In a system, a call may be holding the resources while other calls wait for the same resources. These resources are released when the other one finishes using them.
Let's come to the technical words--actually, blocking a call means some operations in the application or system that take a longer time to complete, such as file I/O operations and database access using blocking drives. The following is a diagram of blocking calls for the JDBC operation in a system:
As you can see in the preceding diagram, the blocking operations, shown here in red, are the ones where the user calls the servlet to fetch data, then that moves to the JDBC and DB connection with the DB server. Until that time, the current thread waits for the result set from the DB server. If the DB server has latency, then this wait time can increase. That means that thread execution depends on the DB server latency.
Let's look at how to make this a non-blocking execution.