What is the approach taken by Oracle to pass ACID test of a database transaction?

A user process is created and maintained to execute the software code of an application  program.  It  is  a  shadow  process  created automatically to facilitate communication between the user and the server process.

Server  Process  handle  requests  from  connected  user  process. A server process is in charge of communicating with the user process and interacting with ORACLE carry out requests of the associated user process.

Dedicated Server Configuration and Multi-threaded Server Configuration.

In a Dedicated Server Configuration a Server Process handles requests for a Single User Process.

In  a Multi-threaded Server Configuration many user processes share a group of server process.

A configuration for loosely coupled systems where multiple instance share a single physical database is called Parallel Server.

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

Process Monitor (PMON)  performs process recovery when a user process fails PMON  is  responsible  for cleaning up the cache and Freeing resources that the  process  was using PMON also checks on dispatcher and server processes and restarts them if they have failed.

Archiver  (ARCH) copies the on-line redo log files to archival storage when they  are  full.  ARCH is active only when a database's redo log is used in ARCHIVELOG mode.

RECOver (RECO) is used to resolve distributed transactions that are pending due  to  a  network  or  system failure in a distributed database. At timed intervals,the  local  RECO  attempts  to  connect  to  remote databases and automatically  complete  the commit or rollback of the local portion of any pending distributed transactions.

Dispatcher   (Dnnn)  process  is  responsible  for  routing  requests  from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.

Atleast one Dispatcher process is created for every communication protocol in use.

Lock  (LCKn)  are  used for inter-instance locking when the ORACLE Parallel Server option is used.

Though  a single LCK process is sufficient for most Parallel Server systemsupto Ten Locks (LCK0,....LCK9) are used for inter-instance locking.