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123 lines
5.5 KiB
123 lines
5.5 KiB
11 months ago
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<!DOCTYPE html>
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<html lang="en">
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<title>Julio Biason .Me 4.3</title>
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<link rel="stylesheet" href="https://blog.juliobiason.me/print.css" media="print">
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<body class=" ">
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<div class="sidebar">
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<div class="container sidebar-sticky">
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<div class="sidebar-about">
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<a href="https://blog.juliobiason.me"><h1>Julio Biason .Me 4.3</h1></a>
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<p class="lead">Old school dev living in a 2.0 dev world</p>
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</div>
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<ul class="sidebar-nav">
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<li class="sidebar-nav-item"><a href="/">English</a></li>
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<li class="sidebar-nav-item"><a href="/pt">Português</a></li>
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<li class="sidebar-nav-item"><a href="/tags">Tags (EN)</a></li>
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<li class="sidebar-nav-item"><a href="/pt/tags">Tags (PT)</a></li>
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</ul>
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<div class="content container">
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<div class="post">
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<h1 class="post-title">Microservices: Where Is the Source of Truth?</h1>
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<span class="post-date">
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2020-02-17
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<a href="https://blog.juliobiason.me/tags/microservices/">#microservices</a>
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<a href="https://blog.juliobiason.me/tags/state/">#state</a>
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<a href="https://blog.juliobiason.me/tags/source-of-truth/">#source of truth</a>
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</span>
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<p>When we are talking about a system, there is one point that we need to discuss
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where the "Source of Truth" of it is. In Monoliths, the source of truth is the
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storage itself. But where does it sit in a system that is composed by multiple
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parts?</p>
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<span id="continue-reading"></span>
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<p>Just to be clear here: What I mean by "source of truth" is whatever you can
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use to check if the system is producing the correct values. For example, in a
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pipeline to count how much users transfer in a CDN, the source of truth could
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be the logs (you can read the logs and check if the service responsible for
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summing the results is correct); in an e-commerce site, we could assume the
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source of truth to be the items in every invoice for the total revenue (if you
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sum all the items quantities by their prices, you'll get the revenue back).</p>
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<p>One important factor of a source of truth is that not only you can use it to
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check if your services are correct, but you can also re-apply its content back
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to the system to rebuild artifacts.</p>
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<p>But here is a problem: If your system is distributed and <a href="https://blog.juliobiason.me/code/microservices-source-of-truth/@./microservices-artifact-input-state.md">each microservice is
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building a different artifact</a>, where
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is the source of truth?</p>
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<p>Previously, I mentioned that microservices could keep an state in order to
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produce the artifact. That could be the source of truth for that microservice,
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as long as it doesn't delete old data -- in which case, how would you rebuild
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old artifacts if you can't return the state to an old... state?</p>
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<p>Another solution is to make the insertion microservice (the one that captures
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data from an external source) to build this source of truth; in case of the
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need to rebuild the data, you can just add an API in that microservice for it
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to republish data related to a certain object, which will go down the pipeline
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and every microservice will rebuild their artifacts.</p>
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<p>And, finally, if you build an event source pipeline correctly, you can build a
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single service that will listed to <em>every</em> event and keep a global event
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source, which you can again expose an API to replay all events of an object.</p>
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<div style="border:1px solid grey; margin:7px; padding: 7px">
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<p>.. although I have a feeling that certain events should be changed; for example,
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if you make the event source replay everything related to an invoice -- in
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order to reproduce the complete invoice artifact -- you may have to replay a
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"create user" event, which won't make sense 'cause the object already exists,
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and it shouldn't create a new user with the same information.</p>
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<p>Either that, or you could make the invoice service request the costumer
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service the data if it doesn't have it already.</p>
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</div>
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<p>Anyway, one recommendation that I have is to always build something that can
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keep track of your data, in case you need to rebuild your artifacts and your
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state removes old data (which is perfectly normal, I must add).</p>
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</div>
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</div>
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</body>
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</html>
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