Merge pull request #3049 from aanand/clarify-depends-on

Clarify depends_on logic

docs/compose-file.md

@@ -203,6 +203,11 @@ Simple example:
       db:
         image: postgres
 
+> **Note:** `depends_on` will not wait for `db` and `redis` to be "ready" before
+> starting `web` - only until they have been started. If you need to wait
+> for a service to be ready, see [Controlling startup order](startup-order.md)
+> for more on this problem and strategies for solving it.
+
 ### dns
 
 Custom DNS servers. Can be a single value or a list.

docs/faq.md

@@ -16,73 +16,9 @@ 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`.
+## Can I control service startup order?
+
+Yes - see [Controlling startup order](startup-order.md).
 
 
 ## Why do my services take 10 seconds to recreate or stop?

docs/startup-order.md

@@ -0,0 +1,88 @@
+<!--[metadata]>
++++
+title = "Controlling startup order"
+description = "How to control service startup order in Docker Compose"
+keywords = "documentation, docs,  docker, compose, startup, order"
+[menu.main]
+parent="workw_compose"
+weight=90
++++
+<![end-metadata]-->
+
+# Controlling startup order in Compose
+
+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 (for example) 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`.
+
+
+## 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)