Sente for iPad Preview: Download on Demand

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In this post I want to discuss something that, while not strictly speaking a new feature, is something very important about Sente for iPad: download on demand for attachments.

As most of you will know, when you create a synchronized copy of a library, you have to specify when attachments should be downloaded from the sync servers in the new copy.  You choices are (a) as soon as they are available, or (b) on demand. 

If you select "as soon as available" the synchronized copy will attempt to download every PDF from our server as soon as they are available.  If you select "download on demand" the synchronized copy is told about each attachment, and a placeholder is added to the appropriate reference, but the file itself is not actually downloaded until the user requests it.

Here are three important reasons why you should consider using Download on Demand for iPad libraries:

  1. It saves on space on the iPad.  PDFs are by far the largest items in a Sente library.  Keeping only those you actually want to read on the device can make your library much smaller.
  2. Performance.  Updates to hundreds of references can be received from the sync servers in the time it takes to download a single PDF.  Downloading dozens or hundreds of PDFs can take a long time, particularly on 3G.  Unlike desktop Sente, Sente for iPad cannot just take care of this for you in the background -- it has to be the current application for the entire process.  You can, of course, be doing other operation in Sente while it is working, but performance will suffer, and you will never catch up if you don't leave Sente running enough on your iPad.
  3. Staying current.  Updates to synchronized libraries are queued up in roughly the order that they occur.  If there are a hundred PDFs in the queue for downloading, small edits that happened later cannot be processed until after the downloads are complete.  This means that dozens or hundreds of small changes may get backed up behind a bunch of large PDF downloads.

What does Download on Demand look like?  Below is a screen shot of a single reference view with an attachment that has not yet been downloaded.  The "No Local Copy" box is telling you that there is an attachment available, but it has not yet been downloaded into this copy of the library.  To download the attachment, just tap on the placeholder icon; a short while later the placeholder will be replaced with the thumbnail of the PDF, indicating that the file is present locally.

Download on Demand.PNG
I should note that regardless of the settings for when attachments should be downloaded, you can always press-and-hold on the attachment icon to get a menu with options for deleting the attachment from the library entirely, or for just removing the local copy.  If you remove the local copy, the placeholder returns, letting you download the file again from our server, should you ever need it.

So, what is the downside of using Download on Demand?  There are really only two that I can think of.  First, you will not be able to download a new attachment onto your iPad without an Internet connection.  All your previously downloaded attachments will be there, but new ones must be requested.  (You do not need to be able to contact any other copy of your library -- just our servers.)  Second, there is a brief delay for the download before you can begin reading.  In most cases this delay will be pretty brief, but large files on slow network connections can be more of a problem.

Here is what I would recommend.  Think about your likely usage pattern.  Are you likely to use Sente for iPad several times a day (so it is always up-to-date) or do you often need access to random PDFs in your library when you are in locations without network access?  If so, then you may need to set the library to download attachments as soon as they are available.

On the other hand, if you expect to use Sente for iPad only every few days, and you expect many PDFs to first enter your library on your desktop copy of Sente, then you should strongly consider setting up your library to download attachments only as needed.  Otherwise, every time you launch it, it will start downloading a lot of data that you may never need, and this will slow everything down that you are trying to do.

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