[WIP] making executor ha

crates/common/src/repositories/execution.rs

@@ -41,6 +41,12 @@ pub struct ExecutionSearchResult {
     pub total: u64,
 }
 
+#[derive(Debug, Clone)]
+pub struct WorkflowTaskExecutionCreateOrGetResult {
+    pub execution: Execution,
+    pub created: bool,
+}
+
 /// An execution row with optional `rule_ref` / `trigger_ref` populated from
 /// the joined `enforcement` table. This avoids a separate in-memory lookup.
 #[derive(Debug, Clone, sqlx::FromRow)]
@@ -209,6 +215,134 @@ impl Update for ExecutionRepository {
 }
 
 impl ExecutionRepository {
+    pub async fn create_workflow_task_if_absent<'e, E>(
+        executor: E,
+        input: CreateExecutionInput,
+        workflow_execution_id: Id,
+        task_name: &str,
+        task_index: Option<i32>,
+    ) -> Result<WorkflowTaskExecutionCreateOrGetResult>
+    where
+        E: Executor<'e, Database = Postgres> + Copy + 'e,
+    {
+        if let Some(execution) =
+            Self::find_by_workflow_task(executor, workflow_execution_id, task_name, task_index)
+                .await?
+        {
+            return Ok(WorkflowTaskExecutionCreateOrGetResult {
+                execution,
+                created: false,
+            });
+        }
+
+        let execution = Self::create(executor, input).await?;
+
+        Ok(WorkflowTaskExecutionCreateOrGetResult {
+            execution,
+            created: true,
+        })
+    }
+
+    pub async fn claim_for_scheduling<'e, E>(
+        executor: E,
+        id: Id,
+        claiming_executor: Option<Id>,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+    {
+        let sql = format!(
+            "UPDATE execution \
+             SET status = $2, executor = COALESCE($3, executor), updated = NOW() \
+             WHERE id = $1 AND status = $4 \
+             RETURNING {SELECT_COLUMNS}"
+        );
+
+        sqlx::query_as::<_, Execution>(&sql)
+            .bind(id)
+            .bind(ExecutionStatus::Scheduling)
+            .bind(claiming_executor)
+            .bind(ExecutionStatus::Requested)
+            .fetch_optional(executor)
+            .await
+            .map_err(Into::into)
+    }
+
+    pub async fn reclaim_stale_scheduling<'e, E>(
+        executor: E,
+        id: Id,
+        claiming_executor: Option<Id>,
+        stale_before: DateTime<Utc>,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+    {
+        let sql = format!(
+            "UPDATE execution \
+             SET executor = COALESCE($2, executor), updated = NOW() \
+             WHERE id = $1 AND status = $3 AND updated <= $4 \
+             RETURNING {SELECT_COLUMNS}"
+        );
+
+        sqlx::query_as::<_, Execution>(&sql)
+            .bind(id)
+            .bind(claiming_executor)
+            .bind(ExecutionStatus::Scheduling)
+            .bind(stale_before)
+            .fetch_optional(executor)
+            .await
+            .map_err(Into::into)
+    }
+
+    pub async fn update_if_status<'e, E>(
+        executor: E,
+        id: Id,
+        expected_status: ExecutionStatus,
+        input: UpdateExecutionInput,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+    {
+        if input.status.is_none()
+            && input.result.is_none()
+            && input.executor.is_none()
+            && input.worker.is_none()
+            && input.started_at.is_none()
+            && input.workflow_task.is_none()
+        {
+            return Self::find_by_id(executor, id).await;
+        }
+
+        Self::update_with_locator_optional(executor, input, |query| {
+            query.push(" WHERE id = ").push_bind(id);
+            query.push(" AND status = ").push_bind(expected_status);
+        })
+        .await
+    }
+
+    pub async fn revert_scheduled_to_requested<'e, E>(
+        executor: E,
+        id: Id,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+    {
+        let sql = format!(
+            "UPDATE execution \
+             SET status = $2, worker = NULL, executor = NULL, updated = NOW() \
+             WHERE id = $1 AND status = $3 \
+             RETURNING {SELECT_COLUMNS}"
+        );
+
+        sqlx::query_as::<_, Execution>(&sql)
+            .bind(id)
+            .bind(ExecutionStatus::Requested)
+            .bind(ExecutionStatus::Scheduled)
+            .fetch_optional(executor)
+            .await
+            .map_err(Into::into)
+    }
+
     async fn update_with_locator<'e, E, F>(
         executor: E,
         input: UpdateExecutionInput,
@@ -274,6 +408,71 @@ impl ExecutionRepository {
             .map_err(Into::into)
     }
 
