.. _blob_support:

============
Blob Support
============

Crate Data includes support to store `binary large objects`_. By utilizing
Crate Data's cluster features the files can be replicated and sharded just
like regular data.

Creating a table for blobs
--------------------------

Before adding blobs a ``blob table`` must be created. Lets use the crate shell ``crash`` to issue
the SQL statement::

    sh$ crash -c "create blob table myblobs clustered into 3 shards with (number_of_replicas=1)"
    CREATE OK (... sec)

Now crate is configured to allow blobs to be management under the ``/_blobs/myblobs`` endpoint.

Altering a blob table
---------------------

The number of replicas a blob table has can be changed using the ``ALTER BLOB
TABLE`` clause::

    sh$ crash -c "alter blob table myblobs set (number_of_replicas=0)"
    ALTER OK (... sec)

Uploading
---------

To upload a blob the sha1 hash of the blob has to be known upfront
since this has to be used as the id of the new blob. For this example
we use a fancy python one-liner to compute the shasum::

    sh$ python -c 'import hashlib;print(hashlib.sha1("contents".encode("utf-8")).hexdigest())'
    4a756ca07e9487f482465a99e8286abc86ba4dc7

The blob can now be uploaded by issuing a PUT request::

    sh$ curl -isSX PUT '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7' -d 'contents'
    HTTP/1.1 201 Created
    Content-Length: 0

If a blob already exists with the given hash a 409 Conflict is returned::

    sh$ curl -isSX PUT '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7' -d 'contents'
    HTTP/1.1 409 Conflict
    Content-Length: 0

Download
--------

To download a blob simply use a GET request::

    sh$ curl -sS '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7'
    contents

.. note::

    Since the blobs are sharded throughout the cluster not every node has all
    the blobs. In case that the GET request has been sent to a node that
    doesn't contain the requested file it will respond with a 307 Temporary
    Redirect which will lead to a node that does contain the file.

If the blob doesn't exist a 404 Not Found error is returned::

    sh$ curl -isS '127.0.0.1:4200/_blobs/myblobs/e5fa44f2b31c1fb553b6021e7360d07d5d91ff5e'
    HTTP/1.1 404 Not Found
    Content-Length: 0

To determine if a blob exists without downloading it, a HEAD request can be
used::

    sh$ curl -sS -I '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7'
    HTTP/1.1 200 OK
    Content-Length: 8
    Accept-Ranges: bytes
    Expires: Thu, 31 Dec 2037 23:59:59 GMT
    Cache-Control: max-age=315360000

.. note::

    The cache headers for blobs are static and basically allows clients to
    cache the response forever since the blob is immutable.

Delete
------

To delete a blob simply use a DELETE request::

    sh$ curl -isS -XDELETE '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7'
    HTTP/1.1 204 No Content
    Content-Length: 0

If the blob doesn't exist a 404 Not Found error is returned::

    sh$ curl -isS -XDELETE '127.0.0.1:4200/_blobs/myblobs/4a756ca07e9487f482465a99e8286abc86ba4dc7'
    HTTP/1.1 404 Not Found
    Content-Length: 0


Deleting a blob table
---------------------

Blob tables can be deleted similar to normal tables (again using the crate shell here)::

    sh$ crash -c "drop blob table myblobs"
    DROP OK (... sec)

.. Hidden: Re-create the blob table so information_schema will show it::

    sh$ crash -c "create blob table myblobs clustered into 3 shards with (number_of_replicas=1)"
    CREATE OK (... sec)


.. _`binary large objects`: http://en.wikipedia.org/wiki/Binary_large_object
