Update 82-code-structure.md

docs/_docs/82-code-structure.md

@@ -8,3 +8,20 @@ classes: wide
 ---
 
 Check out https://github.com/microsoft/FASTER. More details will be added in future.
+
+## Lifecycle of a Read operation for disk data
+
+Here is the sequence of steps from FASTER side, during a Read operation for disk data:
+1.	User issues `Read` operation
+2.	We create a `PendingContext` object for the read
+3.	The `HandleOperationStatus()` call adds `PendingContext` object to `ioPendingRequests` dictionary
+4.	Create `AsyncIOContext`  which has pointer to a `readyResponses` queue (also called `callbackQueue`), for that session
+5.	Issue IO with `AsyncIOContext` as param
+6.	…
+7.	IO callback accesses `readyResponses`queue, and enqueues the result in there (result is just the `AsyncIOContext` itself)
+8.	…
+9.	User calls `CompletePending` on session, which does this:
+    * Dequeue entry from `readyResponses` queue
+    * Remove item from `ioPendingRequests` dictionary
+    * Finally return result to user via `ReadCompletionCallback`
+