+    async fn update_with_locator_optional<'e, E, F>(
+        executor: E,
+        input: UpdateExecutionInput,
+        where_clause: F,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+        F: FnOnce(&mut QueryBuilder<'_, Postgres>),
+    {
+        let mut query = QueryBuilder::new("UPDATE execution SET ");
+        let mut has_updates = false;
+
+        if let Some(status) = input.status {
+            query.push("status = ").push_bind(status);
+            has_updates = true;
+        }
+        if let Some(result) = &input.result {
+            if has_updates {
+                query.push(", ");
+            }
+            query.push("result = ").push_bind(result);
+            has_updates = true;
+        }
+        if let Some(executor_id) = input.executor {
+            if has_updates {
+                query.push(", ");
+            }
+            query.push("executor = ").push_bind(executor_id);
+            has_updates = true;
+        }
+        if let Some(worker_id) = input.worker {
+            if has_updates {
+                query.push(", ");
+            }
+            query.push("worker = ").push_bind(worker_id);
+            has_updates = true;
+        }
+        if let Some(started_at) = input.started_at {
+            if has_updates {
+                query.push(", ");
+            }
+            query.push("started_at = ").push_bind(started_at);
+            has_updates = true;
+        }
+        if let Some(workflow_task) = &input.workflow_task {
+            if has_updates {
+                query.push(", ");
+            }
+            query
+                .push("workflow_task = ")
+                .push_bind(sqlx::types::Json(workflow_task));
+        }
+
+        query.push(", updated = NOW()");
+        where_clause(&mut query);
+        query.push(" RETURNING ");
+        query.push(SELECT_COLUMNS);
+
+        query
+            .build_query_as::<Execution>()
+            .fetch_optional(executor)
+            .await
+            .map_err(Into::into)
+    }
+
     /// Update an execution using the loaded row's hypertable keys.
     ///
     /// Including both the partition key (`created`) and compression segment key
@@ -356,6 +555,34 @@ impl ExecutionRepository {
             .map_err(Into::into)
     }
 
+    pub async fn find_by_workflow_task<'e, E>(
+        executor: E,
+        workflow_execution_id: Id,
+        task_name: &str,
+        task_index: Option<i32>,
+    ) -> Result<Option<Execution>>
+    where
+        E: Executor<'e, Database = Postgres> + 'e,
+    {
+        let sql = format!(
+            "SELECT {SELECT_COLUMNS} \
+             FROM execution \
+             WHERE workflow_task->>'workflow_execution' = $1::text \
+               AND workflow_task->>'task_name' = $2 \
+               AND (workflow_task->>'task_index')::int IS NOT DISTINCT FROM $3 \
+             ORDER BY created ASC \
+             LIMIT 1"
+        );
+
+        sqlx::query_as::<_, Execution>(&sql)
+            .bind(workflow_execution_id.to_string())
+            .bind(task_name)
+            .bind(task_index)
+            .fetch_optional(executor)
+            .await
+            .map_err(Into::into)
+    }
+
     /// Find all child executions for a given parent execution ID.
     ///
     /// Returns child executions ordered by creation time (ascending),

crates/common/src/repositories/workflow.rs

@@ -411,6 +411,12 @@ impl WorkflowDefinitionRepository {
 
 pub struct WorkflowExecutionRepository;
 
+#[derive(Debug, Clone)]
+pub struct WorkflowExecutionCreateOrGetResult {
+    pub workflow_execution: WorkflowExecution,
+    pub created: bool,
+}
+
 impl Repository for WorkflowExecutionRepository {
     type Entity = WorkflowExecution;
     fn table_name() -> &'static str {
@@ -606,6 +612,51 @@ impl Delete for WorkflowExecutionRepository {
 }
 
