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Couchstore Data Types

Struct members not listed should be considered internal to Couchstore.

sized_buf

struct sized_buf {
    char* buf;
    size_t size;
}

Couchstore uses sized_bufs to point to data buffers.

DocInfo

struct DocInfo {
    sized_buf id;
    uint64_t db_seq;
    uint64_t rev_seq;
    sized_buf rev_meta;
    int deleted;
    uint8_t content_meta;
}
  • id - Document ID
  • db_seq - Change sequence number the document was inserted at.
  • rev_seq - The version number of the document
  • rev_meta - Revision metadata. Used by ep-engine to store CAS value, expiry time, and flags
  • deleted - 1 if document should be considered "deleted" and not subject to indexing, otherwise 0.
  • content_meta - Number field used to store flags indicating metadata about the document content (is it JSON, etc.)

The content_meta field

  • The least-significant two bits are used to encode why/whether the body is or is not JSON
    • 0 - Document body is JSON
    • 1 - Inserted data was not valid JSON
    • 2 - Inserted data was valid JSON, but contained a key reserved for internal use, such as one with a $ prefix.
    • 3 - The document was inserted in non-JSON mode.
  • If the most-significant bit is set the document body data has been compressed with snappy.

When saving documents with save_doc or save_docs, id, rev_seq, rev_meta, deleted, and content_meta must be set on the DocInfos passed to Couchstore. The db_seq is determined at insert time.

The rev_meta field

[128 bit binary value] | 64 bits | 32 bits | 32 bits | -----------|------------|------------| CAS Value | Expiration | User flags |

All values are Big Endian.

  • CAS Value - Random nonce value set by Memcached, used to allow ensuring value has not been modified since it was read when writing.
  • Expiration - 32 bit timestamp for value expiry time. 0 if item does not expire.
  • User flags - Memcache API allows users to set this 32 bit number along with values, it is not used by memcached internally.

Doc

struct Doc {
    sized_buf id;
    sized_buf data;
}

id contains the document ID, data contains the document body data. Couchstore does not compress or modify the body data in any way, unless the COMPRESS_DOC_BODIES or DECOMPRESS_DOC_BODIES flags are passed to the read and write functions, then it will check the content_meta of the corresponding doc info to see if it should compress or decompress the doc data.

LocalDoc

struct LocalDoc {
    sized_buf id;
    sized_buf json;
    int deleted;
}
  • id - The local document ID, it must start with _local/
  • json - The local document body.
  • deleted - if set to 1 on a LocalDoc passed to save_local_doc, json will be ignored, and the local document with the ID in id will be removed, if it exists.

C API

Documented in include/libcouchstore/couch_db.h

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