A flow diagram demonstrating how a workflow is processed at high level (

cadence-workflow#5801)

docs/flow.md

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+# 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.
+
+[comment]: <> (To visualize mermaid flowchart below, install Mermaid plugin for your IDE. Works in github out of the box)
+
+```mermaid
+flowchart TD
+    User((Domain X Admin)) -->|"0. Deploy worker with config: domain, tasklist(s)"| Worker[[\nDomain X Worker\n\n]]
+    ExternalInitiator(("Domain X\nAnother Service/CLI")) -->|"1. Start/Signal Workflow"| Frontend
+    Worker -->|2. Poll tasks for domain: X, tasklist: tl1| Frontend[[\nCadence Frontend\n\n]]
+    Frontend -->|3. Forward Poll request| Matching[[\nCadence Matching\n\n]]
+    Matching -->|4. Poll| TaskListManager(Tasklist Manager for tl1)
+    TaskListManager -->|5. Long Poll| WaitUntilTaskFound
+    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| ExecutionsStore[(Executions Store)]
+    History -->|10.a. Notify| HistoryQueue(History Queues - per shard)
+    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| 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| 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}