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<meta name="description" content="I spent a couple hours yesterday debugging what I thought was a Protobuf serialization issue, which turns out to be an unlawful Monoid-like use of aggregateByKey in Scio. The Problem Both Scio and Spark have aggregate and aggregateByKey transformations that look like this: // on SCollection[V] def aggregate[U](zeroValue: U)(seqOp: (U, V) => U, combOp: (U, U) => U): SCollection[U] // on SCollection[(K, V)] def aggregateByKey[U](zeroValue: U)(seqOp: (U, V) => U, combOp: (U, U) => U): SCollection[(K, U)] And we have some business logic that looks like this: case class Count(id: String, count: Int) val z = Count("", 0) // zeroValue def seqOp(acc: Count, v: Count) = Count(v.id, acc.count + v.count) def combOp(x: Count, y: Count) = Count(x.id, x.count + y.count) sc.parallelize(Seq(Count("a", 10), Count("a", 100), Count("b", 5), Count("b", 50))) .groupBy(_.id) .aggregateByKey(z)(seqOp, combOp) This code however, only works correctly locally with DirectRunner and always produces results with id == "" when running on Dataflow service with the DataflowRunner. Can you spot the bug? Monoid laws You might notice that zeroValue and combOp together resemble a Monoid, which should satisfy the identity law: combOp(zeroValue …" />
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<meta property="og:description" content="I spent a couple hours yesterday debugging what I thought was a Protobuf serialization issue, which turns out to be an unlawful Monoid-like use of aggregateByKey in Scio. The Problem Both Scio and Spark have aggregate and aggregateByKey transformations that look like this: // on SCollection[V] def aggregate[U](zeroValue: U)(seqOp: (U, V) => U, combOp: (U, U) => U): SCollection[U] // on SCollection[(K, V)] def aggregateByKey[U](zeroValue: U)(seqOp: (U, V) => U, combOp: (U, U) => U): SCollection[(K, U)] And we have some business logic that looks like this: case class Count(id: String, count: Int) val z = Count("", 0) // zeroValue def seqOp(acc: Count, v: Count) = Count(v.id, acc.count + v.count) def combOp(x: Count, y: Count) = Count(x.id, x.count + y.count) sc.parallelize(Seq(Count("a", 10), Count("a", 100), Count("b", 5), Count("b", 50))) .groupBy(_.id) .aggregateByKey(z)(seqOp, combOp) This code however, only works correctly locally with DirectRunner and always produces results with id == "" when running on Dataflow service with the DataflowRunner. Can you spot the bug? Monoid laws You might notice that zeroValue and combOp together resemble a Monoid, which should satisfy the identity law: combOp(zeroValue …"/>
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<p>I spent a couple hours yesterday debugging what I thought was a Protobuf serialization issue, which turns out to be an unlawful Monoid-like use of <code>aggregateByKey</code> in <a href="https://github.com/spotify/scio">Scio</a>.</p>
<h2>The Problem</h2>
<p>Both Scio and Spark have <code>aggregate</code> and <code>aggregateByKey</code> transformations that look like this:</p>
<div class="highlight"><pre><span></span><span class="c1">// on SCollection[V]</span>
<span class="k">def</span> <span class="n">aggregate</span><span class="o">[</span><span class="kt">U</span><span class="o">](</span><span class="n">zeroValue</span><span class="k">:</span> <span class="kt">U</span><span class="o">)(</span><span class="n">seqOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">V</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">,</span> <span class="n">combOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">U</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">)</span><span class="k">:</span> <span class="kt">SCollection</span><span class="o">[</span><span class="kt">U</span><span class="o">]</span>
<span class="c1">// on SCollection[(K, V)]</span>
<span class="k">def</span> <span class="n">aggregateByKey</span><span class="o">[</span><span class="kt">U</span><span class="o">](</span><span class="n">zeroValue</span><span class="k">:</span> <span class="kt">U</span><span class="o">)(</span><span class="n">seqOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">V</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">,</span> <span class="n">combOp</span><span class="k">:</span> <span class="o">(</span><span class="kt">U</span><span class="o">,</span> <span class="kt">U</span><span class="o">)</span> <span class="k">=></span> <span class="n">U</span><span class="o">)</span><span class="k">:</span> <span class="kt">SCollection</span><span class="o">[(</span><span class="kt">K</span>, <span class="kt">U</span><span class="o">)]</span>
