FEATURE: Partitioned Wait Lists

README.md

@@ -19,7 +19,7 @@ This is NOT a fully featured CI pipeline solution.
 <!-- TOC -->
   * [Badges](#badges)
   * [Components](#components)
-    * [prunner (this repository)](#prunner--this-repository-)
+    * [prunner (this repository)](#prunner-this-repository)
     * [prunner-ui](#prunner-ui)
     * [Flowpack.Prunner](#flowpackprunner)
   * [User guide](#user-guide)
@@ -32,7 +32,10 @@ This is NOT a fully featured CI pipeline solution.
     * [Limiting concurrency](#limiting-concurrency)
     * [The wait list](#the-wait-list)
     * [Debounce jobs with a start delay](#debounce-jobs-with-a-start-delay)
+    * [Partitioned Wait Lists](#partitioned-wait-lists)
     * [Disabling fail-fast behavior](#disabling-fail-fast-behavior)
+    * [Error handling with on_error](#error-handling-with-on_error)
+      * [Important notes:](#important-notes)
     * [Configuring retention period](#configuring-retention-period)
     * [Handling of child processes](#handling-of-child-processes)
     * [Graceful shutdown](#graceful-shutdown)
@@ -44,11 +47,11 @@ This is NOT a fully featured CI pipeline solution.
   * [Development](#development)
     * [Requirements](#requirements)
     * [Running locally](#running-locally)
-    * [IDE Setup (IntelliJ/GoLand)](#ide-setup--intellijgoland-)
+    * [IDE Setup (IntelliJ/GoLand)](#ide-setup-intellijgoland)
     * [Building for different operating systems.](#building-for-different-operating-systems)
     * [Running Tests](#running-tests)
     * [Memory Leak Debugging](#memory-leak-debugging)
-    * [Generate OpenAPI (Swagger) spec](#generate-openapi--swagger--spec)
+    * [Generate OpenAPI (Swagger) spec](#generate-openapi-swagger-spec)
     * [Releasing](#releasing)
   * [Security concept](#security-concept)
   * [License](#license)
@@ -222,7 +225,8 @@ pipelines:
 
 To deactivate the queuing altogether, set `queue_limit: 0`.
 
-Now, if the queue is limited, an error occurs when it is full and you try to add a new job.
+Now, if the queue is limited (and default `queue_strategy: append` is configured),
+an error occurs when it is full and you try to add a new job.
 
 Alternatively, you can also set `queue_strategy: replace` to replace the last job in the
 queue by the newly added one:
@@ -257,6 +261,7 @@ in form of a zero or positive decimal value with a time unit ("ms", "s", "m", "h
 ```yaml
 pipelines:
   do_something:
+    # NOTE: to prevent starvation, use queue_limit >= 2x
     queue_limit: 1
     queue_strategy: replace
     concurrency: 1
@@ -265,6 +270,52 @@ pipelines:
     tasks: # as usual
 ```
 
+NOTE: If you use `queue_limit: 1` and `start_delay`, you will run into **starvation** (=the job never starts)
+if jobs are submitted quicker than `start_delay`. If you instead use `queue_limit: 2` or higher, you can
+avoid this issue: Then, the 1st slot will always be worked on after `start_delay`, while the 2nd slot will
+be replaced quickly.
+
+### Partitioned Wait Lists
+
+If you have a multi-tenant application, you might want to use **one wait-list per tenant** (e.g. for import jobs),
+combined with global `concurrency` limits (depending on the globally available server resources).
+
+To enable this, do the following:
+
+- `queue_strategy: partitioned_replace`: Enabled partitioned wait list
+- `queue_partition_limit: 1` (or higher): Configure the number of wait-list slots per tenant. The last slot gets replaced
+  when the wait-list is full.
+- can be combined with arbitrary `start_delay` and `concurrency` as expected
+
+Full example:
+
+```
+pipelines:
+  do_something:
+    queue_strategy: partitioned_replace
+    # prevent starvation
+    queue_partition_limit: 2
+    concurrency: 1
+    # Queues a run of the job and only starts it after 10 seconds have passed (if no other run was triggered which replaced the queued job)
+    start_delay: 10s
+    tasks: # as usual
+
+```
+
+Additionally, when submitting a job, you need to specify the `queuePartition` argument:
+
+```
+POST /pipelines/schedule
+
+{
+  "pipeline": "my_pipeline",
+  "variables": {
+    ...
+  },
+  "queuePartition": "tenant_foo"
+}
+```
+
 
