Concurrent editors need presence and location
Aliases: collaborator cursors · presence · who is here
What it is
When two people have the same content open, a conflict may not have happened yet, but crossing edits already can. Presence and location means the surface shows who else is in this object, and roughly where their focus sits—a roster, colored cursors, a highlighted block—so someone can yield or talk before rewriting the same sentence. This is the prevention layer. Choosing which side to keep after a clash, and a later write silently covering an earlier one, come after. Looking up who changed what in a version list is after-the-fact history, not presence now.
Why it happens
People assume “this open copy is mine alone.” With no presence, both treat the same passage as their workspace, interrupt each other’s typing, and watch a sentence become the other person’s words. Location beats a roster: “Li is in the doc” still leads to the same paragraph; “Li’s cursor is on the section 3 heading” is what makes someone edit elsewhere. Too coarse (file-level only) prevents little; too fine (every keystroke broadcast) feels like surveillance and people will not pause on a sentence to think. A stale cursor is a lie: the other person has left the line and the local cursor still draws them there, so people dodge an empty stretch. Presence must also split viewing from writing, or a read-only reviewer looks like someone grabbing the edit.
Studying it
Two people (or one person plus a scripted second) edit one document at once. Compare no presence, avatars only, roster plus paragraph location, roster plus caret location.
Independent variables: presence grain, update delay, whether read-only and writable are distinct. Dependent variables: same-sentence dual edits, voluntary yields, error in “where are they,” reports of feeling watched.
Lab partners sitting together will coordinate by voice; isolate sight and speech or you are not measuring the UI. A scripted second whose path is too regular will be spotted. Do not substitute after-the-fact version diffs for presence—that is history, conflict that already happened, not prevention.
Where it stops holding
Turn-taking with an explicit lock does not need live carets; a roster of “last edited” is enough. A class-sized document with hundreds of cursors destroys the body; collapse to “n viewing, k editing” with expansion on demand. Location-sensitive work (exams, anonymous review) may forbid presence; then prevent with paragraph locks or read-only, not a hidden identity. On a weak network, mark “location delayed” or fall back to a roster rather than draw a stale caret.
Applying it
- Show who has the object open; mark writers and readers differently.
- Place writers at paragraph or caret grain, with delay still usable as “now.” If delay is larger, say location may be wrong.
- Allow hiding others’ carets while keeping a block-level “someone is editing section 3,” so the choice is not full surveillance versus nothing.
- Verify: isolate two people, have them edit different sections, then plant a sentence both will want. If neither can say where the other is before typing, presence failed. If a caret remains on a paragraph the other person has left, grain or delay failed.