.. currentmodule:: dallinger.bots
When you run an experiment using the bot recruiter,
it will look for a class named
Bot in your
The Bot class should typically be a subclass of either
:py:class:`~BotBase` (for bots that interact with the
experiment by controlling a real browser using
:py:class:`~HighPerformanceBotBase` (for bots that
interact with the experiment server directly via HTTP or websockets).
The interaction of the base bots with the experiment takes place in several phases:
- Signup (including creating a Participant)
- Participation in the experiment
- Signoff (including completing the questionnaire)
- Recording completion (complete or failed)
To build a bot, you will definitely need to implement the
which will be called once the bot has navigated to the main experiment page.
If the structure of your ad, consent, instructions or questionnaire pages
differs significantly from the demo experiments, you may need to override other
The :py:class:`HighPerformanceBotBase` can be used as a basis for a bot that interacts with the experiment server directly over HTTP rather than using a real browser. This scales better than using Selenium bots, but requires expressing the bot's behavior in terms of HTTP requests rather than in terms of DOM interactions.
For a guide to Dallinger's web API, see :doc:`web_api`.
For an example of a high-performance bot implementation, see the Griduniverse bots. These bots interact primarily via websockets rather than HTTP.
.. autoclass:: HighPerformanceBotBase :members:
The :py:class:`BotBase` provides a basis for a bot that interacts with an experiment using Selenium, which means that a separate, real browser session is controlled by each bot. This approach does not scale very well because there is a lot of overhead to running a browser, but it does allow for interacting with the experiment in a way similar to real participants.
By default, Selenium will try to run PhantomJS, a headless browser meant for scripting. However, it also supports using Firefox and Chrome through configuration variables.
webdriver_type = firefox
We recommend using Firefox when writing bots, as it allows you to visually see its output and allows you to attach the development console directly to the bot's browser session.
For an example of a selenium bot implementation, see the Bartlett1932 bots.
For documentation of the Python Selenium WebDriver API, see Selenium with Python.
.. autoclass:: BotBase :members:
Scaling Selenium bots
For example you may want to run a dedicated computer on your lab network to host bots, without slowing down experimenter computers. It is recommended that you run Selenium in a hub configuration, as a single Selenium instance will limit the number of concurrent sessions.
You can also provide a URL to a Selenium WebDriver instance using the
webdriver_url configuration setting. This is required if you're running
Selenium in a hub configuration. The hub does not need to be on the same computer
as Dallinger, but it does need to be able to access the computer running
Dallinger directly by its IP address.
On Apple macOS, we recommend using Homebrew to install and run selenium, using:
brew install selenium-server-standalone selenium-server -port 4444
On other platforms, download the latest
from SeleniumHQ and run a hub using:
java -jar selenium-server-standalone-3.3.1.jar -role hub
and attach multiple nodes by running:
java -jar selenium-server-standalone-3.3.1.jar -role node -hub http://hubcomputer.example.com:4444/grid/register
These nodes may be on other computers on the local network or on the same host
machine. If they are on the same host you will need to add
-port 4446 (for
some port number) such that each Selenium node on the same server is listening
on a different port.
You will also need to set up the browser interfaces on each computer that's running a node. This requires being able to run the browser and having the correct driver available in the system path, so the Selenium server can run it.
We recommend using Chrome when running large numbers of bots, as it is more feature-complete than PhantomJS but with better performance at scale than Firefox. It is best to run at most three Firefox sessions on commodity hardware, so for best results 16 bots should be run over 6 Selenium servers. This will depend on how processor intensive your experiment is. It may be possible to run more sessions without performance degradation.