For Users: Accessing CARS Data

What is CARS Cloud Data?

The CARS Cloud Data Service gives users of the CARS beamlines access to the data collected at CARS using a cloud-based synchronization tool similar to Dropbox, Box, Google Drive. That is, data files will be sent automatically from specific folders on the CARS data collection servers to your remote machine as the data is being collected. Depending on the speed of your internet connection and the speed of data collection at the beamline, files should show up within a few minutes of being collected.

The CARS Cloud Data service uses the Nextcloud software which we run on our own web server at CARS. Thie data will be transferred from our data collection servers directly to your computers, without any other servers being involved. This means that we don’t need to put any limits on the amount of data transferred, and also gives us control about who we share the data with. If you have any concerns about the security of your data using this service, please contact us.

Getting Access

To get access to the CARS Cloud Data, you will receive an email from the email address Nextcloud <> with the subject line of Your Nextcloud account was created telling you that your account was created and giving you links to both set your password and install the client application.

This email message will show your user name and give links to set your password and Install Client. Start by following the set your password link to finish creating your account. The login and data transfer is encrypted, but please use a good password that is not one that you use for other important sites. If you have not received such an email and would like access to the CARS Cloud Data, or would like other people in your group to get access to the data, please contact your beamline scientist and they should be able to help you get access.

Just to be clear, the CARS Nextcloud is not related to any other accounts from CARS, U Chicago, or ANL, but access is restricted to legitimate CARS users with real access credentials to the beamline. That is, we can share data with CARS users, but if you want to share it with some other collaborators, we’ll leave that up to you.

The web client

Logging in to the CARS Cloud Data Service for the first time you will see some introductory pop-up screens, including more links to Install Client apps, and then you will see a screen that looks something like this:


Basic view of the CARS Cloud Data

On the top Blue menu, you will a U. Chicago logo uchicago_icon labeled ‘CARS Help’ that leads to this document, a File Folder icon showing your files, and a few other icons. In the upper right corner you will see a round icon with the initials of your user name from which you can get to your user settings (including changing your password) or log out.

In the File view, you will see a few documentation files for Nextcloud and standard folders for Documents and Photos. You should see a folder icon with a arrow in it: external_folder_icon. In the example above this folder is called xas_testdata but in your folder it will have a name related to your data collection (probably including the last name of the PI for the beam time). The arrow indicates that this folder is really a link to our data collection servers accessed from our webserver, and not directly on that webserver itself.

From this web interface, you can download any individual file. Like other file-sharing apps, Nextcloud has many additional features. There are note-taking apps, you can create links for any file to be able to share selected files with others, and so forth. You may find some of these features useful, but the main point is to automatically transfer your data from the beamline data acquisition systems to your computers.

Using WebDav: View remote files

From the settings icon settings_icon in the lower left, you will see a link for WebDAV, an address that will be something like You can use this address to mount your Nextcloud data folder with your operating systems file explorer.

On Microsoft Windows, you can use the Map Network Drive feature of Explorer and enter this address. On MacOS, you can use Connect to Server (at the bottom of the Go menu in Finder) Finder. Linux users can mount this with davs://millenia…. from the Gnome Command Center or using their desktop folder of choice.

This gives you access to the files on the CARS server from your computer, but it does not automatically copy the files to your local machine. Each time you open a file, it will be read over the network.

WebDAV with Windows

On Windows, there is a setting that limits the maximum size of a file that can be transferred. By default, this setting is 500 Mb. If you’re using the WebDAV client of Windows and see a message like:

Error 0x800700DF: The file size exceeds the limit allowed and cannot be saved

You increase this maximum file size (but only up to 4 Gb!) by changing a Windows Registry setting - you may need Administrator privileges to do this. From “Start->Run” or “Cmd” you can run the “regedit” utility. From there, search for:


Then find FileSizeLimitInBytes and click on it to Modify it. You can set the value up to 4,294,967,295, then click OK and restart your computer.

