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‎blogs/volumes_in_docker.html

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<h5 class="w3-center w3-padding-64"><span class="w3-tag w3-wide">Docker Volumes</span></h5>
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</p>
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<p class="w3-content" style="max-width:900px">
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When a container writes files, it writes them inside of the container.
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Which means that when the container dies (the host machine restarts or the container is moved from one node
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to another in a cluster, it simply fails, etc.) all of that data is lost. It also means that if you run the
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same container several times in a load-balancing scenario, each container will have its own data, which may
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result in inconsistent user experience.<br><br>
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A rule of thumb for the sake of simplicity is to ensure that containers are stateless, for instance, storing
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their data in an external database (relational like an SQL Server or document-based like MongoDB) or
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distributed cache (like Redis). However, sometimes you want to store files in a place where they are
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persisted; this is done using volumes.
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<br><br>
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Using a volume, you map a directory inside the container to a persistent storage. Persistent storages are
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managed through drivers, and they depend on the actual Docker host. They may be an Azure File Storage on
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Azure or Amazon S3 on AWS. With Docker Desktop, you can map volumes to actual directories on the host system;
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this is done using the -v switch on the docker run command or if you are using Docker-compose you can simply
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set the volume there.
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<br><br>
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Suppose you run a Oracle database with no volume:<br>
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<code>docker run -d oraclelinux:7-slim</code>
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<br>
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Any data stored in that database will be lost when the container is stopped or restarted.
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In order to avoid data loss, you can use a volume mount:<br>
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<code>
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docker run -v /your/dir:/var/lib/oraclelinux -d oraclelinux:7-slim
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</code><br>
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<br>
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It will ensure that any data written to the <i>/var/lib/oraclelinux</i> directory inside the container is actually
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written to the <i>/your/dir</i> directory on the host system.
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This ensures that the data is not lost when the container is restarted.
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Another very big advantage is that it allows to edit the code inside your volume which takes effect upon
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restarting the container. This means that we can actually make code changes persistent.
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</p>
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<blockquote>
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<cite>References:<br> [1] <a href="https://docs.docker.com/develop/develop-images/multistage-build/">https://docs.docker.com/develop/develop-images/multistage-build/</a></cite>
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</blockquote>
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<p>
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Copyright &#x000A9;&nbsp;2020 &#x000B7; Akash Singh
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