Advanced usage patterns

B2 server API allows for creation of an object from existing objects. This allows to avoid transferring data from the source machine if the desired outcome can be (at least partially) constructed from what is already on the server.

The way b2sdk exposes this functionality is through a few functions that allow the user to express the desired outcome and then the library takes care of planning and executing the work. Please refer to the table below to compare the support of object creation methods for various usage patterns.

Available methods

Method / supported options

Source

Range
overlap

Streaming
interface

Continuation

b2sdk.v2.Bucket.upload()

local

no

no

automatic

b2sdk.v2.Bucket.copy()

remote

no

no

automatic

b2sdk.v2.Bucket.concatenate()

any

no

no

automatic

b2sdk.v2.Bucket.concatenate_stream()

any

no

yes

manual

b2sdk.v2.Bucket.create_file()

any

yes

no

automatic

b2sdk.v2.Bucket.create_file_stream()

any

yes

yes

manual

Range overlap

Some methods support overlapping ranges between local and remote files. b2sdk tries to use the remote ranges as much as possible, but due to limitations of b2_copy_part (specifically the minimum size of a part) that may not be always possible. A possible solution for such case is to download a (small) range and then upload it along with another one, to meet the b2_copy_part requirements. This can be improved if the same data is already available locally - in such case b2sdk will use the local range rather than downloading it.

Streaming interface

Some object creation methods start writing data before reading the whole input (iterator). This can be used to write objects that do not have fully known contents without writing them first locally, so that they could be copied. Such usage pattern can be relevant to small devices which stream data to B2 from an external NAS, where caching large files such as media files or virtual machine images is not an option.

Please see advanced method support table to see where streaming interface is supported.

Continuation

Please see here

Concatenate files

b2sdk.v2.Bucket.concatenate() accepts an iterable of upload sources (either local or remote). It can be used to glue remote files together, back-to-back, into a new file.

b2sdk.v2.Bucket.concatenate_stream() does not create and validate a plan before starting the transfer, so it can be used to process a large input iterator, at a cost of limited automated continuation.

Concatenate files of known size

>>> bucket = b2_api.get_bucket_by_name(bucket_name)
>>> input_sources = [
...     CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=100, length=100),
...     UploadSourceLocalFile('my_local_path/to_file.txt'),
...     CopySource('4_z5485a1682662eb3e60980d10_f1022e2320daf707f_d20190620_m122848_c002_v0001123_t0020', length=2123456789),
... ]
>>> file_info = {'how': 'good-file'}
>>> bucket.concatenate(input_sources, remote_name, file_info)
<b2sdk.file_version.FileVersion at 0x7fc8cd560551>

If one of remote source has length smaller than absoluteMinimumPartSize then it cannot be copied into large file part. Such remote source would be downloaded and concatenated locally with local source or with other downloaded remote source.

Please note that this method only allows checksum verification for local upload sources. Checksum verification for remote sources is available only when local copy is available. In such case b2sdk.v2.Bucket.create_file() can be used with overalapping ranges in input.

For more information about concatenate please see b2sdk.v2.Bucket.concatenate() and b2sdk.v2.CopySource.

Concatenate files of known size (streamed version)

b2sdk.v2.Bucket.concatenate() accepts an iterable of upload sources (either local or remote). The operation would not be planned ahead so it supports very large output objects, but continuation is only possible for local only sources and provided unfinished large file id. See more about continuation in b2sdk.v2.Bucket.create_file() paragraph about continuation.

>>> bucket = b2_api.get_bucket_by_name(bucket_name)
>>> input_sources = [
...     CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=100, length=100),
...     UploadSourceLocalFile('my_local_path/to_file.txt'),
...     CopySource('4_z5485a1682662eb3e60980d10_f1022e2320daf707f_d20190620_m122848_c002_v0001123_t0020', length=2123456789),
... ]
>>> file_info = {'how': 'good-file'}
>>> bucket.concatenate_stream(input_sources, remote_name, file_info)
<b2sdk.file_version.FileVersion at 0x7fc8cd560551>

Concatenate files of unknown size

While it is supported by B2 server, this pattern is currently not supported by b2sdk.

Synthethize an object

Using methods described below an object can be created from both local and remote sources while avoiding downloading small ranges when such range is already present on a local drive.

Update a file efficiently

b2sdk.v2.Bucket.create_file() accepts an iterable which can contain overlapping destination ranges.

Note

Following examples create new file - data in bucket is immutable, but b2sdk can create a new file version with the same name and updated content

Append to the end of a file

The assumption here is that the file has been appended to since it was last uploaded to. This assumption is verified by b2sdk when possible by recalculating checksums of the overlapping remote and local ranges. If copied remote part sha does not match with locally available source, file creation process would be interrupted and an exception would be raised.