The Nextcloud App: syncing data

This is the recommended way to fetch your data from CARS. It is pretty easy to set up and will automatically and efficiently copy new data directly from the CARS data collection servers to your local computer.

In order to automatically synchronize data from CARS to your local machine, you will need to download and install the Nextcloud client application. These are available for all major operating systems (including mobile devices) at

When you install and run this, you do not need to “Register with a provider”, but you do need to “Log in” and provide the server address which is:

The use of https here is important: http will not work. Clicking “Next” should bring up your web browser with an interface to log in (if you have not already logged in) and then “Connect to your account” and finally “Grant access”.

Once your account is connected, a folder – probably called “Nextcloud” in your home folder – will be created and start to synchronize with the files on the CARS servers. There will also be a Nextcloud app that you can access (usually from the Toolbar or from your normal File Browser) to check activity and refine settings such as which folders to sub-folders synchronize and set files to optionally ignore.


If you have trouble downloading files larger than 500 Mb, check the Settings in your Nextcloud client application and make sure this suggested default value is not selected.

How to use your CARS Cloud Data Share

The files in the top-level Nextcloud folder work just like other cloud services: updating a file in the Nextcloud folder on your local computer will push those changes “to the cloud” (that is, back to the CARS webserver) and any other folder tied to that account will soon see those changes. You can install the client app on multiple machines to synchronize files across them all.

The files in the “live beamline data folder” (marked with external_folder_icon) are shared with you as “read only”. You can delete or move files in this folder, but those changes will not be pushed back to the beamline server. It is a good idea to think of the files in this folder as a snapshot of the files at the beamline.

We have a few tips and suggestions for working effectively with these data files as they are being collected. First, as data is being collected, you can always make a copy of the data from “live beamline data” folder to another folder or to your own backup drive.

Nextcloud uses the timestamps of files on the server to decide which files need to be fetched, and it will look for conflicts between the “local” and the “server” versions of the file. This has a few consequences.

  1. If a file on the beamline data server is partially written or gets updated, the changed file will be pushed to your local computer. Such an update can cause large-ish files to be sent more than once, but ensures you have the latest version.

  2. If you delete a file from your local folder and it does not change on the server it will not be re-copied unless you explicitly ask to re-sync (which you can do from the Nextcloud app).

  3. If you have modified a file locally and it also changes on the server, the Nextcloud app will warn you that there is a conflict, and you might end up with two copies of the file: file.dat as exists on the server and file (conflicted…).dat as was modified locally.

For X-ray microprobe users working with XRF maps, and for GPD Users, this last point may be especially important as the data files for “XRF Maps” (ID-E user) and “spec files” and “Data Shell” (ID-C users) are largish files that may be automatically updated on the beamline-side as the data is being collected but that you may also want to use on your own computer. For the XRF Maps and “Data Shell” files that use the “HDF5” format (typically with the extension .h5), this can be particularly noticeable as these files can become corrupted if two processes write to them.

That is, if you are processing live data files for GSE MapViewer (ID-E users) or Data Shell (ID-C users) as the data are coming in, you may see corrupted HDF5. If this happens, you can simply delete the broken file and re-sync the file from the beamline server. Note that this will lose any changes you made to the file. For microprobe users, this will include “saved positions” or “named areas” which you will have to recreate.

Cleaning up your data

The Nextcloud folder on your local machine will remain synchronized with the CARS location as long as we keep that share alive and as long as you run the Nextcloud application (and point it to that folder). We intend to keep these shares alive during your data collection and for a few weeks after that.


Please make a copy of the data in your Nextcloud folder when your beam time is over!

Although we will keep your data, we don’t keep it where it was collected – our fastest and most robust disk servers. At the end of each run cycle, we will move your data from the folder we originally shared with you. We will also stop sharing data with you at this point. The data will stay in your Nextcloud folder but you should also copy the data because if you return for beamtime and we share a different folder with you for your next run, it will remove and replace the data on your local Nextcloud folder. If this does happen, we can help restore lost original data, but we hope we don’t have to do this very often.