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Update compose networking for container-container communication #21481

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Update compose networking for container-container communication #21481

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8 changes: 7 additions & 1 deletion content/manuals/compose/how-tos/networking.md
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Expand Up @@ -59,6 +59,12 @@ Within the `web` container, your connection string to `db` would look like
`postgres://db:5432`, and from the host machine, the connection string would
look like `postgres://{DOCKER_IP}:8001` for example `postgres://localhost:8001` if your container is running locally.

## Container to container communication

Those familiar with using `http://localhost:3000` to reference a local development URL, use `http://servicename:3000` instead when using docker compose.

Example: In local development you have a frontend node app running at `http://localhost:3000` which references a backend api running at `http://localhost:8080/api`. In Docker networking, you would replace the node app api reference to the backend api with `http://apiservice:8080/api`. Similarly, the frontend app would be referenced by service name `http://myapp:3000`
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We should clarify that this DNS resolution only works between compose services. E.g., if you would like to view your node frontend app in your browser, you would still need to publish the port from the container to the host and access it via loopback.

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This is a great idea, wouldnt want to confuse learners further by having people type the wrong port or something likehttp://servicename:3000. I think the sentence you wrote should be good, because the content just above in /compose/how-tos/networking/ describes host ports and container ports. Should I add a small example if it helps?

We should clarify that this DNS resolution only works between compose services. For example, if you would like to view your node frontend app in your browser running at container port 80 and host port 3000, you would still need to publish the port from the container to the host then you can view the node app at http://localhost:3000

Also I haven't heard of loopback before, what should I look into and should I mention loopback as well?


## Update containers on the network

If you make a configuration change to a service and run `docker compose up` to update it, the old container is removed and the new one joins the network under a different IP address but the same name. Running containers can look up that name and connect to the new address, but the old address stops working.
Expand Down Expand Up @@ -178,7 +184,7 @@ networks:

Instead of attempting to create a network called `[projectname]_default`, Compose looks for a network called `my-pre-existing-network` and connects your app's containers to it.

## Further reference information
## Further reference information

For full details of the network configuration options available, see the following references:

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2 changes: 2 additions & 0 deletions content/manuals/engine/network/drivers/bridge.md
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Expand Up @@ -37,6 +37,8 @@ network.**

- **User-defined bridges provide automatic DNS resolution between containers**.

> **See also**: Learn how to communicate between containers by name (containername:port instead of localhost:port) in: [Container to container communication in Docker Compose](/manuals/compose/how-tos/networking.md#container-to-container-communication).

Containers on the default bridge network can only access each other by IP
addresses, unless you use the [`--link` option](../links.md), which is
considered legacy. On a user-defined bridge network, containers can resolve
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