Question

I want to write a android application which needs data from the web. This information is stored in a json-file. The data from the json-file is saved on the device. To keep it up to date, I need to check for changes in the file every hour.

As the remote File can get quite large I want to download it only if it is different from the version which was previously downloaded. I thought about using the Last-Modified-Header of HTTP for this.

I came up with the following workflow (pseudo-code):

• data = null; data_timestamp = null;
• Every hour repeat:
  • Issue a HTTP Head-Request to the URL and option new_timestamp from Last-Modified Header.
  • If either data==null or new_timestamp > data_timestamp then
    • Issue a normal HTTP-Request to the URL
    • Save to data and set data_timestamp = new_timestamp

Do you think this is a reasonable approach? I could use the if-modified-since HTTP Header to get the data only if it has changed since the last download. This would save me one request. If it has changed, a body containing the new data is provided, if it hasn’t, the body is empty.

I also thought about using ETags, as I typically want to download if the file has new content (and not if the modified-date has changed), but my webserver (nginx) doesn’t support the creation of etags and I don’t want to involve another layer on the server-side for performance-reasons.

Answer 1

I solved the problem as described by me above: Download the file by using the if-modified-since HTTP Header. The ngnix webserver can be configured to return the right information regarding this header.