 impl WorkflowExecutionRepository {
+    pub async fn create_or_get_by_execution<'e, E>(
+        executor: E,
+        input: CreateWorkflowExecutionInput,
+    ) -> Result<WorkflowExecutionCreateOrGetResult>
+    where
+        E: Executor<'e, Database = Postgres> + Copy + 'e,
+    {
+        let inserted = sqlx::query_as::<_, WorkflowExecution>(
+            "INSERT INTO workflow_execution
+             (execution, workflow_def, task_graph, variables, status)
+             VALUES ($1, $2, $3, $4, $5)
+             ON CONFLICT (execution) DO NOTHING
+             RETURNING id, execution, workflow_def, current_tasks, completed_tasks, failed_tasks, skipped_tasks,
+                       variables, task_graph, status, error_message, paused, pause_reason, created, updated"
+        )
+        .bind(input.execution)
+        .bind(input.workflow_def)
+        .bind(&input.task_graph)
+        .bind(&input.variables)
+        .bind(input.status)
+        .fetch_optional(executor)
+        .await?;
+
+        if let Some(workflow_execution) = inserted {
+            return Ok(WorkflowExecutionCreateOrGetResult {
+                workflow_execution,
+                created: true,
+            });
+        }
+
+        let workflow_execution = Self::find_by_execution(executor, input.execution)
+            .await?
+            .ok_or_else(|| {
+                anyhow::anyhow!(
+                    "workflow_execution for parent execution {} disappeared after conflict",
+                    input.execution
+                )
+            })?;
+
+        Ok(WorkflowExecutionCreateOrGetResult {
+            workflow_execution,
+            created: false,
+        })
+    }
+
     /// Find workflow execution by the parent execution ID
     pub async fn find_by_execution<'e, E>(
         executor: E,

crates/common/tests/execution_repository_tests.rs

@@ -1153,3 +1153,108 @@ async fn test_execution_result_json() {
 
     assert_eq!(updated.result, Some(complex_result));
 }
+
+#[tokio::test]
+#[ignore = "integration test — requires database"]
+async fn test_claim_for_scheduling_succeeds_once() {
+    let pool = create_test_pool().await.unwrap();
+
+    let pack = PackFixture::new_unique("claim_pack")
+        .create(&pool)
+        .await
+        .unwrap();
+    let action = ActionFixture::new_unique(pack.id, &pack.r#ref, "claim_action")
+        .create(&pool)
+        .await
+        .unwrap();
+
+    let created = ExecutionRepository::create(
+        &pool,
+        CreateExecutionInput {
+            action: Some(action.id),
+            action_ref: action.r#ref.clone(),
+            config: None,
+            env_vars: None,
+            parent: None,
+            enforcement: None,
+            executor: None,
+            worker: None,
+            status: ExecutionStatus::Requested,
+            result: None,
+            workflow_task: None,
+        },
+    )
+    .await
+    .unwrap();
+
+    let first = ExecutionRepository::claim_for_scheduling(&pool, created.id, None)
+        .await
+        .unwrap();
+    let second = ExecutionRepository::claim_for_scheduling(&pool, created.id, None)
+        .await
+        .unwrap();
+
+    assert_eq!(first.unwrap().status, ExecutionStatus::Scheduling);
+    assert!(second.is_none());
+}
+
+#[tokio::test]
+#[ignore = "integration test — requires database"]
+async fn test_update_if_status_only_updates_matching_row() {
+    let pool = create_test_pool().await.unwrap();
+
+    let pack = PackFixture::new_unique("conditional_pack")
+        .create(&pool)
+        .await
+        .unwrap();
+    let action = ActionFixture::new_unique(pack.id, &pack.r#ref, "conditional_action")
+        .create(&pool)
+        .await
+        .unwrap();
+
+    let created = ExecutionRepository::create(
+        &pool,
+        CreateExecutionInput {
+            action: Some(action.id),
+            action_ref: action.r#ref.clone(),
+            config: None,
+            env_vars: None,
+            parent: None,
+            enforcement: None,
+            executor: None,
+            worker: None,
+            status: ExecutionStatus::Scheduling,
+            result: None,
+            workflow_task: None,
+        },
+    )
+    .await
+    .unwrap();
+
+    let updated = ExecutionRepository::update_if_status(
+        &pool,
+        created.id,
+        ExecutionStatus::Scheduling,
+        UpdateExecutionInput {
+            status: Some(ExecutionStatus::Scheduled),
+            worker: Some(77),
+            ..Default::default()
+        },
+    )
+    .await
+    .unwrap();
+    let skipped = ExecutionRepository::update_if_status(
+        &pool,
+        created.id,
+        ExecutionStatus::Scheduling,
+        UpdateExecutionInput {
+            status: Some(ExecutionStatus::Failed),
+            ..Default::default()
+        },
+    )
+    .await
+    .unwrap();
+
+    assert_eq!(updated.unwrap().status, ExecutionStatus::Scheduled);
+    assert!(skipped.is_none());
+}