 ### Disabling fail-fast behavior
 

definition/loader_test.go

@@ -23,3 +23,28 @@ func TestLoadRecursively_WithMissingDependency(t *testing.T) {
 	_, err := LoadRecursively("../test/fixtures/missingDep.yml")
 	require.EqualError(t, err, `loading ../test/fixtures/missingDep.yml: invalid pipeline definition "test_it": missing task "not_existing" referenced in depends_on of task "test"`)
 }
+
+func TestPartitionedWaitlist_OK(t *testing.T) {
+	_, err := LoadRecursively("../test/fixtures/partitioned_waitlist.yml")
+	require.NoError(t, err)
+}
+
+func TestPartitionedWaitlist_err_0_partition_limit(t *testing.T) {
+	_, err := LoadRecursively("../test/fixtures/partitioned_waitlist_err_0_partition_limit.yml")
+	require.EqualError(t, err, `loading ../test/fixtures/partitioned_waitlist_err_0_partition_limit.yml: invalid pipeline definition "test_it": queue_partition_limit must be defined and >=1 if queue_strategy=partitioned_replace`)
+}
+
+func TestPartitionedWaitlist_err_no_partition_limit(t *testing.T) {
+	_, err := LoadRecursively("../test/fixtures/partitioned_waitlist_err_no_partition_limit.yml")
+	require.EqualError(t, err, `loading ../test/fixtures/partitioned_waitlist_err_no_partition_limit.yml: invalid pipeline definition "test_it": queue_partition_limit must be defined and >=1 if queue_strategy=partitioned_replace`)
+}
+
+func TestPartitionedWaitlist_err_queue_limit(t *testing.T) {
+	_, err := LoadRecursively("../test/fixtures/partitioned_waitlist_err_queue_limit.yml")
+	require.EqualError(t, err, `loading ../test/fixtures/partitioned_waitlist_err_queue_limit.yml: invalid pipeline definition "test_it": queue_limit is not allowed if queue_strategy=partitioned_replace, use queue_partition_limit instead`)
+}
+
+func TestWaitlist_err_partitioned_queue_limit(t *testing.T) {
+	_, err := LoadRecursively("../test/fixtures/waitlist_err_partitioned_queue_limit.yml")
+	require.EqualError(t, err, `loading ../test/fixtures/waitlist_err_partitioned_queue_limit.yml: invalid pipeline definition "test_it": queue_partition_limit is not allowed if queue_strategy=append|replace, use queue_limit instead`)
+}

definition/pipelines.go

@@ -52,10 +52,15 @@ type OnErrorTaskDef struct {
 type PipelineDef struct {
 	// Concurrency declares how many instances of this pipeline are allowed to execute concurrently (defaults to 1)
 	Concurrency int `yaml:"concurrency"`
-	// QueueLimit is the number of slots for queueing jobs if the allowed concurrency is exceeded, defaults to unbounded (nil)
+	// QueueLimit is the number of slots for queueing jobs if the allowed concurrency is exceeded, defaults to unbounded (nil). Only allowed with queue_strategy=append|replace, not with partitioned_replace (there, use queue_partition_limit instead)
 	QueueLimit *int `yaml:"queue_limit"`
+
+	// QueuePartitionLimit is the number of slots for queueing jobs per partition, if queue_strategy=partitioned_replace is used.
+	QueuePartitionLimit *int `yaml:"queue_partition_limit"`
+
 	// QueueStrategy to use when adding jobs to the queue (defaults to append)
 	QueueStrategy QueueStrategy `yaml:"queue_strategy"`
+
 	// StartDelay will delay the start of a job if the value is greater than zero (defaults to 0)
 	StartDelay time.Duration `yaml:"start_delay"`
 
