# Core Concepts

About DropSync
DropSync is a fast, flexible, folder updating tool. It's useful for things like backing up photos to a local NAS, or pushing changes to a website. These help pages describe DropSync and its features. We hope you find what you're looking for here but if not, then please just get in touch.
Folder Pairs
A Folder Pair is like a bookmark that remembers two top level (root) folders. It also holds all the settings and logs associated with transfers between those top level folders. When creating a new Folder Pair you are asked to provide a name (which must be unique) and a connection mode (Finder or SSH). Use the Finder connection mode for anything you can browse to on your mac (e.g., USB drives, local network shares, or folders on your Mac). SSH mode allows you to enter login credentials to connect to a remote server.

##### Setup a folder pair

1. Select New from the Folder Pairs menu
2. In the window that appears, type a name for the Folder Pair. Any name is fine provided that it is unique.
3. Select the access mode, either Finder or Remote Login
4. Click Finish
5. If you selected Finder as the access mode you should now see a split window inviting you to drop a folder on each side.
6. If you selected Remote Login as the access mode you should see a drop field on the left pane and a login field on the right. Drop a folder from Finder into the left pane and enter your connection details at right

Syncing
DropSync offers three syncing modes, Left to Right, Right to Left and Bidirectional. It displays arrows to indicate one of the two unidirectional cases. For example an arrow pointing towards the right indicates an update where the left is treated as source and the right as the destination. The default mode for DropSync is to update content at the destination if a newer version is present at the source. Unidirectional syncs never alter files at the source, whereas a bidirectional sync will update both sides to the newest version of a file.

##### Run a left to right sync

1. Set up a new folder pair or select an existing pair
2. Use the browser to select items for inclusion
3. Click the arrow that points from left to right in the centre of the browser

4. Wait for a preview window to appear. Unless your selections contain a huge number of files this should take just a few seconds.
5. Inspect the preview and click proceed if everything looks as you expect

Case Sensitivity

By default, Mac OSX preserves the case of filenames, but does not distinguish files on the basis of case. For example a file named `MyFile` is considered the same as `myfile` and cannot exist in the same folder as it. Unfortunately, many other common filesystems behave differently and regard these files as different. In order to avoid unexpected results when syncing with other filesystems DropSync ignores differences on the basis of case. This means that if you sync with a case sensitive filesystem as the source and a case insensitive one as the destination only one of the two source files will be synced (both cannot exist at the destination). Importantly, when syncing in the opposite direction the alternate file (on the basis of case) will be ignored and therefore not subject to deletion. An additional complication is that it is not possible to use the ignore case mode for all SSH Folder Pairs because most remote hosts do not support it.

If all this seems horribly complicated the best advice we can give is simply to avoid using case to distinguish files.

Deletions
When DropSync performs a unidirectional sync it updates the files at the destination if they have changed at the source. If a file has been deleted at the source, the default setting is not to delete that file at the destination (i.e. deletion is turned off). If deletion has been turned on then a file that is present at the destination but absent at the source will be deleted. Since this method of deletion propagation is inherently unidirectional it cannot be enabled for bidirectional syncs.

##### Turn on deletion

1. Open the Configure window for selected Folder Pair (Folder Pairs > Configure Selected).
2. Select the General tab
3. Locate the item labelled Delete extraneous files at the destination
4. Click the button corresponding to direction for which deletion should be enabled

Deletion cannot be used for bidirectional syncs

When using the deletion option take care to inspect previews to avoid accidentally deleting items. If you do accidentally delete something, try looking in the Folder Pair's rescue folder.

Previews
DropSync is able to preview its actions without actually altering or deleting any files. It displays a preview whenever a sync is initiated using the left and right arrow buttons in the main browser window. Previews display all files and folders being changed, are searchable, and provide general summary information such as the total transfer size.

Previews are not displayed for automatic syncs, or for some syncs initiated using the run menu or status menu.

The total transfer size displayed in a preview is an upper-limit estimate only. Actual syncs might transfer much less data if files at the destination only need updating rather than copying from scratch.

Logs
A record of every change made is recorded in DropSync's log. The last 30 days of logs can be accessed using the Activity window.

##### Access the full log details for a previous sync

1. Open the Activity Window
2. Switch to History View using the toggle button at the bottom right
3. Select the item of interest and click its magnifying glass icon

##### Save a complete log to a file

1. Open the log details of interest (see above)
2. Click the Save Report button