Application Logging

SAP provides a method to use a standard logging facility for all of your custom ABAP programs. It consists of several transaction codes, tables, and function modules. By using the SAP functionality, it is possible to have a standard way to store error messages, making the handling of errors much simpler, and increasing the maintainablity of code.
Transaction Codes: SLG0 is used to configure categories for the error messages, and SLG1 is used to view the messages.
Tables: BALHDR is the main (header table), and BALM contains all of the detailed messages.
Function Modules: Much like the SAP function modules used to send email, these function modules would benefit from having a simpler interface wrapped around them. The function modules can be found in Function Group SLG0. Look for the INIT, HEADER, and MESSAGE function modules, as well as the WRITE_DB (I think that is the name...). If you want examples of how these function modules are used, please do a where used on them. I am unable to post the SAP examples here.
All APPL_LOG function modules are from 3.0. Starting at 4.6, see function groups that start with SBAL.
3.0 to 4.5 Function Modules
Create application log
Function group SLG0
You write the application log records with these function modules.
APPL_LOG_WRITE_HEADER	With this function module, you write the log header data in local memory.
APPL_LOG_WRITE_LOG_PARAMETERS	With this function module, you write the name of the log parameters and the associated values for the specified object or sub-object in local memory. If this function module is called repeatedly for the same object or sub-object, the existing parameters are updated accordingly. If you do not specify an object or sub-object with the call, the most recently used is assumed.
APPL_LOG_WRITE_MESSAGES	With this function module you write one or more messages, without parameters, in local memory.
APPL_LOG_WRITE_SINGLE_MESSAGE	With this function module you write a single message, without parameters, in local memory. If no header entry has yet been written for the object or sub-object, it is created. If you do not specify an object or sub-object with the call, the most recently used is assumed.
APPL_LOG_WRITE_MESSAGE_PARAMS	With this function module you write a single message, with parameters, in local memory. Otherwise the function module works like APPL_LOG_WRITE_SINGLE_MESSAGE.
APPL_LOG_SET_OBJECT	With this function module, you create a new object or sub-object for writing in local memory. With a flag you can control whether the APPL_LOG_WRITE_... messages are written in local memory or are output on the screen.
APPL_LOG_INIT	This function module checks whether the specified object or sub-object exists and deletes all existing associated data in local memory.
APPL_LOG_WRITE_DB	With this function module you write all data for the specified object or sub-object in local memory to the database. If the log for the object or sub-object in question is new, the log number is returned to the calling program.
Display application log
Function group SLG3
With these function modules you display logs for analysis.
APPL_LOG_DISPLAY	With this function module you can analyze logs in the database.
APPL_LOG_DISPLAY_INTERN	With this function module you can analyze logs in local memory, e.g. when you have only collected log records at runtime and do not want to write to the database.
Read application log
Function group SLG1
If you want to analyze the log yourself, you can read the logs with these function modules.
APPL_LOG_READ_DB	With this function module you read the log data in the database for an object or sub-object according to specified selection conditions.
APPL_LOG_READ_INTERN	With this function module you read all log data whose log class has at least the specified value, from local memory, for the specified object or sub-object.
Delete application log
Function group SLG2
With this function module you delete logs.
APPL_LOG_DELETE	With this function module you delete logs in the database according to specified selection conditions
4.6 Function modules for writing application logs.
Much more flexible than old functions. Modules are relatively well documented. Read the documentation.
Run SBAL_DOCUMENTATION to see detailed documentation.