</pre></div>
<p>And we have some business logic that looks like this:</p>
<div class="highlight"><pre><span></span><span class="k">case</span> <span class="k">class</span> <span class="nc">Count</span><span class="o">(</span><span class="n">id</span><span class="k">:</span> <span class="kt">String</span><span class="o">,</span> <span class="n">count</span><span class="k">:</span> <span class="kt">Int</span><span class="o">)</span>
<span class="k">val</span> <span class="n">z</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="s">""</span><span class="o">,</span> <span class="mi">0</span><span class="o">)</span> <span class="c1">// zeroValue</span>
<span class="k">def</span> <span class="n">seqOp</span><span class="o">(</span><span class="n">acc</span><span class="k">:</span> <span class="kt">Count</span><span class="o">,</span> <span class="n">v</span><span class="k">:</span> <span class="kt">Count</span><span class="o">)</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="n">v</span><span class="o">.</span><span class="n">id</span><span class="o">,</span> <span class="n">acc</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="n">v</span><span class="o">.</span><span class="n">count</span><span class="o">)</span>
<span class="k">def</span> <span class="n">combOp</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Count</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Count</span><span class="o">)</span> <span class="k">=</span> <span class="nc">Count</span><span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">id</span><span class="o">,</span> <span class="n">x</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="n">y</span><span class="o">.</span><span class="n">count</span><span class="o">)</span>
<span class="n">sc</span><span class="o">.</span><span class="n">parallelize</span><span class="o">(</span><span class="nc">Seq</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">10</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">100</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"b"</span><span class="o">,</span> <span class="mi">5</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"b"</span><span class="o">,</span> <span class="mi">50</span><span class="o">)))</span>
<span class="o">.</span><span class="n">groupBy</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">id</span><span class="o">)</span>
<span class="o">.</span><span class="n">aggregateByKey</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">,</span> <span class="n">combOp</span><span class="o">)</span>
</pre></div>
<p>This code however, only works correctly locally with <code>DirectRunner</code> and always produces results with <code>id == ""</code> when running on Dataflow service with the <code>DataflowRunner</code>. Can you spot the bug?</p>
<h2>Monoid laws</h2>
<p>You might notice that <code>zeroValue</code> and <code>combOp</code> together resemble a <a href="https://en.wikipedia.org/wiki/Monoid">Monoid</a>, which should satisfy the identity law:</p>
<div class="highlight"><pre><span></span><span class="n">combOp</span><span class="o">(</span><span class="n">zeroValue</span><span class="o">,</span> <span class="n">x</span><span class="o">)</span> <span class="o">==</span> <span class="n">combOp</span><span class="o">(</span><span class="n">x</span><span class="o">,</span> <span class="n">zeroValue</span><span class="o">)</span> <span class="o">==</span> <span class="n">x</span>
</pre></div>
<p>Since elements in Scio, Spark, etc. are unordered, <code>combOp</code> should also be commutative to make computation deterministic.</p>
<div class="highlight"><pre><span></span><span class="n">combOp</span><span class="o">(</span><span class="n">x</span><span class="o">,</span> <span class="n">y</span><span class="o">)</span> <span class="o">==</span> <span class="n">combOp</span><span class="o">(</span><span class="n">y</span><span class="o">,</span> <span class="n">x</span><span class="o">)</span>
</pre></div>
<h2>Under the hood</h2>
<p>Here are 3 scenarios of what could happen to our code. Let’s assume our data is split up into 2 partitions.</p>
