Leveling Resources#

This guide shows how to remove resource overallocation from a plan. It covers what Project.Level changes, how it picks the task that gives way, how to limit it to part of the plan, and how to read, preview and undo its result.

How leveling works#

A resource is overallocated when the tasks booked on it need more than its capacity at the same time. Solve only reports this, as Overallocations windows on the result. Level fixes it by moving work later. It never changes assignments, resources or capacities. It changes two task fields:

• LevelingDelay delays the whole task.
• Pauses splits a task: it stops working for a while and picks up again later.

Leveling looks at the plan in buckets: day buckets when Config.DurationUnit is TimeUnit.Day, hour buckets for Hour and Minute. The load of a resource in a bucket is the sum of Value over the assignments that book working time in that bucket. A task counts with its full Value as soon as it works any time in the bucket. When the load is higher than the resource's Capacity (by default Config.DefaultResourceCapacity, which is 1), the bucket is overallocated. A run of overallocated buckets in a row is a window.

Level works through the plan from left to right, in rounds:

1. Find the earliest overallocated bucket.
2. Collect the tasks that contribute to it, the contenders, and sort them by the selection order.
3. Move the weakest contender just past the bucket, with a delay or a pause, until the load fits.
4. Reschedule the project, so links and constraints push the successors along.
5. Repeat until no overallocated bucket with a movable contender is left.

Some tasks never move. Leveling leaves these alone and calls them the fixed set:

• tasks with AutoScheduling = false
• completed tasks (Progress == 1, when Config.UseProgress is on)
• tasks with an Mso or Mfo constraint (when Config.ApplyConstraints is on)
• tasks with Priority == 1000
• summaries (their dates follow their children)
• tasks outside LevelingScope.Tasks, when you set it

Sometimes a constraint or another hard rule pulls a moved task back. Leveling then reverts the move and freezes the task for the rest of the run. Hard requirements always win. Whatever leveling can't resolve stays in Overallocations, and each remaining window gets a Reason.

A few rules hold for every run:

• Level commits. It solves the current state first, writes the delays and pauses into the project and accepts the leveled dates as the new baseline. To show leveling as a proposal, run it on a clone. See Previewing and clearing.
• Leveling is forward-only. For a project with Config.ScheduleFromEnd = true, Level throws InputException at the path config.schedule_from_end.
• The output is deterministic. Every tie ends in input order, so the same data and options always give the same result.
• There's no round limit. A run ends when nothing is left to resolve. Level is synchronous and CPU-bound, and on large projects it can take a while. In a server or UI app, run it off the request or UI thread, for example with Task.Run. See Call from async code.

Leveling a project#

Call Level with a default LevelingOptions after the project is solved. The examples on this page use the default calendar (Monday to Friday, 08:00-12:00 and 13:00-17:00) and start on Monday, August 10, 2026.

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

// Two 2-day tasks, both booked full time on dev1 from Monday.
var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0 },
        new TaskData("B") { StartDate = Aug(10), Duration = 2.0 },
    ],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};

var project = new Project();
var solved = project.Solve(data, new SolveOptions());
foreach (var over in solved.Overallocations ?? [])
    foreach (var window in over.Windows)
        // dev1: 2026-08-10 08:00 .. 2026-08-11 17:00, peak 2 of 1
        Console.WriteLine($"{over.Resource}: {window.From} .. {window.To}, peak {window.Peak} of {window.Capacity}");

var leveled = project.Level(new LevelingOptions());
foreach (var update in leveled.Schedule.Updates)
    // B: 2026-08-12 08:00 .. 2026-08-14 08:00, delay 2
    Console.WriteLine($"{update.Id}: {update.StartDate} .. {update.EndDate}, delay {update.LevelingDelay}");

Solve reports one window on dev1 over Monday and Tuesday, with a peak load of 2. Level keeps A and moves B to Wednesday. Both tasks are equal, so the input-order tie-break decides: the later task moves.

Leveling diffs its Updates against the state the call started from, so you can apply them to the model you already hold. Next to the new dates, each entry carries the task's new LevelingDelay and, when it changed, its complete Pauses list. A Mutation maps field-for-field onto a TaskUpdate.

Delays and pauses#

LevelingDelay and Pauses are elapsed time, in the task's duration unit: with day units, 1 is 24 hours of calendar time, weekends and nights included. Duration and Lag stay working time.

