The HTTP spec defines a Content-Encoding header, which signifies whether the entity body of an HTTP message is “encoded” and, if so, by which algorithm. The only commonly used content encodings, apart from identity (i.e. plain text), are compression algorithms.
Currently spray supports the compression and decompression of HTTP requests and responses with the gzip or deflate encodings. The core logic for this, which is shared by the spray-client and spray-routing modules for the client- and server-side (respectively), lives in the spray.httpx.encoding package.
The support is not enabled by default, but must be explicitly requested. For server configuration, see When to use which decompression directive?. For client configuration, see spray.client.pipelining.decode and spray.httpx.ResponseTransformation.
Compression of Chunk Streams
Properly combining HTTP compression with the chunked HTTP/1.1 Transfer-Encoding can be a little tricky. For optimal results the peer sending the message (i.e. the client or the server) should use a single compression context across all chunks, so that common patterns shared by several chunks contribute to a high compression ratio. At the same time the decompressor at the other end must be able to properly decompress each chunk as it arrives.
In order to achieve this the compressor must properly flush its compression stream after each chunk, something that the GZIP- and DeflaterOutputStream implementations of the Java 6 JDK unfortunately do not support correctly (see this JDK bug, fixed only in Java 7). sprays compression implementation jumps through a few hoops to achieve the desired behavior also under Java 6, with no cost to you as the user.