Simple call
BAL_LOG_CREATE	Create log with header data
BAL_LOG_MSG_ADD	Put message in log
BAL_DSP_LOG_DISPLAY	Display messages in memory
Methods of collecting messages
BAL_LOG_CREATE	Create log with header data
BAL_LOG_MSG_ADD	Put message in log
BAL_LOG_MSG_CUMULATE	Add message cumulated
BAL_LOG_MSG_REPLACE	Replace last message
BAL_GLB_MSG_CURRENT_HANDLE_GET	Get current message handle
BAL_LOG_MSG_DELETE	Delete message
BAL_LOG_MSG_CHANGE	Change message
BAL_GLB_MSG_DEFAULTS_GET	Get message defaults
Program SBAL_DEMO_02
Find and read messages in memory
BAL_GLB_SEARCH_LOG	Find logs in memory
BAL_GLB_SEARCH_MSG	Find messages in memory
BAL_LOG_HDR_READ	Read log header and other data
BAL_LOG_MSG_READ	Read message and other data
Program SBAL_DEMO_03
Log display and display profiles
BAL_DSP_LOG_DISPLAY	Display Log
BAL_DSP_PROFILE_STANDARD_GET	Standard profile (SLG1) for a lot of logs
BAL_DSP_PROFILE_SINGLE_LOG_GET	Standard profile (SLG1) for one log
BAL_DSP_PROFILE_NO_TREE_GET	Display without tree (fullscreen)
BAL_DSP_PROFILE_POPUP_GET	Display without tree (popup)
BAL_DSP_PROFILE_DETLEVEL_GET	Message hierarchy in DETLEVEL
Program SBAL_DEMO_04
Display log in subscreen
BAL_DSP_OUTPUT_INIT	Initialize output
BAL_DSP_OUTPUT_SET_DATA	Set dataset to be displayed
BAL_DSP_OUTPUT_FREE	End output
Program SBAL_DEMO_04_SUBSCREEN
Log display in Control Container
BAL_CNTL_CREATE	Create Control for log display
BAL_CNTL_REFRESH	Put new data in log display
BAL_CNTL_FREE	Release Control
Program SBAL_DEMO_04_CONTROL
Save, load, find and delete logs in the database
BAL_DB_SAVE	Save logs in the database
BAL_DB_SAVE_PREPARE	Prepare save
BAL_DB_SEARCH	Find logs in the database
BAL_DB_LOAD	Load logs from the database
BAL_LOG_REFRESH	Delete log from memory
BAL_GLB_MEMORY_REFRESH	(Partially) reset global memory
BAL_DB_DELETE	Delete logs from the database
Program SBAL_DEMO_05
Change logs
BAL_DB_ENQUEUE	Lock log
BAL_DB_LOAD	Load log(s)
BAL_DB_SAVE	Save log(s)
BAL_DB_DEQUEUE	Unlock log
BAL_LOG_MSG_CHANGE	Change message
BAL_LOG_MSG_DELETE	Delete message
BAL_LOG_HDR_CHANGE	Change log header
BAL_LOG_DELETE	Delete log (from database also at Save)
BAL_LOG_REFRESH	Delete log from memory
Start transaction: Initializations
BAL_GLB_CONFIG_SET	Set configuration
BAL_GLB_CONFIG_GET	Read configuration
BAL_GLB_AUTHORIZATION_GET	Assign authorization
BAL_GLB_AUTHORIZATION_RESET	Reset authorization
BAL_GLB_MEMORY_REFRESH	(Partially) initialize memory
BAL_MSG_DISPLAY_ABAP	Output message as ABAP-MESSAGE
Working roll area-independently
BAL_GLB_MEMORY_EXPORT	Put function group memory in ABAP-MEMORY
BAL_GLB_MEMORY_IMPORT	Get function group memory from ABAP-MEMORY
Data and existence checks
BAL_LOG_HDR_CHECK	Check log header data for consistency
BAL_LOG_MSG_CHECK	Check message data for consistency
BAL_LOG_EXIST	Check existence of a log in memory
BAL_LOG_MSG_EXIST	Check existence of a message in memory
Read or check Application Log object and subobject
BAL_OBJECT_SELECT	Read Application Log objects table record
BAL_SUBOBJECT_SELECT	Read subobject table record
BAL_OBJECT_SUBOBJECT	Check whether object and subobject exist and the combination is allowed
Log Display: Detail screens
BAL_DSP_MSG_LONGTEXT	Display message long text
BAL_DSP_MSG_PARAMETERS	Either output extended long text or call a callback routine (based on the data in BAL_S_MSG-PARAMS)
BAL_DSP_MSG_TECHNICAL_DATA	Output technical data of a message such as work area, error number, etc.
BAL_DSP_LOG_PARAMETERS	Either output extended long text or call a callback routine (based on the data in BAL_S_LOG-PARAMS)
BAL_DSP_LOG_TECHNICAL_DATA	Output all log header data
Here is some documentation on SAP's logging:
PDF format -
HTML document -