[Canonical page](<https://docs.dhtmlx.com/planning-engine/java/live-projects/editing-a-project/>)
[Full documentation index](<https://docs.dhtmlx.com/planning-engine/llms.txt>)


# Data Updates

This guide covers how to change a live [`Project`](<https://docs.dhtmlx.com/planning-engine/java/api/project/project.md>) as the user works: adding, updating and removing tasks, links, resources and assignments, rescheduling after the edit, and recovering when part of an edit is refused.

## How editing works

A [`Project`](<https://docs.dhtmlx.com/planning-engine/java/api/project/project.md>) holds one accepted state: the data from the last [`solve(data, …)`](<https://docs.dhtmlx.com/planning-engine/java/api/project-solve.md>) plus every edit since, with the dates the last solve accepted. You don't send the whole plan again for each change. You describe the change as a [`SchedulingChanges`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/scheduling-changes.md>) object and pass it to [`Project#apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>).

Editing takes two separate calls:

1. `apply(changes)` writes the edit into the accepted state and marks the project unsolved. It doesn't schedule anything.
2. `solve(null, options)` schedules the edited state, accepts the new dates and returns what moved.

Between the two calls the project has no baseline. `preview`, `loadProfile`, `evaluate`, `suggestAssignments` and `roster` throw [`ProjectException`](<https://docs.dhtmlx.com/planning-engine/java/api/errors/project-exception.md>) with code `NO_BASELINE` until you solve. You can call `apply` several times before one `solve`, and the edits accumulate.

`apply` accepts what it can. Each operation in the change set applies or is rejected on its own. A rejection isn't an exception. It's listed in the returned [`ApplyReport`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md>) and sets [`needsResync`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md#needsresync>). From then on the project refuses `solve(null, …)`, `apply` and `level` with `NEEDS_RESYNC`. Your model and the project's state may now disagree, and the only way back is to send the full data with `solve(data, …)`.

When `apply` itself throws a `ProjectException`, the call didn't fit the project's state: `NEEDS_RESYNC` after an earlier rejection, or `STALE_BASELINE` when the change set names a baseline that isn't current anymore. In both cases nothing was applied. Both exceptions are unchecked. See [Error handling](<https://docs.dhtmlx.com/planning-engine/java/error-handling.md>).

## Change sets

A [`SchedulingChanges`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/scheduling-changes.md>) groups the edits by record kind. Each group is a [`Changes<TAdd, TUpdate>`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/changes.md>) with three lists: `add`, `update` and `remove`. All of them start empty, so you add only what the edit needs.

| Group | `add` | `update` | `remove` |
|---|---|---|---|
| `tasks` | [`TaskData`](<https://docs.dhtmlx.com/planning-engine/java/api/data/task-data.md>) | [`TaskUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/task-update.md>) | task [`Id`](<https://docs.dhtmlx.com/planning-engine/java/api/data/id.md>) |
| `links` | [`LinkAdd`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/link-add.md>) | [`LinkUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/link-update.md>) | link `Id` |
| `resources` | [`ResourceData`](<https://docs.dhtmlx.com/planning-engine/java/api/data/resource-data.md>) | [`ResourceUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/resource-update.md>) | resource `Id` |
| `assignments` | [`AssignmentData`](<https://docs.dhtmlx.com/planning-engine/java/api/data/assignment-data.md>) | [`AssignmentUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/assignment-update.md>) | assignment `Id` |

Additions use the same record types as [`ProjectData`](<https://docs.dhtmlx.com/planning-engine/java/api/data/project-data.md>), except links: a `LinkAdd` may leave `id` as `null`. Leave a link or assignment id `null` and [`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>) generates one (`$generated_link_1`, `$generated_assignment_1`, ...). Generated ids come back in [`ApplyReport.effects`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md#effects>), so you can store them in your model.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class ChangeSet {
  public static void main(String[] args) throws Exception {
    Moment monday = Moment.fromYmdHm(2026, 8, 10, 8, 0);
    ProjectData data = new ProjectData()
        .withTasks(List.of(
            new TaskData(Id.num(1)).withStartDate(monday).withDuration(2.0),
            new TaskData(Id.num(2)).withStartDate(monday).withDuration(3.0)))
        .withLinks(List.of(
            new LinkData().withId(Id.num(1)).withSource(Id.num(1)).withTarget(Id.num(2))));

