This will depend entirely on your particular server implementations behavior, and is not fully defined in RFC959.
In General, a file is not available on the server until its upload has returned a 226, and its status is 150. if the file has not yet been fully uploaded, its status response will not be 550, and it will not transfer the file to the client.
In order to do this, the STOU verb will know that a file upload is complete when the EOF byte arrives, so at that point the entire file has been uploaded, so it returns 226.
The RECV verb will first return the status of the file requested. a response of 150 is expected if the file is in an acceptable status, and will then start the transfer.
Additionally, remember most server services are implemented as multithreaded applications, and thread-safety for files usually relies on filesystem and server OS locks on the files. On most platforms, when a file is being actively written to, other threads can neither read or write to that file, until the lock is released.
possible duplicate of requesting webpage while uploading files via ftp
– Ƭᴇcʜιᴇ007 – 2015-02-20T15:59:24.667@Techie007, this process may be different when using an HTTP server instead of a second FTP call to retrieve the file, so unless the op is specifically concerned about HTTP, there will be technical differences in the result. – Frank Thomas – 2015-02-20T16:05:22.790