">
<h3 id="sec-10-2"><span class="section-number-3">10.2</span> Examples <span class="tag"><span class="slide">slide</span></span></h3>
<div class="outline-text-3" id="text-10-2">
<ul>
<li>PageRank: Multiple steps till solution converges
</li>
<li>Multi-level summaries
</li>
</ul>
</div>
</div>
<div id="outline-container-10-3" class="outline-3">
<h3 id="sec-10-3"><span class="section-number-3">10.3</span> PageRank <span class="tag"><span class="notes">notes</span></span></h3>
<div class="outline-text-3" id="text-10-3">
<ul>
<li>PageRank is an algorithm for calculating the important of a page
</li>
<li>But it depends on the importance of every page pointing to it!
</li>
<li>So iteratively calculate the important of all pages
</li>
<li>Find average presidential donations by candidate, then normalize averages
</li>
</ul>
</div>
</div>
</div>
<div id="outline-container-11" class="outline-2">
<h2 id="sec-11"><span class="section-number-2">11</span> Unique Review <span class="tag"><span class="slide">slide</span> <span class="animate">animate</span></span></h2>
<div class="outline-text-2" id="text-11">