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 Oracle Database Architecture

     Without knowing the Oracle database architecture is hard to be an Oracle DBA. Knowing the Oracle database architecture helps you to manage and to troubleshoot the Oracle database.


      The first things you have to know about the Oracle database architecture are:

     a) the main Oracle processes:

    Oracle Process     Tasks performed
  PMON (Process Monitor)

 - when a session is broken:                                                                           

  • the rollback is performed  (PROCESS  RECOVERY)

  • the memory used became available 

- restart the Dispatcher and server processes if they are stopped


- registers information about the instance and dispatcher processes with the network listener

  SMON (System Monitor) - when an instance is broken or restarted:
  • a rollback is performed  (INSTANCE  RECOVERY). In an Oracle Real Application Clusters database, the SMON process of one instance can perform instance recovery for other instances that have failed.

- cleans up temporary segments that are no longer in use

- coalesces contiguous free extents in dictionary managed tablespaces that have PCTINCREASE set to a non-zero value.

  DBWn (Database Writer)


- writes modified blocks from the database buffer cache to the data files. 

 The database selects an appropriate default setting for this initialization parameter or adjusts a user-specified setting based on the number of CPUs and the number of processor groups. However, the DB_WRITER_PROCESSES initialization parameter could be used to specifies the number of DBWn processes. Oracle Database allows a maximum of 20 database writer processes (DBW0-DBW9 and DBWa-DBWj).

  LGWR (Log Writer) - write to disk the content of redo log buffer
  CKPT (Checkpoint) - tell to the DBWn to write the buffer cache information to the disk files
  CJQ0 (Coordinator Job Queue) - check for scheduled jobs create processes Jnnn processes which will run the particular jobs.
  Jnnn (Job slave process) - run different scheduled jobs. For more information about the Oracle jobs follow the link.
  Dnnn (Dispatcher process) - receive requests from the listener and acts like a requests coordinator for the Oracle Shared (MTS) Server processes. For more information about the Shared (MTS) Server architecture follow the link.
  ARCn (Archiver Process) - archive the log files (if database is in ARCHIVELOG mode).



      b) Which is the difference between an Oracle database and an Oracle instance

          Also, I want to note that an instance has two major memory structures:   System Global Area (SGA): stores information in memory shared by processes in Oracle and Program Global Area (PGA): contains information private to a particular process.


      c) What is an Oracle Listener


     d) Oracle Shared vs. Dedicated Server Architecture


     e) Oracle Service Name

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