One of the killer features of computing in the cloud is that it (should) seamlessly allow you to scale up or down your services to meet your needs and whims. Juju not only makes it simple to deploy services, but crucially makes it easy to manage them too. It won't anticipate you getting slashdotted or on the front page of hacker news (yet), but it does mean that when you do you can reliably scale your services to meet the demand.
The general usage to scale a service up is via the
juju add-unit [options] <service-name>
The command options are:
#juju environment to operate in -e, --environment <environment_name> # number of service units to add -n, --num-units [integer] # the machine or container to deploy the unit in, bypasses constraints --to <machine>
Usually you just can't add more units to a service and have it magically scale - you need to use a load balancer. In this case you can just deploy a proxy in front of your units; let's deploy a load balanced mediawiki:
juju deploy haproxy juju deploy mediawiki juju deploy mysql juju add-relation mediawiki:db mysql juju add-relation mediawiki haproxy juju expose haproxy
The haproxy charm configures and installs an HAProxy (http://haproxy.1wt.eu/) service, the widely used TCP/HTTP load balancer. When you add a relation between the MediaWiki instance and HAProxy, it will be configured to load balance requests to that service. Note that this means the web traffic should be directed to the HAProxy instance. Running:
juju status haproxy
will return the public IP for the load balancer. This is the IP you want to point your DNS to.
Now that you are behind a load balancer, you can grow the MediaWiki instances behind the proxy as you see fit, let's add 5 more:
juju add-unit -n5 mediawiki
You don't need to worry about manually adding your units to the load balancer. You've made the relationship at the service level, so the new units know exactly how to relate. Juju is also smart enough to ensure that the new units are installed and configured before adding them to the load balancer, ensuring minimal user disruption of the service.
Some charms have native scaling built in. For instance, the WordPress charm has built in load balancing. In this case, scaling up services is really as simple as requesting more instances. Note that this feature is charm specific, not all charms can scale this way. Consider the following setup for a WordPress:
juju deploy mysql juju deploy wordpress juju add-relation mysql wordpress juju expose wordpress
When you notice the WordPress instance is struggling under the load, you can simply scale up the service:
juju add-unit wordpress
This will cause a new instance to be run and configured to work alongside the currently running one. Behind the scenes, Juju is adding an instance to the environment (also called a 'machine') and provisioning the specified service onto that instance/machine.
Suppose your MySQL service needs hyperscale, you can use the
units options to
add-unit to specify the desired number of units you want
added to the service. For example, to scale up your service by 100 units simply
juju add-unit -n 100 mysql
or you can use
--num-unit which has the same result, but is more readable:
juju add-unit --num-unit 100 mysql
If you would like to add a unit to a specific machine just append the
# add unit to machine 23 juju add-unit mysql --to 23 # add unit to lxc container 3 on host machine 24 juju add-unit mysql --to 24/lxc/3 # add unit to a new lxc container on host machine 25 juju add-unit mysql --to lxc:25
add-unit command deploys a machine matching the constraints of the
initially deployed service. For example, if MySQL was deployed with the
defaults (i.e. no
--constraints option) you would have MySQL on an instance
that matches the closest to 1 Gigabyte of memory and 1 CPU available. If you
would like to add a unit with more resources to the MySQL service you will
first need to issue a
add-machine with the desired constraint followed by a
add-unit. For example, the following command adds a 16 Gigabyte unit to the
MySQL service (note in this example
juju status returns machine 3 for the
juju add-machine --constraints="mem=16G" juju add-unit mysql --to 3
Note: Keep in mind you can always use the
to specify which environment/cloud you would like the command to run against.
In the following example the
-e hpcloud adds 100 units to the mysql service
in HP's cloud:
juju add-unit -n 100 mysql -e hpcloud
Sometimes you may want to scale back some of your services, and this too is easy with Juju.
The general usage to scale down a service is with the
juju remove-unit [options] <unit> [...]
For example, the following scales down the MediaWiki service by one unit:
juju remove-unit mediawiki/1
If you have scaled-up the MediaWiki service by more than one unit you can
remove multiple units in the same command as long as you know the unit name (ie
juju remove-unit mediawiki/1 mediawiki/2 mediawiki/3 mediawiki/4 mediawiki/5
remove-unit command can be run to remove running units safely. The
running services should automatically adjust to the change.
Note: After removing a service the machine will still be running. In order
to completely remove the machine that once housed the service you need to issue
destroy-machine. For example, to remove machine 1 that the unit
mediawiki/1 was housed on use the command:
juju destroy-machine 1
For more information on removing services, please see the section on destroying services.