• A delay shifts the task's start from its driven position (the start that links and constraints give it). Draw it as the stretch [start - delay, start) before the bar.
• A Pause starts At after the task start and lasts Length. The leveler sets By to PauseSource.Leveling on every pause it writes. Draw pauses as gaps in the bar. They need no calendar.

Which one leveling picks depends on the contender. A task that starts inside the overallocated bucket has no work before it, so it gets a whole-task delay. A task that's already running in that bucket can be paused: it keeps the work it has done and resumes after the contention. Pausing a running task leaves no idle hole behind it. Delaying it would. Set AllowSplits = false to get whole-task delays only.

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task, string resource) =>
    new() { Task = task, Resource = resource, Value = 1.0 };

// A runs Monday and Tuesday. B joins on Tuesday and feeds C, so it has less slack.
var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0 },
        new TaskData("B") { StartDate = Aug(11), Duration = 2.0 },
        new TaskData("C") { StartDate = Aug(13), Duration = 5.0 },
    ],
    Links = [new LinkData { Id = "bc", Source = "B", Target = "C" }],
    Resources = [new ResourceData { Id = "dev1" }, new ResourceData { Id = "dev2" }],
    Assignments = [Booking("A", "dev1"), Booking("B", "dev1"), Booking("C", "dev2")],
};

var project = new Project();
project.Solve(data, new SolveOptions());

// Splits allowed (the default): A is paused, B and C keep their dates.
var split = project.Clone().Level(new LevelingOptions());
foreach (var update in split.Schedule.Updates)
{
    // A: end 2026-08-14 08:00, pauses [1 + 2]
    var pauses = string.Join(", ", (update.Pauses ?? []).Select(p => $"{p.At} + {p.Length}"));
    Console.WriteLine($"{update.Id}: end {update.EndDate}, pauses [{pauses}]");
}

// Whole-task delays only: B moves past A, and C moves with it.
var whole = project.Clone().Level(new LevelingOptions { AllowSplits = false });
foreach (var update in whole.Schedule.Updates)
    // B: 2026-08-12 08:00 .. 2026-08-14 08:00
    // C: 2026-08-14 08:00 .. 2026-08-21 08:00
    Console.WriteLine($"{update.Id}: {update.StartDate} .. {update.EndDate}");

With splits, A keeps its start and works Monday, pauses for two days (At 1, Length 2), and finishes its second day on Thursday. B works in the pause, so the chain into C doesn't move. Without splits, A can't be paused and stays in place, so B is delayed whole and the link pushes C along. The update for A has a null StartDate, because its start didn't change.

Pauses only come from leveling: there are no user-defined pauses. A pause takes up no load, and a task without pauses gets a null Pauses in its update. The field rules are on the Pause reference.

Leveling options#

LevelingOptions has a default for every property. Set the ones you need with an object initializer:

The full list is on the LevelingOptions reference.

Leveling part of a plan#

LevelingScope limits a run in three ways, and you can combine them:

• Tasks: only these tasks may move. Everything else acts like a fixed task.
• Resources: only contention on these resources triggers a move.
• From / To: only buckets that start in [From, To) trigger a move. The range limits where leveling looks for contention, not where tasks land: a moved task may end up past To.

using Plen;

static Moment Aug(int day, int hour) =>
    Moment.FromYmdHm(2026, 8, day, hour, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10, 8), Duration = 2.0 },
        new TaskData("B") { StartDate = Aug(10, 8), Duration = 2.0 },
    ],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};
var project = new Project();
project.Solve(data, new SolveOptions());

// Only A may move, so A gives way even though B is later in input order.
var onlyA = new LevelingOptions { Scope = new LevelingScope { Tasks = ["A"] } };
var result = project.Clone().Level(onlyA);
foreach (var update in result.Schedule.Updates)
    // A: 2026-08-12 08:00 .. 2026-08-14 08:00
    Console.WriteLine($"{update.Id}: {update.StartDate} .. {update.EndDate}");

// Only contention from Wednesday on. The Monday-Tuesday window stays.
var fromWednesday = new LevelingOptions { Scope = new LevelingScope { From = Aug(12, 0) } };
result = project.Clone().Level(fromWednesday);
foreach (var over in result.Schedule.Overallocations ?? [])
    foreach (var window in over.Windows)
        // dev1: 2026-08-10 08:00 .. 2026-08-11 17:00, out-of-scope
        Console.WriteLine($"{over.Resource}: {window.From} .. {window.To}, {window.Reason}");

The second run leaves the Monday window alone and reports it with the reason out-of-scope. The result still says Converged == true, because the run did everything it was asked to do. The reason tells you why the window is still there.

