The framework has two distinctive parts.

• A core that is developed by the Fetch.ai team as well as external contributors.
• Extensions (also known as packages) developed by any developer.

Currently, the framework supports four types of packages which can be added to the core as modules:

• Skills
• Protocols
• Connections
• Contracts

The following figure illustrates the framework's architecture:

The execution is broken down in more detail below:

The agent operation breaks down into three parts:

• Setup: calls the setup() method of all registered resources
• Operation:
  • Main loop (Thread 1 - Synchronous):
    • react(): this function grabs all Envelopes waiting in the InBox queue and calls the handle() function on the Handler(s) responsible for them. As such it consumes and potentially produces Messages.
    • act(): this function calls the act() function of all registered Behaviours. As such it potentially produces Messages.
    • update(): this function enqueues scheduled tasks for execution with the TaskManager and executes the decision maker.
  • Task loop (Thread 2- Synchronous): executes available tasks
  • Decision maker loop (Thread 3- Synchronous): processes internal messages
  • Multiplexer (Thread 4 - Asynchronous event loop): the multiplexer has an event loop which processes incoming and outgoing messages across several connections asynchronously.
• Teardown: calls the teardown() method of all registered resources

To prevent a developer from blocking the main loop with custom skill code, an execution time limit is applied to every Behaviour.act and Handler.handle call.

By default, the execution limit is set to 0 seconds, which disables the feature. You can set the limit to a strictly positive value (e.g. 0.1 seconds) to test your AEA for production readiness. If the act or handle time exceed this limit, the call will be terminated.

An appropriate message is added to the logs in the case of some code execution being terminated.

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