add notes about cassandra schema

docs/flow.md

@@ -1,4 +1,6 @@
-# Flow of tasks
+# Flow
+
+## Workflow processing flow
 
 Below diagram demonstrates how a workflow is processed by Cadence workers and backend components.
 Note that this is a partial diagram that only highlights selected components and their interactions. There is more happening behind the scenes.
@@ -16,18 +18,25 @@ flowchart TD
     WaitUntilTaskFound --> |6. Return task to worker| Worker
     Worker -->|7. Generate decisions\n& respond| Frontend
     Frontend -->|8. Respond decisions| History[[\nCadence History\n\n]]
-    History -->|9. Mutable state update| ExecutionsTable[(Executions Table)]
+    History -->|9. Mutable state update| ExecutionsStore[(Executions Store)]
     History -->|10.a. Notify| HistoryQueue(History Queues - per shard)
-    HistoryQueue -->|Periodic task fetch| ExecutionsTable
+    HistoryQueue -->|Periodic task fetch| ExecutionsStore
     HistoryQueue -->|11.b. Execute transfer task| TransferTaskExecutor(Transfer Task Executor)
     HistoryQueue -->|11.a. Execute timer task| TimerTaskExecutor(Timer Task Executor)
-    HistoryQueue -->|Periodic per-shard offset save| ShardsTable[(Shards Table)]
+    HistoryQueue -->|Periodic per-shard offset save| ShardsStore[(Shards Store)]
     TransferTaskExecutor -->|12.b. Add task| Matching
     TimerTaskExecutor -->|12.a. Add task| Matching
     Matching -->|13. Add task| TaskListManager
     TaskListManager -->|14. Add task| SyncMatch{Check sync match}
     SyncMatch -->|15.b. Found poller - Do sync match| SyncMatched
-    SyncMatch -->|15.a. No pollers - Save task| TasksTable[(Tasks Table)]
-    TaskListManager -->|Periodic task fetch| TasksTable
+    SyncMatch -->|15.a. No pollers - Save task| TasksStore[(Tasks Store)]
+    TaskListManager -->|Periodic task fetch| TasksStore
 
 ```
+
+### Cassandra Notes:
+
+In order to leverage [Cassandra Light Weight Transactions](https://www.yugabyte.com/blog/apache-cassandra-lightweight-transactions-secondary-indexes-tunable-consistency/), Cadence stores multiple types of records in the same `executions` table. ([ref](https://github.com/uber/cadence/blob/51758676ce9d9609e736c64f94dc387ed2c75b7c/schema/cassandra/cadence/schema.cql#L338))
+- **Shards Store**: Shard records are stored in `executions` table with type=0.
+- **Executions Store**: Execution records are stored in `executions` table with type=1
+- **Tasks Store**: Task records are stored in `executions` table with type={2,3,4,5}