Flavio Poletti bio photo

Flavio Poletti

Irreducible Perler.

Email Comics Twitter LinkedIn MetaCPAN Github Stackoverflow Pinterest

Telegram is a messagging platform/App much like WhatsApp and companions. One nifty feature is that it allows integrating bots, contarily to WhatsApp… let’s see an example in Perl.

Telegram Bot HowTo

There are a few steps to get your bot up to speed, full instructions here. The gist is:

  • register yourself in Telegram
    • this allows you to register your bot
  • open a chat with BotFather and follow instructions
    • you will need to provide a name for your bot, we’ll call it prova_bot in the following;
    • at the end of the process you are provided with a token, that is a string like nnnnnnnnn:ssssssssssssssssssssssss-ssssssssss where n are digits and s are alphanumeric characters
  • code your bot
  • connect to Telegram authenticating with the token obtained in a previous step

In the following, we will assume that you set your token in the environment variable TOKEN, e.g. in a bash shell:

$ export TOKEN='nnnnnnnnn:ssssssssssssssssssssssss-ssssssssss'

A Very Simple Telegram Bot

For building our bot we will rely upon Bot::ChatBots::Telegram from CPAN. We’ll just take the example:

#!/usr/bin/env perl
use strict;
use warnings;
use Bot::ChatBots::Telegram::LongPoll;

my $token = shift || $ENV{TOKEN};

my $lp = Bot::ChatBots::Telegram::LongPoll->new(
   token     => $token,
   processor => \&processor,
   start     => 1,
);

sub processor { # tube-compliant
   my $record = shift;
   my $text = $record->{payload}{text};
   print "$text\n";
   $record->{send_response} = "you said: '$text'";
   return $record; # follow on..
}

Save this as longpoll. Now you can just start the bot (make sure to have Bot::ChatBots::Telegram installed and reachable!):

$ perl longpoll "$TOKEN"

If you exported the environment variable TOKEN you don’t need to pass it on the command line of course.

At this point you’re ready to connect to your bot! Assuming, again, that your bot’s name is prova_bot, you can look for it in Telegram (its address would be https://telegram.me/prova_bot if you use the web application in a browser). Start chatting with it and it will echo everything back! Very useful!

What is happening:

  • your application is running a loop and constantly querying Telegram’s API to seek new incoming messages. This method is called polling and this is why we named our file longpoll;
  • as soon as any message is received, Bot::ChatBots::Telegram::LongPoll puts it in a data structure and calls your processor sub defined in the file. This is the sense of the line processor => \&processor;
  • you process the incoming message as you like in your own callback function. It is supposed to return the record itself or something else, see Data::Tubes for more details. If you “just” want to send a simple textual response, just populate the send_response key as in the example file above, but of course you might want to explore sending photos, videos, documents, etc. etc.

A few considerations:

  • PRO: extremely simple to setup
  • PRO: allows testing your own logic very quickly, just stop and restart the process
  • PRO: works well from anywhere with access to the Telegram API endpoint as a client (hence also behind proxies)
  • CON: the polling mechanism is suboptimal

Growing Up: Web Hooks

The Telegram API also allows using a different mechanism for connecting a bot, called webhook. In this alternative you can provide a callback URL served by your bot: whenever one or more messages are available for it, Telegram will take care to send them along to that endpoint. No more polling, yay!

Here is an example that we will put in a file webhook:

#!/usr/bin/env perl
use strict;
use warnings;
use Log::Any qw< $log >;
use Log::Any::Adapter;
use Mojolicious::Lite;
Log::Any::Adapter->set('Stderr');

my $token   = $ENV{TOKEN};
my $bot_url = $ENV{BOT_URL} || 'https://example.com/mybot';

plugin 'Bot::ChatBots::Telegram' => instances => [
   [
      'WebHook',
      processor  => \&processor,
      register   => 1,
      token      => $token,
      unregister => 0,
      url        => $bot_url,
   ],
];

app->start;

sub processor { # tube-compliant
   my $record = shift;
   my $text = $record->{payload}{text};
   print "$text\n";
   $record->{send_response} = "you said: '$text'";
   return $record; # follow on..
}

The processor function is exactly the same as before, which is good: as soon as you are ready to step up your bot as a web hook, you don’t need to change your logic, just switch gears in the interface towards Telegram.

The script is started like this:

$ # set TOKEN
$ # set BOT_URL
$ perl webhook daemon

We are relying upon Mojolicious here, that will start a server to implement the bot endpoint; see the documentation for all options upon starting the program.

We are supposed to provide the exact location of this endpoint in the Internet, in a place reachable by the Telegram API, which is why we have to set the variable BOT_URL.

One requirement when using web hooks is that the URL MUST be secure, i.e. the scheme must be HTTPS. This means that you will have to setup certificates and the proper encryption layer, which is in any case quite easy to do with Mojolicious. You will probably have to read a bit more if you plan on running the application behind a reverse proxy.

Summarizing:

  • PRO: production level, allows you to start multiple instances and distribute the load instead of relying on polling
  • CON: quite more difficult to set up:
    • requires setting up TLS, which means either generating certificates or getting valid ones (e.g. see Let’s Encrypt for this)
    • requires a place that is visible from Telegram’s servers, i.e. exposed on the Internet (with all security implications…)

Security Caveat

You have to do your homework and assess the security of Telegram for your case. I read around that they basically baked their own security layer which is normally suboptimal. Additionally, it’s not possible to use bots in private encrypted chats.

Release Your Bots!

I hope you were intrigued by the simplicity to build a bot for Telegram. We did just a little more than Hello, World! of course, but the logic of your bot is… yours! You know that you can do without much of the scaffolding, because it’s already there for you.

Have fun!