Explain background processes in Oracle 19c.
In Oracle Database 19c, the architecture has evolved to be more "intelligent" and autonomous, but the core engine still relies on a specialized team of Background Processes.
If the Oracle Instance is a high-performance restaurant, the SGA is the kitchen, and these processes are the specialized chefs, cleaners, and managers working behind the scenes to ensure the "diners" (your users) get their data fast and fresh.
These are the "mandatory" processes. If one of these stops, the entire instance usually crashes to protect data integrity.
The Mover: Its job is to write "dirty buffers" (data that has been changed in memory) from the SGA to the physical Data Files on disk.
19c Fact: In 19c, you can have up to 100 database writer processes (DBW0 through DBW9 and DBWa through DBWj) to handle massive I/O throughput.
The Recorder: This is the most time-critical process. It flushes the Redo Log Buffer to the Online Redo Log files on disk.
The Rule: It writes when a user commits, when the buffer is 1/3 full, or every 3 seconds. Data isn't "safe" until LGWR finishes its job.
The Synchronizer: It doesn't write data; it signals when data should be written. It updates data file headers and control files to record the "checkpoint," ensuring Oracle knows the exact point from which to start recovery if a crash occurs.
These processes manage the health of the instance and handle unexpected errors.
The Janitor: If a user's session crashes (e.g., their laptop dies), PMON detects the failure. It rolls back their uncommitted changes, releases their row locks, and cleans up their memory (PGA).
The Medic: When you start the database after a crash, SMON performs Instance Recovery. It also handles housekeeping, like cleaning up temporary segments that are no longer needed.
The Receptionist: In older versions, PMON did this. In 19c, LREG is a dedicated process that notifies the Oracle Listener that the database is "Open" and ready for connections.
19c introduces or refines processes that make the database "Self-Managing."
The Analysts: These processes are the engines behind AWR (Automatic Workload Repository). They take "snapshots" of performance data every hour. MMON also triggers alerts if the database hits a performance threshold.
The Historian: Only active in ARCHIVELOG mode. When an Online Redo Log file fills up, ARCn copies it to a permanent archive. This is what allows you to recover data from five years ago.
| Process | Role | Criticality | Analogy |
| DBWn | Writes Data to Disk | High | The Delivery Truck |
| LGWR | Writes Logs to Disk | Extreme | The Legal Notary |
| PMON | Cleans up after users | High | The Cleanup Crew |
| SMON | Recovers the system | High | The First Responder |
| LREG | Registers with Listener | Medium | The Signage |
| MMON | Performance Monitoring | Low | The Data Scientist |
One of the major shifts in 19c (specifically on Linux/Unix) is the move toward Threaded Architecture. By setting the parameter THREADED_EXECUTION = TRUE, Oracle can run these background processes as threads within a few large operating system processes, rather than hundreds of individual OS-level processes. This reduces the memory overhead and CPU context switching.
If you ever see a process named CJQ0, don't panic! That’s the Job Queue Coordinator. In 19c, it's busier than ever because so many maintenance tasks (like statistics gathering) are now automated.