@SnapshotSave

Description

The @SnapshotSave system procedure saves the contents of the current in-memory database to disk. Each node of the database cluster saves its portion of the database locally. The snapshot consists of multiple files saved to the directory specified by directory-path using unique-ID as a filename prefix.

The fourth parameter, blocking-flag, specifies whether the save is performed synchronously (thereby blocking any following transactions until the save completes) or asynchronously. If this parameter is set to any non-zero value, the save operation will block any following transactions. If it is zero, others transactions will be executed in parallel.

Note that it is normal to perform manual saves synchronously, to ensure the snapshot represents a known state of the database. However, automatic snapshots are performed asynchronously to reduce the impact on ongoing database activity.

See Chapter 9, Saving & Restoring a VoltDB Database for more information about saving and restoring VoltDB databases.

Return Values

The @SnapshotSave system procedure returns two different VoltTables, depending on the outcome of the request.

Option #1: one VoltTable with a row for every execution site. (That is, the number of hosts multiplied by the number of sites per host.).

Option #2: one VoltTable with a variable number of rows.

Example

The following example uses @SnapshotSave to save the current database content to the path /tmp/voltdb/backup/ using the unique identifier flight on each node of the cluster.

Note that the procedure call will return successfully even if the save was not entirely successful. The information returned in the VoltTable array tells you what parts of the operation were successful or not. For example, save may succeed on one node but not on another.

The following example checks the return values and notifies the operator when portions of the save operation are not successful.

VoltTable[] results = null;

try { results = client.callProcedure("@SnapshotSave",
                                     "/tmp/voltdb/backup/",
                                     "flight", 1).getResults();
}
catch (Exception e) { e.printStackTrace(); }

for (int table=0; table<results.length; table++) {
    for (int r=0;r<results[table].getRowCount();r++) {
        VoltTableRow row = results[table].fetchRow(r);
        if (row.getString("RESULT").compareTo("SUCCESS") != 0) {
            System.out.printf("Site %s failed to write " +
                   "table %s because %s.\n",
                   row.getString("HOSTNAME"), row.getString("TABLE"),
                   row.getString("ERR_MSG"));
        }
    }
}