MongoDB 3.4:

• Enable compression with

  mongod --networkMessageCompressors snappy
  

• Only supports snappy and noop
• Compresses all responses if a supported compressor was provided in the initial isMaster (including the initial response to the isMaster!), irregardless if the request was compressed or not
• It will compress using the first compressor configured in --networkMessageCompressors snappy,noop that was also present in the isMaster response

MongoDB 3.5-current-RC:

• Adds support for zlib
• snappy enabled by default (no need for --networkMessageCompressors)
• To enable zlib or noop, use --networkMessageCompressors (remember to include snappy in the list if desired)
• Compresses the response if the request was compressed. uses the first compressor configured in the --networkMessageCompressors (defaults to snappy if none provided) that was also present in the isMaster response

MongoDB 3.6:

• Probably, maybe, will only compress responses using the same compressor as the request
• Probably, maybe, adds zlib to its default --networkMessageCompressors, making the default value snappy,zlib

OP_COMPRESSED

The new opcode looks like:

struct OP_COMPRESSED {
    struct MsgHeader {
        int32  messageLength;
        int32  requestID;
        int32  responseTo;
        int32  opCode = 2012;
    };
    int32_t  originalOpcode;
    int32_t  uncompressedSize;
    uint8_t  compressorId;
    char    *compressedMessage;
};

Currently supported/defined compressorIds are:

Negotiation / handshake

Compression is negotiated during the MongoDB Handshake:

    {
        isMaster: 1,
        client: {}, /* See MongoDB Handshake */
        compression: ["snappy", "zlib"]
    }

New Connection String Options

zlibCompressionLevel meaning (informative, this is how zlib defines them)

Blacklisted commands

The following commands MUST NOT be compressed:

• isMaster
• saslStart
• saslContinue
• getnonce
• authenticate
• createUser
• updateUser
• copydbSaslStart
• copydbgetnonce
• copydb