docs/plans/executor-ha-horizontal-scaling.md

@@ -0,0 +1,317 @@
+# Executor HA Horizontal Scaling Plan
+
+## Overview
+
+This plan describes the changes required to make the Attune executor service safe to run with multiple replicas. The current implementation already uses RabbitMQ competing consumers for most work distribution, but several correctness-critical parts of the executor still rely on process-local memory or non-atomic database updates. Those assumptions make the service unsafe under horizontal scaling, message replay, or partial failure.
+
+The goal of this plan is to make executor behavior correct under:
+
+- Multiple executor replicas running concurrently
+- Message redelivery from RabbitMQ
+- Replica crash/restart during scheduling or workflow advancement
+- Background recovery loops running on more than one replica
+
+## Problem Summary
+
+The current executor has five HA blockers:
+
+1. **Concurrency/FIFO control is process-local.**
+   `ExecutionQueueManager` stores active slots and waiting queues in memory. That means one replica can admit work while another replica receives the completion notification, causing slot release and queue advancement to fail.
+
+2. **Execution scheduling has no atomic claim step.**
+   The scheduler reads an execution in `requested`, does policy checks and worker selection, then updates it later. Two replicas can both observe the same row as schedulable and both dispatch it.
+
+3. **Workflow orchestration is not serialized.**
+   Workflow start and workflow advancement perform read-check-create-update sequences without a distributed lock or optimistic version check, so duplicate successor tasks can be created.
+
+4. **Event/enforcement/inquiry handlers are not idempotent.**
+   Duplicate message delivery can create duplicate enforcements, duplicate executions, duplicate workflow starts, and duplicate inquiries.
+
+5. **Timeout and DLQ handlers use non-conditional updates.**
+   Recovery loops can overwrite newer worker-owned state if the execution changes between the initial read and the final update.
+
+## Goals
+
+- Make execution scheduling single-winner under multiple executor replicas
+- Make policy-based concurrency control and FIFO queueing shared across replicas
+- Make workflow start and workflow advancement idempotent and serialized
+- Make duplicate message delivery safe for executor-owned entities
+- Make recovery loops safe to run on every replica
+
+## Non-Goals
+
+- Re-architecting RabbitMQ usage across the whole platform
+- Replacing PostgreSQL with a dedicated distributed lock service
+- Solving general worker autoscaling or Kubernetes deployment concerns in this phase
+- Reworking unrelated executor features like retry policy design unless needed for HA correctness
+
+## Design Principles
+
+### Database is the source of truth
+
+All coordination state that affects correctness must live in PostgreSQL, not in executor memory. In-memory caches and metrics are fine as optimization or observability layers, but correctness cannot depend on them.
+
+### State transitions must be compare-and-swap
+
+Any executor action that changes ownership or lifecycle state must use an atomic update that verifies the prior state in the same statement or transaction.
+
+### Handlers must be idempotent on domain keys
+
+RabbitMQ gives at-least-once delivery semantics. The executor must therefore tolerate duplicate delivery even when the same message is processed more than once or by different replicas.
+
+### Workflow orchestration must be serialized per workflow
+
+A workflow execution should have exactly one active mutator at a time when evaluating transitions and dispatching successor tasks.
+
+## Proposed Implementation Phases
+
+## Phase 1: Atomic Execution Claiming
+
+### Objective
+
+Ensure only one executor replica can claim a `requested` execution for scheduling.
+
+### Changes
+
+**`crates/common/src/repositories/execution.rs`**
+
+- Add a repository method for atomic status transition, for example:
+  - `claim_for_scheduling(id, expected_status, executor_id) -> Option<Execution>`
+  - or a more general compare-and-swap helper
+- Implement the claim as a single `UPDATE ... WHERE id = $1 AND status = 'requested' RETURNING ...`
+- Optionally persist the claiming replica identity in `execution.executor` for debugging and traceability
+
+**`crates/executor/src/scheduler.rs`**
+
+- Claim the execution before policy enforcement, worker selection, workflow start, or any side effects
+- Use `scheduling` as the claimed intermediate state
