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| <div class="subTitle">org.apache.beam.sdk.extensions.sketching</div> |
| <h2 title="Class ApproximateDistinct" class="title">Class ApproximateDistinct</h2> |
| </div> |
| <div class="contentContainer"> |
| <ul class="inheritance"> |
| <li>java.lang.Object</li> |
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| <ul class="inheritance"> |
| <li>org.apache.beam.sdk.extensions.sketching.ApproximateDistinct</li> |
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| <pre><a href="../../../../../../org/apache/beam/sdk/annotations/Experimental.html" title="annotation in org.apache.beam.sdk.annotations">@Experimental</a> |
| public final class <span class="typeNameLabel">ApproximateDistinct</span> |
| extends java.lang.Object</pre> |
| <div class="block"><a href="../../../../../../org/apache/beam/sdk/transforms/PTransform.html" title="class in org.apache.beam.sdk.transforms"><code>PTransform</code></a>s for computing the approximate number of distinct elements in a stream. |
| |
| <p>This class relies on the HyperLogLog algorithm, and more precisely HyperLogLog+, the improved |
| version of Google. |
| |
| <h2>References</h2> |
| |
| <p>The implementation comes from <a href="https://github.com/addthis/stream-lib">Addthis' |
| Stream-lib library</a>. <br> |
| The original paper of the HyperLogLog is available <a |
| href="http://algo.inria.fr/flajolet/Publications/FlFuGaMe07.pdf">here</a>. <br> |
| A paper from the same authors to have a clearer view of the algorithm is available <a |
| href="http://cscubs.cs.uni-bonn.de/2016/proceedings/paper-03.pdf">here</a>. <br> |
| Google's HyperLogLog+ version is detailed in <a |
| href="https://research.google.com/pubs/pub40671.html">this paper</a>. |
| |
| <h2>Parameters</h2> |
| |
| <p>Two parameters can be tuned in order to control the computation's accuracy: |
| |
| <ul> |
| <li><b>Precision: <code>p</code></b> <br> |
| Controls the accuracy of the estimation. The precision value will have an impact on the |
| number of buckets used to store information about the distinct elements. <br> |
| In general one can expect a relative error of about <code>1.1 / sqrt(2^p)</code>. The value |
| should be of at least 4 to guarantee a minimal accuracy. <br> |
| By default, the precision is set to <code>12</code> for a relative error of around <code>2%</code>. |
| <li><b>Sparse Precision: <code>sp</code></b> <br> |
| Used to create a sparse representation in order to optimize memory and improve accuracy at |
| small cardinalities. <br> |
| The value of <code>sp</code> should be greater than <code>p</code>, but lower than 32. <br> |
| By default, the sparse representation is not used (<code>sp = 0</code>). One should use it if |
| the cardinality may be less than <code>12000</code>. |
| </ul> |
| |
| <h2>Examples</h2> |
| |
| <p>There are 2 ways of using this class: |
| |
| <ul> |
| <li>Use the <a href="../../../../../../org/apache/beam/sdk/transforms/PTransform.html" title="class in org.apache.beam.sdk.transforms"><code>PTransform</code></a>s that return <code>PCollection<Long></code> corresponding to the |
| estimate number of distinct elements in the input <a href="../../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a> of objects or for |
| each key in a <a href="../../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a> of <a href="../../../../../../org/apache/beam/sdk/values/KV.html" title="class in org.apache.beam.sdk.values"><code>KV</code></a>s. |
| <li>Use the <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct.ApproximateDistinctFn</code></a> <code>CombineFn</code> that is exposed in order to make |
| advanced processing involving the <code>HyperLogLogPlus</code> structure which resumes the |
| stream. |
| </ul> |
| |
| <h3>Using the Transforms</h3> |
| |
| <h4>Example 1: globally default use</h4> |
| |
| <pre><code> |
| PCollection<Integer> input = ...; |
| PCollection<Long> hllSketch = input.apply(ApproximateDistinct.<Integer>globally()); |
| </code></pre> |
| |
| <h4>Example 2: per key default use</h4> |
| |
| <pre><code> |
| PCollection<Integer, String> input = ...; |
| PCollection<Integer, Long> hllSketches = input.apply(ApproximateDistinct |
| .<Integer, String>perKey()); |
| </code></pre> |
| |
| <h4>Example 3: tune precision and use sparse representation</h4> |
| |
| <p>One can tune the precision and sparse precision parameters in order to control the accuracy |
