Saturday Oct 03, 2009

A Dashboard Like No Other: The OpenDS Weather Station

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 Doing so many benchmarks, profiling and other various performance related activities, I had to find a way to "keep an eye" on things while fetching emails, chatting on IM and the like. Having some experience in past projects with microcontrollers, although on Windows, I figured I could put together a little gizmo to help me keep tabs on my Directory Server.

Bird's Eye View

This is basically a simple setup with a USB Bit Whacker controlled by a Python script, feeding it data crunched from various sources, mainly the Directory Server access log, the garbage collection log and kstats... the result is a useful dashboard where I can see things happen at a glance.

The Meat

Everything starts with the USB Bit Whacker. It's a long story, but to cut short, a couple a years ago, Kohsuke Kawaguchi put together an orb that could be used to monitor the status of a build / unit tests in Hudson. Such devices are also know as eXtreme Feedback Devices or XFDs. Kohsuke chose to go with the USB Bit Whacker (UBW) for it is a USB 'aware' microcontroller that also draws power from the bus, and is therefore very versatile while remaining affordable ($25 soldered and tested from sparkfun but you can easily assemble your own). A quick search will tell you that this is a widely popular platform for hobbyists.

 On the software side, going all java would have been quite easy except for the part where you need platform specific libraries from the serial communication. Sun's javacomm library or rxtx have pros and cons but in my case, the cons were just too much of a hindrance. What's more, I am not one to inflict myself pain unless it is absolutely necessary. For that reason, I chose to go with Python. While apparently not as good on cross-platformedness compared to Java, installing the Python libraries for serial communication with the UBW is trivial and has worked for me right off the bat on every platform I have tried, namely: Mac OS, Linux and Solaris. For example, on OpenSolaris all there is to it is:

 $ pfexec easy_install-2.4 pySerial
Searching for pySerial
Best match: pyserial 2.4
Processing pyserial-2.4.tar.gz
Running pyserial-2.4/ -q bdist_egg --dist-dir /tmp/easy_install-Y8iJv9/pyserial-2.4/egg-dist-tmp-WYKpjg
zip_safe flag not set; analyzing archive contents...
Adding pyserial 2.4 to easy-install.pth file

Installed /usr/lib/python2.4/site-packages/pyserial-2.4-py2.4.egg
Processing dependencies for pySerial
Finished processing dependencies for pySerial

 that's it! Of course, having easy_install is a prerequisite. If you don't, simply install setuptools for your python distro, which is a 400kB thing to install. You'll be glad you have it anyway.

Then, communicating with the UBW is mind boggingly easy. But let's not get ahead of ourselves, first things first:

Pluging The USB Bit Whacker On OpenSolaris For The First Tim

The controller will appear as a modem of the old days and communicating with equates to sending AT commands. For those of you who are used to accessing Load Balancers or other network equipment through the serial port, this is no big deal.

In the screenshot below, the first ls command output shows that nothing in /dev/term is an actual link, however, the second -which I issued after plugging the UBW on the usb port- shows a new '0' link has been created by the operating system.

Remember which link your ubw appeared as for our next step: talking to the board.

Your First Python Script To Talk To The UBW

I will show below how to send the UBW the 'V' command which instructs it to return the firmware version, and we'll see how to grab the return value and display it. Once you have that down, the sky is the limit. Here is how:

from serial import \*
ubw = Serial("/dev/term/0")
print "Requesting UBW Firmware Version"
print "Result=["+ubw.readline().strip() + "]\\n"

Below is the output for my board:


That really is all there is to it, you are now one step away from your dream device. And it really is only a matter of imagination. Check out the documentation of current firmware to see what commands the board supports and you will realize all the neat things you can use it for: driving LEDs, Servos, LCD displays, acquiring data, ...

Concrete Example: The OpenDS Weather Station

As I said at the beginning of this post, my initial goal was to craft a monitoring device for OpenDS. Now you have a good idea of how I dealt with the hardware part, but an image is worth a thousand words so here is a snap...

On the software front, well, being a software engineer by trade, that was the easy part so that's almost not fun and I won't go inot as much detail but here is a 10,000ft view:

  • data is collected in a matrix of hash tables.
  • each hash table represent a population of data points for a sampling period
  • an individual time thread pushes a fresh list of hash tables in the matrix so as to reset the counters for a new sampling period

So for example, if we want to track CPU utilization, we only need to keep one metric. The hash table will only have one key pair. Easy. Slightly overkill but easy. Now if you want to keep track of transactions response times, the hash table will keep the response time (in ms) as a key and the number of transactions that were processed in that particular response time as the associated value. Therefore, if you have within one sampling period, 10,000 operations processed with 6,000 in 0 ms, 3,999 in 1ms and 1 in 15 ms, your hashtable will only have 3 entries as follows: [ 0 => 6000; 1=>3999; 15=>1 ]

This allows for a dramatic compression of the data compared to having a single line with etime for each operation, which would result in 10,000 lines of about 100 bytes.

What's more is that this representation of the same information allows to easily compute the average, extract the maximum value and calculate the standard deviation.

All that said, the weather station is only sent the last of the samples, so it always shows the current state of the server. And as it turns out, it is very useful, I like it very much just the way it worked out.

 Well, I'm glad to close down the shop, it's 7:30pm .... another busy Saturday


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