Best explained using diagrams
Resources
http://www-01.ibm.com/support/knowledgecenter/SSPK3V_6.3.0/com.ibm.swg.im.soliddb.sql.doc/doc/pessimistic.vs.optimistic.concurrency.control.html
http://stackoverflow.com/questions/129329/optimistic-vs-pessimistic-locking?lq=1
http://www.toadworld.com/platforms/oracle/b/weblog/archive/2014/10/03/dealing-with-oracle-lock-contention.aspx
https://petermeinl.wordpress.com/2011/03/14/db-concurrency-control-patterns-for-applications/
