unable to connect to MySQL Or MySql is not Allowing more Connections

some times aborting connections or interrupting process lead to another problem to server from stop giving connections to client. that is
server do not allow any more connections to client.
probable causes would be.
client is idle more than wait_timeout or
due to long queries getting aborted from client or
client makes mistake in authentications while connecting to database.
all these lead to increase in aborted_client counter of server variable.

and if max_connect_errors= (default is 10) is set to very low will cause a server to give more connection to client when it goes beyond limit
so its better to set it something more (normally 5000-10000) which will minimize this error.

please see more ref. I found for same

http://dev.mysql.com/doc/refman/5.0/en/blocked-host.html
http://dev.mysql.com/doc/refman/5.0/en/communication-errors.html
http://lists.mysql.com/internals/33476

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException

I have noticed this error in my MySql Outage, suddenly going down due to overload.
after my observation I found that this kind of error mostly will cause because of
system variables settings of wait_timeout and interactive_timeout on mysql server is set lower than datasource
idle timeout period.
my datasource pooling property for conection time out is set to 1 min
on server variable wait_timeout is set to 28 sec. (28800 milisec)

in most cases where connection is idle from host and if server wait_timeout is less than client datasource timeout
server closes connection after its sleep time is more than wait_timeout or interactive_timeout period. and client is unaware of this
and when he fires query again on same connection gets above error.

to avoid this there are following ways
1. if you are using plain connection using jconnector set autoconnect property to true.
2. if you have custom connection poolilng machanism try to validate your connection before using it from pool.
3. in other case try to set your server variable values for wait_timeout, interactive_timeout greater than your datasource timeout.

Upgrading MySQL 4.x to 5.x

This note give brief about how to uninstall MYSQL 4.1.xx and install MYSQL 5.1.xx on Linux OS
lately I have found similar steps on following site you can take help of this as well
http://confluence.atlassian.com/display/DISC/Prerequisites+and+installation+on+Fedora+or+RHEL+Linux

first steps to find where current version of MYSQL has been installed
its location of binaries, datafiles and its config file (my.inf/cnf) if any

1. give following command to check MYSQL processes running

 
[root@RAJ ~]# ps ax | grep mysqld

from its output note down location of
mysqld running (/home/raj/mysql/bin/mysqld)
--basedir=/home/raj/mysql (location where mysql binaries are located)
--datadir=/var/lib/mysql (data files of databases created)
--defaults-extra-file =/home/raj/mysql/data/my.cnf (config file if any specifed other than /etc path)

2. now stop the server

 
[root@RAJ ~]# mysqladmin shutdown

3. now fire command in step one again to verify that no process is running.

4. now check which packages are installed of MYSQL

 
[root@RAJ ~]# rpm -qa | grep mysql
mysql-4.1.22-2.el4
mysqlclient10-3.23.58-4.RHEL4.1

5. try to uninstall with rpm command for the above displayaed packags

 
[root@RAJ ~]#  rpm -e mysql-4.1.22-2.el4
error: Failed dependencies:
libmysqlclient.so.14 is needed by (installed) perl-DBD-MySQL-2.9004-3.1.centos4.i386
libmysqlclient.so.14 is needed by (installed) cyrus-sasl-sql-2.1.19-14.i386
libmysqlclient.so.14(libmysqlclient_14) is needed by (installed) cyrus-sasl-sql-2.1.19-14.i386

perl-DBD-MySQL = A bundle to install Perl drivers for MySQL 4 since for mysql5 we will not need it
cyrus-sasl-sql = SASL is the Simple Authentication and Security Layer, mysql5 will have in its own package installation

by looking at error we know that there is dependancy by other files so try to remove those files first
if you require pearl for mysql5 there are newer version of perl driver you need to install.
NOTE: I could have removed it with --nodeps parameter which i come to know later so you may try with this parameter as follows
rpm --nodeps --allmatches -e mysql-4.1.22-2.el4
rpm --nodeps --allmatches -e mysqlclient10-3.23.58-4.RHEL4.1


6. remove dependencies (pls check note above)

 
[root@RAJ ~]#  rpm -e perl-DBD-MySQL-2.9004-3.1.centos4.i386
[root@RAJ ~]# rpm -e cyrus-sasl-sql-2.1.19-14.i386

7. now try again step 5 again (pls check note above)

 
[root@RAJ ~]#  rpm -e mysql-4.1.22-2.el4
warning: /etc/my.cnf saved as /etc/my.cnf.rpmsave

8. remove mysqlclient (pls check note above)

 
[root@RAJ ~]# rpm -e dovecot-0.99.14-1.1.el3.rf.i386
[root@RAJ ~]#
[root@RAJ ~]# rpm -e  mysqlclient10-3.23.58-4.RHEL4.1

9. after this is done pls check data and binary folder of mysql is removed or not if not removed then rename it if you require them or can safely remove it

