Here comes a nice quote from the latest Steve Yegge post (read it entirely if you have the time, it’s both fun and educational – at least for me). So, there:

I made the famously, horribly, career-shatteringly bad mistake of trying to use Ruby at Google, for this project. And I became, very quickly, I mean almost overnight, the Most Hated Person At Google. And, uh, and I’d have arguments with people about it, and they’d be like Nooooooo, WHAT IF… And ultimately, you know, ultimately they actually convinced me that they were right, in the sense that there actually were a few things. There were some taxes that I was imposing on the systems people, where they were gonna have to have some maintenance issues that they wouldn’t have. [...] But, you know, Google’s all about getting stuff done.

[...]

Is it allowed at Google to use Lisp and other languages?

No. No, it’s not OK. At Google you can use C++, Java, Python, JavaScript… I actually found a legal loophole and used server-side JavaScript for a project.

Mmmmm … key ?

In 2007, the number of sold Ruby books was larger than the number of Python books. In their article they qualify Ruby as being a “mid-major programming language” and Python as “mid-minor programming language”. However, after the announcement of Google App Engine the number of Python downloads from ActiveState has tripled in May. This should become visible in the book sales statistics, pretty soon.

My original contribution here is a comparison between the original memcached server from Danga and the tugela fork from the MediaWiki programmers. I’ve also tried memcachedb but the pre 1.0 version (from Google Code) in November 2007 was quite unstable and unpredictible.

In a nutshell, these memcache versions are using BerkeleyDB instead of memory slab allocator. There are 2 direct consequences:

• when the memory is large enough for the whole cache, database-backed servers will be slower (my tests shown 10-15% which might be tolerable – or not – for your app)
• when you’ve got lots of data to cache and your server’s memory is low, relying on bdb is significantly better than letting the swap mechanism to do its job (from my benchmarks, the difference can go up to 10 times faster especially under very high concurrency conditions)

Tugela will prove especially useful when running it on virtualized servers with very low memory.

My tests were performed with the “Tummy” Python client and Stackless for the multithreaded version. In one of the following weeks I’ll update the benchmarks for memcachedb 1.0.x – and I promise never ever to wait 4 months for a presentation, again …

As for watering your mouth, this is a Python code fragment that I found on the site, for the classic inline conditional which does not exist “such as” in Python:

n = ['no', 'yes'][thing == 1]

Obviously it has the big disadvantage of having to compute both values no matter what the condition thing is, but is very short and elegant. Simple but nice code sugar.

• from faemalia.net, a script which is integrated with the MySQL server templates for Cacti. Uses the Perl client.
• from dealnews.com, a dedicated memcached template for Cacti and some scripts based on the Python client. The installation is thoroughly described here.

These two solutions have the same approach – provide a specialized Cacti template. The charts drawn by these templates are based on data extracted by the execution of memcached client scripts. Maybe very elegant, but could become a pain in the dorsal area. Futzing with Cacti templates was never my favorite pasttime. Just try to import a template exported from a different version of Cacti and you’ll know what I mean. In my opinion, there is a simple way, which consists in installing a memcached client on all the memcached servers, then extracting the statistical values using a script. We’ll use the technique described in one of my previous posts, to expose script results as SNMP OID values. Then, track these values in Cacti via the generic existing mechanism. My approach has the disadvantage of installing a memcached client on all the servers. However, it is very simple to build your own charts and data source templates, as for any generic SNMP data. All you need now a simple script which will print the memcached statistics, one per line. I will provide one-liners for Python, which will obviously work only on machines having Python and the “tummy” client installed. This is the recipe (default location of Python executable on Debian is /usr/bin/python but YMMV):

1. first use this one liner as snmpd exec :

/usr/bin/python -c “import memcache; print (‘%s’%[memcache.Client(['127.0.0.1:11211'], debug=0).get_stats()[0][1],]).replace(\”‘\”,”).replace(‘,’,'\n’).replace(‘[','')
.replace(']‘,”).replace(‘{‘,”).replace(‘}’,”)”

This will display the name of the memcached statistic along with its value and will allow you to hand pick the OIDs that you want to track. Yes, I know it could be done simpler with translate instead of multiple replace. Left as an exercise for the Python-aware reader.

2. after having a complete list of OIDs use this one-liner:

/usr/bin/python -c “import memcache; print ‘##’.join(memcache.Client(['127.0.0.1:11211'], debug=0).get_stats()[0][1].values()).replace(‘##’,'\n’)”

The memcached statistics will be displayed in the same order, but only their values not their names.

