What is zlib compression

Using zlib encoding can be expensive, and the results ought to be cached. By default, the zlib methods will throw an error when decompressing truncated data. However, if it is known that the data is incomplete, or the desire is to inspect only the beginning of a compressed file, it is possible to suppress the default error handling by changing the flushing method that is used to decompress the last chunk of input data:.

This will not change the behavior in other error-throwing situations, e. Using this method, it will not be possible to determine whether the input ended prematurely or lacks the integrity checks, making it necessary to manually check that the decompressed result is valid. For example, to reduce the default memory requirements from K to K, the options should be set to:.

This is in addition to a single internal output slab buffer of size chunkSize , which defaults to 16K. The speed of zlib compression is affected most dramatically by the level setting. A higher level will result in better compression, but will take longer to complete. A lower level will result in less compression, but will be much faster. In general, greater memory usage options will mean that Node.

So, this is another factor that affects the speed, at the cost of memory usage. There are equivalents to the zlib options for Brotli-based streams, although these options have different ranges than the zlib ones:. This may come at the cost of degraded compression quality, but can be useful when data needs to be available as soon as possible.

In the following example, flush is used to write a compressed partial HTTP response to the client:. All of the constants defined in zlib. In the normal course of operations, it will not be necessary to use these constants. They are documented so that their presence is not surprising. This section is taken almost directly from the zlib documentation.

Previously, the constants were available directly from require 'zlib' , for instance zlib. Accessing the constants directly from the module is currently still possible but is deprecated. Negative values are errors, positive values are used for special but normal events. There are several options that can be set on Brotli encoders, affecting compression efficiency and speed.

Both the keys and the values can be accessed as properties of the zlib. The following flags can be set for advanced control over the compression algorithm and memory usage tuning:. See the deflateInit2 and inflateInit2 documentation for more information. Let me share some more info on ZLIB compressed data. This byte is very often equal to 0x The 4 least significant bits identify the compression method 8 is DEFLATE and 15 is reserved , the 4 most significant bits are used to encode the size of the window when the compression method is 8.

This value is often 7 32K window size. My tool translate. And depending on these flags, there might be some more data, but usually, it's the compressed data that follows. If you mix-up data format and functions, you get an error. Authors Gailly, Jean-loup. Publication Date Metadata Show full item record. Citation Gailly, J. The zlib data format is itself portable across platforms. Unlike the LZW compression method used in Unix compress 1 and in the GIF image format, the compression method currently used in zlib essentially never expands the data.

LZW can double or triple the file size in extreme cases.