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, and how to limit it to part of the plan. It also shows how to read, preview and undo the 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:

• leveling_delay delays the whole task.
• pauses splits a task: the task stops working for a while and picks up again later.

Leveling looks at the plan in buckets: day buckets when config.duration_unit is "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 (default config.default_resource_capacity, 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 auto_scheduling: false
• completed tasks (progress: 1, when config.use_progress is on)
• tasks with an "mso" or "mfo" constraint (when config.apply_constraints is on)
• tasks with priority: 1000
• summaries (their dates follow their children)
• tasks outside scope.tasks, when you set it

Sometimes a proposed move doesn't stick, because a constraint or another hard rule pulls the 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. A project with config.schedule_from_end: true is refused with an InputError 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, so a long run blocks the event loop. On large projects, move it to a worker: see Run in a worker.

Leveling a project#

Call level() without options 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.

import { Project } from "plen-js";

// Two 2-day tasks, both booked full time on dev1 from Monday.
const data = {
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
};

const project = new Project();
const solved = project.solve(data);
for (const over of solved.overallocations ?? []) {
  for (const w of over.windows ?? []) {
    // dev1: 2026-08-10 08:00 .. 2026-08-11 17:00, peak 2 of 1
    console.log(`${over.resource}: ${w.from} .. ${w.to}, peak ${w.peak} of ${w.capacity}`);
  }
}

const leveled = project.level();
for (const u of leveled.updates) {
  // B: 2026-08-12 08:00 .. 2026-08-14 08:00, delay 2
  console.log(`${u.id}: ${u.start_date} .. ${u.end_date}, delay ${u.leveling_delay}`);
}

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 leveling_delay and, when it changed, its complete pauses list. An update entry can be fed back as a TaskUpdate as is.

Delays and pauses#

leveling_delay 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 { at, length, by } starts at after the task start and lasts length. The leveler sets by: "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 instead. 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.

import { Project } from "plen-js";

// A runs Monday and Tuesday. B joins on Tuesday and feeds C, so it has less slack.
const data = {
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-11 08:00", duration: 2 },
    { id: "C", start_date: "2026-08-13 08:00", duration: 5 },
  ],
  links: [{ id: "bc", source: "B", target: "C", type: "fs" }],
  resources: [{ id: "dev1" }, { id: "dev2" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
    { task: "C", resource: "dev2", value: 1 },
  ],
};

const project = new Project();
project.solve(data);

// Splits allowed (the default): A is paused, B and C keep their dates.
const split = project.clone().level();
for (const u of split.updates) {
  // A: end 2026-08-14 08:00, pauses [{"at":1,"by":"leveling","length":2}]
  console.log(`${u.id}: end ${u.end_date}, pauses ${JSON.stringify(u.pauses)}`);
}

// Whole-task delays only: B moves past A, and C moves with it.
const whole = project.clone().level({ allowSplits: false });
for (const u of whole.updates) {
  // B: 2026-08-12 08:00 .. 2026-08-14 08:00
  // C: 2026-08-14 08:00 .. 2026-08-21 08:00
  console.log(`${u.id}: ${u.start_date} .. ${u.end_date}`);
}

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. B is delayed whole, and the link pushes C along. The update for A has no start_date, 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 never gets a pauses field in its update. The field rules are on the TaskData reference.

Leveling options#

Pass a LevelingOptions object to level. Every field is optional. Unknown keys throw an InputError.

The full list is on the LevelingOptions reference.

Leveling part of a plan#

scope 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.

import { Project } from "plen-js";

const project = new Project();
project.solve({
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});

// Only A may move, so A gives way even though B is later in input order.
const onlyA = project.clone().level({ scope: { tasks: ["A"] } });
for (const u of onlyA.updates) {
  // A: 2026-08-12 08:00 .. 2026-08-14 08:00
  console.log(`${u.id}: ${u.start_date} .. ${u.end_date}`);
}

// Only contention from Wednesday on. The Monday-Tuesday window stays.
const fromWednesday = project.clone().level({ scope: { from: "2026-08-12 00:00" } });
for (const over of fromWednesday.overallocations ?? []) {
  for (const w of over.windows ?? []) {
    // dev1: 2026-08-10 08:00 .. 2026-08-11 17:00, out-of-scope
    console.log(`${over.resource}: ${w.from} .. ${w.to}, ${w.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 throws an InputError with kind: "validation" and 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.

import { Project } from "plen-js";

const project = new Project();
project.solve({
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});

const result = project.level({ withinSlackOnly: true });
console.log(result.updates.length); // 0
for (const over of result.overallocations ?? []) {
  for (const w of over.windows ?? []) {
    // dev1: 2026-08-10 08:00 .. 2026-08-11 17:00, slack-exhausted
    console.log(`${over.resource}: ${w.from} .. ${w.to}, ${w.reason}`);
  }
}

Both tasks end the project, so neither has slack to give, and the window stays.

