Data Updates#

This guide covers how to change a live Project 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 holds one accepted state: the data from the last solve(Some(data), ..) 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 value and pass it to Project::apply.

Editing happens in two steps, and they're separate calls:

1. apply(&changes) writes the edit into the accepted state and marks the project unsolved. It doesn't schedule anything.
2. solve(None, &options) schedules the edited state, accepts the new dates and returns what moved.

Between the two calls the project has no baseline. preview, load_profile, evaluate, suggest_assignments and roster fail with ProjectError::NoBaseline until you solve. You can call apply several times before one solve. The edits accumulate.

apply accepts what it can. Each operation in the change set either applies or is rejected on its own. A rejection isn't an Err: it's listed in the returned ApplyReport, and it sets needs_resync. From then on the project refuses solve(None, ..), apply and level with ProjectError::NeedsResync. Your model and the project's state may now disagree, and the only way back is to send the full data with solve(Some(data), ..).

Err from apply itself means the call didn't fit the project's state: NeedsResync from an earlier rejection, or StaleBaseline when the change set names a baseline that isn't current anymore. In both cases nothing was applied. See Error handling.

Change sets#

A SchedulingChanges groups the edits by record kind. Each group is a Changes<A, U> with three lists: add, update and remove.

Additions use the same record types as ProjectData, except links: LinkAdd has an optional id. Leave a link or assignment id as None and apply generates one ($generated_link_1, $generated_assignment_1, ...). Generated ids come back in ApplyReport.effects, so you can store them in your model.

use plen_core::{
    DateTime, LinkAdd, LinkData, LinkType, Patch, Project, ProjectData, SchedulingChanges,
    SolveOptions, TaskData, TaskUpdate,
};

fn task(id: i32, start: DateTime, duration: f64) -> TaskData {
    let mut task = TaskData::new(id);
    task.start_date = Some(start);
    task.duration = Some(duration);
    task
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let monday = DateTime::from_ymd_hm(2026, 8, 10, 8, 0).ok_or("invalid date")?;
    let data = ProjectData {
        tasks: vec![task(1, monday, 2.0), task(2, monday, 3.0)],
        links: vec![LinkData {
            id: 1.into(),
            source: 1.into(),
            target: 2.into(),
            link_type: LinkType::Fs,
            lag: 0.0,
            lag_unit: None,
        }],
        ..Default::default()
    };
    let mut project = Project::default();
    let options = SolveOptions::default();
    project.solve(Some(data), &options)?;

    let mut changes = SchedulingChanges::default();
    // A new task, linked after task 2. The link has no id: apply generates one.
    changes.tasks.add.push(task(3, monday, 1.0));
    changes.links.add.push(LinkAdd {
        id: None,
        source: 2.into(),
        target: 3.into(),
        link_type: LinkType::Fs,
        lag: 0.0,
        lag_unit: None,
    });
    // Task 1 now takes three days.
    let mut longer = TaskUpdate::new(1);
    longer.duration = Patch::Value(3.0);
    changes.tasks.update.push(longer);

    let report = project.apply(&changes)?;
    for op in &report.applied {
        println!("applied {}", op.path);
    }
    println!("generated links: {:?}", report.effects.generated_links);

    let result = project.solve(None, &options)?;
    for update in &result.updates {
        if let (Some(start), Some(end)) = (update.start_date, update.end_date) {
            println!("task {}: {start} .. {end}", update.id);
        }
    }
    Ok(())
}

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.

Operations don't run in the order you pushed 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 and ApplyReport.rejected.

Update records and Patch#

Update records (TaskUpdate, LinkUpdate, ResourceUpdate, AssignmentUpdate) name the record by id and carry every other field as a Patch<T>. A patch has three states:

• Patch::Missing (the default) leaves the field as it is.
• Patch::Value(v) sets it.
• Patch::Null clears an optional field, or restores the default of a field that always has a value: auto_scheduling goes back to true, unscheduled to false, progress to 0, priority to 500, leveling_delay to 0, pauses to empty, link_type to FS and lag to 0.

TaskUpdate::new(id) and the other new constructors set every field to Missing, so you only touch the fields you mean to change.

use plen_core::{
    ConstraintType, DateTime, Patch, Project, ProjectData, SchedulingChanges, SchedulingConfig,
    SolveOptions, TaskData, TaskUpdate,
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let monday = DateTime::from_ymd_hm(2026, 8, 10, 8, 0).ok_or("invalid date")?;
    let thursday = DateTime::from_ymd_hm(2026, 8, 13, 8, 0).ok_or("invalid date")?;

    let mut review = TaskData::new("review");
    review.start_date = Some(monday);
    review.duration = Some(1.0);

    let data = ProjectData {
        config: SchedulingConfig {
            apply_constraints: true,
            ..Default::default()
        },
        tasks: vec![review],
        ..Default::default()
    };
    let mut project = Project::default();
    let options = SolveOptions::default();
    project.solve(Some(data), &options)?;

