Event time versus processing time
There are many reasons why processing may be delayed: queueing, network faults (see “Unreliable Networks” on page 277), a performance issue leading to contention in the message broker or processor, a restart of the stream consumer, or reprocessing of past events (see “Replaying old messages” on page 451) while recovering from a fault or after fixing a bug in the code.
Moreover, message delays can also lead to unpredictable ordering of messages. For example, say a user first makes one web request (which is handled by web server A), and then a second request (which is handled by server B). A and B emit events describing the requests they handled, but B’s event reaches the message broker before A’s event does. Now stream processors will first see the B event and then the A event, even though they actually occurred in the opposite order.
If it helps to have an analogy, consider the Star Wars movies: Episode IV was released in 1977, Episode V in 1980, and Episode VI in 1983, followed by Episodes I, II, and III in 1999, 2002, and 2005, respectively, and Episode VII in 2015 .“ If you watched the movies in the order they came out, the order in which you processed the movies is inconsistent with the order of their narrative. (The episode number is like the event timestamp, and the date when you watched the movie is the processing time.) As humans, we are able to cope with such discontinuities, but stream processing algorithms need to be specifically written to accommodate such timing and ordering issues.
Confusing event time and processing time leads to bad data. For example, say you have a stream processor that measures the rate of requests (counting the number of requests per second). If you redeploy the stream processor, it may be shut down for a minute and process the backlog of events when it comes back up. If you measure the rate based on the processing time, it will look as if there was a sudden anomalous spike of requests while processing the backlog, when in fact the real rate of requests was steady (Figure 11-7). 
Figure 11-7. Windowing by processing time introduces artifacts due to variations in processing rate.
-  Thank you to Kostas Kloudas from the Flink community for coming up with this analogy.