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23 lines
3.1 KiB
Markdown
23 lines
3.1 KiB
Markdown
go-unsnap-stream
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This is a small golang library for decoding and encoding the snappy *streaming* format, specified here: https://github.com/google/snappy/blob/master/framing_format.txt
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Note that the *streaming or framing format* for snappy is different from snappy itself. Think of it as a train of boxcars: the streaming format breaks your data in chunks, applies snappy to each chunk alone, then puts a thin wrapper around the chunk, and sends it along in turn. You can begin decoding before receiving everything. And memory requirements for decoding are sane.
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Strangely, though the streaming format was first proposed in Go[1][2], it was never upated, and I could not locate any other library for Go that would handle the streaming/framed snappy format. Hence this implementation of the spec. There is a command line tool[3] that has a C implementation, but this is the only Go implementation that I am aware of. The reference for the framing/streaming spec seems to be the python implementation[4].
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Update to the previous paragraph: Horray! Good news: Thanks to @nigeltao, we have since learned that the [github.com/golang/snappy](https://github.com/golang/snappy) package now provides the snappy streaming format too. Even though the type level descriptions are a little misleading because they don't mention that they are for the stream format, the [snappy package header documentation](https://godoc.org/github.com/golang/snappy) points out that the [snappy.Reader](https://godoc.org/github.com/golang/snappy#Reader) and [snappy.Writer](https://godoc.org/github.com/golang/snappy#Writer) types do indeed provide stream (vs block) handling. Although I have not benchmarked, you should probably prefer that package as it will likely be maintained more than I have time to devote, and also perhaps better integrated with the underlying snappy as they share the same repo.
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For binary compatibility with the [python implementation](https://pypi.python.org/pypi/python-snappy) in [4], one could use the C-snappy compressor/decompressor code directly; using github.com/dgryski/go-csnappy. In fact we did this for a while to verify byte-for-byte compatiblity, as the native Go implementation produces slightly different binary compression (still conformant with the standard of course), which made test-diffs harder, and some have complained about it being slower than the C.
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However, while the c-snappy was useful for checking compatibility, it introduced dependencies on external C libraries (both the c-snappy library and the C standard library). Our go binary executable that used the go-unsnap-stream library was no longer standalone, and deployment was painful if not impossible if the target had a different C standard library. So we've gone back to using the snappy-go implementation (entirely in Go) for ease of deployment. See the comments at the top of unsnap.go if you wish to use c-snappy instead.
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[1] https://groups.google.com/forum/#!msg/snappy-compression/qvLNe2cSH9s/R19oBC-p7g4J
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[2] https://codereview.appspot.com/5167058
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[3] https://github.com/kubo/snzip
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[4] https://pypi.python.org/pypi/python-snappy
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