How to pass container config

slides/intro-fullday.yml

@@ -43,6 +43,7 @@ chapters:
   - intro/Compose_For_Dev_Stacks.md
   - intro/Docker_Machine.md
   - intro/Advanced_Dockerfiles.md
+  - intro/Application_Configuration.md
 - - intro/Namespaces_Cgroups.md
   - intro/Copy_On_Write.md
   #- intro/Containers_From_Scratch.md

slides/intro/Application_Configuration.md

@@ -0,0 +1,201 @@
+# Application Configuration
+
+There are many ways to provide configuration to containerized applications.
+
+There is no "best way" — it depends on factors like:
+
+* configuration size,
+
+* mandatory and optional parameters,
+
+* scope of configuration (per container, per app, per customer, per site, etc),
+
+* frequency of changes in the configuration.
+
+---
+
+## Command-line parameters
+
+```bash
+docker run jpetazzo/hamba 80 www1:80 www2:80
+```
+
+* Configuration is provided through command-line parameters.
+
+* In the above example, the `ENTRYPOINT` is a script that will:
+
+  - parse the parameters,
+
+  - generate a configuration file,
+
+  - start the actual service.
+
+---
+
+## Command-line parameters pros and cons
+
+* Appropriate for mandatory parameters (without which the service cannot start).
+
+* Convenient for "toolbelt" services instanciated many times.
+
+  (Because there is no extra step: just run it!)
+
+* Not great for dynamic configurations or bigger configurations.
+
+  (These things are still possible, but more cumbersome.)
+
+---
+
+## Environment variables
+
+```bash
+docker run -e ELASTICSEARCH_URL=http://es42:9201/ kibana
+```
+
+* Configuration is provided through environment variables.
+
+* The environment variable can be used straight by the program,
+  <br/>or by a script generating a configuration file.
+
+---
+
+## Environment variables pros and cons
+
+* Appropriate for optional parameters (since the image can provide default values).
+
+* Also convenient for services instanciated many times.
+
+  (It's as easy as command-line parameters.)
+
+* Great for services with lots of parameters, but you only want to specify a few.
+
+  (And use default values for everything else.)
+
+* Ability to introspect possible parameters and their default values.
+
+* Not great for dynamic configurations.
+
+---
+
+## Baked-in configuration
+
+```
+FROM prometheus
+COPY prometheus.conf /etc
+```
+
+* The configuration is added to the image.
+
+* The image may have a default configuration; the new configuration can:
+
+  - replace the default configuration,
+
+  - extend it (if the code can read multiple configuration files).
+
+---
+
+## Baked-in configuration pros and cons
+
+* Allows arbitrary customization and complex configuration files.
+
+* Requires to write a configuration file. (Obviously!)
+
+* Requires to build an image to start the service.
+
+* Requires to rebuild the image to reconfigure the service.
+
+* Requires to rebuild the image to upgrade the service.
+
+* Configured images can be stored in registries.
+
+  (Which is great, but requires a registry.)
+
+---
+
+## Configuration volume
+
+```bash
+docker run -v appconfig:/etc/appconfig myapp
+```
+
+* The configuration is stored in a volume.
+
+* The volume is attached to the container.
+
+* The image may have a default configuration.
+
+  (But this results in a less "obvious" setup, that needs more documentation.)
+
+---
+
+## Configuration volume pros and cons
+
+* Allows arbitrary customization and complex configuration files.
+
+* Requires to create a volume for each different configuration.
+
+* Services with identical configurations can use the same volume.
+
+* Doesn't require to build / rebuild an image when upgrading / reconfiguring.
+
+* Configuration can be generated or edited through another container.
+
+---
+
+## Dynamic configuration volume
+
+* This is a powerful pattern for dynamic, complex configurations.
+
+* The configuration is stored in a volume.
+
+* The configuration is generated / updated by a special container.
+
+* The application container detects when the configuration is changed.
+
+  (And automatically reloads the configuration when necessary.)
+
+* The configuration can be shared between multiple services if needed.
+
+---
+
+## Dynamic configuration volume example
+
+In a first terminal, start a load balancer with an initial configuration:
+
+```bash
+$ docker run --name loadbalancer jpetazzo/hamba \
+  80 goo.gl:80
+```
+
+In another terminal, reconfigure that load balancer:
+
+```bash
+$ docker run --rm --volumes-from loadbalancer jpetazzo/hamba reconfigure \
+  80 google.com:80
+```
+
+The configuration could also be updated through e.g. a REST API.
+
+(The REST API being itself served from another container.)
+
+---
+
+## Keeping secrets
+
+.warning[Ideally, you should not put secrets (passwords, tokens...) in:]
+
+* command-line or environment variables (anyone with Docker API access can get them),
+
+* images, especially stored in a registry.
+
+Secrets management is better handled with an orchestrator (like Swarm or Kubernetes).
+
+Orchestrators will allow to pass secrets in a "one-way" manner.
+
+Managing secrets securely without an orchestrator can be contrived.
+
+E.g.:
+
+- read the secret on stdin when the service starts,
+
+- pass the secret using an API endpoint.