An unknown id in Scope.Tasks or Scope.Resources fails the call with InputException, at the path scope.tasks or scope.resources.

Staying within slack#

With WithinSlackOnly = true, leveling never pushes a task further than its total slack. The slack is measured against the project end of the plan the run started from, so leveling can't make the project finish later. If a contender can't clear the bucket within its slack, leveling skips it and tries the next one. If no contender fits, the window stays with the reason slack-exhausted.

The slack leveling uses is internal. It ignores the existing delays and uses that fixed project end, so it can differ from Analytics.Slack in a Solve result.

Fewer rounds with the skip step#

By default a move clears one bucket. Then leveling reschedules and re-selects at the next bucket, so in a long window the task that gives way can change from bucket to bucket. Step = LevelingStep.Skip moves the task past the whole window in one move. That takes fewer rounds and reschedules less often. Use it for large projects where Level is slow.

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0 },
        new TaskData("B") { StartDate = Aug(10), Duration = 2.0 },
    ],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};
var project = new Project();
project.Solve(data, new SolveOptions());

var minimal = project.Clone().Level(new LevelingOptions());
var skip = project.Clone().Level(new LevelingOptions { Step = LevelingStep.Skip });

// Same dates for B (Wednesday to Friday), in 2 rounds and in 1.
Console.WriteLine($"minimal: {minimal.Rounds} rounds, skip: {skip.Rounds} rounds");
Console.WriteLine(minimal.Schedule.Updates.SequenceEqual(skip.Schedule.Updates)); // True

Here both steps give the same plan. That's not guaranteed: buckets the skip step jumps over never get a new selection, so on real plans the two can level differently. If a jump hits a constraint or doesn't fit the slack, leveling falls back to the one-bucket move for that task.

Selection order#

When several contenders share an overallocated bucket, the order decides which one stays and which moves first. Every order ends in the same tie-break: the task later in input order moves.

All three orders also apply the running rule. A task already working before the bucket stays, and the task that starts in the bucket moves, unless the running task can be paused instead. This keeps whole-task delays from leaving gaps in work that was already scheduled. Under Priority, a higher priority beats this rule: the lower-priority running task is paused, or delayed whole when splits are off.

Priority is a ushort on TaskData, 500 by default. At 1000 the task never moves.

The dependency check exists because delaying a predecessor would push its successor along and resolve nothing. In this plan B comes first in the input, but an SS link from A makes A its predecessor, so B moves:

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

var data = new ProjectData
{
    Tasks =
    [
        new TaskData("B") { StartDate = Aug(10), Duration = 2.0 },
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0 },
    ],
    Links = [new LinkData { Id = "ab", Source = "A", Target = "B", LinkType = LinkType.Ss }],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};
var project = new Project();
project.Solve(data, new SolveOptions());

var result = project.Level(new LevelingOptions());
foreach (var update in result.Schedule.Updates)
    // B: 2026-08-12 08:00 .. 2026-08-14 08:00
    Console.WriteLine($"{update.Id}: {update.StartDate} .. {update.EndDate}");

To protect a task from leveling, set its Priority to 1000 or its AutoScheduling to false. With Priority = 1000 on B in the first example, A moves to Wednesday instead.

Reading the result#

Level returns a LevelingResult:

• Schedule: a ScheduleResult, the same shape Solve returns. Its Updates carry the new dates, delays and pauses. BaselineId names the new baseline.
• Rounds: the number of rounds that proposed a move. Rounds that only skipped an unresolvable bucket don't count.

Treat Schedule as read-only. The project keeps it as its baseline result and can hand the same object to later calls and clones. Call DeepClone() before you modify it.

Schedule.Converged is the scheduler's convergence on the final plan, not a leveling flag. Leveling itself always runs until nothing is left to resolve.

Schedule.Overallocations lists the windows that remain (see ResourceOverallocation). Every window from Level has a Reason. When a window has several immovable contenders, the first reason in this table wins:

A reason is data, not an error. It tells the user what to change to resolve the window: unpin a task, loosen a constraint, widen the scope.