>>> bucket = b2_api.get_bucket_by_name(bucket_name)
>>> input_sources = [
...     WriteIntent(
...         data=CopySource(
...             '4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044',
...             offset=0,
...             length=5000000,
...         ),
...         destination_offset=0,
...     ),
...     WriteIntent(
...         data=UploadSourceLocalFile('my_local_path/to_file.txt'),  # of length 60000000
...         destination_offset=0,
...     ),
... ]
>>> file_info = {'how': 'good-file'}
>>> bucket.create_file(input_sources, remote_name, file_info)
<b2sdk.file_version.FileVersion at 0x7fc8cd560552>

LocalUploadSource has the size determined automatically in this case. This is more efficient than b2sdk.v2.Bucket.concatenate(), as it can use the overlapping ranges when a remote part is smaller than absoluteMinimumPartSize to prevent downloading a range (when concatenating, local source would have destination offset at the end of remote source)

For more information see b2sdk.v2.Bucket.create_file().

Change the middle of the remote file

>>> bucket = b2_api.get_bucket_by_name(bucket_name)
>>> input_sources = [
...     WriteIntent(
...         CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=0, length=4000000),
...         destination_offset=0,
...     ),
...     WriteIntent(
...         UploadSourceLocalFile('my_local_path/to_file.txt'),  # length=1024, here not passed and just checked from local source using seek
...         destination_offset=4000000,
...     ),
...     WriteIntent(
...         CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=4001024, length=123456789),
...         destination_offset=4001024,
...     ),
... ]
>>> file_info = {'how': 'good-file'}
>>> bucket.create_file(input_sources, remote_name, file_info)
<b2sdk.file_version.FileVersion at 0x7fc8cd560552>

LocalUploadSource has the size determined automatically in this case. This is more efficient than b2sdk.v2.Bucket.concatenate(), as it can use the overlapping ranges when a remote part is smaller than absoluteMinimumPartSize to prevent downloading a range.

For more information see b2sdk.v2.Bucket.create_file().

Synthesize a file from local and remote parts

This is useful for expert usage patterns such as:
  • synthetic backup

  • reverse synthetic backup

  • mostly-server-side cutting and gluing uncompressed media files such as wav and avi with rewriting of file headers

  • various deduplicated backup scenarios

Please note that b2sdk.v2.Bucket.create_file_stream() accepts an ordered iterable which can contain overlapping ranges, so the operation does not need to be planned ahead, but can be streamed, which supports very large output objects.

Scenarios such as below are then possible:

A          C       D           G
|          |       |           |
| cloud-AC |       | cloud-DG  |
|          |       |           |
v          v       v           v
############       #############
^                              ^
|                              |
+---- desired file A-G --------+
|                              |
|                              |
|    ######################### |
|    ^                       ^ |
|    |                       | |
|    |      local file-BF    | |
|    |                       | |
A    B     C       D       E F G
>>> bucket = b2_api.get_bucket_by_name(bucket_name)
>>> def generate_input():
...     yield WriteIntent(
...         CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=0, length=lengthC),
...         destination_offset=0,
...     )
...     yield WriteIntent(
...         UploadSourceLocalFile('my_local_path/to_file.txt'), # length = offsetF - offsetB
...         destination_offset=offsetB,
...     )
...     yield WriteIntent(
...         CopySource('4_z5485a1682662eb3e60980d10_f113f963288e711a6_d20190404_m065910_c002_v0001095_t0044', offset=0, length=offsetG-offsetD),
...         destination_offset=offsetD,
...     )
...
>>> file_info = {'how': 'good-file'}
>>> bucket.create_file(generate_input(), remote_name, file_info)
<b2sdk.file_version.FileVersion at 0x7fc8cd560552>

In such case, if the sizes allow for it (there would be no parts smaller than absoluteMinimumPartSize), the only uploaded part will be C-D. Otherwise, more data will be uploaded, but the data transfer will be reduced in most cases. b2sdk.v2.Bucket.create_file() does not guarantee that outbound transfer usage would be optimal, it uses a simple greedy algorithm with as small look-aheads as possible.

For more information see b2sdk.v2.Bucket.create_file().

Encryption

Even if files A-C and D-G are encrypted using SSE-C with different keys, they can still be used in a single b2sdk.v2.Bucket.create_file() call, because b2sdk.v2.CopySource accepts an optional b2sdk.v2.EncryptionSetting.

Prioritize remote or local sources

b2sdk.v2.Bucket.create_file() and b2sdk.v2.Bucket.create_file_stream() support source/origin prioritization, so that planner would know which sources should be used for overlapping ranges. Supported values are: local, remote and local_verification.