@@ -100,6 +105,19 @@ func (d PipelineDef) validate() error {
 		return errors.New("start_delay needs queue_limit > 0")
 	}
 
+	if d.QueueStrategy == QueueStrategyPartitionedReplace {
+		if d.QueueLimit != nil {
+			return errors.New("queue_limit is not allowed if queue_strategy=partitioned_replace, use queue_partition_limit instead")
+		}
+		if d.QueuePartitionLimit == nil || *d.QueuePartitionLimit < 1 {
+			return errors.New("queue_partition_limit must be defined and >=1 if queue_strategy=partitioned_replace")
+		}
+	} else {
+		if d.QueuePartitionLimit != nil {
+			return errors.New("queue_partition_limit is not allowed if queue_strategy=append|replace, use queue_limit instead")
+		}
+	}
+
 	for taskName, taskDef := range d.Tasks {
 		for _, dependentTask := range taskDef.DependsOn {
 			_, exists := d.Tasks[dependentTask]
@@ -164,13 +182,21 @@ func (d PipelineDef) Equals(otherDef PipelineDef) bool {
 	return true
 }
 
+// QueueStrategy defines the behavior when jobs wait (=are queued) before pipeline execution.
 type QueueStrategy int
 
 const (
-	// QueueStrategyAppend appends jobs to the queue until queue limit is reached
+	// QueueStrategyAppend appends jobs to the queue until the queue limit is reached (FIFO)
 	QueueStrategyAppend QueueStrategy = 0
-	// QueueStrategyReplace replaces pending jobs (with same variables) instead of appending to the queue
+
+	// QueueStrategyReplace replaces the **LAST** pending job if the queue limit is reached. If the queue is not yet full, the job is appended.
+	// NOTE: if using queue_limit=1 + replace, this can lead to starvation if rapidly enqueuing jobs. If using queue_limit >= 2, this cannot happen anymore.
+	// (see 2025_08_14_partitioned_waitlist.md for detailed description)
 	QueueStrategyReplace QueueStrategy = 1
+
+	// QueueStrategyPartitionedReplace implements the "partitioned waitlist" strategy, as explained in 2025_08_14_partitioned_waitlist.md.
+	// -> it replaces the **LAST** pending job of a given partition, if the partition is full (=queue_partition_limit).
+	QueueStrategyPartitionedReplace QueueStrategy = 2
 )
 
 func (s *QueueStrategy) UnmarshalYAML(unmarshal func(interface{}) error) error {
@@ -185,6 +211,8 @@ func (s *QueueStrategy) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		*s = QueueStrategyAppend
 	case "replace":
 		*s = QueueStrategyReplace
+	case "partitioned_replace":
+		*s = QueueStrategyPartitionedReplace
 	default:
 		return errors.Errorf("unknown queue strategy: %q", strategyName)
 	}

