Hacking so Existing

Mapping port 80 to inner port 8080:
/sbin/iptables -t nat -A PREROUTING -i eth+ -p tcp –dport 80 -j REDIRECT –to-port 8080

Well, I use this trick to avoid firewall blocking.

I struggled to find out why my server broke down frequently these days. And I think I found it.

My Apache HTTP server was configured with MaxClients at 50, the default AJP connector’s connections number was about 15, and the Tomcat’s maxThreads was 150. So the bottleneck would be AJP connector, especially all my connection-heavy Comet application was serving about 20 Gtalk/MSNLive simultaneous connections. It is obviously it will break down the servers. And after switching my Gtalk/MSNLive into query modes, it still broke down the server. The reason may be the bottleneck of Apache HTTP server’s 50 maximum connections.

I updated all the configuration to 256 for maximum simultaneous connections. Hope server won’t break down for Comet application from now on.

For more about extreme maximum simultaneous connections, please read articles:
http://jha.rajeev.googlepages.com/web2push
http://www.stdlib.net/~colmmacc/Apachecon-EU2005/scaling-apache-handout.pdf

http.conf

<IfModule prefork.c>
#StartServers 1
#MinSpareServers 1
#MaxSpareServers 5
StartServers 3
MinSpareServers 5
MaxSpareServers 10
#ServerLimit 50
ServerLimit 256
#MaxClients 50
MaxClients 256
MaxRequestsPerChild 4000
</IfModule>


server.xml

<!– Define an AJP 1.3 Connector on port 8009 –>
<Connector port="8009"
maxThreads="256" minSpareThreads="25" maxSpareThreads="75"
enableLookups="false" protocol="AJP/1.3" />

Here is an example to replace string with a different delimiter in Windows’ *.bat file.

for /f "tokens=1-15 delims=/" %%i in ("%1") do (
set _X1_=%%i
set _X2_=%%j
set _X3_=%%k
set _X4_=%%l
set _X5_=%%m
set _X6_=%%n
set _X7_=%%o
set _X8_=%%p
set _X9_=%%q
set _XA_=%%r
set _XB_=%%s
set _XC_=%%t
set _XD_=%%u
set _XE_=%%v
set _XF_=%%w)
set _XPATH_=%_X1_%
if not "%_X2_%" == "" set _XPATH_=%_XPATH_%\%_X2_%
if not "%_X3_%" == "" set _XPATH_=%_XPATH_%\%_X3_%
if not "%_X4_%" == "" set _XPATH_=%_XPATH_%\%_X4_%
if not "%_X5_%" == "" set _XPATH_=%_XPATH_%\%_X5_%
if not "%_X6_%" == "" set _XPATH_=%_XPATH_%\%_X6_%
if not "%_X7_%" == "" set _XPATH_=%_XPATH_%\%_X7_%
if not "%_X8_%" == "" set _XPATH_=%_XPATH_%\%_X8_%
if not "%_X9_%" == "" set _XPATH_=%_XPATH_%\%_X9_%
if not "%_XA_%" == "" set _XPATH_=%_XPATH_%\%_XA_%
if not "%_XB_%" == "" set _XPATH_=%_XPATH_%\%_XB_%
if not "%_XC_%" == "" set _XPATH_=%_XPATH_%\%_XC_%
if not "%_XD_%" == "" set _XPATH_=%_XPATH_%\%_XD_%
if not "%_XE_%" == "" set _XPATH_=%_XPATH_%\%_XE_%
if not "%_XF_%" == "" set _XPATH_=%_XPATH_%\%_XF_%

For more information, please key in “for /?” in command line.

If you ever visit Eclipse homepage http://www.eclipse.org/, you may notice that Eclipse homepage embeds plugin updates news from Eclipse Plugin Central (http://www.eclipseplugincentral.org/). If you try to visit the news by the given link, you may find that the page loads very very very SLOW!

Why? Open the the page source, you will see that it generates all 900+ plugins in the source! And the *.html file size is up to 200+k. And worse thing is that every page in Eclipse Plugin Central is the same with 200+k HTML file! Every page!

