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<p>Heron provides support for multiple delivery semantics, and you can select delivery semantics on a topology-by-topology basis. Thus, if you have topologies for which <a href="#available-semantics">at-most-once</a> semantics are perfectly acceptable, for example, you can run them alongside topologies with more stringent semantics (such as effectively once).</p>
<h2><a class="anchor" aria-hidden="true" id="available-semantics"></a><a href="#available-semantics" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Available semantics</h2>
<p>Heron supports three delivery semantics:</p>
<table>
<thead>
<tr><th style="text-align:left">Semantics</th><th style="text-align:left">Description</th><th style="text-align:left">When to use?</th></tr>
</thead>
<tbody>
<tr><td style="text-align:left">At most once</td><td style="text-align:left">Heron processes tuples using a best-effort strategy. With at-most-once semantics, it's possible that some of the tuples delivered into the system may be lost due to some combination of processing, machine, and network failures. What sets at-most-once semantics apart from the others is that Heron will not attempt to retry a processing step upon failure, which means that the tuple may fail to be delivered.</td><td style="text-align:left">When some amount of data loss is acceptable</td></tr>
<tr><td style="text-align:left">At least once</td><td style="text-align:left">Tuples injected into the Heron topology are guaranteed to be processed at least once; no tuple will fail to be processed. It's possible, however, that any given tuple is processed more than once in the presence of various failures, retries, or other contingencies.</td><td style="text-align:left">When you need to guarantee no data loss</td></tr>
<tr><td style="text-align:left">Effectively once</td><td style="text-align:left">Heron ensures that the data it receives is processed effectively once---even in the presence of various failures---leading to accurate results. <strong>This applies only to <a href="#stateful-topologies">stateful topologies</a></strong>. &quot;Effectively&quot; in this case means that there's a guarantee that tuples that cause <a href="#stateful-topologies">state changes</a> will be processed once (that is, they will have <em>an effect</em> on state once).</td><td style="text-align:left">When you're using <a href="#stateful-processing">stateful topologies</a> and need strong</td></tr>
</tbody>
</table>
<p>You can see a visual representation of these different delivery semantics in the figure below:</p>
<p><img src="https://www.lucidchart.com/publicSegments/view/f35df5fd-bfc1-4270-aad6-40766abae024/image.png" alt="Heron delivery semantics"></p>
<p>In this diagram, you see three Heron topologies, each of which is processing a series of tuples (<code>(1,2,3)</code>, <code>(7,8,11)</code>, etc.).</p>
<ul>
<li>The topology in the upper left offers at-most-once semantics, which means that each tuple will either be delivered once or fail to be processed. In this case, the <code>(1,5)</code> tuple fails to be processed.</li>
<li>The topology in the lower left offers at-least-once semantics, which means that each tuple will be delivered either once or more than once. In this case, the <code>(7,8,11)</code> tuple is processed more than once (perhaps due to a network glitch or a retry).</li>
<li>The topology in the upper right offers effectively once semantics, which means that every tuple is delivered one time and one time only. This does <em>not</em> mean that every tuple is processed exactly one time. Some tuples may be processed multiple times <em>within the topology</em>, but we use the &quot;effectively once&quot; terminology here to express that</li>
</ul>
<h2><a class="anchor" aria-hidden="true" id="requirements-for-effectively-once"></a><a href="#requirements-for-effectively-once" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Requirements for effectively once</h2>
<p>In order to use effectively-once semantics with a topology, that topology must satisfy two conditions:</p>
<ol>
<li><p>It must be a <a href="#stateful-topologies">stateful, idempotent topology</a>.</p></li>
<li><p>The input stream into the topology must be strongly consistent. In order to provide effectively-once semantics, topologies need to be able to &quot;rewind&quot; state in case of failure. The state that it &quot;rewinds&quot; needs to be reliable state---preferably durably stored.</p>
<p>If the input to the topology is, for example, a messaging system that cannot ensure stream consistency, then effectively-once semantics cannot be applied, as the state &quot;rewind&quot; may return differing results. To put it somewhat differently, Heron can only provide delivery semantics as stringent as its data input sources can themselves provide.</p></li>
</ol>
<h3><a class="anchor" aria-hidden="true" id="exactly-once-semantics"></a><a href="#exactly-once-semantics" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Exactly-once semantics?</h3>
<p>There has been a lot of discussion recently surrounding so-called &quot;exactly-once&quot; processing semantics. We'll avoid this term in the Heron documentation because we feel that it's misleading. &quot;Exactly-once&quot; semantics would mean that no processing step is ever performed more than once---and thus that no processing step is ever retried.</p>
<p>It's important to always keep in mind that <em>no system</em> can provide exactly-once semantics in the face of failures (as <a href="http://bravenewgeek.com/you-cannot-have-exactly-once-delivery">this article</a> argues). But that's okay because they don't really need to; the truly important thing is that a stream processing system be able to recover from failures by &quot;rewinding&quot; state to a previous, pre-failure point and to re-attempt to apply processing logic. We use the tern <strong>effectively once</strong>, following <a href="https://twitter.com/viktorklang/status/789036133434978304">Victor Klang</a>, for this style of semantics.</p>
<p>Heron <em>can</em> provide effectively-once guarantees if a topology meets the conditions <a href="#requirements-for-effectively-once">outlined above</a>, but it cannot provide &quot;exactly-once&quot; semantics.</p>
<h2><a class="anchor" aria-hidden="true" id="stateful-topologies"></a><a href="#stateful-topologies" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Stateful topologies</h2>
<p>The Heron topologies that you create can be either stateful or non stateful.</p>
<ul>
<li>In <strong>stateful topologies</strong>, each component must implement an interface that requires it to store its state every time it processes a tuple (both spouts <em>and</em> bolts must do so).</li>
<li>In <strong>non-stateful topologies</strong>, there is no requirement that any processing components store a state snapshot. Non-stateful topologies can provide at-most-once or at-least-once semantics, but never effectively-once semantics.</li>
</ul>
<blockquote>
<p>Heron currently supports two state managers: <a href="state-managers-zookeeper">ZooKeeper</a> and the <a href="state-managers-local-fs">local filesystem</a>, although others are currently under development.</p>
</blockquote>
<p>Stateful topologies, in turn, are of two types:</p>
<ul>
<li><p><strong>Idempotent</strong> stateful topologies are stateful topologies in which applying the processing graph to an input more than once, it will continue to return the same result. A basic example is multiplying a number by 0. The first time you do so, the number will change (always to 0), but if you apply that transformation again and again, it will not change.</p>
<p>For topologies to provide effectively-once semantics, they need to transform tuple inputs idempotently as well. If they don't, and applying the topology's processing graph multiple times yields different results, then effectively-once semantics <em>cannot</em> be achieved.</p>
<p>If you'd like to create idempotent stateful topologies, make sure to write tests to ensure that idempotency requirements are being met.</p></li>
<li><p><strong>Non-idempotent</strong> stateful topologies are stateful topologies that do not apply processing logic along the model of &quot;multiply by zero&quot; and thus cannot provide effectively-once semantics. An example of a non-idempotent</p></li>
</ul>
<blockquote>
<p>Remember: effectively-once semantics can only be applied to topologies that are: (a) stateful and (b) idempotent.</p>
</blockquote>
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