Hello everyone! I have just upgraded to OCP Java SE 11.
My Java certification path went through 1Z0-808 (97%) -> 1Z0-809 (82%) -> 1Z0-817 (88%)
First of all: the information on Oracle 1Z0-817 official page
1Z0-817 has 80 questions for a total of 180 minutes! Beware!!
In terms of difficulty it is similar to 809, only that it has the concurency, jdbs questions replaced with modularity.
Questions about java modules where mostly ok, some of them included questions about command line parameters for different tools: javac, java, jar, jdeps, jlink
For the exam, I read almost entirely The Java Module System written by Nicolai Parlog, Mannning Publishing. I bought it directly on their site, it's early access program now. This book has a very good difficulty curve, it takes from the basics and goes into deep details.
Trust me, you will not understand modules only from reading online tutorials, they rarely explain the important details like module graph resolution, reading. Without them you are stuck not understanding why JVM throws module not found error even when you have added it in the module-path.
The exam had tons of questions about Lambda, Streams, Functional Interfaces and Comparator interfaces. This can be safely considered the core of the exam. If you have taken 809 exam, take this one as well. You are already 50-60% qualified for it.
And most important, read everything Mikalai Zaikin
has written about 1Z0-817 exam