    // Value sets a field: "start no earlier than Thursday".
    let mut pin = TaskUpdate::new("review");
    pin.constraint_type = Patch::Value(ConstraintType::Snet);
    pin.constraint_date = Patch::Value(thursday);
    let mut changes = SchedulingChanges::default();
    changes.tasks.update.push(pin);
    project.apply(&changes)?;
    for update in &project.solve(None, &options)?.updates {
        if let Some(start) = update.start_date {
            println!("pinned: {} starts {start}", update.id);
        }
    }

    // Null clears the constraint; start_date moves the task back.
    // Every field left Missing keeps its value.
    let mut unpin = TaskUpdate::new("review");
    unpin.constraint_type = Patch::Null;
    unpin.constraint_date = Patch::Null;
    unpin.start_date = Patch::Value(monday);
    let mut changes = SchedulingChanges::default();
    changes.tasks.update.push(unpin);
    project.apply(&changes)?;
    for update in &project.solve(None, &options)?.updates {
        if let Some(start) = update.start_date {
            println!("unpinned: {} starts {start}", update.id);
        }
    }
    Ok(())
}

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 start_date.

Which dates a task update keeps#

A task has three linked date fields: start_date, end_date and duration. When an update sets some of them, apply keeps what you sent and derives the rest:

• start_date alone keeps the duration and recomputes the end (a drag).
• end_date alone keeps the start and recomputes the duration (a resize from the right edge).
• duration alone keeps the start and recomputes the end.
• start_date and end_date together recompute the duration.
• A new calendar_id, or a new parent while calendars are inherited (CalendarMode::TaskInherited), 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.

An explicit start_date that falls on non-working time is kept as sent when the task has no predecessor. 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 from a result uses the same field names as TaskUpdate, so a result entry maps field by field onto an update. That's useful when you want to commit dates you got from preview or a leveled clone:

use plen_core::{Mutation, Patch, TaskUpdate};

fn patch<T: Clone>(value: &Option<T>) -> Patch<T> {
    match value {
        Some(value) => Patch::Value(value.clone()),
        None => Patch::Missing,
    }
}

fn to_update(mutation: &Mutation) -> TaskUpdate {
    let mut update = TaskUpdate::new(mutation.id.clone());
    update.start_date = patch(&mutation.start_date);
    update.end_date = patch(&mutation.end_date);
    update.duration = patch(&mutation.duration);
    update.leveling_delay = patch(&mutation.leveling_delay);
    update.pauses = patch(&mutation.pauses);
    update
}

pauses replaces the whole list. Set it to Patch::Value(vec![]) or Patch::Null to remove every pause. See Clear leveling.

Applying#

apply returns an ApplyReport:

• applied lists the path of every accepted operation.
• rejected lists the path and message of every refused one.
• effects lists what the edit changed beyond the records you named (below).
• needs_resync is true when anything was rejected.
• revision is the project's revision number after this edit.

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.

use plen_core::{
    AssignmentData, DateTime, LinkData, LinkType, Project, ProjectData, ResourceData,
    SchedulingChanges, SolveOptions, TaskData, TaskType,
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let monday = DateTime::from_ymd_hm(2026, 8, 10, 8, 0).ok_or("invalid date")?;
    let mut phase = TaskData::new("phase");
    phase.task_type = TaskType::Summary;
    let mut spec = TaskData::new("spec");
    spec.parent = Some("phase".into());
    spec.start_date = Some(monday);
    spec.duration = Some(2.0);
    let mut release = TaskData::new("release");
    release.start_date = Some(monday);
    release.duration = Some(1.0);

    let data = ProjectData {
        tasks: vec![phase, spec, release],
        links: vec![LinkData {
            id: "spec-release".into(),
            source: "spec".into(),
            target: "release".into(),
            link_type: LinkType::Fs,
            lag: 0.0,
            lag_unit: None,
        }],
        resources: vec![ResourceData {
            id: "ann".into(),
            capacity: None,
            calendar_id: None,
        }],
        assignments: vec![AssignmentData {
            id: Some("ann-spec".into()),
            task: "spec".into(),
            resource: "ann".into(),
            value: 1.0,
            offset: None,
            duration: None,
        }],
        ..Default::default()
    };
    let mut project = Project::default();
    project.solve(Some(data), &SolveOptions::default())?;

    let mut changes = SchedulingChanges::default();
    changes.tasks.remove.push("phase".into());
    let report = project.apply(&changes)?;

    let effects = &report.effects;
    println!("tasks: {:?}", effects.removed_tasks);
    println!("links: {:?}", effects.removed_links);
    println!("assignments: {:?}", effects.removed_assignments);
    Ok(())
}

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:

• generated_links and generated_assignments hold the ids apply made up for additions without one.
• dropped_links and dropped_assignments 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 takes &mut self and edits the project in place. Clones made earlier don't see the edit, so you can clone a project, apply a multi-step edit to the clone and drop it if the user cancels. For a single edit, preview does the same job without a clone. See Previewing changes.