docs/2025_08_14_partitioned_waitlist.md

@@ -0,0 +1,111 @@
+# FEATURE: Partitioned Waitlists
+
+
+# Problem description
+
+## Current state
+
+> **FROM README**
+>
+> By default, if you limit concurrency, and the limit is exceeded, further jobs are added to the
+> waitlist of the pipeline.
+> 
+> However, you have some options to configure this as well:
+> 
+> The waitlist can have a maximum size, denoted by `queue_limit`:
+> 
+> ```yaml
+> pipelines:
+>   do_something:
+>     queue_limit: 1
+>     concurrency: 1
+>     tasks: # as usual
+> ```
+> 
+> To deactivate the queuing altogether, set `queue_limit: 0`.
+> 
+> Now, if the queue is limited, an error occurs when it is full and you try to add a new job.
+> 
+> Alternatively, you can also set `queue_strategy: replace` to replace the last job in the
+> queue by the newly added one:
+
+## Current state -> Problem "Starvation" with "Debounce" ??
+
+> Queues a run of the job and only starts it after 10 seconds have passed (if no other run was triggered which replaced the queued job)
+
+```
+--> time (1 char = 1 s)
+
+# 1 waitlist slot, delay 10 s
+
+a____ _____ <-- here the job A is queued
+          A <-- here job A starts
+
+
+# 1 waitlist slot, delay 10 s
+# STARVATION CAN HAPPEN
+a____ b____ c_____ _____
+                       C
+
+
+# 2 waitlist slots, delay 10 s
+# !!! PREVENTS STARVATION !!!
+a____ b__c_ ______ _____
+[a_]  [ab]  A              <-- here, a starts. NO STARVATION 
+         [ac]              
+            [c_]
+```
+
+SUMMARY: 
+- Starvation can only happen if waitlist size=1; if waitlist size=2 (or bigger) cannot happen because always the LAST job gets replaced.
+- We cannot debounce immediately; so we ALWAYS wait at least for start_delay. (not a problem for us right now).
+
+## problem description
+
+In a project, we have 50 Neos instances, which use prunner for background tasks (some are short, some are long).
+
+Currently, we have 4 pipelines globally
+- concurrency 1
+- concurrency 8
+- concurrency 4
+- concurrency 4 (import job)
+  - -> needs the global concurrency to limit the needed server resources
+
+Now, a new pipeline should be added for "irregular import jobs" triggered by webhooks.
+- can happen very quickly after each other
+- -> Pipeline should start after a certain amount of time (newer should override older pipelines)
+- StartDelay combined with QueueStrategy "Replace"
+  - `waitList[len(waitList)-1] = job` -> *LAST* Element is replaced of wait list.
+- -> GLOBAL replacement does not work, because each job has arguments (which are relevant, i.e. tenant ID).
+
+We still want to have a GLOBAL CONCURRENCY LIMIT (per pipeline), but a WAITLIST per instance.
+
+
+## Solution Idea:
+
+we want to be able to SEGMENT the waitlist into different partitions. The `queue_strategy` and `queue_limit` should be per partition.
+`concurrency` stays per pipeline (as before)
+
+```
+**LOGICAL VIEW** (Idea)
+┌──────────────────────┐      ┌──────────────────────────────────────────────────┐
+│ Waitlist Instance 1  │      │          Pipeline (with concurrency 2)           │
+├──────────────────────┤      │                                                  │
+│ Waitlist Instance 2  │  ->  ├──────────────────────────────────────────────────┤
+├──────────────────────┤      │                                                  │
+│ Waitlist Instance 3  │      │                                                  │
+└──────────────────────┘      └──────────────────────────────────────────────────┘
+
+          if job is *delayed*,                                                    
+           stays in wait list                                                     
+            for this duration                                                     
+
+```
+
+Technically, we still have a SINGLE Wait List per pipeline, but the jobs can be partitioned by `waitlist_partition_id`.
+
+-> In this case, the `replace` strategy will replace the last element of the given partition.
+
+-> we create a new queueStrategy for the partitioned waitlist: `partitioned_replace`
+
+If we partition the waitlist, the waitlist can grow up to queue_limit * number of partitions.

helper/slice_utils/sliceUtils.go

@@ -0,0 +1,11 @@
+package slice_utils
+
+func Filter[T any](s []T, p func(i T) bool) []T {
+	var result []T
+	for _, i := range s {
+		if p(i) {
+			result = append(result, i)
+		}
+	}
+	return result
+}