+- If the claim returns no row, treat the execution as already claimed/handled and acknowledge the message
+- Convert all later scheduler writes to conditional transitions from claimed state
+
+### Success Criteria
+
+- Two schedulers racing on the same execution cannot both dispatch it
+- Redelivered `execution.requested` messages become harmless no-ops after the first successful claim
+
+## Phase 2: Shared Concurrency Control and FIFO Queueing
+
+### Objective
+
+Replace the in-memory `ExecutionQueueManager` as the source of truth for concurrency slots and waiting order.
+
+### Changes
+
+**New schema**
+
+Add database-backed coordination tables, likely along these lines:
+
+- `execution_admission_slot`
+  - active slot ownership for an action/group key
+- `execution_admission_queue`
+  - ordered waiting executions for an action/group key
+
+Alternative naming is fine, but the design needs to support:
+
+- Action-level concurrency limits
+- Parameter-group concurrency keys
+- FIFO ordering within each action/group
+- Deterministic advancement when a slot is released
+
+**`crates/executor/src/policy_enforcer.rs`**
+
+- Replace `ExecutionQueueManager` slot acquisition with DB-backed admission logic
+- Keep existing policy semantics:
+  - `enqueue`
+  - `cancel`
+  - parameter-based concurrency grouping
+
+**`crates/executor/src/completion_listener.rs`**
+
+- Release the shared slot transactionally on completion
+- Select and wake the next queued execution in the same transaction
+- Republish only after the DB state is committed
+
+**`crates/executor/src/queue_manager.rs`**
+
+- Either remove it entirely or reduce it to a thin adapter over DB-backed coordination
+- Do not keep active slot ownership in process-local `DashMap`
+
+**`crates/common/src/repositories/queue_stats.rs`**
+
+- Keep `queue_stats` as derived telemetry only
+- Do not rely on it for correctness
+
+### Success Criteria
+
+- Completion processed by a different executor replica still releases the correct slot
+- FIFO ordering holds across multiple executor replicas
+- Restarting an executor does not lose queue ownership state
+
+## Phase 3: Workflow Start Idempotency and Serialized Advancement
+
+### Objective
+
+Ensure workflow orchestration is safe under concurrent replicas and duplicate completion messages.
+
+### Changes
+
+**Migration**
+
+Add a uniqueness constraint to guarantee one workflow state row per parent execution:
+
+```sql
+ALTER TABLE workflow_execution
+ADD CONSTRAINT uq_workflow_execution_execution UNIQUE (execution);
+```
+
+**`crates/executor/src/scheduler.rs`**
+
+- Change workflow start to be idempotent:
+  - either `INSERT ... ON CONFLICT ...`
+  - or claim parent execution first and only create workflow state once
+- When advancing a workflow:
+  - wrap read/decide/write logic in a transaction
+  - lock the `workflow_execution` row with `SELECT ... FOR UPDATE`
+  - or use advisory locks keyed by workflow execution id
+
+**Successor dispatch dedupe**
+
+Add a durable uniqueness guarantee for child task dispatch, for example:
+
+- one unique key for regular workflow tasks:
+  - `(workflow_execution_id, task_name, task_index IS NULL)`
+- one unique key for `with_items` children:
+  - `(workflow_execution_id, task_name, task_index)`
+
+This may be implemented with explicit columns or a dedupe table if indexing the current JSONB layout is awkward.
+
+**Repository support**
+
+- Add workflow repository helpers that support transactional locking and conditional updates
+- Avoid blind overwrite of `completed_tasks`, `failed_tasks`, and `variables` outside a serialized transaction
+
+### Success Criteria
+
+- Starting the same workflow twice cannot create two `workflow_execution` rows
+- Duplicate `execution.completed` delivery for a workflow child cannot create duplicate successor executions
+- Two executor replicas cannot concurrently mutate the same workflow state
+
+## Phase 4: Idempotent Event, Enforcement, and Inquiry Handling
+
+### Objective
+
+Make duplicate delivery safe for all earlier and later executor-owned side effects.
+
+### Changes
+
+**Enforcement dedupe**
+
+Add a uniqueness rule so one event/rule pair produces at most one enforcement when `event` is present.
+
+Example:
+
+```sql
+CREATE UNIQUE INDEX uq_enforcement_rule_event
+ON enforcement(rule, event)
+WHERE event IS NOT NULL;
+```
+
+**`crates/executor/src/event_processor.rs`**
+
+- Use upsert-or-ignore semantics for enforcement creation
+- Treat uniqueness conflicts as idempotent success
+
+**`crates/executor/src/enforcement_processor.rs`**
+
+- Check current enforcement status before creating an execution
