[Cassandra Wiki] Update of "CassandraLimitations" by JonathanEllis

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The following page has been changed by JonathanEllis:
http://wiki.apache.org/cassandra/CassandraLimitations

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  From easiest to fix to hardest:
  
   * Cassandra's compaction code currently deserializes an entire row (per columnfamily) at a time.  So all the data from a given columnfamily/key pair must fit in memory.  Fixing this is relatively easy since columns are stored in-order on disk so there is really no reason you have to deserialize row-at-a-time except that that is easier with the current encapsulation of functionality.
-  * Cassandra does not currently fsync the commitlog before acking a write.  Most of the time this is Good Enough when you are writing to multiple replicas since the odds are slim of all replicas dying before the data actually hits the disk, but the truly paranoid will want real fsync-before-ack.  Just adding fsync would be just a few lines (to CommitLog, naturally), but we want to do this without killing performance, so what we want is an Executor that fsyncs after writing batches of commitlog entries (and then asynchronously notifies the write threads).
   * Cassandra has two levels of indexes: key and column.  But in super columnfamilies there is a third level of subcolumns; these are not indexed, and any request for a subcolumn deserializes _all_ the subcolumns in that supercolumn.  So you want to avoid a data model that requires large numbers of subcolumns.  This can be fixed; the core classes involved are SuperColumn and SequenceFile.
-  * Cassandra's public API is based on Thrift, which offers no streaming abilities -- any value written or fetched has to fit in memory.  This is inherent to Thrift's design; I don't see it changing.  So (similar to traditional rdbmses) you're better off storing large blobs directly in the filesystem with a pointer to machine:path, than storing the blobs directly in Cassandra.
+  * Cassandra's public API is based on Thrift, which offers no streaming abilities -- any value written or fetched has to fit in memory.  This is inherent to Thrift's design; I don't see it changing.  So adding large object support to Cassandra would need a special API that manually split the large objects up into pieces.  Jonathan Ellis sketched out one approach in https://issues.apache.org/jira/browse/CASSANDRA-265.
  
+ == Obsolete Limitations ==
+  * Prior to version 0.4, Cassandra did not fsync the commitlog before acking a write.  Most of the time this is Good Enough when you are writing to multiple replicas since the odds are slim of all replicas dying before the data actually hits the disk, but the truly paranoid will want real fsync-before-ack.  This is now an option.
+