Concurrency is the execution of multiple computations at the same time, and potentially on the same piece of data. These computations interleave with each other. This introduces additional complexity since these computations can interfere with each other and create unexpected behaviors. This is, again, in contrast to simplistic applications with no concurrency, where the program runs in the order the sequence of commands in the source code defined.

Two processes writing to the same resource concurrently

The chapter that talks about isolation explains the various types of problematic behaviors that arise from concurrency.

Network asynchrony, partial failures, and concurrency are the major contributors to complexity in the field of distributed systems. So, we should keep them in mind when we build distributed systems in real life. Doing so would help us anticipate edge cases and handle them appropriately.