+- Add a durable relation that prevents an enforcement from creating more than one top-level execution
+- Options:
+  - unique partial index on `execution(enforcement)` for top-level executions
+  - or a separate coordination record
+
+**Inquiry dedupe**
+
+- Prevent duplicate inquiry creation per execution result/completion path
+- Add a unique invariant such as one active inquiry per execution, if that matches product semantics
+- Update completion handling to tolerate duplicate `execution.completed`
+
+### Success Criteria
+
+- Duplicate `event.created` does not create duplicate enforcements
+- Duplicate `enforcement.created` does not create duplicate executions
+- Duplicate completion handling does not create duplicate inquiries
+
+## Phase 5: Safe Recovery Loops
+
+### Objective
+
+Make timeout and DLQ processing safe under races and multiple replicas.
+
+### Changes
+
+**`crates/executor/src/timeout_monitor.rs`**
+
+- Replace unconditional updates with conditional state transitions:
+  - `UPDATE execution SET ... WHERE id = $1 AND status = 'scheduled' ... RETURNING ...`
+- Only publish completion side effects when a row was actually updated
+- Consider including `updated < cutoff` in the same update statement
+
+**`crates/executor/src/dead_letter_handler.rs`**
+
+- Change failure transition to conditional update based on current state
+- Do not overwrite executions that have already moved to `running` or terminal state
+- Only emit side effects when the row transition succeeded
+
+**`crates/executor/src/service.rs`**
+
+- It is acceptable for these loops to run on every replica once updates are conditional
+- Optional future optimization: leader election for janitor loops to reduce duplicate scans and log noise
+
+### Success Criteria
+
+- Timeout monitor cannot fail an execution that has already moved to `running`
+- DLQ handler cannot overwrite newer state
+- Running multiple timeout monitors produces no conflicting state transitions
+
+## Testing Plan
+
+Add focused HA tests after the repository and scheduler primitives are in place.
+
+### Repository tests
+
+- Compare-and-swap execution claim succeeds exactly once
+- Conditional timeout/DLQ transition updates exactly one row or zero rows as expected
+- Workflow uniqueness constraint prevents duplicate workflow state rows
+
+### Executor integration tests
+
+- Two scheduler instances processing the same `execution.requested` message only dispatch once
+- Completion consumed by a different executor replica still advances the shared queue
+- Duplicate workflow child completion does not create duplicate successor tasks
+- Duplicate `event.created` and `enforcement.created` messages do not create duplicate downstream records
+
+### Failure-injection tests
+
+- Executor crashes after claiming but before publish
+- Executor crashes after slot release but before republish
+- Duplicate `execution.completed` delivery after successful workflow advancement
+
+## Recommended Execution Order for Next Session
+
+1. Add migrations and repository primitives for atomic execution claim
+2. Convert scheduler to claim-first semantics
+3. Implement shared DB-backed concurrency/FIFO coordination
+4. Add workflow uniqueness and serialized advancement
+5. Add idempotency to event/enforcement/inquiry paths
+6. Fix timeout and DLQ handlers to use conditional transitions
+7. Add HA-focused tests
+
+## Expected Outcome
+
+After this plan is implemented, the executor should be able to scale horizontally without relying on singleton behavior. Multiple executor replicas should be able to process work concurrently while preserving:
+
+- exactly-once scheduling semantics at the execution state level
+- shared concurrency limits and FIFO behavior
+- correct workflow orchestration
+- safe replay handling
+- safe recovery behavior during failures and redelivery

migrations/20250101000006_workflow_system.sql

@@ -67,11 +67,11 @@ CREATE TABLE workflow_execution (
     paused BOOLEAN DEFAULT false NOT NULL,
     pause_reason TEXT,
     created TIMESTAMPTZ DEFAULT NOW() NOT NULL,
-    updated TIMESTAMPTZ DEFAULT NOW() NOT NULL
+    updated TIMESTAMPTZ DEFAULT NOW() NOT NULL,
+    CONSTRAINT uq_workflow_execution_execution UNIQUE (execution)
 );
 
 -- Indexes
-CREATE INDEX idx_workflow_exec_execution ON workflow_execution(execution);
 CREATE INDEX idx_workflow_exec_workflow_def ON workflow_execution(workflow_def);
 CREATE INDEX idx_workflow_exec_status ON workflow_execution(status);
 CREATE INDEX idx_workflow_exec_paused ON workflow_execution(paused) WHERE paused = true;