.. _blobs-sync:

Blobs Synchronization
=====================

Because blobs are immutable, synchronization is much simpler than the
JSON-based :ref:`document-sync`. The synchronization process is as follows:

1. The client asks the server for a list of Blobs and compares it with the local list.
2. A local list is updated with pending downloads and uploads.
3. Downloads and uploads are triggered.

Immutability brings some characteristics to the blobs system:

- There's no need for storage of versions or revisions.
- Updating is not possible (you would need to delete and recreate a blob).

Client-side encryption
----------------------

Blobs are encrypted before being sent to the server and decrypted after being
received. A MAC is also calculated to authenticate the contents of each blob.
See :ref:`client-encryption` for more details of how this is done.

Synchronization status
----------------------

In the client-side, each blob has an associated synchronization status, which
can be one of:

- ``SYNCED``: The blob exists both in this client and in the server.
- ``PENDING_UPLOAD``: The blob was inserted locally, but has not yet been uploaded.
- ``PENDING_DOWNLOAD``: The blob exists in the server, but has not yet been downloaded.
- ``FAILED_DOWNLOAD``: A download attempt has been made but the content is corrupted for some reason.

Concurrency limits
------------------

In order to increase the speed of synchronization on the client-size,
concurrent database operations and transfers to the server are allowed. Despite
that, to prevent indiscriminated use of client resources (cpu, memory,
bandwith), concurrenty limits are set both for database operations and data
transfer.

Transfer retries
----------------

When a blob is queded for download or upload, it will stay in that queue until
the transfer has been successful or until there has been an unrecoverable
transfer error. Currently, the only unrecoverable transfer error is a failed
verification of the blob tag (i.e. a failed MAC verification).

Successive failed transfer attempts of the same blob are separated by an
increasing time interval to minimize competition for resources used by other
concurrent transfer attempts. The interaval starts at 10 seconds and increases
to 20, 30, 40, 50, and finally 60 seconds. All further retries will be
separated by a 60 seconds time interval.