*:cpp:func:`esp_http_client_init`: Creates an :cpp:type:`esp_http_client_handle_t` instance, i.e., an HTTP client handle based on the given :cpp:type:`esp_http_client_config_t` configuration. This function must be the first to be called; default values will be assumed for the configuration values that are not explicitly defined by the user.
*:cpp:func:`esp_http_client_perform`: Performs all operations of the ``esp_http_client`` - opening the connection, exchanging data, and closing the connection (as required), while blocking the current task before its completion. All related events will be invoked through the event handler (as specified in :cpp:type:`esp_http_client_config_t`).
*:cpp:func:`esp_http_client_cleanup`: Closes the connection (if any) and frees up all the memory allocated to the HTTP client instance. This must be the last function to be called after the completion of operations.
Persistent connection means that the HTTP client can re-use the same connection for several exchanges. If the server does not request to close the connection with the ``Connection: close`` header, the connection is not dropped but is instead kept open and used for further requests.
To allow ESP HTTP client to take full advantage of persistent connections, one should make as many requests as possible using the same handle instance. Check out the example functions ``http_rest_with_url`` and ``http_rest_with_hostname_path`` in the application example. Here, once the connection is created, multiple requests (``GET``, ``POST``, ``PUT``, etc.) are made before the connection is closed.
A secure element (ATECC608) can be also used for the underlying TLS connection in the HTTP client connection. Please refer to the *ATECC608A (Secure Element) with ESP-TLS* section in the :doc:`ESP-TLS documentation </api-reference/protocols/esp_tls>` for more details. The secure element support has to be first enabled in menuconfig through :ref:`CONFIG_ESP_TLS_USE_SECURE_ELEMENT`. Then the HTTP client can be configured to use secure element as follows:
ESP HTTP client supports SSL connections using **mbedTLS**, with the ``url`` configuration starting with ``https`` scheme or ``transport_type`` set to ``HTTP_TRANSPORT_OVER_SSL``. HTTPS support can be configured via :ref:`CONFIG_ESP_HTTP_CLIENT_ENABLE_HTTPS` (enabled by default).
..note:: While making HTTPS requests, if server verification is needed, an additional root certificate (in PEM format) needs to be provided to the ``cert_pem`` member in the ``esp_http_client_config_t`` configuration. Users can also use the ``ESP x509 Certificate Bundle`` for server verification using the ``crt_bundle_attach`` member of the ``esp_http_client_config_t`` configuration.
Check out the example functions ``https_with_url`` and ``https_with_hostname_path`` in the application example for implementation details of the above note.
Some applications need to open the connection and control the exchange of data actively (data streaming). In such cases, the application flow is different from regular requests. Example flow is given below:
*:cpp:func:`esp_http_client_init`: Create a HTTP client handle.
*``esp_http_client_set_*`` or ``esp_http_client_delete_*``: Modify the HTTP connection parameters (optional).
*:cpp:func:`esp_http_client_open`: Open the HTTP connection with ``write_len`` parameter (content length that needs to be written to server), set ``write_len=0`` for read-only connection.
*:cpp:func:`esp_http_client_write`: Write data to server with a maximum length equal to ``write_len`` of :cpp:func:`esp_http_client_open` function; no need to call this function for ``write_len=0``.
*:cpp:func:`esp_http_client_fetch_headers`: Read the HTTP Server response headers, after sending the request headers and server data (if any). Returns the ``content-length`` from the server and can be succeeded by :cpp:func:`esp_http_client_get_status_code` for getting the HTTP status of the connection.
* Users can provide the username and password in the ``url`` or the ``username`` and ``password`` members of the ``esp_http_client_config_t`` configuration. For ``auth_type = HTTP_AUTH_TYPE_BASIC``, the HTTP client takes only one perform operation to pass the authentication process.
* If ``auth_type = HTTP_AUTH_TYPE_NONE``, but the ``username`` and ``password`` fields are present in the configuration, the HTTP client takes two perform operations. The client will receive the ``401 Unauthorized`` header in its first attempt to connect to the server. Based on this information, it decides which authentication method to choose and performs it in the second operation.
* Check out the example functions ``http_auth_basic``, ``http_auth_basic_redirect`` (for Basic authentication) and ``http_auth_digest`` (for Digest authentication) in the application example for implementation details.
ESP HTTP Client supports event handling by triggering an event handler corresponding to the event which took place.
:cpp:enum:`esp_http_client_event_id_t` contains all the events which could occur while performing an HTTP request using the ESP HTTP Client.
To enable event handling, you just need to set a callback function using the :cpp:member:`esp_http_client_config_t::event_handler` member.
ESP HTTP Client Diagnostic Information
--------------------------------------
Diagnostic information could be helpful to gain insights into a problem. In the case of ESP HTTP Client, this diagnostic information can be collected by registering an event handler with :doc:`the Event Loop library <../system/esp_event>`.
This feature has been added by keeping in mind the `ESP Insights <https://github.com/espressif/esp-insights>`_ framework which collects the diagnostic information. However, this feature can also be used without any dependency on ESP Insights framework for the diagnostic purpose.
Event handler can be registered to the event loop using the :cpp:func:`esp_event_handler_register` function.
Expected data types for different HTTP Client events in the event loop:
The :cpp:type:`esp_http_client_handle_t` received along with the event data will be valid until :cpp:enumerator:`HTTP_EVENT_DISCONNECTED <esp_http_client_event_id_t::HTTP_EVENT_DISCONNECTED>` is not received. This handle has been sent primarily to differentiate between different client
connections and must not be used for any other purpose (as it may change based on client connection state).
