Stop rsync from deleting files that are being uploaded

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http://serverfault.com – I have a script which will upload files from a specific directory (with rsync) to the server, but in the mean time another script will pull files from the server (also with rsync and with the --delete flag). This means, that when I add a lot of files to it and rsync is in the process of uploading files and the download script is also being run, some files (which are not yet uploaded to the server) are deleted. Is there any way I can prevent this? I want both my upload script and download script work simultaneously. (HowTos)