Chapter 13. Binary Large Objects

Chapter 13. + Binary Large Objects +
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Chapter 13. + Binary Large Objects +

+ Binary Large Objects (BLOB) support is designed for efficient + storage of large objects. An object is considered to be large + if it is more than a third of the size of a page. Without BLOB + support, large objects must be broken up into smaller pieces, + and then reassembled and/or disassembled every time the record + is read or updated. Berkeley DB BLOB support avoids this + assembly/disassembly process by storing the large object in a + special directory set aside for the purpose. The data itself + is not kept in the database, nor is it placed into the + in-memory cache. +

+ BLOBs can only be stored using the data portion of a key/data + pair. They are supported only for Btree, Hash, and Heap + databases, and only so long as the database is not configured + for checksums, encryption, duplicate records, or duplicate + sorted records. In addition, the DBT that you use to + access the BLOB data cannot be configured as a partial + DBT if you want to access the data using the BLOB's + streaming interface (introduced below). +

+ Note that if the environment is transactionally-protected, then + all access to the BLOB is also transactionally protected. +

+ BLOBs are not supported for environments configured for + replication. Nor can BLOBs be used with in-memory-only + databases. +

+ In order to use Berkeley DB's BLOB support, you must set the + BLOB threshold to a non-zero positive value. (The default value + is 0, which means that BLOBs are not + enabled). You set the BLOB threshold using either + DB->set_blob_threshold() or DB_ENV->set_blob_threshold(). +

+ Once BLOBs are enabled, there are two ways to create a BLOB record: +

+
+ Configure the DBT used to access the BLOB data + (that is, the DBT used for the data portion of + the record) with the + DB_DBT_BLOB + flag. This causes the data to be stored as a BLOB + regardless of its size, so long as the database + otherwise supports BLOBs. +
+
+
+ Alternatively, creating a data item with a size greater + than the BLOB threshold will cause that data item to be + stored as a BLOB. Of course, for this method to work, + the BLOB threshold must be greater than + 0. +
+

+ BLOBs may be accessed in the same way as other DBT + data, so long as the data itself will fit into memory. More + likely, you will find it necessary to use the BLOB streaming + API to read and write BLOB data. You open a BLOB stream using + the DBC->db_stream() method, close it with the DB_STREAM->close() + method, write to it using the the DB_STREAM->write() method, and + read it using the DB_STREAM->read() method. +

BLOBs and Related Methods

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BLOB Operations	Description
+ DBC->db_stream() +	Create a BLOB stream
+ DB_STREAM->close() +	Close a BLOB stream
+ DB_STREAM->read() +	Read a BLOB stream
+ DB_STREAM->size() +	Returns the size of a BLOB
+ DB_STREAM->write() +	Write a BLOB stream
+ + BLOB Configuration + +
DB->set_blob_dir(), DB->get_blob_dir()	Sets/gets the location where blob data is stored
DB->set_blob_threshold(), DB->get_blob_threshold()	Sets/gets the size when data records are stored as blobs
DB_ENV->set_blob_dir(), DB_ENV->get_blob_dir()	Sets/gets the location where blob data is stored
DB_ENV->set_blob_threshold(), DB_ENV->get_blob_threshold()	Sets/gets the default size for the environment when BLOBs are used