A              D               G
|              |               |
| cloud-AD     |               |
|              |               |
v              v               |
################               |
^                              |
|                              |
+---- desired file A-G --------+
|                              |
|                              |
|    #######   #################
|    ^     ^   ^               |
|    |     |   |               |
|    |   local file BC and DE  |
|    |     |   |               |
A    B     C   D               E

A=0, B=50M, C=80M, D=100M, E=200
>>> bucket.create_file(input_sources, remote_name, file_info, prioritize='local')
# planner parts: cloud[A, B], local[B, C], remote[C, D], local[D, E]

Here the planner has only used a remote source where remote range was not available, minimizing downloads.

>>> planner.create_file(input_sources, remote_name, file_info, prioritize='remote')
# planner parts: cloud[A, D], local[D, E]

Here the planner has only used a local source where remote range was not available, minimizing uploads.

>>> bucket.create_file(input_sources, remote_name, file_info)
# or
>>> bucket.create_file(input_sources, remote_name, file_info, prioritize='local_verification')
# planner parts: cloud[A, B], cloud[B, C], cloud[C, D], local[D, E]

In local_verification mode the remote range was artificially split into three parts to allow for checksum verification against matching local ranges.

Note

prioritize is just a planner setting - remote parts are always verified if matching local parts exists.

Continuation

Continuation of upload

In order to continue a simple upload session, b2sdk checks for any available sessions with of the same file name, file_info and media type, verifying the size of an object as much as possible.

To support automatic continuation, some advanced methods create a plan before starting copy/upload operations, saving the hash of that plan in file_info for increased reliability.

If that is not available, large_file_id can be extracted via callback during the operation start. It can then be passed into the subsequent call to continue the same task, though the responsibility for passing the exact same input is then on the user of the function. Please see advanced method support table to see where automatic continuation is supported. large_file_id can also be passed if automatic continuation is available in order to avoid issues where multiple matching upload sessions are matching the transfer.

Continuation of create/concantenate

b2sdk.v2.Bucket.create_file() supports automatic continuation or manual continuation. b2sdk.v2.Bucket.create_file_stream() supports only manual continuation for local-only inputs. The situation looks the same for b2sdk.v2.Bucket.concatenate() and b2sdk.v2.Bucket.concatenate_stream() (streamed version supports only manual continuation of local sources). Also b2sdk.v2.Bucket.upload() and b2sdk.v2.Bucket.copy() support both automatic and manual continuation.

Manual continuation

>>> def large_file_callback(large_file):
...     # storage is not part of the interface - here only for demonstration purposes
...     storage.store({'name': remote_name, 'large_file_id': large_file.id})
>>> bucket.create_file(input_sources, remote_name, file_info, large_file_callback=large_file_callback)
# ...
>>> large_file_id = storage.query({'name': remote_name})[0]['large_file_id']
>>> bucket.create_file(input_sources, remote_name, file_info, large_file_id=large_file_id)

Manual continuation (streamed version)

>>> def large_file_callback(large_file):
...     # storage is not part of the interface - here only for demonstration purposes
...     storage.store({'name': remote_name, 'large_file_id': large_file.id})
>>> bucket.create_file_stream(input_sources, remote_name, file_info, large_file_callback=large_file_callback)
# ...
>>> large_file_id = storage.query({'name': remote_name})[0]['large_file_id']
>>> bucket.create_file_stream(input_sources, remote_name, file_info, large_file_id=large_file_id)

Streams that contains remote sources cannot be continued with b2sdk.v2.Bucket.create_file() - internally b2sdk.v2.Bucket.create_file() stores plan information in file info for such inputs, and verifies it before any copy/upload and b2sdk.v2.Bucket.create_file_stream() cannot store this information. Local source only inputs can be safely continued with b2sdk.v2.Bucket.create_file() in auto continue mode or manual continue mode (because plan information is not stored in file info in such case).

Auto continuation

>>> bucket.create_file(input_sources, remote_name, file_info)

For local source only input, b2sdk.v2.Bucket.create_file() would try to find matching unfinished large file. It will verify uploaded parts checksums with local sources - the most completed, having all uploaded parts matched candidate would be automatically selected as file to continue. If there is no matching candidate (even if there are unfinished files for the same file name) new large file would be started.

In other cases plan information would be generated and b2sdk.v2.Bucket.create_file() would try to find unfinished large file with matching plan info in its file info. If there is one or more such unfinished large files, b2sdk.v2.Bucket.create_file() would verify checksums for all locally available parts and choose any matching candidate. If all candidates fails on uploaded parts checksums verification, process is interrupted and error raises. In such case corrupted unfinished large files should be cancelled manullay and b2sdk.v2.Bucket.create_file() should be retried, or auto continuation should be turned off with auto_continue=False

No continuation

>>> bucket.create_file(input_sources, remote_name, file_info, auto_continue=False)

Note, that this only forces start of a new large file - it is still possible to continue the process with either auto or manual modes.