RFC: Add execution concurrency

rfc/system/RFC-0000-execution-concurrency.md

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+# [RFC Template] Title
+
+**Authors:**
+
+- @eapolinario
+- @katrogan
+
+## 1 Executive Summary
+
+This is a proposal to implement workflow execution concurrency, defined at the launch plan level.
+
+## 2 Motivation
+
+See the following issues
+1. https://github.com/flyteorg/flyte/issues/267
+2. https://github.com/flyteorg/flyte/issues/420
+3. https://github.com/flyteorg/flyte/discussions/3754
+4. https://github.com/flyteorg/flyte/issues/5125
+
+## 3 Proposed Implementation
+
+Introduce a new attribute in [LaunchPlan.get_or_create](https://github.com/flyteorg/flytekit/blob/bc2e000cc8d710ed3d135cdbf3cbf257c5da8100/flytekit/core/launch_plan.py#L195) to allow specifying execution concurrency
+
+e.g.     
+```python
+my_lp = LaunchPlan.get_or_create(
+    name="my_serial_lp",
+    workflow=my_wf,
+    ...
+    concurrency=Concurrency(
+        max=1,  # defines how many executions with this launch plan can run in parallel
+        policy=ConcurrencyPolicy.WAIT  # defines the policy to apply when the max concurrency is reached
+    )
+)
+```
+
+### FlyteIDL
+We propose adding a new IDL message to capture concurrency behavior at CreateExecutionTime
+
+```protobuf
+message Concurrency {
+  // Defines how many executions with this launch plan can run in parallel
+  uint32 max = 1;
+
+  // Defines how to handle the execution when the max concurrency is reached.
+  ConcurrencyPolicy policy = 2;
+}
+
+enum ConcurrencyPolicy {
+  UNSPECIFIED = 0;
+
+  // wait for previous executions to terminate before starting a new one
+  WAIT = 1;
+
+  // fail the CreateExecution request and do not permit the execution to start
+  ABORT = 2;
+}
+
+message LaunchPlanSpec {
+  ...
+
+  Concurrency concurrency = X; 
+}
+
+// embedded in the ExecutionClosure
+message ExecutionStateChangeDetails  {
+  ...
+
+  // Includes the reason for the `PENDING` phase
+  string description = X;
+
+
+}
+
+// Can also add to ExecutionSpec to specify execution time overrides
+
+```
+
+### FlyteAdmin
+At a broad level
+1. At CreateExecution time, if the launch plan in the ExecutionSpec has a concurrency policy
+   1. Create the execution in the database with a new `PENDING` execution phase and reason populated in `ExecutionStateChangeDetails`.
+      1. or fail the request when the concurrency policy is set to `ABORT`
+   1. Do not create the workflow CRD
+
+Introduce an async reconciliation loop in FlyteAdmin to poll for all pending executions:
+1. 1x a minute: Query all pending executions by timestamp ascending, grouped by launch plan ID, roughly something like
+```sql
+SELECT e.*
+FROM   executions AS e
+WHERE  ( launch_plan_id, created_at ) IN (SELECT launch_plan_id,
+                                                 Min(created_at)
+                                          FROM   executions
+                                          WHERE  phase = 'PENDING'
+                                          GROUP  BY launch_plan_id); 
+```
+2. For each execution returned by the above query, `Add()` the pending execution to a [rate limiting workqueue](https://github.com/kubernetes/client-go/blob/master/util/workqueue/rate_limiting_queue.go#L27-L40) (as a suggestion)
+3. In a separate goroutine, fetch items from the workqueue and individually process each execution entry
+   1. Check the database to see if there are fewer than `MAX_CONCURRENCY` non-terminal executions matching the launch plan ID in the pending execution model
+   ```sql
+   select count(launch_plan_id) from executions where phase not in ('SUCCEEDED', 'FAILED', 'ABORTED', 'TIMED_OUT') group by launch_plan_id;
+   ```
+   1. If there are fewer than `MAX_CONCURRENCY` executions running
+      1. check that the execution is still in `PENDING`
+      1. create the workflow CRD
+          1. if the CRD already exists because we've previously processed this pending execution before we had a chance to update the DB state, swallow the already exists error gracefully
+      1. conditionally mark the execution as `QUEUED` in the db if it's already in `PENDING` or  `QUEUED` to not overwrite any events should the execution have already reported progress in the interim
+      1. If creating the workflow CRD fails: mark the execution as `FAILED` in the db (and swallow any errors if the workflow is already in a terminal state should we have previously reported the failure after re-enqueuing the pending execution a previous loop). This will remove its eligiblity from the pending loop
+   1. If there are already `MAX_CONCURRENCY` executions running, simply proceed to (iii.)
+   1. Finally, always mark the queue item as [Done()](https://github.com/kubernetes/client-go/blob/master/util/workqueue/queue.go#L33) 
+
+If we wanted further parallelization here, we could introduce a worker pool rather than having one async process read from the workqueue.
