Document ways to make services wait for dependencies

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docs/faq.md

@@ -15,6 +15,76 @@ weight=90
 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
@@ -90,30 +160,6 @@ specify the filename to use, for example:
 docker-compose -f docker-compose.json up
 ```
 
-## How do I get Compose to wait for my database to be ready before starting my application?
-
-Unfortunately, Compose won't do that for you but for a good reason.
-
-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.  The application needs to be
-resilient to these types of failures.
-
-To handle this, the application would 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.
-
-To wait for the application to be in a good state, you can implement a
-healthcheck. A healthcheck makes a request to the application and checks
-the response for a success status code. If it is not successful it waits
-for a short period of time, and tries again. After some timeout value, the check
-stops trying and report a failure.
-
-If you need to run tests against your application, you can start by running a
-healthcheck. Once the healthcheck gets a successful response, you can start
-running your tests.
-
-
 ## 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