<div class="highlight"><pre><span></span><span class="k">val</span> <span class="n">xs1</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">10</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">15</span><span class="o">))</span>
<span class="k">val</span> <span class="n">xs2</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">100</span><span class="o">),</span> <span class="nc">Count</span><span class="o">(</span><span class="s">"a"</span><span class="o">,</span> <span class="mi">150</span><span class="o">))</span>
</pre></div>
<p>The first scenario is the most intuitive given the method signature of <code>aggregateByKey</code>. Elements in every partition are accumulated into <code>zeroValue</code> with <code>seqOp</code>, and the accumulated values are reduced with <code>combOp</code>. This produces the correct result.</p>
<div class="highlight"><pre><span></span><span class="k">val</span> <span class="n">r1</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="n">xs1</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">),</span> <span class="n">xs2</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">)).</span><span class="n">reduce</span><span class="o">(</span><span class="n">combOp</span><span class="o">)</span>
</pre></div>
<p>The next 2 scenarios are similar, except that <code>zeroValue</code> is appended or prepended to the list of accumulated results of <code>xs1</code> and <code>xs2</code> as input to <code>combOp</code>.</p>
<div class="highlight"><pre><span></span><span class="k">val</span> <span class="n">r2</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="n">z</span><span class="o">,</span> <span class="n">xs1</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">),</span> <span class="n">xs2</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">)).</span><span class="n">reduce</span><span class="o">(</span><span class="n">combOp</span><span class="o">)</span> <span class="c1">// incorrect</span>
<span class="k">val</span> <span class="n">r3</span> <span class="k">=</span> <span class="nc">List</span><span class="o">(</span><span class="n">xs1</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">),</span> <span class="n">xs2</span><span class="o">.</span><span class="n">foldLeft</span><span class="o">(</span><span class="n">z</span><span class="o">)(</span><span class="n">seqOp</span><span class="o">),</span> <span class="n">z</span><span class="o">).</span><span class="n">reduce</span><span class="o">(</span><span class="n">combOp</span><span class="o">)</span> <span class="c1">// correct</span>
</pre></div>
<p>We realize that only <code>r3</code> is correct but <code>r2</code> is not. This is because <code>combOp</code> breaks the identity law and since <code>List#reduce</code> starts from the left, <code>combOp(z, x)</code> will not populate <code>id</code> correctly while <code>combOp(x, z)</code> does. We made the wrong assumption in our code that <code>zeroValue</code> is only used in <code>seqOp</code> and never fed directly into <code>combOp</code>, and didn’t implement it correctly. Any of these 3 scenarios could happen depending on the platform implementation. We need to fix <code>combOp</code> to satisfy the identity law:</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="n">combOp</span><span class="o">(</span><span class="n">x</span><span class="k">:</span> <span class="kt">Count</span><span class="o">,</span> <span class="n">y</span><span class="k">:</span> <span class="kt">Count</span><span class="o">)</span><span class="k">:</span> <span class="kt">Count</span> <span class="o">=</span> <span class="nc">Count</span><span class="o">(</span><span class="k">if</span> <span class="o">(</span><span class="n">x</span><span class="o">.</span><span class="n">id</span><span class="o">.</span><span class="n">nonEmpty</span><span class="o">)</span> <span class="n">x</span><span class="o">.</span><span class="n">id</span> <span class="k">else</span> <span class="n">y</span><span class="o">.</span><span class="n">id</span><span class="o">,</span> <span class="n">x</span><span class="o">.</span><span class="n">count</span> <span class="o">+</span> <span class="n">y</span><span class="o">.</span><span class="n">count</span><span class="o">)</span>
</pre></div>
<p>For more details see <a href="https://github.com/spotify/scio/issues/729">spotify/scio#729</a> and <a href="https://issues.apache.org/jira/browse/BEAM-2453"><span class="caps">BEAM</span>-2453</a>.</p>
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