Oracle Applications/E-Business Suite Release 12 introduces a new concept of a top-level directory for an Applications Instance which is known as Instance Home and is denoted the environment variable $INST_TOP.
Instance Home contains all the config files, log files, SSL certificates etc.
Advantages of new INSTANCE HOME
The additional Instance Home makes the middle tier more easy to manage and organised since the data is kept separate from the config files. The Instance Home also has the ability to share the Applications and Technology stack code across multiple instances.
Another advantage of the Instance Home is that the Autoconfig no longer writes anything to the APPL_TOP and ORACLE_HOME directories, everything is now written in the INST_TOP as a result APPL_TOP and ORACLE_HOME can also be made read only file system if required . Earlier say the adpatch used to write the log file in APPL_TOP/admin directory but with the new model the APPL_CONFIG_HOME/admin is used.
To create a new instance that shares an existing middle-tier, just create a new instance_top with proper config files and NFS Mount the middle tier in the server.The basic structure of the Instance Home is:$APPS_BASE/inst/apps/$CONTEXT_NAME or also defined by environment variable $INST_TOP, where APPS_BASE (which does not have or need a corresponding environment variable) is the top level of the Applications installation and $CONTEXT_NAME is the highest level at which the Applications context exists (format $SID_$hostname).
Instance Home Directory Structure
$INST_TOP/admin/scripts ($ADMIN_SCRIPTS_HOME): All AD scripts are located here
$INST_TOP/appl ($APPL_CONFIG_HOME) : For standalone envs, this is set to $APPL_TOP
$INST_TOP/appl/fnd/12.0.0/secure FND_SECURE: The dbc file is located here
$INST_TOP/appl/admin All the env config files are located here
$INST_TOP/certs SSL Certificates go here
$INST_TOP/logs LOG_HOME: Central log file location. All log files are placed here (except adconfig)
$INST_TOP/logs/ora/10.1.2 ‘C’ Oracle home config, Contains tnsnames and forms listener servlet config files
$INST_TOP/logs/ora/10.1.3 Apache & OC4J config home, Apache, OC4J and opmn. This is the ‘Java’ oracle home configuration for OPMN, Apache and OC4J
$INST_TOP/pids Apache/Forms server PID files are located here
$INST_TOP/portal Apache’s DocumentRoot folder