Simon Spero created COMPRESS-414:
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Summary: Provide API for Channel(-ish) Seekable, Possibly
Read/Write, Possibly Transactional Compressors & Archivers,
Key: COMPRESS-414
URL: https://issues.apache.org/jira/browse/COMPRESS-414
Project: Commons Compress
Issue Type: Improvement
Reporter: Simon Spero
It would be very useful to have APIs to support random-access, possibly
read-write archives & compressors.
These interfaces should probably be based around Seekable, Scatter, and Gather
ByteChannels.
It might be necessary or desirable to have these classes extend FileChannel,
since there are some important methods that first make their appearance as
abstract methods on that class.
The methods of interest are the absolute positioned read and write methods,
plus force/sync.
The use case I'm thinking of for the the non-relative read/write would be for
a random-access compressed "file", where it may be useful to keep a cache of
decompressed blocks, and where a task may require access to several parts of
the file simultaneously. Nesting this deeper, there might be (well, there
are) a tar.xz file containing a sequence of versions of a maven artifact jar,
and with an analysis process that needs access to two or more of the files
simultaneously.
A use case requiring read/write + transactional + sync support would be to
support in-place appending new items to a tar.xz file. It can be quite useful
to be able to perform such updates "in-place"; for safety this requires a log
the parts of the existing file that are going to be changed, so that the
contents can be recovered in the event of failure.
If there is related metadata in a separate transactional resource, it is good
to have both stores remain in sync. The complicated parts are the job of a
transaction manager (e.g. Aries); basic XAResource support is relatively
simple. There are other bits of glue that are useful for handling simple
transactions with no transaction manager around, but those can be optional
imports.
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