1. Process Architecture
Process Architecture - A portion of a program that can run independently of and concurrently with other portions of the program. Some operating systems use the terms job or task
An Oracle database server has:
User processes
Oracle processes
*(basically the ability to run a series of steps)

2. User and Oracle Processes
User (Client)Processes
Used to run the software code of an application program
Manage communication with the server process through the program interface
Two types:
Connection
Session
Oracle Processes
Used to run Oracle database server code
Two Types:
Server Processes
Background Process

3. Connection- Definition
A physical path between your client process and the database instance.
The connection is made between the client process and either a dedicated server or dispatcher.

4. Session- Definition
A session is a logical entity that exists in the instance. It is a†collection of data structures in memory that represent a unique session.
Sessions are where you execute SQL commands, commit transactions and run stored procedures.

5. Differentiating between Connection and Session
One or many Sessions may share the same physical connection to the database OR
There could be a connection to the database with no associated sessions
A physical connection could be drop leaving a session intact (idle) and would remain so until the connection was reestablished

6. Sever Process
Handles requests of user processes connected to the instance
Perform system functions:
Run SQL statements
Read data blocks from data file into the shared database buffers of the SGA
Returns results in a readable format for the application
Two configurations for Server Processes:
Dedicated server configuration
Shared server configuration

7. Dedicated Server Configuration
For each user the database application is run by a different user processes that one that runs the Oracle process

8. Dedicated Server Configuration- Diagram

9. Shared Server
The database application is run by a different user process than the one that runs the Oracle database server code

10. Shared Server- Diagram

11. Dedicated Server vs. Shared Server
No waiting for long transactions
Scalability (Provided sufficient CPU and RAM are available)
Recommended configuration
Shared Server
Reduces the number of threads
Reduces the memory needed
Maximize concurrent users

12. Background Processes Database Writer (DBWn) Log Writer (LGWR)
Checkpoint (CKPT)
System Monitor (SMON)
Process Monitor (PMON)
Archiver (ARCn)
Recoverer (RECO)
Dispatcher(Dnnn)

13. Background Processes - Diagram

14. Database Writer (DBWn)

15. Database Writer (DBWn)
Manages the buffer cache so that users processes can always find free buffers
Writes dirty buffers from the data buffer cache to the database when:
# of free buffers gets too low
Directed by the checkpoint process
Utilizes the Least Recently Used (LRU) algorithm for buffer management

16. Checkpoint (CKPT)

17. Checkpoint (CKPT)
Coordinates the process of seeing that all transactions completed or rolled back prior to the checkpoint time have been written to back-up storage
Writes a checkpoint number to the redo log file
Writes a checkpoint number to the header of each tablespace and to the control file

18. Log Writer (LGWR)

19. Log Writer (LGWR)
Clears the Redo Log Buffer by writing its contents to a Redo Log File
LGWR initiates a checkpoint when redo log file is full and then switches to the next Redo Log File
Switch occurs only if the prior checkpoint for the next Redo Log File is complete

20. Archiver (ARCn)

21. Archiver (ARCn)
Copies redo log files to a designated storage device after a log switch occurs
ARCn begins writing when a log file becomes inactive
The system will hang if the ARCn has not completed writing before that redo log file is used again

22. System Monitor (SMON)

23. System Monitor (SMON) Performs recovery at start-up
Responsible for cleaning up temporary segments no loner in use
Recovers terminated transactions skipped during start-up, once system is available
SMON regularly checks if it is needed or can be called by other processes when needed

24. Process monitor (PMON)

25. Process monitor (PMON)
Performs automatic processes recovery when a user process fails
Responsible for cleaning up the database buffer cache
Reclaims memory and other resources allocated to the failed process
Checks status or dispatcher and server, restarts if necessary
PMON regularly checks if it is needed or can be called by other processes when needed

26. Recoverer (RECO)

27. Recoverer (RECO)
Used to resolve failures involving distribute transactions
Can automatically complete the Commit or Rollback Commands, locally
Remove all related pending transactions, globally
RECO fails to connect to remote serve it automatically tries again at timed interval

28. Dispatcher (Dnnn)

29. Dispatcher (Dnnn)

30. Dispatcher (Dnnn)
Directs multiple incoming network session requests to a pool of shared server processes
Places a single program interface call on to a common request queue
Removes the result from its own response queue
Returns completed request to appropriate user

31. References Oracle Database Concepts - Chapter 9 Process Architecture
Oracle Database Developer’s Guide- Chapter 4 Managing Oracle Database Processes
Expert Oracle Database Architecture - Thomas Kyle