Running bots as participants

Dallinger supports using the Selenium framework to write bots that participate in experiments. Not all experiments will have bots available; the Bartlett (1932), stories and Networked chatroom-based coordination game demos are the only built-in experiments that do.

Writing a bot

In your you will need to create a subclass of BotBase called Bot. This class should implement the participate method, which will be called once the bot has navigated to the main experiment. Note, the BotBase class makes some assumptions about HTML structure, based on the demo experiments. If your HTML differs significantly you may need to override other methods too.

class dallinger.bots.BotBase(URL, assignment_id='', worker_id='')[source]

A base class for Bots that works with the built-in demos.


Complete the standard debriefing form.


Returns a Selenium WebDriver instance of the type requested in the configuration.


Participate in the experiment.


Sign up, run the participate method, then sign off and close the driver.


Submit questionnaire and finish.


Accept HIT, give consent and start experiment.

Running bots locally

You must set the configuration value recruiter='bots' to run an experiment using its bot. As usual, this can be set in local or global configurations, as an environment variable or as a keyword argument to run(). You should also set max_participants to the number of bots you want to run at once. num_dynos_worker should be more than max_participants, as a bot takes up a worker process while it is running. In addition, you may want to increase num_dynos_web to improve performance.

Dallinger uses Selenium to run bots locally. By default, it will try to run phantomJS directly, however it 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.

Running an experiment with the API may look like:

participants = 4
data =
    num_dynos_web=int(participants/4) + 1,

Running a single bot

If you want to run a single bot as part of an ongoing experiment, you can use the bot command. This is useful for testing a single bot’s behavior as part of a longer-running experiment, and allows easy access to the Python pdb debugger.

Scaling bots locally

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 selenium-server-standalone.jar file 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

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.