How to read a part of amazon s3 key, assuming that "multipart upload complete" is yet to happen for that key?
There is no API in S3 to retrieve a part of a multi-part upload.  You can list the parts but I don't believe there is any way to retrieve an individual part once it has been uploaded.
You can re-upload a part.  S3 will just throw away the previous part and use the new one in it's place.  So, if you had the old and new versions of the file locally and were keeping track of the parts yourself, I suppose you could, in theory, replace individual parts that had been modified after the multipart upload was initiated.   However, it seems to me that this would be a very complicated and error-prone process.  What if the change made to a file was to add several MB's of data to it?  Wouldn't that change  your boundaries?  Would that potentially affect other parts, as well?
I'm not saying it can't be done but I am saying it seems complicated and would require you to do a lot of bookkeeping on the client side.
I'm working on aws S3 multipart upload, And I am facing following issue.
Basically I am uploading a file chunk by chunk to s3, And during the time if any write happens to the file locally, I would like to reflect that change to the s3 object which is in current upload process.
Here is the procedure that I am following,

Initiate multipart upload operation.
upload the parts one by one [5 mb chunk size.] [do not complete that   operation yet.]
During the time if a write goes to that file, [assuming i have the details for the write [offset, no_bytes_written] ].
I will calculate the part no for that write happen locally, And read that chunk from the s3 uploaded object.
Read the same chunk from the local file and write to read part from s3.
Upload the same part to s3 object.
This will be an a-sync operation. I will complete the multipart operation at the end.

I am facing an issue in reading the uploaded part that is in multipart uploading process. Is there any API available for the same?
Any help would be greatly appreciated.