10. I have downloaded following rpm from mysql site

 
[root@RAJ installers]# ls -l
total 25864
-rw-r--r--  1 root root  7256227 Aug 18 06:02 MySQL-client-community-5.1.37-0.rhel4.i386.rpm
-rw-r--r--  1 root root 19179293 Aug 18 01:50 MySQL-server-community-5.1.37-0.rhel4.i386.rpm
[root@RAJ installers]# chmod 755 *

11. Install new version of mysql 5 server and client

 
[root@RAJ installers]# rpm -i MySQL-server-community-5_1.1.37-0.rhel4.i386.rpm
warning: MySQL-server-community-5_1.1.37-0.rhel4.i386.rpm: V3 DSA signature: NOKEY, key ID 5072e1f5
090818  7:52:19 [Warning] Forcing shutdown of 2 plugins
090818  7:52:20 [Warning] Forcing shutdown of 2 plugins

It shows some more message on screen and automatically starts mysql server.

 
[root@RAJ installers]# rpm -i MySQL-client-community-5.1.37-0.rhel4.i386.rpm
warning: MySQL-client-community-5.1.37-0.rhel4.i386.rpm: V3 DSA signature: NOKEY, key ID 5072e1f5

this completes the installation of mysql server and clients
there are some post installation steps to be folllowed to configure mysql if needed
like setting up my.cnf file to /etc location or creating auto start script of mysql on start of server. etc.

12. check mysql version now

 
[root@RAJ Whitehedge]# mysql -V
mysql  Ver 14.14 Distrib 5.1.37, for pc-linux-gnu (i686) using readline 5.1

Getting started with Ajax: Basics.

Background:

I had used XmlHttp in my javascripts for quite long and extensively. espically dealing with grid control... but lately I come to know that it was called as an AJAX... which has much more than that... to make your UI richer. A common way of calling ajax function is to create an object of XMLHttpRequest. this you can either use it directly or can use wrapper libraries available in the market like extjs, JQuery, Prototype etc. to begin with I will present the internal coding of actual ajax code how it works. please see the following code...

Basic Code: First to check which browser we are using and accordingly activate XmlHttp object as

function GetXmlHttpObject(){ 
   var xmlHttp=null;
   try{ 
     // Firefox, Opera 8.0+, Safari
     xmlHttp=new XMLHttpRequest();
   } catch (e){
   // Internet Explorer
   try{
     xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
   }catch (e){
     xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
     }
   }
   return xmlHttp;
} 

It just that I have saparated code. creating of XMLHttp object in different function as above and calling it in other function as below

function getAjaxResponse(sURL){
   xmlHttp = GetXmlHttpObject();
   xmlHttp.onreadystatechange=stateChanged(){
     if (xmlHttp.readyState==4){ 
  document.getElementById(divID).innerHTML=xmlHttp.responseText;
      };
   };
   xmlHttp.open("GET",sURL ,true);
   xmlHttp.send(null);
}

Lets observe the code above we have specifed call back function to receive response from server.

xmlHttp.onreadystatechange=stateChanged(); 

... here you can either mention function name or you can inline the function itself. this function block will get executed when you receive response from server. Here actually you can do more based on the status code that you receive from server, for e.g. you can show process status while you receive response or please wait messgage/image to user till response received etc. please see the table below for the status code that you get on this function.
Response that you get from the server can be of Html code or xml or simply a string, and it depends how you want to show it to user. in above code I have shown that it returns html response which I directly paste into div block.
fire http GET/POST request to server

xmlHttp.open("GET",url,true);
xmlHttp.send(null); 

Please note the following things when calling open,

• Always set Async to true.
• always set call back function before calling send()
• To send form data/POST use endcodeURI() for all data values and set mime type of request to "application/x-www-form-urlencoded"
  

basic advantage of using javascript to post the request to server is that we don't have to wait till server response arrives. instead we can perform other task. I am listing below the XMLHttpRequest objects method available here.

Method/ Property	Description.
abort()	stops listining for current response.
getAllResponseHeader()	Get array of response header as key value pair.
getRespoonseHeader(name)	Get response header by name passed as key.
open(method, url[,asyncFlag[, "username"[, "password"]]])	This prepares the xmlHTTPRequest object to make call to the server. Method: HTTP; can be either GET, PUT or POST. url: is url string to open can be either obsolute or relate may include querystring
send(content)	perform http request. content: string to be used as request body, i.e. to form POST body, you can format your POST parameters just like querystring.
setRequestHeader(header, value)	Add header to Http requset, normally a key value pair and mostly used to set content type of request.
onreadystatechange	used to set as callback fucntion that handles request state change.(similar to onclick, onload)
readyState	returns the status of the request which can be on of following at a time 0:= uninitialised 1:= loading 2:= loaded 3:= interactive 4:= complete
responseText	returns server response as String
ResponseXML	returns server response as XML Document
status	returns HTTP status code like 200 for OK etc
statusText	return String representation of HTTP status code like OK for 200, not found for 404 etc.