    Project project = new Project();
    SolveOptions options = new SolveOptions();
    project.solve(data, options);

    SchedulingChanges changes = new SchedulingChanges();
    // A new task, linked after task 2. The link has no id: apply generates one.
    changes.tasks.add.add(new TaskData(Id.num(3)).withStartDate(monday).withDuration(1.0));
    changes.links.add.add(new LinkAdd().withSource(Id.num(2)).withTarget(Id.num(3)));
    // Task 1 now takes three days.
    changes.tasks.update.add(new TaskUpdate(Id.num(1)).withDuration(Patch.of(3.0)));

    ApplyReport report = project.apply(changes);
    for (AppliedOperation op : report.applied) {
      System.out.println("applied " + op.path);
    }
    System.out.println("generated links: " + report.effects.generatedLinks);

    ScheduleResult result = project.solve(null, options);
    for (Mutation update : result.updates) {
      System.out.println("task " + update.id + ": " + update.startDate + " .. " + update.endDate);
    }
  }
}
```

The report lists `tasks.add[0]`, `tasks.update[0]` and `links.add[0]`, and the new link is `$generated_link_1`. After the solve, task 1 runs until `2026-08-13 08:00`, task 2 moves to `2026-08-13 08:00 .. 2026-08-18 08:00`, and task 3 starts when task 2 ends.

The lists are ordinary `Vec` (`ArrayList`) fields, so `changes.tasks.update.add(...)` appends one operation. To set a whole list at once, use the [`withAdd`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/changes.md#withadd>), [`withUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/changes.md#withupdate>) and [`withRemove`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/changes.md#withremove>) setters, which also take `List.of(...)`.

Operations don't run in the order you added them. `apply` runs them in a fixed order, so every reference comes after the adds it may need, and every removal comes before the edits it could break:

1. tasks: remove, add, update
2. links: remove, update, add
3. resources: remove, add, update
4. assignments: remove, update, add

An operation is named by its group, list and index, such as `tasks.update[0]`. The same path shows up in [`ApplyReport.applied`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md#applied>) and [`ApplyReport.rejected`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md#rejected>).

## Update records and Patch

Update records ([`TaskUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/task-update.md>), [`LinkUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/link-update.md>), [`ResourceUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/resource-update.md>), [`AssignmentUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/assignment-update.md>)) name the record by `id` and carry every other field as a [`Patch<T>`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/patch.md>). A patch has three states:

- [`Patch.missing()`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/patch.md#missing>) (the default) leaves the field as it is.
- [`Patch.of(value)`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/patch.md#of>) sets it.
- [`Patch.nullValue()`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/patch.md#nullvalue>) clears an optional field, or restores the default of a field that always has a value: `autoScheduling` goes back to `true`, `unscheduled` to `false`, `progress` to `0`, `priority` to `500`, `levelingDelay` to `0`, `pauses` to empty, `linkType` to `FS` and `lag` to `0`.

`new TaskUpdate(id)` and the other update constructors set every field to `Patch.missing()`, so you only touch the fields you mean to change. Each field has a fluent `withX(patch)` setter.

`Patch.of(null)` isn't the same as `Patch.nullValue()`. It's a value patch that holds `null`. On a task or resource `calendarId` it clears the calendar. On fields that can't hold `null` (task `parent` and `pauses`, link `source` and `target`, assignment `task` and `resource`, the boxed numbers and flags) it throws `NullPointerException` when applied. Use `Patch.nullValue()` to clear.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class PinUnpin {
  public static void main(String[] args) throws Exception {
    Moment monday = Moment.fromYmdHm(2026, 8, 10, 8, 0);
    Moment thursday = Moment.fromYmdHm(2026, 8, 13, 8, 0);
    Id review = Id.str("review");

    ProjectData data = new ProjectData()
        .withConfig(new SchedulingConfig().withApplyConstraints(true))
        .withTasks(List.of(new TaskData(review).withStartDate(monday).withDuration(1.0)));
    Project project = new Project();
    SolveOptions options = new SolveOptions();
    project.solve(data, options);

    // Patch.of sets a field: "start no earlier than Thursday".
    SchedulingChanges pin = new SchedulingChanges();
    pin.tasks.update.add(new TaskUpdate(review)
        .withConstraintType(Patch.of(ConstraintType.SNET))
        .withConstraintDate(Patch.of(thursday)));
    project.apply(pin);
    for (Mutation update : project.solve(null, options).updates) {
      System.out.println("pinned: " + update.id + " starts " + update.startDate);
    }

    // Patch.nullValue() clears the constraint; startDate moves the task back.
    // Every field left Patch.missing() keeps its value.
    SchedulingChanges unpin = new SchedulingChanges();
    unpin.tasks.update.add(new TaskUpdate(review)
        .withConstraintType(Patch.nullValue())
        .withConstraintDate(Patch.nullValue())
        .withStartDate(Patch.of(monday)));
    project.apply(unpin);
    for (Mutation update : project.solve(null, options).updates) {
      System.out.println("unpinned: " + update.id + " starts " + update.startDate);
    }
  }
}
```

