How to perform an action when a remote (Http) file changed?
A possible solution would be periodically running this algorithm on the client box.
- Create a HTTP request indicating the
If-Modified-Since
header equal to the date of your local file. If the file does not exist yet do not include this header; - The server will either send you the file if it was changed since the
If-Modified-Since
header in the payload or send304 Not Modified
HTTP status. - If you receive a
200 OK
HTTP status simply get the payload from the HTTP body and unzip the file. - If in the other hand you received a
304 Not Modified
you know that your file is up-to-date. - Use the
Last-Modified
header to touch your local file. This way you will be in sync with the server datetime.
Another way would be for the server to push notifications (a broadcast package for example) when the file is changed. When the notification is received the client would then execute the above algorithm. This would imply code to live in the HTTP server that listens for file system changes and then broadcast them to interested parties.
Perhaps this info for the curl
command is of some importance:
TIME CONDITIONS
HTTP allows a client to specify a time condition for the document it requests. It is If-Modified-Since or If-Unmodified-Since. Curl allow you to specify them with the -z/--time-cond flag.
For example, you can easily make a download that only gets performed if the remote file is newer than a local copy. It would be made like:
curl -z local.html http://remote.server.com/remote.html
Or you can download a file only if the local file is newer than the remote one. Do this by prepending the date string with a '-', as in:
curl -z -local.html http://remote.server.com/remote.html
You can specify a "free text" date as condition. Tell curl to only download the file if it was updated since yesterday:
curl -z yesterday http://remote.server.com/remote.html
Curl will then accept a wide range of date formats. You always make the date check the other way around by prepending it with a dash '-'.prepending it with a dash '-'.
To sum up, you will need:
- curl command
- touch command
- some bash scripting
is Java applicable in your case? I did a similar thing in one of my homework using the Apache HTTPcore library, you need to add the header "If-Modified-Since" to your HTTP request before you send it to the server, if the status code of the response that you receive from the server is not 304 then you know that the file has changed since the time value that you're checking against.