In above example I have already shown how I am getting HTML response from server which I am setting back to perticlular html object. see below the example of parsing xml response received from server and setting to html object.
here I assume that I am getting some key-value pair from server as xml response which i need to set to combobox in html so I am passing combobox object to method

xmlHttp.onreadystatechange=usrfunc;
....
....
function processListXML(xmlId, objlist ){
// first remove all existing option values from list box
  for(i=objlist.options.length-1;i>=0;i—){
    objlist.remove(i);
  }
  var t = document.getElementById(xmlId);
  var tableData = t.getElementsByTagName("RList");
  if(tableData != null){
    var t = tableData.item(0);
    var columns = t.getElementsByTagName("ROption");
    if(columns != null){
      for (j = 0; j < columns.length; j++){
        key= columns[j].getAttribute("id");
        value= columns[j].firstChild.data;
        objlist.add(new Option( value ,key ));
      }
    }
  }
}

Now assume that I get following xml
<RList>
<ROption id="01">INDIA </ROption>
<ROption id="02">JAPAN </ROption>
<ROption id="03">AMERICA </ROption>
</RList>

Web Service Quick Start

 

What are Web Services?
Web services are application components
Web services communicate using open protocols
Web services are self-contained and self-describing
Web services can be discovered using UDDI
Web services can be used by other applications
XML is the basis for Web services

How Does it Work?
The basic Web services platform is XML + HTTP.

XML provides a language which can be used between different platforms and programming languages and still express complex messages and functions.

The HTTP protocol is the most used Internet protocol.

below are Web services platform elements:

SOAP (Simple Object Access Protocol)
a framework for exchanging XML-based information in a network
It defines a uniform way of passing XML-encoded data.
In also defines a way to perform remote procedure calls (RPCs) using HTTP as the underlying communication protocol.

UDDI (Universal Description, Discovery and Integration)
provides a registry mechanism for clients and servers to find each other
uses SOAP for communication

WSDL (Web Services Description Language)
an XML-based language for describing network services
WSDL descriptions of capabilities and locations of services
like an interface description language for Web services
communication using SOAP or direct HTTP

Web services uses set of tools that can be used in a number of ways. The three most common styles of use are RPC, SOA and REST.
Remote procedure calls
RPC Web services present a distributed function (or method) call interface that is familiar to many developers.
Typically, the basic unit of RPC Web services is the WSDL operation
The first Web services tools were focused on RPC, and as a result this style is widely deployed and supported. However, it is sometimes criticized for not being loosely coupled, because it was often implemented by mapping services directly to language-specific functions or method calls. Many vendors felt this approach to be a dead end, and pushed for RPC to be disallowed in the WS-I Basic Profile.

Service-oriented architecture
Web services can also be used to implement an architecture according to Service-oriented architecture (SOA) concepts, where the basic unit of communication is a message, rather than an operation. This is often referred to as "message-oriented" services.

SOA Web services are supported by most major software vendors and industry analysts. Unlike RPC Web services, loose coupling is more likely, because the focus is on the "contract" that WSDL provides, rather than the underlying implementation details.

Representational state transfer
Representational State Transfer (REST) attempts to describe architectures which use HTTP or similar protocols by constraining the interface to a set of well-known, standard operations (like GET, POST, PUT, DELETE for HTTP). Here, the focus is on interacting with stateful resources, rather than messages or operations. An architecture based on REST (one that is 'RESTful') can use WSDL to describe SOAP messaging over HTTP, which defines the operations, can be implemented as an abstraction purely on top of SOAP (e.g., WS-Transfer), or can be created without using SOAP at all.

 

Further to understand how RPC model can be defined using the following steps:

1. A client application builds an XML document containing the URI of the server that will service the request, the name of the remote method to execute, and the parameters associated with that method.
2. The targeted server receives and unwinds the XML document. It then executes the named method.
3. After the named method has returned its results, the results are packed into a response XML document, which is sent back to the calling client.
4. The client application receives the response and unwinds the results, which contains the response of the invocated method.

Web Services tools with Java
The most essential Java tools for Web service development:
xml.apache.org/axis - Apache AXIS (Apache Extensible Interaction System)
a Java-based implementation of SOAP+WSDL
largely allows the programmer to forget these technologies  typically used together with Tomcat

www.alphaworks.ibm.com/tech/ettk - alphaWorks's EETK (Emerging Technologies Toolkit)
support for SOAP, WSDL, UDDI and much more...

java.sun.com/webservices/downloads/webservicespack.html - Sun's Java WSDP (Web Services Developer Pack)
support for SOAP, WSDL, UDDI, ...
JAX-RPC maps SOAP/WSDL to RMI (Java Remote Method Invocations)

imp links
Tutorial on what is web service
http://www.w3schools.com/webservices/default.asp

Typical soap RPC based web service example
http://www.onjava.com/pub/a/onjava/2002/02/27/tomcat.html

List of web service framework on which web service can be built
http://en.wikipedia.org/wiki/List_of_Web_service_Frameworks

Wikipedia definition and detail note on web service
http://en.wikipedia.org/wiki/Web_service

Top Ten FAQs for Web Services
http://webservices.xml.com/pub/a/ws/2002/02/12/webservicefaqs.html