telegram bot image from url - undesired cache

2
votes

I am working on a telegram bot that displays images from several webcams upon request. I fetch the images from urls and then send to the user (using bot.sendPhoto() ) My problem is that for any given webcam the filename does not change and it seems that the photo is sent from telegram's cache. So it will display the image from the first time that image was requested.

I have thought about downloading the image from the url, saving with a variable name (like a name with a timestamp in it) then sending it to the chat, this seems like an inelegant solution and was hoping for something better. Like forcing the image not to be cached on the telegram server.

I am using the python-telegram-bot wrapper, but I am not sure that it's specific to that.

Any ideas? I have tried searching but so far am turning up little. Thanks in advance.

3
pythoncachingtelegrampython-telegram-bot
Seems that you have no choice but to use that inelegant way. But I'm afraid Telegram bots were not designed for such tasks :) If I were you I would make a web server with a page with a regularly refreshing image from the webcam. - Andrew Che

3 Answers

9
votes

I had the same problem too, but i've found the simplest solution.

When you call the image, you have to add a parameter with timestamp to the image link. Example:

http://www.example.com/img/img.jpg?a=TIMESTAMP

Where TIMESTAMP is the timestamp function based on the language you are using.

Simple but tricky ;)

2
votes

I think the best way is to do the same as we do in React where also, same URL calls are first checked in the cache.

If you are using Python the best way is:

timestamp = datetime.datetime.now().isoformat()
# Above statement returns like: '2013-11-18T08:18:31.809000'
pic_url = '{0}?a={1}'.format(img_url, timestamp)

Hope that helps!

0
votes

I had the same problem. I wanted to create a bot which sends an image taken by a webcam of a ski slope (webcam.example.com/image.jpg). Unfortunately, the filename and so the url never updates and telegram always sends the cached image. So I decided to alter the url passed to the api. In order to achieve this, I wrote a simple php site (example.com/photo.php) which redirects to the original url of the photo. After that, I created a folder (example.com/getphoto/) on my webspace with a .htaccess file inside. The .htaccess redirects all request in this folder to the photo.php site which redirects to the image (webcam.example.com/image.jpg). So you could add everything to the url of the folder and still get the picture (e. g. example.com/getphoto/42 or example.com/getphoto/hrte8437g). The telegram api seems to cache photos by url, so if you add always another ending to the url passed to the api, telegram doesnt use the cached version and sends the current image instead. The easiest way to always change the url is by adding the current date to it.

example.com/photo.php

<?php
header("Location: http://webcam.example.com/image.jpg");
die();
?>

example.com/getphoto/.htaccess

RewriteEngine on
RewriteRule ^(.*)$ http://example.com/photo.php

in python:

bot.sendPhoto(chat_id, 'example.com/getphoto/' + strftime("%Y-%m-%d_%H-%M-%S", gmtime()))

This workaround should also work in other languages like java or php. You just need to change the way to get the current date.