Add info on exercise 24.3

Author: Stratus3D

chapter_24/README.md

@@ -43,3 +43,22 @@ ok
 ```
 
 **3. Make a key value store where some key-value pairs are persistent and others are transient. Calling `put(Key, memory, Val)` will store a key-value pair in memory, `put(Key, disk, Val)` should store the data on disk. Use a pair of processes to do this, one for the persistent store and one for the disk store. Reuse the earlier code in the chapter.**
+
+Solution in the [exercise_3](exercise_3/) directory. It wasn't clear to me what the significance was between exercise 2 and exercise 3 other than the use of two processes. The processes themselves aren't really doing much as I chose to continue to use ETS and DETS tables to store the key value data. The gen_server implementation is a little unique in that it is effectively two different gen_server implementations in the same module. It worked out well enough, but I'm not sure it's a good pattern because there ended up being two distinct sets of code.
+
+Compile the Erlang files:
+
+```
+erlc *.erl
+```
+
+Run the tests in the shell:
+
+```
+$ erl
+Erlang/OTP 20 [erts-9.3] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
+
+Eshell V9.3  (abort with ^G)
+1> adapter_db1_test:test().
+ok
+```