| and the memory. The tuning works exactly the same for <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#globally--"><code>globally()</code></a> and <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#perKey--"><code>perKey()</code></a>. |
| |
| <pre><code> |
| int precision = 15; |
| int sparsePrecision = 25; |
| PCollection<Double> input = ...; |
| PCollection<Long> hllSketch = input.apply(ApproximateDistinct |
| .<Double>globally() |
| .withPrecision(precision) |
| .withSparsePrecision(sparsePrecision)); |
| </code></pre> |
| |
| <h3>Using the <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct.ApproximateDistinctFn</code></a> CombineFn</h3> |
| |
| <p>The CombineFn does the same thing as the transform but it can be used in cases where you want |
| to manipulate the <code>HyperLogLogPlus</code> sketch, for example if you want to store it in a |
| database to have a backup. It can also be used in stateful processing or in <a href="../../../../../../org/apache/beam/sdk/transforms/CombineFns.ComposedCombineFn.html" title="class in org.apache.beam.sdk.transforms"><code>CombineFns.ComposedCombineFn</code></a>. |
| |
| <h4>Example 1: basic use</h4> |
| |
| <p>This example is not really interesting but show how you can properly create an <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct.ApproximateDistinctFn</code></a>. One must always specify a coder using the <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html#create-org.apache.beam.sdk.coders.Coder-"><code>ApproximateDistinct.ApproximateDistinctFn.create(Coder)</code></a> method. |
| |
| <pre><code> |
| PCollection<Integer> input = ...; |
| PCollection<HyperLogLogPlus> output = input.apply(Combine.globally(ApproximateDistinctFn |
| .<Integer>create(BigEndianIntegerCoder.of())); |
| </code></pre> |
| |
| <h4>Example 2: use the <a href="../../../../../../org/apache/beam/sdk/transforms/Combine.CombineFn.html" title="class in org.apache.beam.sdk.transforms"><code>Combine.CombineFn</code></a> in a stateful <a href="../../../../../../org/apache/beam/sdk/transforms/ParDo.html" title="class in org.apache.beam.sdk.transforms"><code>ParDo</code></a></h4> |
| |
| <p>One may want to use the <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct.ApproximateDistinctFn</code></a> in a stateful ParDo in order to make |
| some processing depending on the current cardinality of the stream. <br> |
| For more information about stateful processing see the blog spot on this topic <a |
| href="https://beam.apache.org/blog/2017/02/13/stateful-processing.html">here</a>. |
| |
| <p>Here is an example of <a href="../../../../../../org/apache/beam/sdk/transforms/DoFn.html" title="class in org.apache.beam.sdk.transforms"><code>DoFn</code></a> using an <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct.ApproximateDistinctFn</code></a> as a <a href="../../../../../../org/apache/beam/sdk/state/CombiningState.html" title="interface in org.apache.beam.sdk.state"><code>CombiningState</code></a>: |
| |
| <pre><code> |
| class StatefulCardinality<V> extends DoFn<V, OutputT> { |
| @StateId("hyperloglog") |
| private final StateSpec<CombiningState<V, HyperLogLogPlus, HyperLogLogPlus>> |
| indexSpec; |
| |
| public StatefulCardinality(ApproximateDistinctFn<V> fn) { |
| indexSpec = StateSpecs.combining(fn); |
| } |
| |
| @ProcessElement |
| public void processElement( |
| ProcessContext context, |
| @StateId("hllSketch") |
| CombiningState<V, HyperLogLogPlus, HyperLogLogPlus> hllSketch) { |
| long current = MoreObjects.firstNonNull(hllSketch.getAccum().cardinality(), 0L); |
| hllSketch.add(context.element()); |
| context.output(...); |
| } |
| } |
| </code></pre> |
| |
| <p>Then the <a href="../../../../../../org/apache/beam/sdk/transforms/DoFn.html" title="class in org.apache.beam.sdk.transforms"><code>DoFn</code></a> can be called like this: |
| |
| <pre><code> |
| PCollection<V> input = ...; |
| ApproximateDistinctFn<V> myFn = ApproximateDistinctFn.create(input.getCoder()); |
| PCollection<V> = input.apply(ParDo.of(new StatefulCardinality<>(myFn))); |
| </code></pre> |
| |
| <h4>Example 3: use the <code>RetrieveCardinality</code> utility class</h4> |
| |
| <p>One may want to retrieve the cardinality as a long after making some advanced processing using |
| the <code>HyperLogLogPlus</code> structure. <br> |
| The <code>RetrieveCardinality</code> utility class provides an easy way to do so: |
| |
| <pre><code> |
| PCollection<MyObject> input = ...; |
| PCollection<HyperLogLogPlus> hll = input.apply(Combine.globally(ApproximateDistinctFn |
