by Samuel Hack and Sebastian Wachter.
Most people assume that the data coming from a database is correct. For most applications this is true, but when the databases are used in systems where the database is at its limit, this is no longer always the case. What happens if during a query of a value exactly this value is changed by another user? Which entry was first when two users create different values at the same time. Especially in systems where one database server is not enough it is very important to choose the right rules for Isolation and Consistency, if this is not the case it can lead to data loss or in worst case to incorrect values in the application.
Most database systems promise virtually nothing by default. Therefore it is important to learn what isolation and consistency level the database promises and then adjust it to fit the application.
In this paper we will give an overview of what isolation and consistency levels are, what different levels are available in databases, and what errors can occur in databases. Afterwards we will give a short introduction to the CAP theorem and discuss the problems that it includes for distributed systems.