Sidecar container pattern in Docker - Step-by-Step Execution

Process Flow - Sidecar container pattern

Main Application Container

↓

Sidecar Container

↓

Shared Resources

↓

Communication & Support

The main container runs the app, the sidecar container runs helper tasks, both share resources and communicate to support the app.

Execution Sample

Docker

version: '3.8'
services:
  app:
    image: myapp:latest
    volumes:
      - shared-data:/data
  sidecar:
    image: log-collector:latest
    volumes:
      - shared-data:/data
volumes:
  shared-data: {}

This Docker Compose file runs an app container and a sidecar container sharing a volume for communication.

Process Table

Step	Action	Container	Resource Used	Result
1	Start main app container	app	shared-data volume	App container running, volume mounted
2	Start sidecar container	sidecar	shared-data volume	Sidecar running, volume mounted
3	App writes logs to /data/log.txt	app	shared-data volume	Log file created in shared volume
4	Sidecar reads /data/log.txt	sidecar	shared-data volume	Sidecar collects logs for processing
5	Sidecar sends logs to external system	sidecar	network	Logs forwarded outside
6	Both containers running	app & sidecar	shared-data volume	App supported by sidecar helper
7	Stop containers	app & sidecar	shared-data volume	Containers stopped, resources freed

💡 Containers stopped after sidecar completes support tasks

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
shared-data volume content	empty	log.txt created	log.txt read	logs sent out	log.txt remains
app container state	stopped	running	running	running	stopped
sidecar container state	stopped	running	running	running	stopped

Key Moments - 3 Insights

Why do both containers share the same volume?

They share the volume to exchange data like logs; the app writes logs there, and the sidecar reads them (see execution_table steps 3 and 4).

Does the sidecar container run the main app?

No, the sidecar runs alongside the main app to support it, not replace it (see execution_table steps 1 and 2).

What happens if the sidecar stops before the app?

The app keeps running but loses sidecar support like log collection; sidecar and app states are tracked separately (see variable_tracker).

Visual Quiz - 3 Questions

Look at the execution table, at which step does the sidecar container start running?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Container' and 'Result' columns in execution_table rows for when sidecar is running

According to variable_tracker, what is the state of the app container after step 3?

A stopped

B starting

C running

D paused

💡 Hint

Look at the 'app container state' row under 'After Step 3' in variable_tracker

If the shared-data volume was not shared, what would change in the execution table?

A Sidecar could not read logs at step 4

B App would not start at step 1

C Sidecar would send logs earlier at step 3

D Containers would stop immediately at step 2

💡 Hint

Check the role of shared-data volume in steps 3 and 4 in execution_table

Concept Snapshot

Sidecar container pattern runs a helper container alongside the main app.
They share resources like volumes to communicate.
Sidecar supports tasks like logging or monitoring.
Both containers run independently but cooperate.
Use shared volumes or network for data exchange.

Full Transcript

The sidecar container pattern runs two containers together: the main application and a helper sidecar. The main app container starts first and mounts a shared volume. Then the sidecar container starts and mounts the same volume. The app writes logs into the shared volume. The sidecar reads these logs and sends them outside. Both containers run independently but share data through the volume. When done, both containers stop and free resources. This pattern helps add features like logging without changing the main app container.

Practice

1. What is the main purpose of the sidecar container pattern in Docker?

easy

A. To run containers only during the build process

B. To replace the main application container with a smaller one

C. To create multiple copies of the main application for load balancing

D. To run helper containers alongside the main application to add features

Solution

1. Step 1: Understand the sidecar pattern concept

   The sidecar pattern involves running a helper container alongside the main app container to provide additional features without modifying the main app.

2. Step 2: Identify the purpose of sidecar containers

   Sidecar containers commonly handle logging, monitoring, or proxy tasks to support the main app.