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, call solve(None, &options). It schedules the edited state, accepts the dates as the new baseline and returns the updates relative to the previous one.

Tasks that a successful operation added or updated are always reported, 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(None, ..) 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 (another edit from another tab, a leveling run), applying the edit can do the wrong thing. Set SchedulingChanges.baseline_id to the token of the result the user saw, and apply refuses the whole change set with StaleBaseline when that token isn't current.

use plen_core::{
    DateTime, Error, Patch, Project, ProjectData, ProjectError, SchedulingChanges, SolveOptions,
    TaskData, TaskUpdate,
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let monday = DateTime::from_ymd_hm(2026, 8, 10, 8, 0).ok_or("invalid date")?;
    let mut design = TaskData::new(1);
    design.start_date = Some(monday);
    design.duration = Some(2.0);

    let mut project = Project::default();
    let options = SolveOptions::default();
    let loaded = project.solve(
        Some(ProjectData {
            tasks: vec![design],
            ..Default::default()
        }),
        &options,
    )?;
    let seen = loaded.baseline_id.clone();

    // The edit was made against the dates the user saw.
    let mut changes = SchedulingChanges {
        baseline_id: seen.clone(),
        ..Default::default()
    };
    let mut longer = TaskUpdate::new(1);
    longer.duration = Patch::Value(3.0);
    changes.tasks.update.push(longer);
    project.apply(&changes)?;

    // A second edit still carrying the old token is refused as a whole.
    match project.apply(&changes) {
        Err(Error::Project(ProjectError::StaleBaseline)) => println!("stale: reload first"),
        other => println!("unexpected: {other:?}"),
    }

    let solved = project.solve(None, &options)?;
    println!("new baseline differs: {}", solved.baseline_id != seen);
    Ok(())
}

A token names a baseline only while the project is solved and clean. After an apply, any baseline_id is stale until the next solve, so take a fresh token from each result. Tokens are opaque: don't store them or compare them across runs. None (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::Null on a required field (a link's source or target, an assignment's task, resource or value), or a record that fails validation after the edit, such as an invalid pause or assignment slice. Each operation is tried on its own copy, so a rejected operation leaves no partial edit behind, and the operations around it still apply.

use plen_core::{
    DateTime, Error, Patch, Project, ProjectData, ProjectError, SchedulingChanges, SolveOptions,
    TaskData, TaskUpdate,
};

fn load() -> Result<ProjectData, Box<dyn std::error::Error>> {
    let monday = DateTime::from_ymd_hm(2026, 8, 10, 8, 0).ok_or("invalid date")?;
    let mut design = TaskData::new("design");
    design.start_date = Some(monday);
    design.duration = Some(2.0);
    Ok(ProjectData {
        tasks: vec![design],
        ..Default::default()
    })
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut project = Project::default();
    let options = SolveOptions::default();
    project.solve(Some(load()?), &options)?;

    let mut changes = SchedulingChanges::default();
    changes.tasks.remove.push("deleted-elsewhere".into());
    let mut longer = TaskUpdate::new("design");
    longer.duration = Patch::Value(4.0);
    changes.tasks.update.push(longer);

    let report = project.apply(&changes)?;
    for op in &report.applied {
        println!("applied {}", op.path);
    }
    for op in &report.rejected {
        println!("rejected {}: {}", op.path, op.message);
    }

    if report.needs_resync {
        // solve(None) and apply now fail until the full data is sent again.
        if let Err(Error::Project(ProjectError::NeedsResync)) = project.solve(None, &options) {
            project.solve(Some(load()?), &options)?;
        }
    }
    Ok(())
}

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(None, ..), apply and level return ProjectError::NeedsResync until a successful solve(Some(data), ..) replaces the whole state with data you know is right.

The resync data must pass validation. If it doesn't, you get Error::Input, and the project still needs a resync. See Recover from rejected changes.

evaluate and suggest_assignments use the same apply on disposable copies, so a rejected candidate shows up as its CandidateResult.error (a RejectedOperation) instead of blocking the project. See Evaluating alternatives.

What change sets can't do#

Some edits have no operation. For these, rebuild the data in your model and send it with solve(Some(data), ..):

• Calendars can't be added, changed or removed. Tasks and resources can switch to another existing calendar through calendar_id.
• SchedulingConfig can't change: duration unit, use_calendars, calendar_mode, apply_constraints and the other settings are fixed until the next full solve.
• A task's task_type and duration_unit aren't in TaskUpdate. 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 changes only capacity and calendar_id.

Calls that don't need a change set also don't use one. level writes its own leveling_delay and pauses edits and accepts the result. See Leveling resources.