This tutorial goes from defining the basics and creation of these elements, to describing their possibilities. If any doubt or issue arises during the reading, the [CARLA forum](https://github.com/carla-simulator/carla/discussions/) is there to solve them.
Clients are one of the main elements in the CARLA architecture. They connect to the server, retrieve information, and command changes. That is done via scripts. The client identifies itself, and connects to the world to then operate with the simulation.
Besides that, clients are able to access advanced CARLA modules, features, and apply command batches. Only command batches will be covered in this section. These are useful for basic things such as spawning lots of actors. The rest of features are more complex, and they will be addressed in their respective pages in __Advanced steps__.
Two things are needed. The __IP__ address identifying it, and __two TCP ports__ to communicate with the server. An optional third parameter sets the amount of working threads. By default this is set to all (`0`). The [carla.Client](python_api.md#carla.Client.__init__) in the Python API reference contains a snipet that shows how to parse these as arguments when running the script.
By default, CARLA uses local host IP, and port 2000 to connect but these can be changed at will. The second port will always be `n+1`, 2001 in this case.
Once the client is created, set its __time-out__. This limits all networking operations so that these don't block the client forever. An error will be returned if connection fails.
It is possible to have many clients connected, as it is common to have more than one script running at a time. Working in a multiclient scheme with advanced CARLA features, such as the traffic manager, is bound to make communication more complex.
Client and server have different `libcarla` modules. If the versions differ, issues may arise. This can be checked using the `get_client_version()` and `get_server_version()` methods.
Every world object has an `id` or episode. Everytime the client calls for `load_world()` or `reload_world()` the previous one is destroyed. A new one is created from scratch with a new episode. Unreal Engine is not rebooted in the process.
__Commands__ are adaptations of some of the most-common CARLA methods, that can be applied in batches. For instance, the [command.SetAutopilot](python_api.md#command.SetAutopilot) is equivalent to [Vehicle.set_autopilot()](python_api.md#carla.Vehicle.set_autopilot), enables the autopilot for a vehicle. However, using the methods [Client.apply_batch](python_api.md#carla.Client.apply_batch) or [Client.apply_batch_sync()](python_api.md#carla.Client.apply_batch_sync), a list of commands can be applied in one single simulation step. This becomes extremely useful for methods that are usually applied to even hundreds of elements.
All the commands available are listed in the [latest section](python_api.md#command.ApplyAngularVelocity) of the Python API reference.
### Other client utilities
The main purpose of the client object is to get or change the world, and apply commands. However, it also provides access to some additional features.
*__Traffic manager.__ This module is in charge of every vehicle set to autopilot to recreate urban traffic.
*__[Recorder](adv_recorder.md).__ Allows to reenact a previous simulation. Uses [snapshots](core_world.md#world-snapshots) summarizing the simulation state per frame.
The major ruler of the simulation. Its instance should be retrieved by the client. It does not contain the model of the world itself, that is part of the [Map](core_map.md) class. Instead, most of the information, and general settings can be accessed from this class.
Some of its most important methods are _getters_, precisely to retrieve information or instances of these elements. Take a look at [carla.World](python_api.md#carla.World) to learn more about it.
The weather is not a class on its own, but a set of parameters accessible from the world. The parametrization includes sun orientation, cloudiness, wind, fog, and much more. The helper class [carla.WeatherParameters](python_api.md#carla.WeatherParameters) is used to define a custom weather.
There are some weather presets that can be directly applied to the world. These are listed in [carla.WeatherParameters](python_api.md#carla.WeatherParameters) and accessible as an enum.
*__Street lights__ automatically turn on when the simulation enters night mode. The lights are placed by the developers of the map, and accessible as [__carla.Light__](python_api.md#carla.Light) objects. Properties such as color and intensity can be changed at will. The variable __light_state__ of type [__carla.LightState__](python_api.md#carla.LightState) allows setting all of these in one call.
Street lights are categorized using their attribute __light_group__, of type [__carla.LightGroup__](python_api.md#carla.LightGroup). This allows to classify lights as street lights, building lights... An instance of [__carla.LightManager__](python_api.md#carla.LightManager) can be retrieved to handle groups of lights in one call.
* __Vehicle lights__ have to be turned on/off by the user. Each vehicle has a set of lights listed in [__carla.VehicleLightState__](python_api.md#carla.VehicleLightState). So far, not all vehicles have lights integrated. Here is a list of those that are available by the time of writing.
*__Bikes.__ All of them have a front and back position light.
*__Motorcycles.__ Yamaha and Harley Davidson models.
*__Cars.__ Audi TT, Chevrolet, Dodge (the police car), Etron, Lincoln, Mustang, Tesla 3S, Wolkswagen T2 and the new guests coming to CARLA.
The lights of a vehicle can be retrieved and updated anytime using the methods [carla.Vehicle.get_light_state](python_api.md#carla.Vehicle.get_light_state) and [carla.Vehicle.set_light_state](#python_api.md#carla.Vehicle.set_light_state). These use binary operations to customize the light setting.
World objects have a [carla.DebugHelper](python_api.md#carla.DebugHelper) object as a public attribute. It allows for different shapes to be drawn during the simulation. These are used to trace the events happening. The following example would draw a red box at an actor's location and rotation.
This example is extended in a snipet in [carla.DebugHelper](python_api.md#carla.DebugHelper.draw_box) that shows how to draw boxes for every actor in a world snapshot.
Contains the state of every actor in the simulation at a single frame. A sort of still image of the world with a time reference. The information comes from the same simulation step, even in asynchronous mode.
A [carla.WorldSnapshot](python_api.md#carla.WorldSnapshot) contains a [carla.Timestamp](python_api.md#carla.Timestamp) and a list of [carla.ActorSnapshot](python_api.md#carla.ActorSnapshot). Actor snapshots can be searched using the `id` of an actor. A snapshot lists the `id` of the actors appearing in it.
The world has access to some advanced configurations for the simulation. These determine rendering conditions, simulation time-steps, and synchrony between clients and server. They are accessible from the helper class [carla.WorldSettings](python_api.md#carla.WorldSettings).
For the time being, default CARLA runs with the best graphics quality, a variable time-step, and asynchronously. To dive further in this matters take a look at the __Advanced steps__ section. The pages on [synchrony and time-step](adv_synchrony_timestep.md), and [rendering options](adv_rendering_options.md) could be a great starting point.