The first solve moves the task to `2026-08-13 08:00`. The second puts it back on Monday. Clearing the constraint alone wouldn't do that: the accepted state already holds the Thursday dates, so the update also sets `startDate`.

### Which dates a task update keeps

A task has three linked date fields: `startDate`, `endDate` and `duration`. When an update sets some of them, [`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>) keeps what you sent and derives the rest:

- `startDate` alone keeps the duration and recomputes the end (a drag).
- `endDate` alone keeps the start and recomputes the duration (a resize from the right edge).
- `duration` alone keeps the start and recomputes the end.
- `startDate` and `endDate` together recompute the duration.
- A new `calendarId`, or a new `parent` while calendars are inherited ([`CalendarMode.TASK_INHERITED`](<https://docs.dhtmlx.com/planning-engine/java/api/data/calendar-mode.md>)), keeps the start and duration and recomputes the end on the new calendar. Under an inherited calendar, the descendants that inherit it re-derive their ends too.

When a task has no predecessor, an explicit `startDate` on non-working time is kept as sent. The end is computed forward from it on the working calendar, so only the start sits outside working time. Snap the date in your UI if you don't want that.

### Feeding results back

A [`Mutation`](<https://docs.dhtmlx.com/planning-engine/java/api/results/mutation.md>) from a result uses the same field names as [`TaskUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/task-update.md>), and sets only the fields that changed (the others are `null`). A result entry maps field by field onto an update. Use this to commit dates you got from [`preview`](<https://docs.dhtmlx.com/planning-engine/java/api/project-preview.md>) or from a leveled copy:

```java
static <T> Patch<T> patch(T value) {
  return value == null ? Patch.missing() : Patch.of(value);
}

static TaskUpdate toUpdate(Mutation mutation) {
  return new TaskUpdate(mutation.id)
      .withStartDate(patch(mutation.startDate))
      .withEndDate(patch(mutation.endDate))
      .withDuration(patch(mutation.duration))
      .withLevelingDelay(patch(mutation.levelingDelay))
      .withPauses(patch(mutation.pauses));
}
```

`pauses` replaces the whole list. Set it to `Patch.of(new Vec<>())` or `Patch.nullValue()` to remove every pause. See [Clear leveling](<https://docs.dhtmlx.com/planning-engine/java/how-to/clear-leveling.md>).

## Applying

[`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>) returns an [`ApplyReport`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md>):

- `applied` lists the path of every accepted operation.
- `rejected` lists the path and message of every refused one.
- `effects` is a [`ChangeEffects`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/apply-report.md#changeeffects>) with what the edit changed beyond the records you named (below).
- `needsResync` is `true` when anything was rejected.
- `revision` is the project's revision number after this edit, as a `long`.

Removals cascade, and `effects` tells you what went with them. Removing a task removes its descendants, every link that touches any of them and every assignment on them. Removing a resource removes its assignments.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class Cascade {
  public static void main(String[] args) throws Exception {
    Moment monday = Moment.fromYmdHm(2026, 8, 10, 8, 0);
    ProjectData data = new ProjectData()
        .withTasks(List.of(
            new TaskData(Id.str("phase")).withTaskType(TaskType.SUMMARY),
            new TaskData(Id.str("spec")).withParent(Id.str("phase"))
                .withStartDate(monday).withDuration(2.0),
            new TaskData(Id.str("release")).withStartDate(monday).withDuration(1.0)))
        .withLinks(List.of(new LinkData()
            .withId(Id.str("spec-release"))
            .withSource(Id.str("spec"))
            .withTarget(Id.str("release"))))
        .withResources(List.of(new ResourceData().withId(Id.str("ann"))))
        .withAssignments(List.of(new AssignmentData()
            .withId(Id.str("ann-spec"))
            .withTask(Id.str("spec"))
            .withResource(Id.str("ann"))
            .withValue(1.0)));
    Project project = new Project();
    project.solve(data, new SolveOptions());

