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‎interview/machine_coding/03.mongodb_coding_round.md

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10. Count the number of documents in a collection.
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11. Sort documents based on a field.
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12. Limit and skip results in a query.
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13. Update one field in one document.
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14. Update multiple documents using a condition.
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15. Use `$inc`, `$set`, `$unset` in update queries.

‎redis/00.docker_redis_installation.md

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# **What is Docker?**
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Docker is a **containerization platform** that packages software (and its dependencies) into lightweight, portable units called **containers**. Think of containers as "shipping containers" for code:
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- They bundle your app, libraries, tools, and runtime into a single standardized unit.
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- Containers run consistently on any machine with Docker installed, eliminating the "it works on my machine" problem.
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---
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### **Key Features of Docker**
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| Feature | Why It Matters |
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|---------|----------------|
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| **Containers** | Isolated, lightweight environments that run apps with their own OS processes, files, and networks. |
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| **Images** | Blueprints for containers (e.g., the `redis` image defines how Redis runs). |
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| **Dockerfile** | A script to automate image creation (e.g., install Redis, configure ports). |
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| **Portability** | Run the same container on Windows, Linux, or macOS. |
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| **Isolation** | Containers don’t interfere with each other or the host system. |
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| **Scalability** | Easily spin up multiple containers (e.g., for microservices). |
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| **Docker Hub** | A public registry to share/pull pre-built images (e.g., `redis`, `nginx`). |
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---
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### **Core Docker Terminologies**
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1. **Container**
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A running instance of a Docker image. Think of it as a lightweight virtual machine (but not a full VM!).
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2. **Image**
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A read-only template with instructions to create a container (e.g., `redis:latest`).
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3. **Dockerfile**
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A text file with commands to build a custom image (e.g., `FROM redis`, `EXPOSE 6379`).
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4. **Docker Compose**
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A tool to define and run multi-container apps using a `docker-compose.yml` file.
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5. **Volume**
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Persistent storage for containers (saves data even if the container stops).
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6. **Network**
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Isolated communication channels between containers (e.g., `bridge`, `host`).
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7. **Registry**
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A repository for Docker images (e.g., Docker Hub, private registries).
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8. **Daemon & Client**
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- **Docker Daemon**: Background service managing containers/images.
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- **Docker Client**: CLI/GUI tool to interact with the daemon (e.g., `docker run`).
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9. **Docker Desktop**
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A GUI tool for Windows/macOS to manage Docker (includes the daemon, CLI, and tools).
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---
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### **Why Use Docker to Run Redis (Especially Offline)?**
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#### 1. **No Dependency Hell**
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- Redis relies on Linux libraries. Docker bundles Redis with its dependencies, so it runs on **Windows/macOS without conflicts**.
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#### 2. **Isolated Environment**
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- Redis runs in a container, separate from your host OS. No risk of breaking your system or other apps.
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#### 3. **Portability**
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- Once you have the Redis image, you can run it offline anywhere (no need for internet after setup).
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#### 4. **Consistent Setup**
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- The same Redis image works on your laptop, server, or teammate’s machine.
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#### 5. **Easy Version Management**
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- Run different Redis versions side-by-side (e.g., `redis:6.2` vs. `redis:7.0`).
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#### 6. **Offline-Friendly**
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- After pulling the Redis image once (e.g., `docker pull redis`), you can reuse it offline indefinitely.
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#### 7. **Persistent Data (Volumes)**
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- Use Docker volumes to save Redis data to your host machine, even when offline:
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```bash
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docker run -v redis_data:/data -d redis
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```
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---
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### **How Docker Enables Offline Redis Usage**
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1. **Pull the Image Once**
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Download the Redis image when you have internet:
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```bash
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docker pull redis
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```
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This image is cached locally and reused offline.
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2. **Run Redis Without Internet**
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Start Redis offline using the cached image:
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```bash
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docker run --name my-redis -d redis
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```
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3. **Persist Data**
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Use volumes to store Redis data on your host machine (no internet needed for storage).
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---
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### **Real-World Analogy**
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Imagine Docker as a **microwave meal**:
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- **Image** = Frozen meal (pre-packaged with everything needed).
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- **Container** = Heated meal (ready to eat).
