Leslie Lamport defined the term distributed system as follows:
"A distributed system is one in which I cannot get any work done because some machine I have never heard of has crashed."
Resource sharing through World Wide Web (aka WWW), a network of connected computers (aka a cluster), is a good example of distributed systems. These distributed environments are often complex and lots of heterogeneity occurs frequently. Testing in these kinds of the heterogeneous environments is also challenging. In this section, at first, we will observe some commons issues that are often raised while working with such system.