docs(crash): Add sequence diagram

Signed-off-by: nkomonen-amazon <[email protected]>

Author: nkomonen-amazon

docs/telemetry-perf.md

@@ -258,3 +258,66 @@ How long it took from when the user stopped pressing a key to when they were sho
         rHandler->>User: add to already shown results
         end
 ```
+
+## Crash Monitoring
+
+We make an attempt to gather information regarding when the IDE crashes. This is the diagram of the steps that take place.
+
+## Sequence Diagram
+
+`Crash Monitoring`: The "service" that allows us to detect and react to crashes
+
+`Heartbeat`: Sends heartbeats which signal that the extension is still running and has not crashed
+
+`Crash Checker`: Observes the heartbeats, reporting a telemetry event if a crash is detected
+
+`File System State`: The user's file system that we store our heartbeat state
+
+```mermaid
+%%{init: {'theme':'default'}}%%
+sequenceDiagram
+  autonumber
+
+  participant VSC as VS Code
+  participant Monitoring as Crash Monitoring
+  participant Checker as Crash Checker
+  participant Heartbeat as Heartbeat
+  participant State as File System State
+
+  rect rgb(121, 210, 121)
+    alt Extension Startup
+        VSC ->> Monitoring: activate() - Start Monitoring
+
+        Monitoring ->> Heartbeat: Start Heartbeats
+        Heartbeat ->> State: Send Initial Heartbeat <br/> (write a file w/ timestamp)
+        rect rgb(64, 191, 64)
+            par every 5 minutes
+                Heartbeat ->> State: Send Heartbeat <br/> (overwrite file w/ new timestamp)
+            end
+        end
+
+        Monitoring ->> Checker: Start Crash Checking
+        rect rgb(64, 191, 64)
+            par every 10 minutes
+                Checker ->> Checker: If computer went to sleep, skip this iteration
+                Checker ->> State: Request all heartbeat timestamps
+                State ->> Checker: Get all heartbeat timestamps
+                loop for each timestamp stale by >= 10 minutes
+                    Checker ->> State: Delete heartbeat file
+                    Checker ->> Checker: Send crash metric: session_end
+                end
+            end
+        end
+    end
+  end
+
+  rect rgb(255, 128, 128)
+    alt Graceful Shutdown
+        VSC ->> Monitoring: deactivate() - Stop Monitoring
+        Monitoring ->> Checker: Stop
+        Monitoring ->> Heartbeat: Stop
+        Heartbeat ->> State: Delete timestamp file <br/> (This is missed when a crash happens)
+    end
+  end
+
+```