compression.adoc

Compression

In response to increasing volumes of data being sent over the wire, Couchbase Data Platform now provides data compression between the SDK and Couchbase Server.

Overview

Documents may already be stored compressed with Snappy. Now documents may be passed from client applications to the server in compressed form, saving around 40% in bandwidth, and also transmission time. These operations take place automatically, after HELLO negotiation with the server, and do not require any changes on the client side, beyond updating to a recent SDK version with compression support (see release notes).

Limits

The document must be below 20MiB in size in both compressed and uncompressed form. Compression is only available in the Enterprise Edition of Couchbase Data Platform.

Tip

This size limit is enforced by Couchbase Server; in practice it will affect very few users, as most JSON documents are considerably smaller. A compressed doument of just under 20MB, which is greater than 20,971,520 bytes (20 MiB) when uncompressed, will be rejected by the server as follows: Couchbase Server decompresses the document to check that it is valid JSON, and is correctly compressed with Snappy, and at this point measures it against max data size (20 MiB). If the decompressed value’s size exceeds this limit, the mutation is failed with a "too big" error code (E2BIG code 3). Therefore, where necessary, enforce document size limits in your application on uncompressed documents.

Operating Modes

The three modes in which compression can be used, “off”, “passive” and “active”, are per bucket configuration settings on the cluster. Depending on how it is set, the HELLO negotiation will fail or succeed. The HELLO flag for compression has the value 0x0a and is defined as:

PROTOCOL_BINARY_FEATURE_SNAPPY = 0x0a

Table 1. Compression Operating Modes

	OFF	PASSIVE	ACTIVE
Sending SNAPPY feature with HELLO	ignore	acknowledge	acknowledge
Sending compressed data when SNAPPY feature enabled	-	accept	accept
Sending compressed data when SNAPPY feature disabled	reject as invalid	reject as invalid	reject as invalid
Receiving data which were previously compressed on the server	server inflates and sends plain data	server sends compressed data	server sends compressed data
Receiving data which were not previously compressed on the server	server sends plain data	server sends plain data	server might send compressed data (the compression is done in the background on the server)

Acceptance Guarantee

The server may change compression settings for the bucket at any time, but it is guaranteed that once the socket negotiates compression through HELLO, the server will never reject compressed data, even if the bucket setting has been changed.

Minimum Size

While the tiniest of documents will not be reduced in size by compressing, there is another category of slightly larger documents in whose case the time overhead of compressing and decompressing outweighs the slight advantage of marginally reduced transmission time from client to server or back.

To safeguard against the case of several thousand documents stealing CPU time to barely discernable advantage, a threshold for minimum doument size to compress is set in the SDK, with a sensible default value - that value can be seen for your chosen SDK in its API documentation (normally 32 bytes), and you can override this to disable compression:

• For Java, to disable compression from client to server, set compressionMinSize to max int (2147483647). Alternatively, to disable compression in both directions, set the Java system property com.couchbase.snappyEnabled to False.

• For .NET, Snappy compression is not currently supported.

• For SDKs using LCB’s connstr (Node.js, PHP, Python), override the default with "compression=off" in your connection string. Additionally, for Python you can set the couchbase.bucket.Bucket.compression property to COMPRESS_NONE.

• For Go, can also be disabled by using compression=off in the connection string.