Besides it was once hacked by someone and its functions were buggy, such 200+k HTML pages design in Eclipse Plugin Central must be listed as #1 of aweful designs that I have ever seen.

Recently, my Apache HTTP server was broken down twice. The scenario was when visiting a page, it was just waiting there, blank. And I checked the server through SSH and saw that there was a lot (about 50) of inner 8009 connections. After restarting Apache HTTP Server and Tomcat Server, these connections decreased to less than 10 connections.

Yes, I deployed Comet applications “Google Talk in JavaScript” and “MSN Live in JavaScript” on my server. Here is the connection routine:

Client <-Kept Connection-> Apache HTTP Server <-AJP Connector (8009 Connection)-> Tomcat

When a user is connected, an AJP Connector is setup for him/her. The connection will not be tore down until the user disconnects from Gtalk or MSNLive service. So if there are more than 50 users online, there are no doubts that the server crashes.

To solve this problem, one solution would be not using Apache HTTP Server in the middle of Tomcat server and browser client. So there is no limitation on AJP connections.

And another solution would be monitoring user connections on Tomcat server side, and if there are more than 50 connections, try to switch application’s Comet-mode into Query-mode smartly.

Providing Comet applications need to solve a lot of unknown problems.

I am puzzled a lot when I got emails with winmail.dat attachment, which is unknown file type attachment in my Thunderbird. Here is the solution for this annoying winmail.dat:

Go to https://addons.mozilla.org/en-US/thunderbird/addon/4433, follow it instructions to download the “LookOut” add-on for Thunderbird, and attached files inside winmail.dat will be listed and be recognized and be opened with existed programs.

Here is a task to challenge myself: Find the first k digits prime number. And k would be 16, 32, 64, 256, 1024, 8096, 65536, or even bigger.

And the most challenging thing would be most of key calculations will be performed in JavaScript engine hosted inside browsers. And a distributed algorithm will be designed. Algorithm which is similar to Google’s Map/Reduce will be designed. And Apache Hadoop’s Map/Reduce is also in references.

Any comments about this challenge? Or join me to design the algorithm! Or join the distributed computing game.

Keep tuned for how big the number k would be!

PS: After searching around Internet for existed prime number theories, I find out that k=16 is already a challenge for me! There are algorithms to find prime number in format of 2^p+1, whose digits may be bigger than 1000000. But there are not the 1st k-digits algorithms. According to theories, I should have to find out all prime numbers less than given n. But the count of prime numbers less than n is about n / ln (n), which is huge number already for k=16! So my first challenge would be 16.

Actually, Google Talk Client’s Java SWT desktop copy is the original source for Google Talk Client’s JavaScript copy.

And now it is available for download. Please take a trial.

Sources of Java2Script Google Talk will be available in the next week. Keep tuned.

If you ever use Eclipse trying to debug into Java’s system APIs, such as java.lang.String or java.math.BigInteger, you may find that even you can step over or step in, or have the correct source file open and the correct line of codes highlighted, you still could not see the local variables’ value and method’s parameters are shown in pattern of “arg0″, “arg1″ and so on.

Yes, you are already using JDK instead of JRE. But you still have the above problem. What’s wrong? It’s not a bug of Eclipse. It is all about the rt.jar along with the JDK (1.5 or 1.6). In Sun’s official JDK release, debug symbols are already packed into the rt.jar. But local variables’ debug information is not packed. Sun may consider that it is not OK to includes those local variables’ debugging information. That is the source of problem.

To solve this problem, you may need to upgrade to JDK 1.6. Download DEBUG jar file from
http://download.java.net/jdk6/binaries/
Double click it to install a new JDK. And configure your Eclipse to use the new JDK.

For JDK 1.5, recompiling rt.jar by yourself, or other information, please visit
http://forums.java.net/jive/thread.jspa?threadID=399&tstart=0
for a detailed discussion.

http://demo.java2script.org/gtalk/

Here is the screenshot:

En, in beta …