Restart policies in Docker - Step-by-Step Execution

Process Flow - Restart policies

Start Container

↓

Container Stops?

No→Keep Running

Yes↓

Check Restart Policy

↓

no

→Do Not Restart

↓

always

→Restart Container

↓

on-failure

→Restart if Exit Code != 0

↓

unless-stopped

→Restart unless Manually Stopped

↓

Restart Container

↓

Container Stops?

This flow shows how Docker checks if a container stops and then applies the chosen restart policy to decide whether to restart it.

Execution Sample

Docker

docker run --restart=on-failure:2 myapp
# Container runs, stops, and Docker restarts it up to 2 times if it fails

Runs a container with a restart policy that restarts it up to 2 times on failure.

Process Table

Step	Container State	Restart Policy	Action	Restart Count	Result
1	Running	on-failure:2	No action	0	Container runs normally
2	Stopped with exit code 1	on-failure:2	Restart container	1	Container restarts
3	Stopped with exit code 1	on-failure:2	Restart container	2	Container restarts again
4	Stopped with exit code 1	on-failure:2	No restart (limit reached)	2	Container stays stopped

💡 Restart limit 2 reached, container stopped with failure exit code, no more restarts

Status Tracker

Variable	Start	After 1	After 2	After 3	Final
Restart Count	0	1	2	2	2
Container State	Running	Stopped (exit 1)	Stopped (exit 1)	Stopped (exit 1)	Stopped

Key Moments - 3 Insights

Why does the container stop restarting after the third failure?

Because the restart policy 'on-failure:2' limits restarts to 2 times, as shown in execution_table step 4 where Restart Count is 2 and no restart happens.

What happens if the container exits with code 0 under 'on-failure' policy?

The container will not restart because 'on-failure' only restarts on non-zero exit codes, so no restart action is taken.

How does 'always' restart policy differ from 'on-failure'?

'always' restarts the container regardless of exit code or failure count, unlike 'on-failure' which restarts only on failure and up to a limit.

Visual Quiz - 3 Questions

Look at the execution table, what is the Restart Count at step 3?

A 1

B 2

C 3

D 0

💡 Hint

Check the Restart Count column in execution_table row for step 3

At which step does the container stop restarting due to reaching the restart limit?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the Action and Result columns in execution_table for when no restart occurs

If the restart policy was changed to 'always', what would happen at step 4?

A Container would not restart

B Container would restart again

C Container would stop permanently

D Container would restart only once more

💡 Hint

Recall that 'always' restarts regardless of exit code or count

Concept Snapshot

Docker Restart Policies:
- no: Do not restart container
- always: Restart container no matter what
- on-failure[:max]: Restart only on failure, up to max times
- unless-stopped: Restart unless container stopped manually
Use with docker run --restart option
Controls container availability after stops

Full Transcript

This visual execution shows how Docker restart policies work. When a container stops, Docker checks the restart policy. For 'on-failure:2', Docker restarts the container up to 2 times if it exits with a failure code. The execution table tracks container state, restart count, and actions taken. Key moments clarify why restarts stop after the limit and how different policies behave. The quiz tests understanding of restart counts and policy effects. The snapshot summarizes the main restart policies and their behavior.