And this is the mandatory eye candy:

• start by adding the default-character-set=utf8 in your MySQL configuration file and restart the database server
• apply this recipe from Activestate Python Cookbook (“guaranteed conversion to unicode or byte string”)
• inside the Cheetah template, use the ReplaceNone filter:


#filter ReplaceNone
${myUnicodeString}
#end filter

in order to prevent escaping non-ASCII characters.

Now. That’s better.

Cacti is built upon an open-source graphing tool called MRTG and a communication protocol SNMP. SNMP is not exactly a developer’s cup of tea, being more of a network administrator’s tool². However, a monitoring server comes extremely handy in performance measurement and tuning, especially for complex performance behavior which can only be benchmarked long-term : such as large caches impact on a web application, or performance of long-running operations.

But is that specific variable you need to monitor, available with SNMP out of the box ? There is a strong chance it is. SNMP being an extensible protocol, lots of organization have recorded their own MIBs and respective implementations. Basically, a MIB is a group of unique identifiers called OIDs. An OID is a sequence of numbers separated by dots, for instance ‘.1.3.6.1.4.1.2021.11′; each number has a special meaning in a standard object tree – this example, the meaning of ‘.1.3.6.1.4.1.2021.11′ is ‘.iso.org.dod.internet.private.enterprises.ucdavis.systemStats’. Even you can have your own MIB in the ‘.iso.org.dod.internet.private.enterprises’ tree, by applying on this page at IANA.

Most probably you don’t really need your own MIB, no matter how ‘exotic’ your monitoring is, because:

a) it’s already there, in the huge list of existing MIBs and implementations

and

b) you are not bound to the existing official MIBs, in fact you can create your own MIB as long as you replicate it in the snmp configuration on all the servers that you want to monitor.

To take a look at existing MIBs, free tools are available on the net, IMHO the best one being MibBrowser. This multiplatform [Java] MIB browser has a free version which should be more than enough for our basic task. The screen capture shown here depicts a “Get Subtree” operation on the ‘.1.3.6.1.4.1.2021.11′ MIB; the result is a list of single value MIBs, such for instance ‘.1.3.6.1.4.1.2021.11.11.0′ which has the alias ’ssCpuIdle.0′ and value 97 [meaning that the CPU is 97% idle]. You can see the alias by loading the corresponding MIB file [select File/Load MIB then choose 'UCD-SNMP-MIB.txt' from the list of predefined MIBs].

From command line, in order to display existing MIB values, you can use snmpwalk:

snmpwalk -Os -c [community_name] -v 1 [hostname] .1.3.6.1.4.1.111111.1

³ and the result is:

.1.3.6.1.4.1.2021.11 OID (.iso.org.dod.internet.private.enterprises.ucdavis.systemStats)
snmpwalk -v 1 -c sncq localhost .1.3.6.1.4.1.2021.11
UCD-SNMP-MIB::ssIndex.0 = INTEGER: 1
UCD-SNMP-MIB::ssErrorName.0 = STRING: systemStats
UCD-SNMP-MIB::ssSwapIn.0 = INTEGER: 0
UCD-SNMP-MIB::ssSwapOut.0 = INTEGER: 0
UCD-SNMP-MIB::ssIOSent.0 = INTEGER: 4
UCD-SNMP-MIB::ssIOReceive.0 = INTEGER: 2
UCD-SNMP-MIB::ssSysInterrupts.0 = INTEGER: 4
UCD-SNMP-MIB::ssSysContext.0 = INTEGER: 1
UCD-SNMP-MIB::ssCpuUser.0 = INTEGER: 2
UCD-SNMP-MIB::ssCpuSystem.0 = INTEGER: 1
UCD-SNMP-MIB::ssCpuIdle.0 = INTEGER: 96
UCD-SNMP-MIB::ssCpuRawUser.0 = Counter32: 17096084
UCD-SNMP-MIB::ssCpuRawNice.0 = Counter32: 24079
UCD-SNMP-MIB::ssCpuRawSystem.0 = Counter32: 6778580
UCD-SNMP-MIB::ssCpuRawIdle.0 = Counter32: 599169454
UCD-SNMP-MIB::ssCpuRawKernel.0 = Counter32: 6778580
UCD-SNMP-MIB::ssIORawSent.0 = Counter32: 998257634
UCD-SNMP-MIB::ssIORawReceived.0 = Counter32: 799700984
UCD-SNMP-MIB::ssRawInterrupts.0 = Counter32: 711143737
UCD-SNMP-MIB::ssRawContexts.0 = Counter32: 1163331309
UCD-SNMP-MIB::ssRawSwapIn.0 = Counter32: 23015
UCD-SNMP-MIB::ssRawSwapOut.0 = Counter32: 13730

Each of this values has its own significance, like for instance ’ssCpuIdle.0′ which announces that the CPU is 96% idle.
In order to retrieve just a single value of the list, use its alias as a parameter to the snmpget command, for instance

snmpget -Os -c [community_name] -v 1 [hostname] UCD-SNMP-MIB::ssCpuIdle.0

Sometimes, you want to monitor something which you do not seem to find in the list of MIBs. Say, for instance, the performance of a MySQL database that your’re pounding pretty hard with your webapp⁴. The easiest way of doing this is to pass through a script – snmp implementations can take the result of any script and expose it through the protocol, line by line.

