[Blog] add blog post on debugging docker container

src/assets/Debugging/debugging.md

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+---
+title: Advanced Docker Container Debugging - A comprehensive guide
+authorName: Prajwol Amatya
+authorAvatar: https://1.gravatar.com/avatar/de64e53c0e2cb393dd0d14ffdd53058ee9c607b35e366dd392425bd1b95a034c?size=256
+authorLink: https://github.com/prajwolamatya
+createdAt: April 29, 2025
+tags: debugging, docker
+banner: https://blog.jankaritech.com/src/assets/Debugging/images/Debugging.png
+---
+
+Docker has revolutionized modern software development by enabling lightweight, portable, and scalable containerized applications. However, as deployments grow in complexity, so do the challenges in debugging and optimizing containers. In this blog, we'll explore powerful debugging techniques using a sample project with Node.js, Nginx, and Redis. You can get the sample project [here](https://github.com/prajwolamatya/debug-docker).
+
+Our setup consists of:
+- A Node.js application (port 3000)
+- Nginx as a reverse proxy (port 80)
+- Redis for caching (port 6379)
+
+## 1. Container Inspection
+### Viewing Running Containers
+First, let's check our running containers:
+
+```bash
+docker-compose ps
+```
+
+**Expected Output:**
+```console
+Name                       Command                         State    Ports
+debug-docker_nginx_1       /docker-entrypoint.sh ngin ...  Up       0.0.0.0:80->80/tcp
+debug-docker_nodejs-app_1  docker-entrypoint.sh node ...   Up       0.0.0.0:3000->3000/tcp
+debug-docker_redis_1       docker-entrypoint.sh redis ...  Up       0.0.0.0:6379->6379/tcp
+```
+
+- **State:** Shows if container is running/stopped
+- **Ports:** Reveals port mappings (host:container)
+
+### Inspecting Container Details
+For deeper inspection of a specific container:
+
+```bash
+docker inspect debug-docker_nodejs-app_1
+```
+
+This returns a JSON with all container details including:
+- Network settings
+- Mounts
+- Environment variables
+- IP addresses
+
+**Pro Tip:** Filter specific information:
+
+```bash
+docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' debug-docker_nodejs-app_1
+```
+
+## 2. Log Analysis
+### Viewing Container Logs
+
+```bash
+docker-compose logs
+```
+
+**Example Output (when accessing the service):**
+
+```console
+debug-docker_nodejs-app_1  | Node.js server running on port 3000
+```
+
+This confirms your Node.js application launched successfully
+
+### Generating Request Logs
+Make a test request to generate logs:
+
+```bash
+curl -v http://localhost:3000
+```
+
+After the request, check logs again to see:
+
+```console
+debug-docker_nodejs-app_1  | Node.js server running on port 3000
+debug-docker_nodejs-app_1  | GET / 200 7.001 ms - 19
+```
+
+To view only GET requests from the last 5 minutes
+```bash
+docker logs --since 5m debug-docker_nodejs-app_1 | grep "GET"
+```
+
+**Expected Output:**
+
+```console
+GET / 200 7.001 ms - 19
+```
+
+## 3. Network Troubleshooting
+### Checking Container Connectivity
+
+Test if Nginx can reach Node.js:
+
+```bash
+docker exec debug-docker_nginx_1 ping nodejs-app
+```
+
+**Expected Output:**
+```console
+PING nodejs-app (172.19.0.3): 56 data bytes
+64 bytes from 172.19.0.3: seq=0 ttl=64 time=0.060 ms
+```
+- **Success:** <1ms response confirms network connectivity
+- **Failure:** Would show "unknown host" or timeout
+
+### Examining Post Accessibility
+Check if Node.js is listening on port 3000 inside its container:
+
+```bash
+docker exec debug-docker_nodejs-app_1 netstat -tuln
+```
+
+**Expected Output:**
+```console
+Active Internet connections (only servers)
+Proto Recv-Q Send-Q Local Address           Foreign Address         State
+tcp        0      0 :::3000                 :::*                    LISTEN
+```
+
+- Shows Node.js listening on port 3000
+- No output means service isn't running properly
+
+## 4. Interactive Debugging
+### Executing into Containers
+
+For Node.js application debugging:
+```bash
+docker exec -it debug-docker_nodejs-app_1 sh
+```
+Now you can:
