Docker Compose

Question 1

Docker Compose Basics

Answer 1

• Markdown tool to specify Docker configurations as code
• Procedural
• Declarative

Question 2

Production System Considerations

Answer 2

• Multiple Systems
• Microservices
• Third-party dependencies

Question 3

Procedural

Answer 3

• Series of ordered steps
• Based on assumptions about the previous step
• Easy to introduce errors

Question 4

Declarative

Answer 4

• Specify end results
• System will determine which steps to execute
• Produces the same result each time

Question 5

Main Benefits of Docker Compose

Answer 5

• Version control
• Self-documenting
• Easy management

Question 6

Designed For

Answer 6

• Local development
• Staging server
• Continuous integration testing environment

Question 7

Not Designed For

Answer 7

• Distributed systems
• Complex production environments
• No autoscaling
• Use orchestration tools like Docker Swarm or Kubernetes in production

Question 8

Docker Compose Configuration

Answer 8

• Create a file “docker-compose.yaml”
• Version: specifies which version of Docker Compose to use
• Services: specifies which containers are required (specify a path to the corresponding Docker build file or image)

Question 9

Start all services defined in the Docker Compose file

Answer 9

• “docker-compose up”
• Pass in a service name to any command to only apply it to that service

Question 10

Stop all containers

Answer 10

• “docker-compose down”
• Deletes all containers and images and removes all artifacts

Question 11

Save battery or free memory

Answer 11

• “docker-compose stop”

Question 12

Stop and restart all containers

Answer 12

• “docker-compose restart”

Question 13

Environment Variables

Answer 13

• Accessible inside a running Docker container
• Specify runtime environment
• Use env files if necessary (pass in env file path)
• Including an env variable name without a value will pass in the value from the host server

Question 14

Build Arguments

Answer 14

• Accessible only at build times
• Build tool versions
• Cloud platform configuration

Question 15

Target Volume

Answer 15

• File directory path inside a container where the volume data lives

Question 16

Source Volume

Answer 16

• File directory path on the host machine outside a container where the volume data lives

Question 17

Volume Access Modes

Answer 17

• rw: read-write by default
• ro: read-only

Question 18

Mounting Volume Syntax

Answer 18

• <source></source>:<TARGET>:<MODE></MODE></TARGET>

Question 19

Named Volumes

Answer 19

• Running “docker-compose up” or “docker-compose restart” will automatically copy old volume data to the new container
• Running “docker-compose down –volumes” will automatically delete named volumes
• Without named volumes you can run out of memory

Question 20

Exposing Ports

Answer 20

• Map the ports for all containers in the Docker Compose file

Question 21

Enforcing Container Start-Up Order

Answer 21

• Add a “depends_on” object under a service definition in the Docker Compose file
• Only guarantees that dependent containers have been started, not that they are healthy and running

Question 22

Advantages of a Single Compose File

Answer 22

• Shared containers
• Easier integration testing
• Conceptually one system

Question 23

Named Subsets of Services

Answer 23

• List one or more profile names under the profile keyword for a service
• A container with no profiles specified is included by default
• Docker Compose commands will only apply to a service if its profile is explicitly enabled
• Add “–profile <NAME>" to enable a non-default profile</NAME>

Question 24

Multiple Compose Files

Answer 24

• Distinct desired behaviors that do not coincide
• Different environments
• By default, Docker Compose will read docker-compose.yaml and docker-compose.override.yaml
• Docker Compose will merge the two files

Question 25

Merge Rules for Compose Files

Answer 25

- Array fields: original + override - Single-value fields: preference to override - Override can be a partial or incomplete configuration

Question 26

Run Docker Compose Override

Answer 26

- "docker-compose -f -f "

Question 27

Example Environment Variables

Answer 27

- Image tags - Software versions - Ports

Question 28

Accessing Environment Variables

Answer 28

- Use the syntax "${NAME}"

Question 29

Environment Variable Defaults

Answer 29

- Empty string (automatic) - Inline in docker-compose configuration - In .env file - Throw error if missing (no default) - The host env variable in the shell will always supersede the default

Question 30

Pass in a .env file

Answer 30

- "--env-file "