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This document describes the proposed programming interface for streaming
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data from and into repositories. This programming interface should allow
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a single interface for pulling data from and inserting data into a Bazaar
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To eliminate the current requirement that extracting data from a
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repository requires either using a slow format, or knowing the format of
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both the source repository and the target repository.
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Here's a brief description of use cases this interface is intended to
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We fetch data between repositories as part of push/pull/branch operations.
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Fetching data is currently an very interactive process with lots of
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requests. For performance having the data be supplied in a stream will
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improve push and pull to remote servers. For purely local operations the
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streaming logic should help reduce memory pressure. In fetch operations
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we always know the formats of both the source and target.
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Smart server operations
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~~~~~~~~~~~~~~~~~~~~~~~
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With the smart server we support one streaming format, but this is only
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usable when both the client and server have the same model of data, and
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requires non-optimal IO ordering for pack to pack operations. Ideally we
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can both provide optimal IO ordering the pack to pack case, and correct
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ordering for pack to knits.
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Bundles also create a stream of data for revisions from a repository.
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Unlike fetch operations we do not know the format of the target at the
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time the stream is created. It would be good to be able to treat bundles
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as frozen branches and repositories, so a serialised stream should be
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At this point we are not trying to integrate data conversion into this
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interface, though it is likely possible.
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Some key aspects of the described interface are discussed in this section.
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All users of this should be able to create an appropriate stream from a
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There should be no need to seek in a stream when inserting data from it
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into a repository. This places an ordering constraint on streams which
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some repositories do not need.
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At this point serialisation of a repository stream has not been specified.
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Some considerations to bear in mind about serialisation are worth noting
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While there shouldn't be too many users of weave repositories anymore,
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avoiding pathological behaviour when a weave is being read is a good idea.
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Having the weave itself embedded in the stream is very straight forward
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and does not need expensive on the fly extraction and re-diffing to take
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Being able to perform random reads from a repository stream which is a
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bundle would allow stacking a bundle and a real repository together. This
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will need the pack container format to be used in such a way that we can
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avoid reading more data than needed within the pack container's readv
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This describes the interface for requesting a stream, and the programming
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interface a stream must provide. Streams that have been serialised should
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expose the same interface.
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To request a stream, three parameters are needed:
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* A revision search to select the revisions to include.
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* A data ordering flag. There are two values for this - 'unordered' and
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'topological'. 'unordered' streams are useful when inserting into
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repositories that have the ability to perform atomic insertions.
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'topological' streams are useful when converting data, or when
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inserting into repositories that cannot perform atomic insertions (such
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as knit or weave based repositories).
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* A complete_inventory flag. When provided this flag signals the stream
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generator to include all the data needed to construct the inventory of
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each revision included in the stream, rather than just deltas. This is
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useful when converting data from a repository with a different
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inventory serialisation, as pure deltas would not be able to be
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Structure of a stream
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---------------------
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A stream is an object. It can be consistency checked via the ``check``
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method (which consumes the stream). The ``iter_contents`` method can be
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used to iterate the contents of the stream. The contents of the stream are
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a series of top level records, each of which contains one or more
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bytestrings (potentially as a delta against another item in the
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repository) and some optional metadata.
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To consume a stream, obtain an iterator from the streams
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``iter_contents`` method. This iterator will yield the top level records.
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Each record has two attributes. One is ``key_prefix`` which is a tuple key
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prefix for the names of each of the bytestrings in the record. The other
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attribute is ``entries``, an iterator of the individual items in the
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record. Each item that the iterator yields is a factory which has metadata
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about the entry and the ability to return the compressed bytes. This
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factory can be decorated to allow obtaining different representations (for
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example from a compressed knit fulltext to a plain fulltext).
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stream = repository.get_repository_stream(search, UNORDERED, False)
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for record in stream.iter_contents():
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for factory in record.entries:
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compression = factory.storage_kind
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print "Object %s, compression type %s, %d bytes long." % (
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record.key_prefix + factory.key,
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compression, len(factory.get_bytes_as(compression)))
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This structure should allow stream adapters to be written which can coerce
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all records to the type of compression that a particular client needs. For
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instance, inserting into weaves requires fulltexts, so a stream would be
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adapted for weaves by an adapter that takes a stream, and the target
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weave, and then uses the target weave to reconstruct full texts (which is
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all that the weave inserter would ask for). In a similar approach, a
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stream could internally delta compress many fulltexts and be able to
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answer both fulltext and compressed record requests without extra IO.
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Valid attributes on the factory are:
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* sha1: Optional ascii representation of the sha1 of the bytestring (after
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delta reconstruction).
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* storage_kind: Required kind of storage compression that has been used
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on the bytestring. One of ``mpdiff``, ``knit-annotated-ft``,
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``knit-annotated-delta``, ``knit-ft``, ``knit-delta``, ``fulltext``.
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* parents: Required graph parents to associate with this bytestring.
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* compressor_data: Required opaque data relevant to the storage_kind.
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(This is set to None when the compressor has no special state needed)
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* key: The key for this bytestring. Like each parent this is a tuple that
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should have the key_prefix prepended to it to give the unified
added
removed
doc/developers/repository-stream.txt
