Rolling updates in Docker - Step-by-Step Execution

Process Flow - Rolling updates

Start with current version running

↓

Deploy new version to some containers

↓

Check new version health

↓

Shift traffic

↓

Update remaining containers

↓

All containers running new version

↓

End

Rolling updates replace old containers with new ones gradually, checking health before continuing.

Execution Sample

Docker

docker service update --image myapp:v2 myapp_service
# Docker replaces containers one by one
# Checks health before moving on

This command updates a Docker service to a new image version using rolling updates.

Process Table

Step	Action	Containers Updated	Health Check	Result	Next Step
1	Start rolling update	0/3	N/A	Old version running	Update first container
2	Update container 1	1/3	Pass	Container 1 running v2	Update container 2
3	Update container 2	2/3	Pass	Container 2 running v2	Update container 3
4	Update container 3	3/3	Pass	All containers running v2	Complete update
5	Complete	3/3	N/A	Rolling update successful	End

💡 All containers updated and passed health checks, rolling update finished successfully.

Status Tracker

Variable	Start	After 1	After 2	After 3	Final
Containers Updated	0	1	2	3	3
Health Status	N/A	Pass	Pass	Pass	Pass
Version Running	v1	v2	v2	v2	v2

Key Moments - 2 Insights

Why does Docker update containers one by one instead of all at once?

Updating one container at a time ensures the service stays available and lets Docker check health before continuing, as shown in steps 2-4 of the execution table.

What happens if a container fails the health check during update?

If a container fails health check, Docker stops the update and can rollback or pause, preventing broken service. This is implied in the flow where 'Fail' leads to rollback or fix.

Visual Quiz - 3 Questions

Look at the execution table, how many containers are updated after step 3?

A 1

B 3

C 2

D 0

💡 Hint

Check the 'Containers Updated' column at step 3 in the execution table.

At which step does the rolling update complete successfully?

A Step 4

B Step 5

C Step 3

D Step 2

💡 Hint

Look for the row where 'Result' says 'Rolling update successful' in the execution table.

If the health check fails at step 2, what would happen next?

A Rollback or fix the issue

B Continue updating next containers

C Ignore and finish update

D Update all containers at once

💡 Hint

Refer to the concept flow where a failed health check leads to rollback or fix.

Concept Snapshot

Rolling updates replace old containers gradually.
Docker updates one container at a time.
Health checks after each update ensure stability.
If health fails, update pauses or rolls back.
This keeps service available during updates.

Full Transcript

Rolling updates in Docker mean updating containers one by one to a new version. First, the current version runs. Then Docker updates one container and checks if it is healthy. If it passes, Docker moves to the next container. This continues until all containers run the new version. If any container fails health check, Docker stops and can rollback. This process keeps the service running without downtime.