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The "WritePathForUsers" page has been changed by MichaelEdge: https://wiki.apache.org/cassandra/WritePathForUsers?action=diff&rev1=21&rev2=22 The local coordinator waits for the appropriate number of acknowledgements (dependent on the consistency level for this write request) before acknowledging back to the client. == Flushing MemTables == MemTables are flushed to disk based on various factors, some of which include: - * commitlog_total_space_in_mb is exceeded + * commitlog_total_space_in_mb is exceeded - * memtable_total_space_in_mb is exceeded + * memtable_total_space_in_mb is exceeded - * ‘Nodetool flush’ command is executed + * ‘Nodetool flush’ command is executed - * Etc. + * Etc. Each flush of a MemTable results in one new, immutable SSTable on disk. After the flush an SSTable (Sorted String Table) is read-only. As with the write to the Commit Log, the write to the SSTable data file is a sequential write operation. An SSTable consists of multiple files, including the following: - * Bloom Filter + * Bloom Filter - * Index + * Index - * Compression File (optional) + * Compression File (optional) - * Statistics File + * Statistics File - * Data File + * Data File - * Summary + * Summary - * TOC.txt + * TOC.txt Each MemTable flush executes the following steps: - 1. Sort the MemTable columns by row key + 1. Sort the MemTable columns by row key - 1. Write the Bloom Filter + 1. Write the Bloom Filter - 1. Write the Index + 1. Write the Index - 1. Serialise and write the data to the SSTable Data File + 1. Serialise and write the data to the SSTable Data File - 1. Write Compression File (if compression is used) + 1. Write Compression File (if compression is used) - 1. Write Statistics File + 1. Write Statistics File - 1. Purge the written data from the Commit Log + 1. Purge the written data from the Commit Log Unavailable Replica Nodes and Hinted Handoff When a local coordinator is unable to send data to a replica node due to the replica node being unavailable, the local coordinator stores the data in its local system.hints table; this process is known as Hinted Handoff. The data is stored for a default period of 3 hours. When the replica node comes back online the coordinator node will send the data to the replica node. Write Path Advantages - * The write path is one of Cassandra’s key strengths: for each write request one sequential disk write plus one in-memory write occur, both of which are extremely fast. + * The write path is one of Cassandra’s key strengths: for each write request one sequential disk write plus one in-memory write occur, both of which are extremely fast. - * During a write operation, Cassandra never reads before writing, never rewrites data, never deletes data and never performs random I/O. + * During a write operation, Cassandra never reads before writing, never rewrites data, never deletes data and never performs random I/O.