+
+We should consider adding an index to the executions table to include
+- launch_plan_id
+- phase
+- created_at
+
+
+
+#### Prior Art
+The flyteadmin native scheduler (https://github.com/flyteorg/flyte/tree/master/flyteadmin/scheduler) already implements a reconciliation loop to catch up on any missed schedules.
+
+#### Caveats
+Executions are always tied to the versioned launch plan that triggered them (see [here](https://github.com/flyteorg/flyte/blob/38883c721dac2875bdd2333f4cd56e757e81ea5f/flyteadmin/pkg/repositories/models/execution.go#L26))
+Therefore, this proposal only applies concurrency at the versioned launch plan level.
+
+If we wanted to support concurrency across launch plan versions we could add another index to the Executions table to include the launch plan named entity, that is the entry in the [NamedEntity table](https://github.com/flyteorg/flyte/blob/38883c721dac2875bdd2333f4cd56e757e81ea5f/flyteadmin/pkg/repositories/models/named_entity.go#L39-L42) corresponding to the launch plan project, domain & name
+
+In models/execution.go:
+```go
+
+type Execution struct {
+
+	...
+	// Already exists
+	LaunchPlanID uint   `gorm:"index"`
+	// New field to make querying on the named entity 
+	LaunchPlanNamedEntityID uint `gorm:"index"`
+
+}
+
+```
+
+Then the reconciliation loop would query executions in a non-terminal phase matching the launch plan named entity ID instead of LaunchPlanID.
+
+## 4 Metrics & Dashboards
+- Time spent in PENDING: It's useful to understand the duration spent in PENDING before a launch plan transitions to RUNNING
+- It may be useful for Flyte platform operators to also configure alerts if an execution stays in PENDING for too long of a threshold
+
+## 5 Drawbacks
+
+The [executions model](https://github.com/flyteorg/flyte/blob/38883c721dac2875bdd2333f4cd56e757e81ea5f/flyteadmin/pkg/repositories/models/execution.go)
+already has indices on
+- primary key index
+- launch plan id
+- workflow id
+- task id (for single task executions)
+- execution created at
+- error kind (code)
+- user who launched the execution
+- state
+
+Database performance suffers as new indices are added (ref [[1](https://use-the-index-luke.com/sql/dml/insert)] [[2](https://www.timescale.com/learn/postgresql-performance-tuning-optimizing-database-indexes)])
+We could as an alternative, repurpose the existing launch plan index to include (launch plan id, phase, created at) to optimize the query for pending executions and not significantly affect queries on launch plan id leveraging the existing index.
+
+
+## 6 Alternatives
+
+### Scheduling
+This proposal purposefully uses FIFO scheduling but there is a chance we may want to define other scheduling orders or catch-up policies.
+
+To accomplish this, we can extend the `ConcurrenyPolicy` proto message to encapsulate scheduling behavior
+
+```protobuf
+message Concurrency {
+  // Defines how many executions with this launch plan can run in parallel
+  uint32 max = 1;
+
+  // Defines how to handle the execution when the max concurrency is reached.
+  ConcurrencyPolicy policy = 2;
+
+  ConcurrencyScheduling scheduling = 3;
+}
+
+
+type ConcurrencyScheduling enum {
+  FIFO = 0;
+  FILO = 1;
+  ...
+}
+```
+
+Furthermore, we may want to introduce a max pending period to fail executions that have been in `PENDING` for too long
+
+
+## 7 Potential Impact and Dependencies
+
+*Here, we aim to be mindful of our environment and generate empathy towards others who may be impacted by our decisions.*
+
+- *What other systems or teams are affected by this proposal?*
+- *How could this be exploited by malicious attackers?*
+
+## 8 Unresolved questions
+
+- Should we always attempt to schedule pending executions in ascending order of creation time?
+    - Decision: We'll use FIFO scheduling by default but can extend scheduling behavior with an enum going forward.
+- Should we propagate concurrency policies to child executions?
+    - Decision: no. Child executions can define concurrency at the child launch plan level if necessary.
+
+## 9 Conclusion
+
+This is a simple and lightweight means for limiting execution concurrency that we can build upon, for flexible scheduling policies and even limiting task execution concurrency.
+
+
+**Checklist:**
+
+- [x]  Copy template
+- [x]  Draft RFC (think of it as a wireframe)
+- [x]  Share as WIP with folks you trust to gut-check
+- [x]  Send pull request when comfortable
+- [ ]  Label accordingly
+- [ ]  Assign reviewers
+- [ ]  Merge PR
+
+**Recommendations**
+
+- Tag RFC title with [WIP] if you're still ironing out details.
+- Tag RFC title with [Newbie] if you're trying out something experimental or you're not entirely convinced of what you're proposing.
+- Tag RFC title with [RR] if you'd like to schedule a review request to discuss the RFC.
+- If there are areas that you're not convinced on, tag people who you consider may know about this and ask for their input.
+- If you have doubts, ask on [#feature-discussions](https://slack.com/app_redirect?channel=CPQ3ZFQ84&team=TN89P6GGK) for help moving something forward.