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- **Volume** = Reusable container storing leftovers.
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- **Docker Hub** = Supermarket where you get frozen meals.
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Once you have the meal, you don’t need the supermarket (internet) to eat it! �
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---
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### Docker simplifies running Redis (or any app) by:
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- Eliminating dependency issues.
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- Providing isolation and portability.
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- Enabling offline use after initial setup.
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- Making deployment consistent across environments.
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## To run Redis on Windows, you can use the following method. Note that Redis is not natively supported on Windows, but these workarounds are effective.
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### **Step 1: Install Docker Desktop**
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1. **Download Docker Desktop**
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Go to [Docker Desktop for Windows](https://www.docker.com/products/docker-desktop/) and download the installer.
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2. **Install and Configure**
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- Run the installer and follow the prompts.
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- Ensure **WSL 2** (Windows Subsystem for Linux) is enabled during installation (Docker will prompt you if not).
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- After installation, launch Docker Desktop and let it run in the background.
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3. **Verify Docker Installation**
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Open PowerShell or Command Prompt and run:
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```bash
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docker --version
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```
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If you see `Docker version XX.XX.X`, you’re good to go!
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---
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### **Step 2: Pull the Redis Image**
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Run this command to download the official Redis image from Docker Hub:
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```bash
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docker pull redis
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```
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---
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### **Step 3: Start a Redis Container**
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Run Redis in a Docker container with this command:
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```bash
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docker run --name my-redis -p 6379:6379 -d redis
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```
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- **`--name my-redis`**: Names the container `my-redis` (you can change this).
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- **`-p 6379:6379`**: Maps port 6379 on your Windows machine to the container’s port 6379.
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- **`-d`**: Runs the container in the background (detached mode).
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- **`redis`**: The image to use.
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---
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### **Step 4: Verify the Container is Running**
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Check if the Redis container is active:
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```bash
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docker ps
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```
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You should see a container named `my-redis` with status `Up`.
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---
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### **Step 5: Connect to Redis**
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#### **Option A: Use `redis-cli` Inside the Container**
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Run this command to access the Redis command-line interface:
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```bash
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docker exec -it my-redis redis-cli
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```
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Test the connection with:
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```bash
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127.0.0.1:6379> PING
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```
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If Redis replies `PONG`, it’s working! 🎉
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#### **Option B: Use a GUI Client (e.g., RedisInsight)**
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Download [RedisInsight](https://redis.com/redis-enterprise/redis-insight/) and connect to `localhost:6379`.
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---
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### **Key Commands for Managing the Container**
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- **Stop the Container**:
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```bash
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docker stop my-redis
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```
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- **Start the Container Again**:
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```bash
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docker start my-redis
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```
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- **Delete the Container**:
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```bash
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docker rm my-redis
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```
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---
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### **Persisting Data (Optional)**
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By default, Redis data in Docker is ephemeral. To save data permanently:
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1. **Create a Volume**:
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```bash
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docker volume create redis_data
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```
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2. **Run Redis with the Volume**:
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```bash
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docker run --name my-redis -p 6379:6379 -v redis_data:/data -d redis redis-server --save 60 1
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```
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- `-v redis_data:/data`: Mounts the volume to the container’s `/data` directory.
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- `--save 60 1`: Saves data to disk every 60 seconds if at least 1 key changes (customize as needed).
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---
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### **Troubleshooting**
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- **Docker Not Running**: Ensure Docker Desktop is open and the whale icon 🐋 is in your system tray.
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- **Port Conflict**: If port `6379` is already in use, change the host port (e.g., `-p 6380:6379`).
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- **Connection Refused**: Restart Docker or check the container logs:
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```bash
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docker logs my-redis
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```
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---

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