Supposing you want to keep track of the values obtained with the following script:

#!/bin/sh
/usr/bin/mysqladmin -uroot status | /usr/bin/awk '{printf("%f\n%d\n%d\n",$4/
10,$6/1000,$9)}'

The mysqladmin command and a bit of simple awk magic display the following three values, each on a separate line:

• number of opened connections / 10
• number of queries / 1000
• number of slow queries

It is interesting to not that, while the first value is instantaneous gauge-like, the following two are incremental, growing and growing as long as new queries and new slow queries are recorded. Will keep this in mind for later, when we will track these values.

But for now, let’s see how these three values are exposed through snmp. The first step is to tell the SNMP daemon that the script has an associated MIB. This is done in the configuration file, usually located at /etc/snmp/snmp.d. The following line attaches the script [for example /home/user/myscript.sh] execution to a certain OID:

exec .1.3.6.1.4.1.111111.1 MySQLParameters /home/user/myscript.sh

the ‘.1.3.6.1.4.1.111111.1′ OID is a branch of ‘.1.3.6.1.4.1′ [meaning '.iso.org.dod.internet.private.enterprises']. We tried to make it look ‘legitimate’ but obviously you can use here any sequence you want to.

After restarting the daemon, let’s interrogate Mibbrowser for the freshly created OID, see the following image snmpwalk -Os -c [community_name] -v 1 [hostname] .1.3.6.1.4.1.111111.1 ; the result is:

enterprises.111111.1.1.1 = INTEGER: 1
enterprises.111111.1.2.1 = STRING: "MySQLParameters"
enterprises.111111.1.3.1 = STRING: "/etc/snmp/mysql_params.sh"
enterprises.111111.1.100.1 = INTEGER: 0
enterprises.111111.1.101.1 = STRING: "0.900000"
enterprises.111111.1.101.2 = STRING: "18551"
enterprises.111111.1.101.3 = STRING: "108"
enterprises.111111.1.102.1 = INTEGER: 0
enterprises.111111.1.103.1 = ""

Great ! Now we have the proof that it really works and our specific values extracted with a custom script are visible through SNMP. Let’s go back to Cacti and see how we can make some nice charts out of them⁵.

Cacti has this nice feature of defining ‘templates’ that you can reuse afterwards. My strategy is to define a data template for each one of the 3 parameters I want to chart, using the ‘Duplicate’ function applied to the ‘SNMP – Generic OID Template’.

On the duplicate datasource template, you have to change the datasource title, name to display in charts, data source type [use DERIVE for incremental counters and GAUGE for instantaneous values], specific OID and the snmp community. Do it for the three values.

Using the three new datasource templates, create a chart template for ‘MySQL Activity’. That’s a bit more complicated, but it boils down to the following procedure, repeated for each of the 3 data sources:

• add a data source and associate a graph [I always use AREA for the first graph as a background and LINE3 for the other, but it's just a matter of taste]
• associate labels with current or computed values: CURRENT, AVERAGE, MAX in this example

All the rest is really fine tuning – deciding for better colors, wether to use autoscale or fixed scale and so on. By now, your graph template should be ready to use.

Note that for the incremental values ['DERIVE' type data sources] I’ve used titles such as ‘Thousands queries/5 min’ – the 5 minutes come from the Cacti poller which is set to query for data each 5 minutes. The end result is something like this one :

On this real production chart you’ll see a few interesting patterns. For instance, at 3 o’clock in the morning, there is a huge spike in all the charted parameters – indeed, a cron’ed script was provoking this spike. From time to time, a small burst of slow queries is recorded – still under investigation. What is interesting here is that these spikes were previously undetectable on the load average chart, which look clean and innocuous:

To conclude, SNMP is a valuable resource for server performance monitoring. Often, investigating specific parameters and displaying them in tools such as Cacti can bring interesting insights upon the behavior of servers.

Some SNMP implementations in different programming languages:

• Java: Westhawk’s Java SNMP stack [free w commercial support], AdventNet SNMP API [commercial, with a feature-restricted un-expiring free version], iREASONING SNMP API [commercial implementation], SNMP4J [free and feature-rich implementation - thank you Mathias for the tip]
• PHP: client-only supported by the php-snmp extension, part of the PHP distribution [free]
• Python: PySNMP is a Python SNMP framework, client+agents [free].
• Ruby: client-only implementation Ruby SNMP [free]

¹ If you’re running Debian, Cacti comes with apt so it’s a breeze to install and run [apt-get install cacti]

² a bit out of the scope of this article, SNMP also allows writing values on remote servers, not only retrieving monitored values.

