What is an oracle instance?

Instance is a combination of memory structure and process structure. Memory structure is SGA and Process structure is background processes. SGA - System or Shared Global Area. Components of SGA:- DBBC - Database Buffer Cache SP - Shared Pool. It is futher divided into Library Cache(LC) and Data Dictionary Cache(DDC) or Row Cache. RLB - Redolog Buffer Background Process:- 1. Mandatory Processes- a. SMON - System Monitor b. PMON - Process Monitor c. DBWR - Database writer d. LGWR - Log Writer e. CKPT - Check point f. RECO - Recoverer 2. Optional Process:- a. ARCN b. RBAC c. MMAN d. MMON e. MMNL