addressed reviews

Author: PrajwolAmatya

src/assets/Debugging/debugging.md

@@ -1,10 +1,10 @@
 ---
-title: Advanced Docker Container Debugging - A comprehensive guide
+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
+createdAt: May 07, 2025
+tags: debugging, docker, docker container
 banner: https://blog.jankaritech.com/src/assets/Debugging/images/Debugging.png
 ---
 
@@ -17,13 +17,13 @@ Our setup consists of:
 
 ## 1. Container Inspection
 ### Viewing Running Containers
-First, let's check our running containers:
+First, let's check whether our containers are actually running:
 
 ```bash
-docker-compose ps
+docker compose ps
 ```
 
-**Expected Output:**
+**Example Output:**
 ```console
 Name                       Command                         State    Ports
 debug-docker_nginx_1       /docker-entrypoint.sh ngin ...  Up       0.0.0.0:80->80/tcp
@@ -47,17 +47,39 @@ This returns a JSON with all container details including:
 - Environment variables
 - IP addresses
 
-**Pro Tip:** Filter specific information:
+**Filter specific information:**
 
 ```bash
 docker inspect -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' debug-docker_nodejs-app_1
 ```
 
+Let's breakdown the command:
+- `-f` or `--format` activates Go template formatting
+- `{{range .NetworkSettings.Networks}}` iterates through networks
+- `{{.IPAddress}}` extracts the IP for each network
+- `{{end}}` closes the loop
+
+**Example Output:**
+
+(*The container's internal IP in Docker's network*)
+```console
+172.19.0.3
+```
+
+Docker uses **Go Templates** for the `--format` filtering in `docker inspect`. You can get more details on how to construct the filter in Go's [text/template](https://pkg.go.dev/text/template) package.
+Also, you can learn more on formatting output in [Format command and log output](https://docs.docker.com/engine/cli/formatting/) section.
+
 ## 2. Log Analysis
 ### Viewing Container Logs
 
 ```bash
-docker-compose logs
+docker compose logs
+```
+
+This will return the logs of containers that are part of the docker compose setup. You can also get the container specific logs using the following command.
+
+```bash
+docker compose logs nodejs-app
 ```
 
 **Example Output (when accessing the service):**
@@ -66,7 +88,7 @@ docker-compose logs
 debug-docker_nodejs-app_1  | Node.js server running on port 3000
 ```
 
-This confirms your Node.js application launched successfully
+This confirms your Node.js application launched successfully.
 
 ### Generating Request Logs
 Make a test request to generate logs:
@@ -75,7 +97,7 @@ Make a test request to generate logs:
 curl -v http://localhost:3000
 ```
 
-After the request, check logs again to see:
+After the request, check the logs again to see:
 
 ```console
 debug-docker_nodejs-app_1  | Node.js server running on port 3000
@@ -84,10 +106,10 @@ 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"
+docker compose logs --since 5m nodejs-app | grep "GET"
 ```
 
-**Expected Output:**
+**Example Output:**
 
 ```console
 GET / 200 7.001 ms - 19
@@ -102,39 +124,89 @@ Test if Nginx can reach Node.js:
 docker exec debug-docker_nginx_1 ping nodejs-app
 ```
 
-**Expected Output:**
+- `docker exec`: Executes a command inside a running container (`debug-docker_nginx_1`)
+- `ping nodejs-app`: Calls the Linux `ping` utility to test network reachability to the hostname `nodejs-app`
+
+**Example 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
 
+Docker Compose automatically assigns hostnames to containers based on the service names defined in `docker-compose.yml`.
+
+Example:
+
+```yaml
+services:
+  nodejs-app: # This becomes the hostname
+    image: node:alpine
+```
+
+Run `docker compose ps` to see the exact service/container names:
+
+```bash
+docker compose ps --format "table {{.Name}}\t{{.Service}}"
+```
+Output:
+
+```console
+NAME                        SERVICE
+debug-docker_nginx_1        nginx
+debug-docker_nodejs-app_1   nodejs-app
+```
+
+Alternatively, you can use service names directly from `docker-compose.yml` instead of full container name.
+
+```bash
+docker compose exec nginx ping nodejs-app
+```
+
+**Example Output:**
+
+```console
+PING nodejs-app (172.19.0.3): 56 data bytes
+64 bytes from 172.19.0.3: seq=0 ttl=64 time=0.042 ms
+```
+
 ### 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
+docker exec nodejs-app netstat -tuln
 ```
 
-**Expected Output:**
+- `docker exec`: Runs a command inside a specific container (`nodejs-app`).
+- `netstat -tuln`: A Linux utility to list all listening network ports with the flags:
+  - `-t`: Show TCP ports
+  - `-u`: Show UDP ports
+  - `-l`: Display only listening ports (services accepting connections)
+  - `-n`: Show numeric addresses/ports
+
+**Example 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
+- No output means the 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
+docker exec -it nodejs-app sh
 ```
-Now you can:
+
+This gives you full shell access inside the container where you can run any Linux command (as long as the tool exists in the container). Following are few things that you can do.
+
 1. Check running processes: `ps aux`
 ```bash
 ps aux
@@ -190,13 +262,14 @@ PONG
 docker exec -it debug-docker_nginx_1 nginx -t
 ```
 
-**Expected Output:**
+**Example 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
+Docker health checks are automated tests that periodically verify if a container is functioning properly. Health checks transform your containers from static processes into self aware services that can catch issues like application crashes, frozen processes, dependency failures, etc.
 Let's enhance our `docker-compose.yml` with health checks:
 
 ```yaml
@@ -218,12 +291,16 @@ services:
       retries: 3
 ```
 
+- Docker runs the `test` command at your specified `interval` (e.g., every 30s)
+- The service is marked `healthy` only if the command succeeds (exit code 0)
+- After `retries` consecutive failures, it's marked `unhealthy`
+
 Now check the container health:
 ```bash
 docker ps --format "table {{.Names}}\t{{.Status}}"
 ```
 
-**Expected Output:**
+**Example Output:**
 ```console
 NAMES                     STATUS
 debug-docker_nginx_1      Up 5 minutes
@@ -232,7 +309,7 @@ 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:
+Sometimes you need additional tools that are not part of the containers you are using, for that create a temporary debug container with the needed tools in the same network:
 ```bash
 docker run -it --rm --network debug-docker_default alpine sh
 ```
@@ -243,7 +320,7 @@ Now from this container you can:
 nslookup nodejs-app
 ```
 
-**Expected Output:**
+**Example Output:**
 ```console
 Server:		127.0.0.11
 Address:	127.0.0.11:53
@@ -261,7 +338,7 @@ Address: 172.19.0.3
 wget -qO- http://nodejs-app:3000
 ```
 
-**Expected Output:**
+**Example Output:**
 ```console
 Hello from Node.js!
 ```
@@ -291,10 +368,10 @@ 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
+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