    SchedulingChanges changes = new SchedulingChanges();
    changes.tasks.remove.add(Id.str("phase"));
    ChangeEffects effects = project.apply(changes).effects;

    System.out.println("tasks: " + effects.removedTasks);
    System.out.println("links: " + effects.removedLinks);
    System.out.println("assignments: " + effects.removedAssignments);
  }
}
```

Removing the summary `phase` also removes `spec`, the link `spec-release` and the assignment `ann-spec`. Delete the same records from your own model.

The other `effects` lists:

- `generatedLinks` and `generatedAssignments` hold the ids `apply` made up for additions without one.
- `droppedLinks` and `droppedAssignments` hold records that were accepted but are now inactive, for example a link to an unscheduled task. They stay in the project and come back into play when the task does.

`apply` edits the project in place. Copies made earlier with [`Project#copy()`](<https://docs.dhtmlx.com/planning-engine/java/api/project/project.md#copy>) don't see the edit, so you can copy a project, apply a multi-step edit to the copy and drop it if the user cancels. A copy is cheap: both sides share state until one of them is edited. For a single edit, [`preview`](<https://docs.dhtmlx.com/planning-engine/java/api/project-preview.md>) does the same job without a copy. See [Previewing changes](<https://docs.dhtmlx.com/planning-engine/java/live-projects/previewing-changes.md>).

Field-only task edits and assignment updates take a fast path that patches the affected records without rebuilding the whole model. Frequent small edits, like drags, stay cheap.

## Rescheduling

After [`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>), call [`solve(null, options)`](<https://docs.dhtmlx.com/planning-engine/java/api/project-solve.md>). It schedules the edited state, accepts the dates as the new baseline and returns the updates relative to the previous one.

`solve` always reports the tasks that a successful operation added or updated, even when the planner leaves them exactly where the edit put them. The same goes for tasks whose working calendars changed because of a resource or assignment edit. Your model gets the derived end or duration for every task you edited, not only for the ones that moved. These reported tasks accumulate across several `apply` calls until the next acyclic `solve(null, …)` or `level`. A cyclic solve keeps them pending.

### Guarding against stale edits

A UI usually edits the dates it showed the user. If the project changed in between (an edit from another tab, a leveling run), applying the edit can do the wrong thing. Set [`SchedulingChanges.baselineId`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/scheduling-changes.md#baselineid>) to the token of the result the user saw. [`apply`](<https://docs.dhtmlx.com/planning-engine/java/api/project-apply.md>) then refuses the whole change set with `STALE_BASELINE` when that token isn't current.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class Stale {
  public static void main(String[] args) throws Exception {
    Moment monday = Moment.fromYmdHm(2026, 8, 10, 8, 0);
    ProjectData data = new ProjectData()
        .withTasks(List.of(new TaskData(Id.num(1)).withStartDate(monday).withDuration(2.0)));

    Project project = new Project();
    SolveOptions options = new SolveOptions();
    String seen = project.solve(data, options).baselineId;

    // The edit was made against the dates the user saw.
    SchedulingChanges changes = new SchedulingChanges().withBaselineId(seen);
    changes.tasks.update.add(new TaskUpdate(Id.num(1)).withDuration(Patch.of(3.0)));
    project.apply(changes);

    // A second edit still carrying the old token is refused as a whole.
    try {
      project.apply(changes);
    } catch (ProjectException e) {
      if (e.code == ProjectError.STALE_BASELINE) {
        System.out.println("stale: reload first");
      } else {
        throw e;
      }
    }

    String fresh = project.solve(null, options).baselineId;
    System.out.println("new baseline differs: " + !fresh.equals(seen));
  }
}
```

A token names a baseline only while the project is solved and clean. After an `apply`, any `baselineId` is stale until the next solve, so take a fresh token from each result. Tokens are opaque and process-unique: don't store them or compare them across runs. `null` (the default) skips the check.

