Docker

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Docker is a platform for developing, packaging, and running applications in lightweight, portable containers that bundle an application with all its dependencies. Containers share the host OS kernel but run in isolated user spaces, making them more resource-efficient than traditional virtual machines while ensuring consistent behavior across environments.

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Docker packs up an app and everything it needs into a neat box, so it runs exactly the same way no matter which computer you open the box on.

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A virtual machine runs a full guest operating system on top of a hypervisor, making it heavier and slower to start. A container shares the host machine's OS kernel and only packages the application and its dependencies, making it much lighter, faster to start, and more resource-efficient than a VM.

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A virtual machine is like building a whole separate house with its own foundation for each guest. A container is like giving each guest their own furnished room in the same building, sharing the same foundation.

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A Docker image is a read-only, immutable template containing an application's code, runtime, libraries, and dependencies, built from a set of instructions in a Dockerfile. Running an image creates a container, which is a live, running instance of that image.

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A Docker image is like a recipe card that describes exactly what should go into a dish. A container is the actual dish you cook using that recipe.

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A Dockerfile is a text file containing a series of instructions, such as FROM, RUN, COPY, and CMD, that Docker uses to automatically build a Docker image in a repeatable, versionable way.

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A Dockerfile is like a step-by-step instruction list that tells Docker exactly how to build your app's box, so anyone can rebuild it exactly the same way.

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COPY simply copies files or directories from the build context into the image. ADD does everything COPY does but also supports extracting local tar archives automatically and fetching files from remote URLs. Because ADD's extra behaviors can be surprising, COPY is generally recommended unless ADD's specific features are needed.

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COPY just moves files from one place to another. ADD can do that too, but it can also unzip archives and grab files from the internet automatically, which can sometimes cause unexpected surprises.

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A Docker container's lifecycle includes states like created, when it's set up but not started; running, when the process inside is actively executing; paused, when execution is temporarily suspended; stopped, when the process has exited; and removed, when the container is deleted entirely along with its writable layer, unless data was stored in a volume.

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A container goes through stages like being built, turned on, sometimes paused, eventually turned off, and finally thrown away, similar to how a toy might be assembled, played with, put away, and later discarded.

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Docker Hub is a cloud-based registry service for finding, storing, and sharing Docker images, hosting both official images maintained by software vendors and public or private images uploaded by the community. Developers can pull existing images or push their own built images for others to use.

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Docker Hub is like an online library of pre-built boxes that anyone can borrow or share, instead of building every single box from scratch.

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A Docker image is a static, read-only template containing everything needed to run an application. A container is a live, running instance of that image, with its own writable layer on top, allowing changes to be made while it's running without affecting the original image.

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An image is like a blueprint. A container is an actual building constructed from that blueprint, which you can live in and even change a little without altering the original blueprint.

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Each instruction in a Dockerfile creates a new read-only layer stacked on top of previous ones using a union filesystem, and Docker caches these layers so unchanged instructions don't need to be rebuilt. This makes builds faster if you order instructions carefully, placing frequently changing steps like copying application code near the end of the Dockerfile.

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Docker builds images like stacking transparent sheets on top of each other, and if a sheet near the bottom hasn't changed, Docker reuses it instead of redrawing it, making rebuilding faster.

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Docker Compose is a tool for defining and running multi-container Docker applications using a single YAML file, docker-compose.yml, where you specify services, networks, and volumes. Running 'docker-compose up' starts all defined services together, simplifying local development and testing of applications with multiple interconnected containers.

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Docker Compose is like a single instruction sheet that starts up several related boxes together at once, like a database box and an app box, instead of starting each one manually one at a time.

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CMD specifies default arguments or a command for a container, which can be easily overridden when running the container. ENTRYPOINT specifies the main command that always runs, treating any CMD or command-line arguments as parameters to it rather than replacing it entirely, making it better suited for containers meant to behave like a fixed executable.

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CMD is like a suggested default action you can easily swap out. ENTRYPOINT is like a fixed main action that always runs, where anything else you provide just becomes extra input to that main action.

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A Docker volume is a persistent storage mechanism managed by Docker that exists outside a container's writable layer, allowing data to survive container restarts or deletion. Volumes are the preferred way to persist data, such as database files, and can be shared between multiple containers.

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A Docker volume is like a storage locker outside the box, so even if you throw away the box, whatever you saved in the locker is still safe.

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The default bridge network provides an isolated private network where containers can communicate with each other via internal IPs. The host network removes network isolation, letting a container share the host's network stack directly for maximum performance at the cost of isolation. The none network disables networking entirely for a container.

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Bridge networking gives containers their own private neighborhood to talk to each other. Host networking means the container just uses the host computer's own network directly. None means the container has no network access at all.

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You can reduce image size by using minimal base images like Alpine Linux, combining related RUN commands to reduce layer count, cleaning up unnecessary files like package caches within the same layer they're created, and using multi-stage builds to exclude build-time dependencies from the final image.

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Shrinking a Docker image is like packing a suitcase efficiently — using a smaller base bag, only packing what you actually need at your destination, and throwing away wrapping and packaging before you close it up.

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Data can be persisted using Docker volumes, which are managed by Docker and stored outside the container's filesystem, or bind mounts, which map a specific host directory into the container. Volumes are generally preferred since they're portable and managed independently of the host's file structure.

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To keep data safe even if a container disappears, you connect it to an outside storage locker (a volume) or a specific folder on the host computer, instead of only storing data inside the container itself.

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docker-compose lets you define an entire multi-container application declaratively in a YAML file, specifying services, networks, environment variables, and dependencies between containers, then start or stop the whole stack with a single command, instead of manually running individual 'docker run' commands with many flags for each container.

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Instead of typing a long, separate command to start each box one at a time, docker-compose lets you write down your whole setup once and start everything together with one simple command.

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You can debug a running container using 'docker logs' to view its console output, 'docker exec -it <container> /bin/bash' to open an interactive shell inside the running container, and 'docker inspect' to view detailed configuration and state information about the container.

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Debugging a container is like peeking inside a running box to check the logs it's writing, or opening a small door to look around and poke at things directly while it's still running.

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Container orchestration automates the deployment, scaling, networking, and management of containers across a cluster of machines, handling tasks like scheduling containers onto available nodes, restarting failed containers, and load balancing traffic between replicas. It's needed because manually managing large numbers of containers across many servers becomes impractical at scale, which is why tools like Kubernetes were created.

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Container orchestration is like having an automatic manager that decides where to place lots of boxes across many shelves, replaces any box that breaks, and makes sure work is spread out evenly, so nobody has to do it all by hand.

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A multi-stage build uses multiple FROM statements in a single Dockerfile, where each stage can perform different tasks, such as compiling code in a build stage with all necessary build tools, and then copying only the compiled output into a minimal final stage. This keeps the final production image small and free of unnecessary build dependencies.

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A multi-stage build is like using a messy workshop to build a piece of furniture, then only bringing the finished furniture into your clean living room, leaving all the sawdust and tools behind in the workshop.

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Linux namespaces provide isolation by giving each container its own view of resources like process IDs, network interfaces, and mount points, so containers can't see or interfere with each other. Control groups (cgroups) limit and monitor the amount of CPU, memory, and other resources a container can use, preventing one container from starving others of resources.

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Namespaces give each container its own private view of the world, so it thinks it's alone even though it's sharing the same computer. Cgroups make sure each container only gets a fair, limited slice of resources like memory and CPU.