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+# Online Transaction Processing Systems
+
+Systems that need real time support for querying and updating of databases by one or more concurrent users.
+
+Data is updated in real time frequently, but must maintain correctness of the database state regardless of hardware and software failures, and multiple users accessing said data.
+
+## Examples of OLTPs
+
+- Banking / Credit Card processing systems
+- Transport reservation systems
+- Trading / Brokerage systems
+- E-commerce
+
+### Example 1
+
+- Two students registering for the same class ( Student Enrolment Database )
+
+NumEnroled = 39
+MaxEnroled = 40
+
+If two students attempt to enrol at the same time, NumEnroled = 41, which is in an inconsistent state, violating semantic constraints, when processing requests from multiple concurrent users.
+
+### Example 2
+
+- Two people attempt to withdraw from two different ATMs in a joint account
+- Person A withdraws £100 at ATM 1
+- Person B withdraws £50 from ATM 2
+- Initial balance £400
+
+Final balance should be £250 no matter who withdraws first.
+However, if they withdraw at the same time, the final balance would be £300 OR £350, depending on various attributes of the ATMs ( physical location, processing speed, current load, etc. ), as £400 total balance would be reported to both ATMs
+
+## Challenges of OLTP
+
+Although the AQL code is written correctly, the database may be in an inconsistent state after processing transactions due to failures of hard / software / multiple users accessing the db.
+A consistent state of the database means it satisfied all the constraints specified in the schema ( overall description of the database: pk, fk, referential integrity, etc. ), and any other constraints on the database that should hold ( eg. semantic constraints. )
+
+# Transaction 
+
+An action / series of actions, carried out by a single user or application program that reads or updates the contents of the database. 
+- A transaction is treated as a logical unit of work on the database. This may be 
+	- An entire program
+	- Part of a program
+	- A single statement 
+
+