OCP exam notes

1. The database server comprises Oracle instances and databases.

      An Oracle database is a set of files on disk.

      The instance is a set of processes and memory structures.

2. Processing model ---- client-server two-tier model

                  Client-side components                                                       Server-side components

                  user  <--->  user process  <-----Oracle Net------->  Server process  <----> Instance <----> Database 

3. Distributed Systems Architectures

    Real Application Cluster(RAC)

    Streams

    Dataguard                            

4. Oracle database Memory Structure

     Mandatory structure:

          The database buffer cache ------ Oracle's work area for executing SQL

          The log buffer ----- a small, short-term staging area for change vectors before they are written to the redo log on disk(The size of the log buffer is static, can't be automatically managed)

          The shared pool

               The library cache / The data dictionary cache / The PL/SQL area / The SQL query and PL/SQL fuction result cache

     Optionally:

          A large pool / A java pool / A streams pool

5. Oracle database Process Structure

     SMON, the System Monitor ---- has the task of mounting and opening a database

     PMON, the Process Monitor ---- when session terminated abnormally, PMON will destory the server process, return PGA, roll back incomplete transaction

     DBWn, the Database Writer ---- writes dirty buffers from the database buffer cache to the datafiles

                                                        ---- What will cause DBWR to write? No free buffers, too many dirty buffers, a three-second timeout, or a checkpoint.

     LGWR, the Log Writer ---- writes the contents of the log buffer to the online log files on disk

                                             ---- What will cause LGWR to write? Session issues a COMMIT, the log buffer is one-third full, DBWn is about to write dirty buffers.

     CKPT, the Checkpoint Process

     MMON, the Manageability Monitor ---- process for many of the self-monitoring and self-tuning capabilities of database

     MMNL, the Manageability Monitor Light

     MMAN, the Memory Manager

     ARCn, the Archiver

     RECO, the Recoverer Process

6. Oracle database Storage Structure

     Physical Database Structure:

         The required files:

               the control file ---- contains pointers to the rest of the database (damage to any controlfile copy will cause the database instance to terminate immediately)

                                         ---- contains the relationship between tablespaces and datafiles

               the online redo log files ---- An oracle database requires at least two groups of at least one member each

               the datafiles

         The external files: the initialization parameter file, the password file, the archive redo log files, the log and trace files.

    Logical Database Structure:

         Oracle abstracts the logical from the physical storage by means of the tablespace, and the relationship between segments and datafiles is many-to-many

         Tablespaces -----< Segments(not contiguous extents) ----< Extents(contiguous blocks) ----< Oracle blocks

         Data dictionary ---- created at database creation time, contains metadata describes the database both physically and logically(system and sysaux tablespaces)

         Views to query the dictionary ---- prefixed DBA_ > ALL_ > USER_