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But what if the long-running job and the short-running job write to the same column family? In that case, a job that needs to see the latest version of every column and does not depend on repeatable reads can read without a snapshot. It will then see all writes that have been made to the table. Optionally, we could allow a snapshot to exclude column families for which repeatable reads are not important, such that a reader can see the latest writes for those column families while relying on repeatable reads for other families. However, this seems to be a very advanced use case, and it is questionable whether it should be implemented until a strong use case emerges.

Implementation

Definitions

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• Table Snapshot - A Snapshot (of a table) is defined as the state of the table at a particular point in time. The state being defined by the list of (column, revision) pairs. Snapshots could be created by the on the fly by requesting the revision manager to take a snapshot of a table.

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Consider an example of a table "Cars" has three columns C1, C2, C3.<insert image>

• The user specifies nothing specific to read. In this mode, the latest snapshot of the table will be taken by default. This snapshot would be used to read the table.
  Example:
  User specification : Read table "Cars".
  Output : (Z,Z,P).

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Code Block

/** * Initialize the revision manager. */ public void initialize(Properties properties); /** * Opens the revision manager. * * @throws IOException */ public void open() throws IOException; /** * Closes the revision manager. * * @throws IOException */ public void close() throws IOException; /** * Start the write transaction. * * @param table The name of table involved in the transaction.  * @param families The column families of table in which data will be written by the transaction.  * @return An instance of Transaction  * @throws IOException */ public Transaction beginWriteTransaction(String table, List<String> families) throws IOException; /** * Start the write transaction. * * @param table The name of table involved in the transaction. * @param families The column families of table in which data will be written by the transaction. * @param keepAlive The duration ( in milliseconds) after which the transaction will expire. * @return An instance of Transaction * @throws IOException */ public Transaction beginWriteTransaction(String table, List<String> families, long keepAlive) throws IOException; /** * Commit the write transaction. * * @param transaction An instance of Transaction to be committed. * @throws IOException */ public void commitWriteTransaction(Transaction transaction) throws IOException; /** * Abort the write transaction. * * @param transaction An instance of Transaction to be aborted. * @throws IOException */ public void abortWriteTransaction(Transaction transaction) throws IOException; /** * Create the latest snapshot of the table. * * @param tableName The name of the table to create a snapshot. * @return An instance of TableSnapshot. * @throws IOException */ public TableSnapshot createSnapshot(String tableName) throws IOException; /** * Create the snapshot of the table using the revision number. * * @param tableName The name of the table to create a snapshot. * @param revision The revision number to be used. * @return An instance of TableSnapshot. * @throws IOException */ public TableSnapshot createSnapshot(String tableName, long revision) throws IOException; /** * Extends the expiration of a transaction by the time indicated by keep alive. * * @param transaction An instance of Transaction. * @throws IOException */ public void keepAlive(Transaction transaction) throws IOException;