Saturday, September 17, 2011

Can I Store BLOBs in Cassandra

Currently Cassandra isn't optimized specifically for large file or BLOB storage. However, files of around 64Mb and smaller can be easily stored in the database without splitting them into smaller chunks. This is primarily due to the fact that Cassandra's public API is based on Thrift, which offers no streaming abilities; any value written or fetched has to fit in to memory. Other non Thrift interfaces may solve this problem in the future, but there are currently no plans to change Thrift's behavior. When planning applications that require storing BLOBS, you should also consider these attributes of Cassandra as well:
  • The main limitation on a column and super column size is that all the data for a single key and column must fit (on disk) on a single machine(node) in the cluster. Because keys alone are used to determine the nodes responsible for replicating their data, the amount of data associated with a single key has this upper bound. This is an inherent limitation of the distribution model.
  • When large columns are created and retrieved, that columns data is loaded into RAM which can get resource intensive quickly. Consider, loading 200 rows with columns that store 10Mb image files each into RAM. That small result set would consume about 2Gb of RAM. Clearly as more and more large columns are loaded, RAM would start to get consumed quickly. This can be worked around, but will take some upfront planning and testing to get a workable solution for most applications. You can find more information regarding this behavior here: memtables, and a possible solution in 0.7 here: CASSANDRA-16.
  • Please refer to the notes in the Cassandra limitations section for more information: Cassandra Limitations

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