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

// A is fixed. B is the only task that could move, but it must start by Monday.
var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0, AutoScheduling = false },
        new TaskData("B")
        {
            StartDate = Aug(10),
            Duration = 2.0,
            ConstraintType = ConstraintType.Snlt,
            ConstraintDate = Aug(10),
        },
    ],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};
data.Config.ApplyConstraints = true;

var project = new Project();
project.Solve(data, new SolveOptions());
var result = project.Level(new LevelingOptions());

// The move was reverted, so nothing changed.
Console.WriteLine(result.Schedule.Updates.Count); // 0
foreach (var over in result.Schedule.Overallocations ?? [])
    foreach (var window in over.Windows)
        // dev1: tasks [A, B], reason pinned-constraint
        Console.WriteLine($"{over.Resource}: tasks [{string.Join(", ", window.Tasks)}], reason {window.Reason}");

Leveling proposed a delay for B, the replan pulled B back to Monday, and leveling reverted the move. Nothing in the project changed. The window stays and reports pinned-constraint. With both tasks fixed and no constraint, the same window would report fixed-task.

The other window properties (From, To, Capacity, Peak, Overload, Tasks) are on the ResourceOverallocation reference. For the load of each bucket, see Resource load.

Level throws InputException for an unknown scope id or a backward-scheduled project, and ProjectException with Code == ProjectError.NeedsResync after a rejected Apply. Both leave the project as it was. See Error handling.

Previewing and clearing#

Because Level commits, run it on a clone to show leveling as a proposal. Project.Clone() is cheap: the data is shared until one copy is edited.

using Plen;

static Project LevelingProposal(Project project)
{
    var draft = project.Clone();
    var proposal = draft.Level(new LevelingOptions());
    foreach (var update in proposal.Schedule.Updates)
        Console.WriteLine($"{update.Id}: delay {update.LevelingDelay}, pauses {update.Pauses?.Count ?? 0}");
    // Accept: use the returned draft in place of project. Reject: drop it.
    return draft;
}

The step-by-step version is in Preview leveling. To score leveling as part of an alternative change set, use Evaluate with EvaluateOptions.Level. See Evaluating alternatives.

There are two ways to remove leveling:

• To undo it, set LevelingDelay to 0 and Pauses to an empty list on the task, then call Solve(null, …). The original contention comes back.
• To level again from scratch, call Level again. ClearFirst is on by default, so the run first clears the delays and pauses of the in-scope tasks and levels from the clean plan. With ClearFirst = false the existing delays and pauses stay, and leveling adds to them. Use that to keep a deliberate gap the user set by hand.

using Plen;

static Moment Aug(int day) =>
    Moment.FromYmdHm(2026, 8, day, 8, 0) ?? throw new InvalidOperationException("invalid date");

static AssignmentData Booking(string task) =>
    new() { Task = task, Resource = "dev1", Value = 1.0 };

var data = new ProjectData
{
    Tasks =
    [
        new TaskData("A") { StartDate = Aug(10), Duration = 2.0 },
        new TaskData("B") { StartDate = Aug(10), Duration = 2.0 },
    ],
    Resources = [new ResourceData { Id = "dev1" }],
    Assignments = [Booking("A"), Booking("B")],
};
var project = new Project();
project.Solve(data, new SolveOptions());
project.Level(new LevelingOptions()); // B moves to Wednesday

// Undo leveling on B.
var changes = new SchedulingChanges();
changes.Tasks.Update.Add(new TaskUpdate("B") { LevelingDelay = 0.0, Pauses = new List<Pause>() });
project.Apply(changes);

var replanned = project.Solve(null, new SolveOptions());
foreach (var update in replanned.Updates)
    // B: 2026-08-10 08:00 .. 2026-08-12 08:00
    Console.WriteLine($"{update.Id}: {update.StartDate} .. {update.EndDate}");

// dev1 is overallocated on Monday and Tuesday again.
Console.WriteLine((replanned.Overallocations ?? []).Any(o => o.Windows.Count > 0)); // True

The LevelingDelay update moves the stored start of B back by the same amount, so B returns to Monday and the window over Monday and Tuesday comes back. To clear every task, put one such update per task with a delay or pauses into the same change set. The full recipe is in Clear leveling.