In the simplest case, a worker implementation is installed and configured manually on a host and executes tasks indefinitely. For most purposes, it is far more cost-effective to dynamically adjust the pool of available workers based on the work to be done. This process entails both provisioning new instances (often referred to simply as "provisioning") and terminating running instances (typically a function of the worker itself).

A provisioner operates by monitoring the pool of running workers and the corresponding queues. Based on the queue length and the configuration of the provisioner, it may create new workers or signal to running workers that they should stop. Of course, provisioners might take into account additional information to make better predictions of future load. A provisioner's configuration can balance performance (many instances to execute tasks in parallel) against cost (from idle workers).

Provisioner IDs

Worker types are scoped within a provisionerId, allowing each provisioner its own "namespace" of managed workers.

The current set of provisioner IDs is visible at

AWS Provisioner

The Taskcluster team provides a provisioner instance, running the AWS Provisioner, which creates EC2 spot instances within the Taskcluster AWS account.

It is identified as provisionerId aws-provisioner-v1. Its administrative interface is available at This interface allows monitoring of current load, as well as management of worker types.

The AWS provisioner only provisions new instances, relying on workers to terminate themselves as appropriate. It does have a safety feature to forcibly terminate instances that have been running for a long time, but that is intended as a cost-saving measure for failed workers, rather than a part of normal operation.

The worker types provided by the AWS provisioner are a mix: some are Firefox-specific, while others are available for other Mozilla uses. Since the service runs with the Taskcluster team's AWS credentials, all costs are borne by the Taskcluster team.

If you are setting up a new worker type for this provisioner, it is your responsibility to ensure that AMI, kernel, security groups, and so on are properly defined. The AWS provisioner has some expectations of the instances it starts:

  • a worker starts on boot;
  • that this worker understands the task payload; and
  • that the worker terminates when it can be considered no longer cost effective to keep running (e.g. after a certain period of inactivity, or a maximum uptime has elapsed)