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When I try to forward any video clips that I downloaded to my phone from any other contact it sends immediately (ie without any uploading) . Also attaching some mp3 files are also sent instantly without showing any delay in uploading? How is this possible? I think they are storing all media files on their server.

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closed as off-topic by Chahk, Izzy, bmdixon, Al E., onik Jun 27 '14 at 10:40

  • Questions on Android Enthusiasts should be asked from a end-user point of view and within the scope defined in the help center.
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So whats the question? – samnaction Jun 26 '14 at 12:28
@sameer The question is clear – Subin Jacob Jun 26 '14 at 12:32
@sameer How whatsapp avoid uploading? – Subin Jacob Jun 26 '14 at 12:32
This question appears to be off-topic because it is about inner-workings of a 3rd-party app. It is best to forward such questions to the app's developers. – Chahk Jun 26 '14 at 14:53
@Chahk That's not one of the reasons listed in [help/off-topic]. It's possible that the answer is explained somewhere online (say, in a development blog, or an interview with one of the developers) so it could be answered here. Even if the answer isn't currently known, if Mr Jacob does what you suggest, he can always come back and answer the question himself. – Dan Hulme Jun 26 '14 at 17:16
up vote 7 down vote accepted

I guess that's because whatsapp stores attachments on their database with its own checksum (every file can be represented by an alphanumeric sequence of characters called like this).
When you try to upload a file whose checksum is already on their database, they just use the uploaded copy of the file.
This leads to several advantages. Indeed, preventing double uploads:

  • Avoids server overloads
  • Makes you save mobile data
  • Make whatsapp save space on their database
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Sometimes, If I share some mp3 files or any music files from my phone, it will be sent immediately even if I didn't receive that file through whatsapp. Same checksum is used, rite? – Subin Jacob Jun 26 '14 at 12:40
Yes, this system works if any whatsapp user ever shared this file – user61191 Jun 26 '14 at 12:42
Dropbox does the same thing. – Brian Jun 26 '14 at 13:34

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