| .<MyObject>create(new MyObjectCoder()) |
| .withSparseRepresentation(20))); |
| |
| // Some advanced processing |
| PCollection<SomeObject> advancedResult = hll.apply(...); |
| |
| PCollection<Long> cardinality = hll.apply(ApproximateDistinct.RetrieveCardinality.globally()); |
| |
| </code></pre> |
| |
| <p><b>Warning: this class is experimental.</b> Its API is subject to change in future versions of |
| Beam. For example, it may be merged with the <a href="../../../../../../org/apache/beam/sdk/transforms/ApproximateUnique.html" title="class in org.apache.beam.sdk.transforms"><code>ApproximateUnique</code></a> transform.</div> |
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| <tr class="altColor"> |
| <td class="colFirst"><code>static class </code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.ApproximateDistinctFn</a><<a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.ApproximateDistinctFn.html" title="type parameter in ApproximateDistinct.ApproximateDistinctFn">InputT</a>></span></code> |
| <div class="block">Implements the <a href="../../../../../../org/apache/beam/sdk/transforms/Combine.CombineFn.html" title="class in org.apache.beam.sdk.transforms"><code>Combine.CombineFn</code></a> of <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching"><code>ApproximateDistinct</code></a> transforms.</div> |
| </td> |
| </tr> |
| <tr class="rowColor"> |
| <td class="colFirst"><code>static class </code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.GloballyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.GloballyDistinct</a><<a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.GloballyDistinct.html" title="type parameter in ApproximateDistinct.GloballyDistinct">InputT</a>></span></code> |
| <div class="block">Implementation of <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#globally--"><code>globally()</code></a>.</div> |
| </td> |
| </tr> |
| <tr class="altColor"> |
| <td class="colFirst"><code>static class </code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.HyperLogLogPlusCoder.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.HyperLogLogPlusCoder</a></span></code> |
| <div class="block">Coder for <code>HyperLogLogPlus</code> class.</div> |
| </td> |
| </tr> |
| <tr class="rowColor"> |
| <td class="colFirst"><code>static class </code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.PerKeyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.PerKeyDistinct</a><<a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.PerKeyDistinct.html" title="type parameter in ApproximateDistinct.PerKeyDistinct">K</a>,<a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.PerKeyDistinct.html" title="type parameter in ApproximateDistinct.PerKeyDistinct">V</a>></span></code> |
| <div class="block">Implementation of <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#perKey--"><code>perKey()</code></a>.</div> |
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| <td class="colOne"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#ApproximateDistinct--">ApproximateDistinct</a></span>()</code> </td> |
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| <td class="colFirst"><code>static <InputT> <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.GloballyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.GloballyDistinct</a><InputT></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#globally--">globally</a></span>()</code> |
| <div class="block">Computes the approximate number of distinct elements in the input <code>PCollection<InputT></code> |
| and returns a <code>PCollection<Long></code>.</div> |
| </td> |
| </tr> |
| <tr id="i1" class="rowColor"> |
| <td class="colFirst"><code>static <K,V> <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.PerKeyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.PerKeyDistinct</a><K,V></code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#perKey--">perKey</a></span>()</code> |
| <div class="block">Like <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#globally--"><code>globally()</code></a> but per key, i.e computes the approximate number of distinct values per |
| key in a <code>PCollection<KV<K, V>></code> and returns <code>PCollection<KV<K, Long>></code>.</div> |
| </td> |
| </tr> |
| <tr id="i2" class="altColor"> |
| <td class="colFirst"><code>static long</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#precisionForRelativeError-double-">precisionForRelativeError</a></span>(double relativeError)</code> |