3. Final Answer:

   To run helper containers alongside the main application to add features -> Option D

4. Quick Check:

   Sidecar pattern = helper containers alongside main app [OK]

Hint: Sidecar means helper container runs with main app [OK]

Common Mistakes:

• Thinking sidecar replaces the main app container
• Confusing sidecar with load balancing
• Believing sidecar runs only during build

2. Which of the following is the correct way to define a sidecar container in a Docker Compose file?

easy

A. Define multiple services in the same docker-compose.yml file, where one service is the main app and another is the sidecar

B. Use the depends_on key to run containers sequentially

C. Run two containers with the same image name in separate Compose files

D. Use docker run --sidecar command to start a sidecar container

Solution

1. Step 1: Review Docker Compose structure for multiple containers

   Docker Compose allows defining multiple services in one file, each representing a container.

2. Step 2: Understand sidecar container definition

   Sidecar containers are defined as separate services alongside the main app service in the same Compose file.

3. Final Answer:

   Define multiple services in the same docker-compose.yml file, where one service is the main app and another is the sidecar -> Option A

4. Quick Check:

   Sidecar in Compose = multiple services in one file [OK]

Hint: Sidecar containers are separate services in one Compose file [OK]

Common Mistakes:

• Using a non-existent --sidecar flag in docker run
• Running containers in separate Compose files without linking
• Misusing depends_on for sidecar definition

3. Given this Docker Compose snippet, what will happen when you run docker-compose up?

version: '3'
services:
  app:
    image: myapp:latest
  logger:
    image: logcollector:latest
    depends_on:
      - app

medium

A. Only app container starts; logger is ignored

B. logger starts before app container

C. Both app and logger containers start; logger waits for app to start first

D. Docker Compose throws an error due to missing networks

Solution

1. Step 1: Analyze the Compose file services and dependencies

   There are two services: app and logger. The logger service depends on app.

2. Step 2: Understand depends_on behavior

   depends_on ensures app starts before logger, but both start when running docker-compose up.

3. Final Answer:

   Both app and logger containers start; logger waits for app to start first -> Option C

4. Quick Check:

   depends_on means start order, both containers run [OK]

Hint: depends_on controls start order, not exclusion [OK]

Common Mistakes:

• Thinking depends_on skips starting dependent container
• Assuming logger starts before app
• Expecting error due to missing networks (default network is created)

4. You have a sidecar container for logging, but logs are not appearing as expected. Which of these is the most likely cause?

medium

A. Docker Compose file version is set to '2' instead of '3'

B. The sidecar container does not share the same volume or network with the main app

C. The main app container image is missing

D. The sidecar container is defined as a separate Dockerfile

Solution

1. Step 1: Identify how sidecar containers share data

   Sidecar containers usually share volumes or networks with the main app to access logs or data.

2. Step 2: Check common causes for missing logs

   If volumes or networks are not shared, the sidecar cannot access the main app's logs, causing missing logs.

3. Final Answer:

   The sidecar container does not share the same volume or network with the main app -> Option B

4. Quick Check:

   Shared volumes/networks needed for sidecar access [OK]

Hint: Sidecar needs shared volumes or network to access data [OK]

Common Mistakes:

• Blaming Compose file version for logging issues
• Confusing missing image with logging problem
• Thinking sidecar must be in same Dockerfile

5. You want to add a monitoring sidecar container to your existing app without changing the app code. Which approach best follows the sidecar pattern?

hard

A. Add a new service in your Docker Compose file that runs the monitoring container sharing the app's network and volumes

B. Modify the app container image to include monitoring tools inside it

C. Run the monitoring container separately on another host

D. Add monitoring commands inside the app's Dockerfile entrypoint script

Solution

1. Step 1: Understand sidecar pattern goal

   The sidecar pattern adds features by running helper containers alongside the main app without modifying the app itself.

2. Step 2: Evaluate options for adding monitoring

   Adding a new service in Compose that shares network and volumes with the app fits the sidecar pattern perfectly.

3. Step 3: Exclude options that modify app or separate hosts

   Modifying app image or running monitoring separately breaks sidecar principles.

4. Final Answer:

   Add a new service in your Docker Compose file that runs the monitoring container sharing the app's network and volumes -> Option A

5. Quick Check:

   Sidecar = helper container alongside app, no app code change [OK]

Hint: Sidecar means add helper container, no app code change [OK]

Common Mistakes:

• Modifying app image instead of adding sidecar
• Running monitoring on separate host breaks sidecar
• Embedding monitoring commands inside app entrypoint