Understanding a LoadRunner Web (HTTP/HTML) script

Once a script is recorded using the Virtual User Generator with the best recording options (please refer to the link ), the next important thing is to understand the complete script and the functions recorded in the script.

At a very high level, the entire script can be classified into three groups i.e.

• Protocol Specific functions
• LoadRunner functions
• Language specific functions

Protocol Specific functions: 
These functions can be used with a specific protocol only and they cannot be used with any other protocol. For a Web (Http/HTML) protocol, the commonly seen functions are:

• web_url(), web_image(), web_link() - All these functions are used to simulate a GET request
• Web_submit_form() is used to simulate a POST request
• web_submit_data() is used to simulate both GET and POST requests. The attribute "Method" tells if that request is a GET or POST.

web_reg_find() and web_reg_save_param() are the service functions used for page verification and correlation respectively.
All the above functions are starting with the word "web" indicating that they are web protocol specific functions and cannot be used outside Web protocol.
Few other web protocol functions are:

web_set_user(), web_set_max_html_param_len(), web_cache_cleanup(), web_cleanup_cookies() etc

LoadRunner functions:
All these functions are loadRunner specific and can be used across any protocol. All these functions start with lr_. Few examples for LoadRunner functions are:

lr_start_transaction() - To start a transaction to measure the response time
lr_end_transaction() - To stop a transaction to measure the response time
lr_think_time() - to wait for a specified duration
lr_eval_string() - To evaluate the value of a LoadRunner parameter placed in {}
lr_save_string() - To save a string to a LoadRunner parameter
lr_save_int() - To save an integer value to a LoadRunner parameter
lr_exit() - exit a loop/iteration/user from execution
lr_set_debug_message() - To control the log settings of the replay log
lr_output_message() - To write to the output log with an information level setting
lr_error_message() - To write to the output message with error level
etc

Language specific functions:
These functions are not a part of the tool, all the functions that can be used in a language(C or Java) can be directly used in the script provided that the protocol is supported in a language.
For example, The Web(HTTP/HTML) protocol is supported by the C language. Hence all the C functions can be used directly in Web script.
The commonly used functions are:

rand() - to generate a random number
atoi()  - converting a string into an integer
sprintf - Saving a formatted output to a string buffer
strcat - string concatenation
strcpy - Copying into a string
strtok - String tokenizer function

The JAVA protocol cannot support the C language and hence these functions cannot be used by the Java based script.

Hope the information provided about the classification a LoadRunner script is helpful in understanding how the scripts work.