# Resource handler model

> A resource handler holds the connection to a cloud or hypervisor and pairs it with the Python class that makes the API calls

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Site [llms.txt](/llms.txt)

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Every resource handler is an instance of `resourcehandlers.models.ResourceHandler`. Each supported technology subclasses it: a vCenter connection is a `VsphereResourceHandler` and an AWS connection is an `AWSHandler`. The base model uses **[HasSubModelsMixin](/docs/cmp/developer-resources/guide/shell-plus/#hassubmodelsmixin)**, meaning that a query against `ResourceHandler` returns base instances. Call `cast()` on one to get the subclass, with the fields and methods specific to that technology.

![Diagram of resource handler subclasses, each paired with a TechnologyWrapper.](/img/cmp/developer-resources/guide/models/resource-handlers/resource-handler-object-model.png)

## How a handler reaches the provider API

Each resource handler is paired with a wrapper class, and `get_api_wrapper()` returns that wrapper, already connected, so a plugin never has to read credentials. Wrappers share a common set of operations, including `validate_connection()`, `create_vm()`, `power_vm()`, and `get_all_templates()`. Check the wrapper for the technology you are working with.

## Capability flags

Handler classes declare what their technology can do as class attributes. Read the flag rather than testing for a specific technology, so your code keeps working when another handler gains the same capability. For example:

| Flag | What it reports |
| --- | --- |
| `can_support_console` | The technology can open a remote console session to one of its servers. |
| `can_sync_vms` | Existing VMs can be discovered and synced into CMP. |
| `can_add_disks_at_prov`, `can_add_disks_after_prov`, `can_add_nics_after_prov` | Whether disks can be added during provisioning, and whether disks or network cards can be added to a server afterward. |
| `can_resources_be_changed` | CPU and memory can be changed on an existing server. |
| `can_import_networks`, `can_import_templates` | Networks and images or templates can be pulled from the provider. Setting `can_import_networks` to False also hides the Import Networks button on the handler's Networks tab. |

## Tech-specific parameters

Each technology contributes its own parameters to an environment, such as an AWS instance type or a vCenter resource pool. These are exposed to users as tech-specific parameters: return available parameters with `tech_parameter_fields()`. These parameters belong to the handler rather than to the environment, so changing an environment's resource handler clears them.

## Related

- [Resource handlers](/docs/cmp/configure/infrastructure/resource-handlers/): Add and configure a connection to a cloud or hypervisor.
- [Environment model](/docs/cmp/developer-resources/guide/models/environments/): The model for the provisioning targets a handler serves.
- [Shell Plus](/docs/cmp/developer-resources/guide/shell-plus/): Query models interactively, including the mixins that make subtyping work.