Container to container communication in Docker - Step-by-Step Execution

Process Flow - Container to container communication

Start Containers

↓

Containers on same network?

No→Create Network

Yes↓

Container A sends request

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Network routes request

↓

Container B receives request

↓

Container B sends response

↓

Container A receives response

↓

Communication successful

Containers must be on the same Docker network to communicate. One container sends a request, the network routes it to the other container, which responds back.

Execution Sample

Docker

docker network create mynet

docker run -d --name containerA --network mynet alpine sleep 1000

docker run -d --name containerB --network mynet alpine sleep 1000

docker exec containerA ping -c 2 containerB

Create a network, start two containers on it, then ping from containerA to containerB to test communication.

Process Table

Step	Action	Command/Condition	Result/Output
1	Create network	docker network create mynet	Network 'mynet' created
2	Start containerA on mynet	docker run -d --name containerA --network mynet alpine sleep 1000	containerA running
3	Start containerB on mynet	docker run -d --name containerB --network mynet alpine sleep 1000	containerB running
4	Ping containerB from containerA	docker exec containerA ping -c 2 containerB	2 packets transmitted, 2 received, 0% packet loss
5	Communication success	Ping replies received	Container to container communication works

💡 Ping test completes successfully, confirming communication between containers on the same network

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
Network 'mynet'	None	Created	Created	Created	Created	Created
containerA status	Not running	Not running	Running	Running	Running	Running
containerB status	Not running	Not running	Not running	Running	Running	Running
Ping result	No ping	No ping	No ping	No ping	2 packets received	Success

Key Moments - 3 Insights

Why do containers need to be on the same network to communicate?

Containers communicate over Docker networks. If they are on different networks, Docker isolates them, so the ping in step 4 would fail. See execution_table step 4 where ping succeeds only because both containers share 'mynet'.

What happens if you try to ping a container by name but it is not on the same network?

The ping command will fail because Docker's internal DNS cannot resolve container names across different networks. This is why in step 4, ping works only when both containers are on 'mynet'.

Why do we use 'docker exec' to run ping inside containerA?

'docker exec' runs commands inside a running container. Here, it lets us test communication from containerA's perspective by running ping inside it, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the status of containerB after step 3?

A Paused

B Not running

C Running

D Exited

💡 Hint

Check the 'containerB status' row in variable_tracker after step 3

At which step does the network 'mynet' get created?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Command' columns in execution_table for network creation

If containerA was not connected to 'mynet', what would happen at step 4?

A Ping would fail because containerB is unreachable

B Ping would succeed with 0% packet loss

C Ping would succeed but with packet loss

D Ping would not run because containerA is stopped

💡 Hint

Refer to key_moments about network isolation and ping failure

Concept Snapshot

Docker containers communicate over networks.
Containers must be on the same Docker network to reach each other by name.
Use 'docker network create' to make a network.
Start containers with '--network <name>' to connect them.
Use 'docker exec <container> ping <other_container>' to test communication.
If containers are on different networks, communication fails.

Full Transcript

To enable communication between Docker containers, they must be connected to the same Docker network. First, create a network using 'docker network create'. Then start containers with the '--network' option to attach them to this network. Inside one container, you can run commands like ping to the other container by its name. This works because Docker provides internal DNS resolution within the same network. If containers are on different networks, they cannot see each other and communication fails. Using 'docker exec' lets you run commands inside a running container to test connectivity. This step-by-step process ensures containers can talk to each other securely and easily.