Links for 2008-04-21

Articles:

1. Simplify Ajax development with jQuery - jQuery allows us to do DOM scripting, event handling, HTML animation, Ajax and more.
2. Don't burn your Bridges - "It is not good to leave in anger. I advise that you don't burn your bridges"?? I think the point is don't express your anger when you leave.
   

Sample Chapter

1. jQuery in Action
   • Chapter 1 - Introducing jQuery: The philosophy of jQuery is "Unobtrusive JavaScript", that is, to separate behavior from structure just as css does to separate style from structure.
   • Chapter 5 - Sprucing up with animations and effects

Comet - Pushing Data over Http

As described in this wiki article, Comet is a term, coined by Alex Russell, to describe the technologies for pushing data over Http. What is Comet? Comet is a web architecture allowing a web server to push data to a web browser using Http protocol. It is realized by taking advantage of the Http persistent connection. There are many solutions available, both open and proprietary. HTML 5 specification even attempts to standardize the Comet transport by adding a new HTML element, event-source and a new data format, called the DOM event stream. However, several issues have to be considered when one tries to adopt the technology.

First, most web servers assign a thread for handling a connection request. Since Comet keeps its http connection with a web server open, a web server can only serve limited amount of Comet connection and will run out of its threads pretty soon. A dedicated comet server should be deployed for pushing data.

Second, the traditional approach of scaling web applications by adding web servers might not be applicable to a Comet server. If a user connect to multiple event sources, it's difficult to distribute users among multiple Comet servers.

Third, proxy servers and firewalls could drop connections that have been open for too long. Though some Comet frameworks would tear down and recreate connection constantly, some proxies could send back buffered data and deceive clients into believing a connection is established.

First Taste of SubSonic

Having heard about SubSonic for almost a year, I finally decided to give it a try. I used SubSonic 2.1 Beta 2 and connected to Oracle9i database. Here were the two errors I encountered in the process, mostly related to stored procedures, and how I worked around the troubles. For the first error, I can use the precompiled SubSonic.dll directly. But to fix the second error, I had to modify the SubSonic source code.

The first error was related to the parameters I pass to a stored procedure. I got the following error after executing the stored procedure. Checking the source code, I found SubSonic add ':' as a prefix for each parameter name of stored procedures. Removing the colon before calling the Execute method solve the problem.Publish Post

System.Data.OracleClient.OracleException: ORA-06550: line 1, column 44:
PLS-00103: Encountered the symbol ":" when expecting one of the following:

( - + case mod new not null <an identifier>
<a double-quoted delimited-identifier> <a bind variable> avg
count current exists max min prior sql stddev sum variance
execute forall merge time timestamp interval date
<a string literal with character set specification>
<a number> <a single-quoted SQL string> pipe
The symbol "( was inserted before ":" to continue.

The second error was related to the output parameter of a stored procedure. The output parameter I expected was a DateTime but I kept getting the following error.

System.Data.OracleClient.OracleException: ORA-06502: PL/SQL: numeric or value error

Checking the source code, I found the AddParams method in OracleDataProvider class didn't set the parameter direction. I checked MySqlDataProvider.cs and made a modification to OracleDataProvider.cs in the same manner. The error was gone. Also, I found the CheckoutOutputParams method in MySqlDataProvider.cs didn't exist in OracleDataProvider. So I also added the same logic to OracleDataProvider.

69 private static void AddParams(OracleCommand cmd, QueryCommand qry)

70 {

71 if(qry.Parameters != null)

72 {

73 foreach(QueryParameter param in qry.Parameters)

74 {

75 OracleParameter sqlParam = new OracleParameter();

76 sqlParam.DbType = param.DataType;

77 sqlParam.OracleType = GetOracleType(param.DataType);

78 sqlParam.ParameterName = param.ParameterName;

79 sqlParam.Value = param.ParameterValue;

80 if (qry.CommandType == CommandType.StoredProcedure)

81 {

82 switch (param.Mode)

83 {

84 case ParameterDirection.InputOutput:

85 sqlParam.Direction = ParameterDirection.InputOutput;

86 break;

87 case ParameterDirection.Output:

88 sqlParam.Direction = ParameterDirection.Output;

89 break;

90 case ParameterDirection.ReturnValue:

91 sqlParam.Direction = ParameterDirection.ReturnValue;

92 break;

93 case ParameterDirection.Input:

94 sqlParam.Direction = ParameterDirection.Input;

95 break;

96 }

97 }

98 cmd.Parameters.Add(sqlParam);

99 }

100 }

101 }