BLOB Transfer Server

.. _bluetooth_mesh_blob_srv:

BLOB Transfer Server
####################

The Binary Large Object (BLOB) Transfer Server model implements reliable receiving of large binary
objects. It serves as the backend of the :ref:`bluetooth_mesh_dfu_srv`, but can also be used for
receiving other binary images.

BLOBs
*****

As described in :ref:`bluetooth_mesh_blob`, the binary objects transferred by the BLOB Transfer
models are divided into blocks, which are divided into chunks. As the transfer is controlled by the
BLOB Transfer Client model, the BLOB Transfer Server must allow blocks to come in any order. The
chunks within a block may also come in any order, but all chunks in a block must be received before
the next block is started.

The BLOB Transfer Server keeps track of the received blocks and chunks, and will process each block
and chunk only once. The BLOB Transfer Server also ensures that any missing chunks are resent by the
BLOB Transfer Client.

Usage
*****

The BLOB Transfer Server is instantiated on an element with a set of event handler callbacks:

.. code-block:: C

   static const struct bt_mesh_blob_srv_cb blob_cb = {
       /* Callbacks */
   };

   static struct bt_mesh_blob_srv blob_srv = {
       .cb = &blob_cb,
   };

   static const struct bt_mesh_model models[] = {
       BT_MESH_MODEL_BLOB_SRV(&blob_srv),
   };

A BLOB Transfer Server is capable of receiving a single BLOB transfer at a time. Before the BLOB
Transfer Server can receive a transfer, it must be prepared by the user. The transfer ID must be
passed to the BLOB Transfer Server through the :c:func:`bt_mesh_blob_srv_recv` function before the
transfer is started by the BLOB Transfer Client. The ID must be shared between the BLOB Transfer
Client and the BLOB Transfer Server through some higher level procedure, like a vendor specific
transfer management model.

Once the transfer has been set up on the BLOB Transfer Server, it's ready for receiving the BLOB.
The application is notified of the transfer progress through the event handler callbacks, and the
BLOB data is sent to the BLOB stream.

The interaction between the BLOB Transfer Server, BLOB stream and application is shown below:

.. figure:: ../../../../../../connectivity/bluetooth/api/mesh/images/blob_srv.svg
   :align: center
   :alt: BLOB Transfer Server model interaction

   BLOB Transfer Server model interaction

Transfer suspension
*******************

The BLOB Transfer Server keeps a running timer during the transfer, that is reset on every received
message. If the BLOB Transfer Client does not send a message before the transfer timer expires, the
transfer is suspended by the BLOB Transfer Server.

The BLOB Transfer Server notifies the user of the suspension by calling the :c:member:`suspended
<bt_mesh_blob_srv_cb.suspended>` callback. If the BLOB Transfer Server is in the middle of receiving
a block, this block is discarded.

The BLOB Transfer Client may resume a suspended transfer by starting a new block transfer. The BLOB
Transfer Server notifies the user by calling the :c:member:`resume <bt_mesh_blob_srv_cb.resume>`
callback.

Transfer recovery
*****************

The state of the BLOB transfer is stored persistently. If a reboot occurs, the BLOB Transfer Server
will attempt to recover the transfer. When the Bluetooth Mesh subsystem is started (for instance by
calling :c:func:`bt_mesh_init`), the BLOB Transfer Server will check for aborted transfers, and call
the :c:member:`recover <bt_mesh_blob_srv_cb.recover>` callback if there is any. In the recover
callback, the user must provide a BLOB stream to use for the rest of the transfer. If the recover
callback doesn't return successfully or does not provide a BLOB stream, the transfer is abandoned.
If no recover callback is implemented, transfers are always abandoned after a reboot.

After a transfer is successfully recovered, the BLOB Transfer Server enters the suspended state. It
will stay suspended until the BLOB Transfer Client resumes the transfer, or the user cancels it.

.. note::
   The BLOB Transfer Client sending the transfer must support transfer recovery for the transfer to
   complete. If the BLOB Transfer Client has already given up the transfer, the BLOB Transfer Server
   will stay suspended until the application calls :c:func:`bt_mesh_blob_srv_cancel`.

API reference
*************

.. doxygengroup:: bt_mesh_blob_srv
   :project: Zephyr
   :members: