Ensemble user tutorial p2

Welcome to the second part of this Ensemble user tutorial. In part 1 we bootstrapped an Ensemble environment, deployed a sample wordpress service and a MySQL service. Related the two services together and got ourselves a working wordpress installation. In this second part, let's check out viewing the debug-log output to understand the asynchronous nature of hook execution. You'll see how easy it is to "scale-up" a service deployment

Tracing hook execution 
An Ensemble user should never have to trace the execution order of hooks, however if you are the kind of person who enjoys looking under the hood, this section is for you. Understanding hook order execution, the parallel nature of hook execution across instances, and how relation-set in a hook can trigger the execution of another hook is quite interesting and provides insight into Ensemble internals

Here are a few important messages from the debug-log of this Ensemble run. The date field has been deliberately left in this log, in order to understand the parallel nature of hook execution.

Things to consider while reading the log include:

• The time the log message was generated
• Which service unit is causing the log message (for example mysql/0)
• The message logging level. In this run DEBUG messages are generated by the Ensemble core engine, while WARNING messages are generated by calling ensemble-log from inside formulas (which you can read in the examples folder)

Let’s view select debug-log messages which can help understand the execution order:

14:29:43,625 unit:mysql/0: hook.scheduler DEBUG: executing hook for wordpress/0:joined
14:29:43,626 unit:mysql/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-joined
14:29:43,660 unit:wordpress/0: hook.scheduler DEBUG: executing hook for mysql/0:joined
14:29:43,660 unit:wordpress/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-joined
14:29:43,661 unit:wordpress/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-changed
14:29:43,789 unit:mysql/0: unit.hook.api WARNING: Creating new database and corresponding security settings
14:29:43,813 unit:wordpress/0: unit.hook.api WARNING: Retrieved hostname: ec2-184-72-156-54.compute-1.amazonaws.com
14:29:43,976 unit:mysql/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-changed
14:29:43,997 unit:wordpress/0: hook.scheduler DEBUG: executing hook for mysql/0:modified
14:29:43,997 unit:wordpress/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-changed
14:29:44,143 unit:wordpress/0: unit.hook.api WARNING: Retrieved hostname: ec2-184-72-156-54.compute-1.amazonaws.com
14:29:44,849 unit:wordpress/0: unit.hook.api WARNING: Creating appropriate upload paths and directories
14:29:44,992 unit:wordpress/0: unit.hook.api WARNING: Writing wordpress config file /etc/wordpress/config-ec2-184-72-156-54.compute-1.amazonaws.com.php
14:29:45,130 unit:wordpress/0: unit.hook.api WARNING: Writing apache config file /etc/apache2/sites-available/ec2-184-72-156-54.compute-1.amazonaws.com
14:29:45,301 unit:wordpress/0: unit.hook.api WARNING: Enabling apache modules: rewrite, vhost_alias
14:29:45,512 unit:wordpress/0: unit.hook.api WARNING: Enabling apache site: ec2-184-72-156-54.compute-1.amazonaws.com
14:29:45,688 unit:wordpress/0: unit.hook.api WARNING: Restarting apache2 service

Scaling the ensemble 
Assuming your blog got really popular, is having high load and you decided to scale it up (it’s a cloud deployment after all). Ensemble makes this magically easy. All what is needed is:

$ bin/ensemble add-unit wordpress
INFO Connecting to environment.
INFO Unit 'wordpress/1' added to service 'wordpress'
INFO 'add_unit' command finished successfully
$ bin/ensemble status
machines:
  0: {dns-name: ec2-50-16-61-111.compute-1.amazonaws.com, instance-id: i-2a702745}
  1: {dns-name: ec2-50-16-117-185.compute-1.amazonaws.com, instance-id: i-227e294d}
  2: {dns-name: ec2-184-72-156-54.compute-1.amazonaws.com, instance-id: i-9c7e29f3}
  3: {dns-name: ec2-50-16-156-106.compute-1.amazonaws.com, instance-id: i-ba6532d5}
services:
  mysql:
    formula: local:mysql-11
    relations: {db: wordpress}
    units:
      mysql/0:
        machine: 1
        relations:
          db: {state: up}
        state: started
  wordpress:
    formula: local:wordpress-29
    relations: {db: mysql}
    units:
      wordpress/0:
        machine: 2
        relations:
          db: {state: up}
        state: started
      wordpress/1:
        machine: 3
        relations:
          db: {state: up}
        state: started

The add-unit command starts a new wordpress instance (wordpress/1), which then joins the relation with the already existing mysql/0 instance. mysql/0 notices the database required has already been created and thus decides all needed configuration has already been done. On the other hand wordpress/1 reads service settings from mysql/0 and starts configuring itself and joining the ensemble. Let’s review a short version of debug-log for adding wordpress/1:

14:36:19,755 unit:mysql/0: hook.scheduler DEBUG: executing hook for wordpress/1:joined
14:36:19,755 unit:mysql/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-joined
14:36:19,810 unit:wordpress/1: hook.scheduler DEBUG: executing hook for mysql/0:joined
14:36:19,811 unit:wordpress/1: unit.relation.lifecycle DEBUG: Executing hook db-relation-joined
14:36:19,811 unit:wordpress/1: unit.relation.lifecycle DEBUG: Executing hook db-relation-changed
14:36:19,918 unit:mysql/0: unit.hook.api WARNING: Database already exists, exiting
14:36:19,938 unit:mysql/0: unit.relation.lifecycle DEBUG: Executing hook db-relation-changed
14:36:19,990 unit:wordpress/1: unit.hook.api WARNING: Retrieved hostname: ec2-50-16-156-106.compute-1.amazonaws.com
14:36:20,757 unit:wordpress/1: unit.hook.api WARNING: Creating appropriate upload paths and directories
14:36:20,916 unit:wordpress/1: unit.hook.api WARNING: Writing wordpress config file /etc/wordpress/config-ec2-50-16-156-106.compute-1.amazonaws.com.php
14:36:21,088 unit:wordpress/1: unit.hook.api WARNING: Writing apache config file /etc/apache2/sites-available/ec2-50-16-156-106.compute-1.amazonaws.com
14:36:21,236 unit:wordpress/1: unit.hook.api WARNING: Enabling apache modules: rewrite, vhost_alias
14:36:21,476 unit:wordpress/1: unit.hook.api WARNING: Enabling apache site: ec2-50-16-156-106.compute-1.amazonaws.com
14:36:21,682 unit:wordpress/1: unit.hook.api WARNING: Restarting apache2 service

Destroying the environment 
Once you are done with an Ensemble deployment, you need to terminate all running instances in order to stop paying for them. The shutdown command helps terminate all running instances:

$ bin/ensemble shutdown

Ensemble will ask for user confirmation of shutdown before proceeding as this will destroy service data in the environment as well.

Hope you enjoyed the article! If you find Ensemble interesting, please do visit us in #ubuntu-ensemble. We're a friendly bunch :) Do you want to write Ensemble formulas? Want to get the satisfaction of Ensemblizing your favorite application, grab me (kim0) on irc, and I will help you do it