How browser profile syncing actually works

What a profile actually is

A browser profile is a directory on disk holding its own bookmarks, history, cookies, saved passwords, extensions, site permissions and settings. Two profiles on one machine share the browser binary and nothing else. That is why a site can see you as two different people, and why nothing crosses between them by accident.

What native sync moves

Sync attaches a profile to an account and mirrors most of that directory to the provider's servers, so signing the same account in on another machine reproduces it. The unit is the account. There is no relationship between two accounts, even when both belong to you.

The three hard problems in any sync system

1. Identity. Deciding when two records are the same thing. Bookmarks are usually matched by URL, which is why the same page saved twice collapses into one and why two URLs that differ by a tracking parameter do not.
2. Merge. Two sides changed between syncs. Additive merges keep both changes, which is almost always right for bookmarks; last-write-wins silently discards someone's work.
3. Deletion. The genuinely hard one, below.

Why deletions are the hard part

A naive sync compares two lists and adds what is missing. Under that rule, deleting an item in profile A looks identical to profile B having added it — so the next sync restores it. This is the resurrection bug that makes home-grown sync scripts unusable within a fortnight.

The fix is to record the deletion as a fact of its own, sometimes called a tombstone: “this item was removed at this time”. The merge then knows the absence was intentional and removes it everywhere rather than restoring it. Any sync tool you evaluate should be asked this one question: what happens when I delete something? Braid Sync records removals explicitly — see how it works.

Scheduling and conflicts

Sync frequency is a trade-off between freshness and load. Continuous syncing costs battery and generates chatter; periodic syncing with a manual trigger covers most real usage, because the moment you actually care is the moment you switch profiles — and that is when you press the button.

Scope is a security property

A sync tool's risk is defined by what it reads, not by what it promises. A design that reads only nominated folders cannot leak history it never touched. When comparing tools, look at requested permissions and the transmitted-data list before anything else — ours is on the security page.