| <div class="block">Computes the precision based on the desired relative error.</div> |
| </td> |
| </tr> |
| <tr id="i3" class="rowColor"> |
| <td class="colFirst"><code>static double</code></td> |
| <td class="colLast"><code><span class="memberNameLink"><a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#relativeErrorForPrecision-int-">relativeErrorForPrecision</a></span>(int p)</code> </td> |
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| <code>clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait</code></li> |
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| <h4>ApproximateDistinct</h4> |
| <pre>public ApproximateDistinct()</pre> |
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| <h4>globally</h4> |
| <pre>public static <InputT> <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.GloballyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.GloballyDistinct</a><InputT> globally()</pre> |
| <div class="block">Computes the approximate number of distinct elements in the input <code>PCollection<InputT></code> |
| and returns a <code>PCollection<Long></code>.</div> |
| <dl> |
| <dt><span class="paramLabel">Type Parameters:</span></dt> |
| <dd><code>InputT</code> - the type of the elements in the input <a href="../../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a></dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="perKey--"> |
| <!-- --> |
| </a> |
| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>perKey</h4> |
| <pre>public static <K,V> <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.PerKeyDistinct.html" title="class in org.apache.beam.sdk.extensions.sketching">ApproximateDistinct.PerKeyDistinct</a><K,V> perKey()</pre> |
| <div class="block">Like <a href="../../../../../../org/apache/beam/sdk/extensions/sketching/ApproximateDistinct.html#globally--"><code>globally()</code></a> but per key, i.e computes the approximate number of distinct values per |
| key in a <code>PCollection<KV<K, V>></code> and returns <code>PCollection<KV<K, Long>></code>.</div> |
| <dl> |
| <dt><span class="paramLabel">Type Parameters:</span></dt> |
| <dd><code>K</code> - type of the keys mapping the elements</dd> |
| <dd><code>V</code> - type of the values being combined per key</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="precisionForRelativeError-double-"> |
| <!-- --> |
| </a> |
| <ul class="blockList"> |
| <li class="blockList"> |
| <h4>precisionForRelativeError</h4> |
| <pre>public static long precisionForRelativeError(double relativeError)</pre> |
| <div class="block">Computes the precision based on the desired relative error. |
| |
| <p>According to the paper, the mean squared error is bounded by the following formula: |
| |
| <pre>b(m) / sqrt(m) |
| Where m is the number of buckets used (<code> p = log2(m)</code>) |
| and <code> b(m) < 1.106</code> for <code> m > 16 (and p > 4)</code>. |
| </pre> |
| |
| <br> |
| <b>WARNING:</b> <br> |
| This does not mean relative error in the estimation <b>can't</b> be higher. <br> |
| This only means that on average the relative error will be lower than the desired relative |
| error. <br> |
| Nevertheless, the more elements arrive in the <a href="../../../../../../org/apache/beam/sdk/values/PCollection.html" title="class in org.apache.beam.sdk.values"><code>PCollection</code></a>, the lower the variation will |
| be. <br> |
| Indeed, this is like when you throw a dice millions of time: the relative frequency of each |
| different result <code>{1,2,3,4,5,6}</code> will get closer to <code>1/6</code>.</div> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>relativeError</code> - the mean squared error should be in the interval ]0,1]</dd> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd>the minimum precision p in order to have the desired relative error on average.</dd> |
| </dl> |
| </li> |
| </ul> |
| <a name="relativeErrorForPrecision-int-"> |
| <!-- --> |
| </a> |
| <ul class="blockListLast"> |
| <li class="blockList"> |
| <h4>relativeErrorForPrecision</h4> |
| <pre>public static double relativeErrorForPrecision(int p)</pre> |
| <dl> |
| <dt><span class="paramLabel">Parameters:</span></dt> |
| <dd><code>p</code> - the precision i.e. the number of bits used for indexing the buckets</dd> |
| <dt><span class="returnLabel">Returns:</span></dt> |
| <dd>the Mean squared error of the Estimation of cardinality to expect for the given value |
| of p.</dd> |
| </dl> |
| </li> |
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