Techno-babble of a Techno-dabbler

Monthly Archives: May 2012

AWS Auto Scaling with boto.

The boto library is awesome for Python people who want to automate Amazon Web Services, but I had a minor struggle with the Auto Scaling interface recently.

Amazon deprecated the old “triggers” mechanism for handling dynamic scaling, and replaced it with CloudWatch “alarms”. While boto supports the new mechanism, it’s hard to find working examples on-line.

UPDATE: This example is now improved and merged into the official boto documentation.

Below is my attempt to define the minimal procedure for creating a dynamically scaled cluster of instances with boto. It’s derived from the excellent work of Liam Friel.

import boto
from boto.ec2.autoscale import ScalingPolicy
from boto.ec2.cloudwatch import MetricAlarm

autoscale = boto.connect_autoscale()
cloudwatch = boto.connect_cloudwatch()

# Let's assume you already have an Auto Scaling group.
# Setting one up is well documented elsewhere.
autoscaling_group = 'as_group_0'

# Define some Scaling Policies. These tell Auto Scaling _how_ to scale
# group, but not when to do it. (We'll define that later).

# We need one policy for scaling up...
scale_up_policy = ScalingPolicy(
    name='scale_up', adjustment_type='ChangeInCapacity',
    as_name=autoscaling_group, scaling_adjustment=1, cooldown=180)

#...and one for scaling down again.
scale_down_policy = ScalingPolicy(
    name='scale_down', adjustment_type='ChangeInCapacity',
    as_name=autoscaling_group, scaling_adjustment=-1, cooldown=180)

# The policy objects are now defined locally.
# Let's submit them to AWS.

# Now that the polices have been digested by AWS, they have extra properties
# that we aren't aware of locally. We need to refresh them by requesting them
# back again. 
# Specifically we'll need the Amazon Resource Name (ARN) of each policy.
scale_up_policy = autoscale.get_all_policies(
    as_group=autoscaling_group, policy_names=['scale_up'])[0]

scale_down_policy = autoscale.get_all_policies(
    as_group=autoscaling_group, policy_names=['scale_down'])[0]

# Now we'll create CloudWatch alarms that will define _when_ to run the
# Auto Scaling policies.

# We want to measure the average CPU usage across the whole Auto Scaling
# group, rather than individual instances. We can define that as a CloudWatch
# "Dimension".
alarm_dimensions = {"AutoScalingGroupName": autoscaling_group}

# One alarm for when to scale up...
scale_up_alarm = MetricAlarm(
        name='scale_up_on_cpu', namespace='AWS/EC2',
        metric='CPUUtilization', statistic='Average',
        comparison='>', threshold='70',
        period='60', evaluation_periods=2,

# ...and one for when to scale down.
scale_down_alarm = MetricAlarm(
        name='scale_down_on_cpu', namespace='AWS/EC2',
        metric='CPUUtilization', statistic='Average',
        comparison='<', threshold='40',
        period='60', evaluation_periods=2,

PCBs – Designing for Hackability

This hack needs help.

Nice board, but it needs a hack

With a heart full of Geek Pride, I recently unwrapped my first commercially fabricated PCB. As a beginner designer, I missed something in the initial design and realised while I was waiting for the board to arrive. Essentially, the board was obsolete by the time it got to me. The new changes would need to be hacked into place on the board. Oh well, no problem…

Hostile Hardware

As soon as I started making modifications, something felt wrong. It felt like I was trying to hack a closed, commercial circuit. It was almost as though I was reverse engineering my own design, just to make a simple change.

Commercial designs are indifferent to hackers at best and are often deliberately hacker-hostile. This board is a prototype for an open-source design. It should actively encourage modification. It should be designed for hackability.

Don’t hide the design

The PCB fabricator, iTeadStudio, offers solder-mask in a range of funky colours. Ordinary transparent green is also an option. Naturally, I just had to have black solder-mask. It looks non-generic, “edgy”, very Death Star chic. Unfortunately, it also makes it very difficult to see the traces on the board, which is not good when you need to cut a trace.

Open hardware should be made to be understood. Black solder mask literally reduces the transparency of the design.

Test-points are hack-points

Inevitably, I brought out the oscilloscope to examine some of the radio frequency signals on the board. A few sensibly placed test points would have been moderately handy here, but I had none in the layout. It would have been extremely handy if the test points had been plated through-holes, then components could have been added to them instead of being haphazardly soldered to already populated holes and IC pins.

Holes are free, dig?

There are veritable fields of unused space on the board. At least some of that space should have been allocated to a prototyping grid. The fabricator doesn’t charge for holes, so why be parsimonious? More hack-points means more hackability.

Hackabillity is for hackers

Ultimately, this is an open-source design. I don’t expect that anyone will use it, but the design should consider that possibility. It should also consider that the hypothetical user is a hacker and will use the design for something I haven’t thought of. The more hackable the design, the more adaptable it is to unintended applications. We are hackers, we like unintended applications.

I’ll try to design my next board for hackabillity. I owe it to myself and to all the hackers out there.

The board in question is a “Tayloe Detector” for Software Defined Radio using a PC.