+1. Check running processes: `ps aux`
+```bash
+ps aux
+```
+
+**Output:**
+
+```console
+PID   USER     TIME  COMMAND
+    1 root      0:00 {MainThread} node app.js
+   28 root      0:00 sh
+   45 root      0:00 ps aux
+```
+- Shows all running processes in the container
+- `PID1`: Your Node.js application (node app.js)
+- `PID28`: The shell session you just started
+- `PID 45`: The `ps aux` command itself
+- Confirms your application is running as the main process
+
+2. Examine environment variables: `printenv`
+```bash
+printenv
+```
+
+**Output:**
+```console
+NODE_VERSION=23.11.0
+HOSTNAME=ada88201c429
+YARN_VERSION=1.22.22
+SHLVL=1
+HOME=/root
+TERM=xterm
+PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+PWD=/app
+```
+- Displays all environment variables to your Node.js app
+
+3. Test Redis connectivity: `redis-cli -h redis ping`
+```bash
+redis-cli -h redis ping
+```
+
+**Output:**
+```console
+PONG
+```
+
+- Tests connectivity to your Redis container
+- `PONG` response confirms network connectivity
+
+### Debugging Nginx Configuration
+```bash
+docker exec -it debug-docker_nginx_1 nginx -t
+```
+
+**Expected Output:**
+```console
+nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
+nginx: configuration file /etc/nginx/nginx.conf test is successful
+```
+
+## 5. Health Checks and Readiness Probes
+Let's enhance our `docker-compose.yml` with health checks:
+
+```yaml
+services:
+  nodejs-app:
+    # ... existing config ...
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:3000"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+
+  redis:
+    # ... existing config ...
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 30s
+      timeout: 5s
+      retries: 3
+```
+
+Now check the container health:
+```bash
+docker ps --format "table {{.Names}}\t{{.Status}}"
+```
+
+**Expected Output:**
+```console
+NAMES                     STATUS
+debug-docker_nginx_1      Up 5 minutes
+debug-docker_nodejs-app_1 Up 5 minutes (healthy)
+debug-docker_redis_1      Up 5 minutes (healthy)
+```
+
+## 6. Temporary Debug Containers
+Sometimes you need additional tools. Create a temporary debug container in the same network:
+```bash
+docker run -it --rm --network debug-docker_default alpine sh
+```
+
+Now from this container you can:
+1. Test DNS resolution:
+```bash
+nslookup nodejs-app
+```
+
+**Expected Output:**
+```console
+Server:		127.0.0.11
+Address:	127.0.0.11:53
+
+Non-authoritative answer:
+Name:	nodejs-app
+Address: 172.19.0.3
+```
+
+- Confirms Docker's internal DNS is working
+- Shows the service name resolves to the correct container IP (172.19.03)
+
+2. Check connectivity:
+```bash
+wget -qO- http://nodejs-app:3000
+```
+
+**Expected Output:**
+```console
+Hello from Node.js!
+```
+
+- Shows successful TCP connection to port 3000
+- Returns the actual HTTP response from your Node.js app
+
+## 7. Docker System Diagnostics
+When facing resource issues:
+```bash
+docker system df
+```
+
+**Example Output:**
+```console
+TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
+Images          17        3         2.374GB   2.295GB (96%)
+Containers      3         3         2B        0B (0%)
+Local Volumes   2         1         88B       88B (100%)
+Build Cache     108       0         37.82MB   37.82MB
+```
+
+Check detailed resource usage:
+```bash
+docker stats
+```
+
+**Example Output:**
+```console
+CONTAINER ID   NAME                        CPU %     MEM USAGE / LIMIT     MEM %     NET I/O           BLOCK I/O     PIDS
+5111f46d640b   debug-docker_nginx_1        0.00%     9.633MiB / 31.06GiB   0.03%     41.9kB / 126B     0B / 4.1kB    13
+e701e4d02bb0   debug-docker_nodejs-app_1   0.00%     13.31MiB / 31.06GiB   0.04%     45.3kB / 3.49kB   0B / 0B       7
+3e0399cc7510   debug-docker_redis_1        0.93%     4.691MiB / 31.06GiB   0.01%     42.3kB / 126B     1.43MB / 0B   6
+```
+
+## Conclusion
+Effective Docker debugging requires a systematic approach. By mastering these techniques, you'll be able to diagnose and resolve even the most complex Docker issues in production environments.