add sequence diagram

readme.md → README.md

@@ -8,7 +8,38 @@ The HPC Client is a self-service solution that allows Flywheel jobs and gears to
 
 ## Architecture
 
-![hpc-client-architecture 20210726](https://user-images.githubusercontent.com/75435671/127048966-af0582f7-10dc-451c-b955-4d5ab50eaf08.png)
+<!-- ![hpc-client-architecture 20210726](https://user-images.githubusercontent.com/75435671/127048966-af0582f7-10dc-451c-b955-4d5ab50eaf08.png) -->
+
+```mermaid
+sequenceDiagram
+    box Flywheel
+       actor User
+       participant Job queue
+       participant Assets
+    end
+    box HPC
+       participant Cast
+       participant Scheduler
+       participant Compute Node
+    end
+
+    User->>Job queue: Submit job
+    Note over Cast: Cast wakes up on cron
+    Cast->>Job queue: Poll for HPC jobs
+    Job queue->>Cast: Return jobs
+    loop
+       Cast->> Scheduler: Submit job
+       Compute Node->>Scheduler: Poll task
+       Scheduler->>Compute Node: Return "fw engine" task
+       Note over Compute Node: Engine starts
+       Assets->>Compute Node: Pull Docker image and job inputs / config
+       Note over Compute Node: Gear execute with singularity runner
+       Compute Node->>Assets: Push job outputs
+       Compute Node->>Job queue: Mark job as complete
+    end
+
+```
+
 
 ## HPC types