³ Replace [hostname] with the server hostname and [community_name] with the SNMP community – default being ‘public’. The SNMP community is a way of authenticating a client to a SNMP server; although the system can be used for pretty sophisticated stuff, most of the time the servers have a read-only passwordless community, visible only in the internal network for monitoring purposes.

⁴ In fact, a commercial implementation of SNMP for MySQL does exist.

⁵ The procedure described here applies to Cacti v0.8.6.c

Wrong ! Sybase ASE is a large, enormous, huge piece of steaming …
ummm … code. Ok, ok, I'm a bit over-reacting. In fact, Sybase is a
very good database – if you are still living in the 90s and the only
Linux flavour that you are able to manage is RedHat ASE. Otherwise,
it's a huge … you know.

Let's not talk about the extreme fragility of this … this product.
You never know when it crashes on you without any specific reason.
Sometimes, it's the bad weather. Crash. Restart. Sometimes, you
flushed twice. Crash. Restart. And maybe, yes maybe, you spent more
than 12 minutes in your lunch break. Crash.

Let's just talk about the JDBC drivers – latest version, downloaded
from the site in a temporary moment of insanity. Man, this is cutting
edge. The sharpest cutting edge you'll ever find – the more advanced
JDBC drivers bar none ! Excepting of course the fact that these
classes were compiled in that memorable day of 6 January 2002 [the day
when the last Sybase JDBC drivers were compiled]. But, don't let such
obscure details ruining your enthusiasm. Just download and use them
and you'll be amazed at their unique features – it's the only JDBC
driver which manages to put down DbVisualizer in different and
innovative ways. I'm restarting the poor thing (dbvis) at least 2 or 3
times per day, when working with Sybase.

Also, as a little quasi-undocumented tip, the letter d from
jconn2d.jar does not mean development [drivers] as some of you would be
inclined to think. In fact, it means debug which is the
abbreviation for put me in production poor lousy bastard and I'll
start spewing reams of useless messages through all the logging APIs
known by man and a few yet to be discovered, therefore instantly
slowing down to a crawl your puny little software.

Ermmm … well, all this nice introduction just to humbly confess that
we do have a couple of them Sybase licences floating around here and
some of our production databases are on Sybase ASE. While I can assure
you that this is going to change at least on the web backend, it's
also true that the beast must be alive in order to keep our company up
and running. And, that's part of my job.

Which is of course very strange for an IT management job. But then again:
when your sole DBA is overwhelmed by a horde of Sybase-specific tasks
(like for instance the log configured to truncate at a specific
threshold which naturally [for Sybase] does not truncate, suddenly
throwing all the users into log suspend in the middle of the peak
production time) – you have to enter into the damn kitchen and do some
cooking !

My endeavour was to perform simple data mining in order to migrate
some reports from a legacy system [no, you don't really want to know
what system]. Due to my limited time, starting a mildly complex Java
reporting project was out of the question, so a scripting language was
the natural choice. Python being the first option – unfortunately,
finding a working Sybase driver for Python is a challenge in itself.
But, thank God for Jython and zxJDBC ! In just a few minutes I was
wiring the tables for reporting. Here's a fake code snippet which lists the 'orders' with amounts < 1000, and you can't go any simpler:

from com.ziclix.python.sql import zxJDBC 

conn = zxJDBC.connect("jdbc:sybase:Tds:myserver.mydomain.com:4110/mydb",\
	"user","pass","com.sybase.jdbc2.jdbc.SybDriver")
cursor = conn.cursor() 

cursor.execute("select count(orderid) from orders")
nb_orders=cursor.fetchone()[0]
print '%.0f total orders ...'%(nb_orders,)

cursor.execute("select orderid, amount from orders where amount <1000")
oids=cursor.fetchall()
for o in oids:
	print 'Order %.0f for amount %.2f'%(o[0],o[1])

cursor.close()
conn.close()

Easy as pie. And once you got one down, you got'em all. For instance,
yesterday evening I wrote in about an hour a small Jython script which
exports some data. The same export process (running about an order of
magnitude slower) needed previously a couple days of development in
FoxPro. Ah, FoxPro - very juicy subject, but I'll keep it for my next
horror story. Until then, don't forget, when you have a monster to
tame and no time at all: try Jython !

*Let's suppose for a very brief instant that having Celine Dion at a certain location is a positive thing.