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 the burden of a long window can switch between contenders bucket by bucket. step: "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.

import { Project } from "plen-js";

const project = new Project();
project.solve({
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});

const minimal = project.clone().level();
const skip = project.clone().level({ step: "skip" });
// Same dates for B (Wednesday to Friday), in 2 rounds and in 1.
console.log(`minimal: ${minimal.rounds} rounds, skip: ${skip.rounds} rounds`);
console.log(JSON.stringify(minimal.updates) === JSON.stringify(skip.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 an integer field 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:

import { Project } from "plen-js";

const project = new Project();
project.solve({
  tasks: [
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  links: [{ id: "ab", source: "A", target: "B", type: "ss" }],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});

const result = project.level();
for (const u of result.updates) {
  // B: 2026-08-12 08:00 .. 2026-08-14 08:00
  console.log(`${u.id}: ${u.start_date} .. ${u.end_date}`);
}

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

Reading the result#

level returns a LevelingResult: the same ScheduleResult shape that solve returns, plus one field.

• updates carry the new dates, delays and pauses. baselineId names the new baseline.
• rounds is the number of rounds that proposed a move. Rounds that only skipped an unresolvable bucket don't count.

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

overallocations lists the windows that remain. 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.

import { Project } from "plen-js";

// A is fixed. B is the only task that could move, but it must start by Monday.
const project = new Project();
project.solve({
  config: { apply_constraints: true },
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2, auto_scheduling: false },
    {
      id: "B",
      start_date: "2026-08-10 08:00",
      duration: 2,
      constraint_type: "snlt",
      constraint_date: "2026-08-10 08:00",
    },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});

const result = project.level();
console.log(result.updates.length); // 0: the move was reverted
for (const over of result.overallocations ?? []) {
  for (const w of over.windows ?? []) {
    // dev1: tasks A,B, reason pinned-constraint
    console.log(`${over.resource}: tasks ${w.tasks}, reason ${w.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 fields (from, to, capacity, peak, overload, tasks) are on the Overallocation reference. For the load of each bucket, see Resource load.

level throws an InputError for an unknown scope id, an unknown option or a backward-scheduled project. It throws a ProjectError with code: "needs-resync" after an apply with rejected operations. Either way, the project stays as it was. See Error handling.

Previewing and clearing#

Because level commits, run it on a clone to show leveling as a proposal. clone() returns an independent project with the same accepted state.

import { Project } from "plen-js";

function levelingProposal(project) {
  const draft = project.clone();
  const proposal = draft.level();
  for (const u of proposal.updates) {
    console.log(`${u.id}: delay ${u.leveling_delay}, pauses ${JSON.stringify(u.pauses ?? [])}`);
  }
  // Accept: keep `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 the level option. See Evaluating alternatives.

There are two ways to remove leveling:

• To undo it, set leveling_delay to 0 and pauses to an empty list on the task, then solve. 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.

import { Project } from "plen-js";

const project = new Project();
project.solve({
  tasks: [
    { id: "A", start_date: "2026-08-10 08:00", duration: 2 },
    { id: "B", start_date: "2026-08-10 08:00", duration: 2 },
  ],
  resources: [{ id: "dev1" }],
  assignments: [
    { task: "A", resource: "dev1", value: 1 },
    { task: "B", resource: "dev1", value: 1 },
  ],
});
project.level(); // B moves to Wednesday

// Undo leveling on B.
project.apply({
  tasks: { update: [{ id: "B", leveling_delay: 0, pauses: [] }] },
});

const replanned = project.solve(null);
for (const u of replanned.updates) {
  // B: 2026-08-10 08:00 .. 2026-08-12 08:00
  console.log(`${u.id}: ${u.start_date} .. ${u.end_date}`);
}
// dev1 is overallocated on Monday and Tuesday again.
const contended = (replanned.overallocations ?? []).some((o) => o.windows?.length);
console.log(contended); // true

The leveling_delay update moves B's stored start 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.