## Rejections and resync

An operation is rejected when it can't apply:

- an unknown id in `update` or `remove`
- a duplicate id in `add`
- `Patch.nullValue()` on a required field (a link's `source` or `target`, an assignment's `task`, `resource` or `value`)
- a record that fails validation after the edit, such as an invalid pause or assignment slice

`apply` tries each operation on its own copy. A rejected operation leaves no partial edit behind, and the operations around it still apply.

```java
import java.util.List;
import org.dhtmlx.plen.*;

public class Resync {
  static ProjectData load() {
    Moment monday = Moment.fromYmdHm(2026, 8, 10, 8, 0);
    return new ProjectData()
        .withTasks(List.of(new TaskData(Id.str("design")).withStartDate(monday).withDuration(2.0)));
  }

  public static void main(String[] args) throws Exception {
    Project project = new Project();
    SolveOptions options = new SolveOptions();
    project.solve(load(), options);

    SchedulingChanges changes = new SchedulingChanges();
    changes.tasks.remove.add(Id.str("deleted-elsewhere"));
    changes.tasks.update.add(new TaskUpdate(Id.str("design")).withDuration(Patch.of(4.0)));

    ApplyReport report = project.apply(changes);
    for (AppliedOperation op : report.applied) {
      System.out.println("applied " + op.path);
    }
    for (RejectedOperation op : report.rejected) {
      System.out.println("rejected " + op.path + ": " + op.message);
    }

    if (report.needsResync) {
      // solve(null, ...) and apply now throw NEEDS_RESYNC until the full data is sent again.
      try {
        project.solve(null, options);
      } catch (ProjectException e) {
        if (e.code != ProjectError.NEEDS_RESYNC) throw e;
        project.solve(load(), options);
      }
    }
  }
}
```

The update applies and the removal is rejected with `rejected tasks.remove[0]: unknown task id`. The project now holds the update but not the removal, which is probably not what your model holds. That's why any rejection blocks further edits: `solve(null, …)`, `apply` and `level` throw [`ProjectException`](<https://docs.dhtmlx.com/planning-engine/java/api/errors/project-exception.md>) with code `NEEDS_RESYNC` until a successful `solve(data, …)` replaces the whole state with data you know is right.

The resync data must pass validation. If it doesn't, you get an [`InputException`](<https://docs.dhtmlx.com/planning-engine/java/api/errors/input-exception.md>), and the project still needs a resync. See [Recover from rejected changes](<https://docs.dhtmlx.com/planning-engine/java/how-to/recover-from-rejected-changes.md>).

[`evaluate`](<https://docs.dhtmlx.com/planning-engine/java/api/project-evaluate.md>) and [`suggestAssignments`](<https://docs.dhtmlx.com/planning-engine/java/api/project-suggest-assignments.md>) use the same apply on disposable copies. A rejected candidate shows up as its [`CandidateResult.error`](<https://docs.dhtmlx.com/planning-engine/java/api/evaluate/candidate-result.md>) (a [`RejectedOperation`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/rejected-operation.md>)) and doesn't block the project. See [Evaluating alternatives](<https://docs.dhtmlx.com/planning-engine/java/resources-and-assignment/evaluating-alternatives.md>).

## What change sets can't do

Some edits have no operation. For these, rebuild the data in your model and send it with [`solve(data, …)`](<https://docs.dhtmlx.com/planning-engine/java/api/project-solve.md>):

- Calendars can't be added, changed or removed. Tasks and resources can switch to another existing calendar through `calendarId`.
- [`SchedulingConfig`](<https://docs.dhtmlx.com/planning-engine/java/api/data/scheduling-config.md>) can't change. The duration unit, `useCalendars`, `calendarMode`, `applyConstraints` and the other settings are fixed until the next full solve.
- A task's `taskType` and `durationUnit` aren't in [`TaskUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/task-update.md>). To turn a task into a summary or a milestone, remove it and add it again (its links and assignments go with the removal), or resync.
- An id can't be renamed. `update` finds the record by its id and never changes it.
- [`ResourceUpdate`](<https://docs.dhtmlx.com/planning-engine/java/api/changes/resource-update.md>) changes only `capacity` and `calendarId`.

Calls that don't need a change set don't take one. [`level`](<https://docs.dhtmlx.com/planning-engine/java/api/project-level.md>) writes its own `levelingDelay` and `pauses` edits and accepts the result. See [Leveling resources](<https://docs.dhtmlx.com/planning-engine/java/resources-and-assignment/leveling-resources.md>).