# Frequently asked questions

If you don’t see your question here, feel free to drop by `#docker-compose` on
freenode IRC and ask the community.

## How do I control the order of service startup? I need my database to be ready before my application starts.

You can control the order of service startup with the
[depends_on](compose-file.md#depends-on) option. Compose always starts
containers in dependency order, where dependencies are determined by
`depends_on`, `links`, `volumes_from` and `network_mode: "service:..."`.

However, Compose will not wait until a container is "ready" (whatever that means
for your particular application) - only until it's running. There's a good
reason for this.

The problem of waiting for a database to be ready is really just a subset of a
much larger problem of distributed systems. In production, your database could
become unavailable or move hosts at any time. Your application needs to be
resilient to these types of failures.

To handle this, your application should attempt to re-establish a connection to
the database after a failure. If the application retries the connection,
it should eventually be able to connect to the database.

The best solution is to perform this check in your application code, both at
startup and whenever a connection is lost for any reason. However, if you don't
need this level of resilience, you can work around the problem with a wrapper
script:

- Use a tool such as [wait-for-it](https://github.com/vishnubob/wait-for-it)
or [dockerize](https://github.com/jwilder/dockerize). These are small
wrapper scripts which you can include in your application's image and will
poll a given host and port until it's accepting TCP connections.

Supposing your application's image has a `CMD` set in its Dockerfile, you
can wrap it by setting the entrypoint in `docker-compose.yml`:

version: "2"
services:
web:
build: .
ports:
- "80:8000"
depends_on:
- "db"
entrypoint: ./wait-for-it.sh db:5432
db:
image: postgres

- Write your own wrapper script to perform a more application-specific health
check. For example, you might want to wait until Postgres is definitely
ready to accept commands:

#!/bin/bash

set -e

host="$1"
shift
cmd="$@"

until psql -h "$host" -U "postgres" -c '\l'; do
>&2 echo "Postgres is unavailable - sleeping"
sleep 1
done

>&2 echo "Postgres is up - executing command"
exec $cmd

You can use this as a wrapper script as in the previous example, by setting
`entrypoint: ./wait-for-postgres.sh db`.

## Why do my services take 10 seconds to recreate or stop?

Compose stop attempts to stop a container by sending a `SIGTERM`. It then waits
for a [default timeout of 10 seconds](./reference/stop.md). After the timeout,
a `SIGKILL` is sent to the container to forcefully kill it. If you
are waiting for this timeout, it means that your containers aren't shutting down
when they receive the `SIGTERM` signal.

There has already been a lot written about this problem of
[processes handling signals](https://medium.com/@gchudnov/trapping-signals-in-docker-containers-7a57fdda7d86)
in containers.

To fix this problem, try the following:

* Make sure you're using the JSON form of `CMD` and `ENTRYPOINT`
in your Dockerfile.

For example use `["program", "arg1", "arg2"]` not `"program arg1 arg2"`.
Using the string form causes Docker to run your process using `bash` which
doesn't handle signals properly. Compose always uses the JSON form, so don't
worry if you override the command or entrypoint in your Compose file.

* If you are able, modify the application that you're running to
add an explicit signal handler for `SIGTERM`.

* Set the `stop_signal` to a signal which the application knows how to handle:

web:
build: .
stop_signal: SIGINT

* If you can't modify the application, wrap the application in a lightweight init
system (like [s6](http://skarnet.org/software/s6/)) or a signal proxy (like
[dumb-init](https://github.com/Yelp/dumb-init) or
[tini](https://github.com/krallin/tini)). Either of these wrappers take care of
handling `SIGTERM` properly.

## How do I run multiple copies of a Compose file on the same host?

Compose uses the project name to create unique identifiers for all of a
project's containers and other resources. To run multiple copies of a project,
set a custom project name using the [`-p` command line
option](./reference/overview.md) or the [`COMPOSE_PROJECT_NAME`
environment variable](./reference/envvars.md#compose-project-name).

## What's the difference between `up`, `run`, and `start`?

Typically, you want `docker-compose up`. Use `up` to start or restart all the
services defined in a `docker-compose.yml`. In the default "attached"
mode, you'll see all the logs from all the containers. In "detached" mode (`-d`),
Compose exits after starting the containers, but the containers continue to run
in the background.

The `docker-compose run` command is for running "one-off" or "adhoc" tasks. It
requires the service name you want to run and only starts containers for services
that the running service depends on. Use `run` to run tests or perform
an administrative task such as removing or adding data to a data volume
container. The `run` command acts like `docker run -ti` in that it opens an
interactive terminal to the container and returns an exit status matching the
exit status of the process in the container.

The `docker-compose start` command is useful only to restart containers
that were previously created, but were stopped. It never creates new
containers.

## Can I use json instead of yaml for my Compose file?

Yes. [Yaml is a superset of json](http://stackoverflow.com/a/1729545/444646) so
any JSON file should be valid Yaml. To use a JSON file with Compose,
specify the filename to use, for example:

```bash
docker-compose -f docker-compose.json up
```

## Should I include my code with `COPY`/`ADD` or a volume?

You can add your code to the image using `COPY` or `ADD` directive in a
`Dockerfile`. This is useful if you need to relocate your code along with the
Docker image, for example when you're sending code to another environment
(production, CI, etc).

You should use a `volume` if you want to make changes to your code and see them
reflected immediately, for example when you're developing code and your server
supports hot code reloading or live-reload.

There may be cases where you'll want to use both. You can have the image
include the code using a `COPY`, and use a `volume` in your Compose file to
include the code from the host during development. The volume overrides
the directory contents of the image.

## Where can I find example compose files?

There are [many examples of Compose files on
github](https://github.com/search?q=in%3Apath+docker-compose.yml+extension%3Ayml&type=Code).

## Compose documentation

- [Installing Compose](install.md)
- [Get started with Django](django.md)
- [Get started with Rails](rails.md)
- [Get started with WordPress](wordpress.md)
- [Command line reference](./reference/index.md)
- [Compose file reference](compose-file.md)