[ACCUMULO-3282] Reduce scans of metadata when assigning tablet

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1.7.0
• Component/s: tserver
• Assignee: Josh Elser
• Reporter: Josh Elser
• Created: 30/Oct/14 22:27
• Updated: 17/Nov/14 18:23
• Resolved: 17/Nov/14 03:04

Description

Noticed the following case digging through the assignment code:

TabletServer.java

  public static Pair<Text,KeyExtent> verifyTabletInformation(KeyExtent extent, TServerInstance instance, SortedMap<Key,Value> tabletsKeyValues,
      String clientAddress, ZooLock lock) throws AccumuloSecurityException, DistributedStoreException, AccumuloException {
...
  public static Pair<Text,KeyExtent> verifyTabletInformation(KeyExtent extent, TServerInstance instance, SortedMap<Key,Value> tabletsKeyValues,
      String clientAddress, ZooLock lock) throws AccumuloSecurityException, DistributedStoreException, AccumuloException {

    log.debug("verifying extent " + extent);
    if (extent.isRootTablet()) {
      return verifyRootTablet(extent, instance);
    }
    String tableToVerify = MetadataTable.ID;
    if (extent.isMeta())
      tableToVerify = RootTable.ID;

    List<ColumnFQ> columnsToFetch = Arrays.asList(new ColumnFQ[] {TabletsSection.ServerColumnFamily.DIRECTORY_COLUMN,
        TabletsSection.TabletColumnFamily.PREV_ROW_COLUMN, TabletsSection.TabletColumnFamily.SPLIT_RATIO_COLUMN,
        TabletsSection.TabletColumnFamily.OLD_PREV_ROW_COLUMN, TabletsSection.ServerColumnFamily.TIME_COLUMN});

    ScannerImpl scanner = new ScannerImpl(HdfsZooInstance.getInstance(), SystemCredentials.get(), tableToVerify, Authorizations.EMPTY);
    scanner.setRange(extent.toMetadataRange());

    TreeMap<Key,Value> tkv = new TreeMap<Key,Value>();
    for (Entry<Key,Value> entry : scanner)
      tkv.put(entry.getKey(), entry.getValue());

    // only populate map after success
    if (tabletsKeyValues == null) {
      tabletsKeyValues = tkv;
    } else {
      tabletsKeyValues.clear();
      tabletsKeyValues.putAll(tkv);

Essentially, we read a few columns for the tablet's row from metadata and (when metadata is in a consistent state) we return those columns to the Tablet constructor.

Tablet.java

  private Tablet(TabletServer tabletServer, Text location, KeyExtent extent, TabletResourceManager trm, Configuration conf, VolumeManager fs,
      SortedMap<Key,Value> tabletsKeyValues) throws IOException {
    this(tabletServer, location, extent, trm, conf, fs, lookupLogEntries(extent, tabletsKeyValues), lookupDatafiles(tabletServer.getSystemConfiguration(), fs,
        extent, tabletsKeyValues), lookupTime(tabletServer.getSystemConfiguration(), extent, tabletsKeyValues), lookupLastServer(extent, tabletsKeyValues),
        lookupScanFiles(extent, tabletsKeyValues, fs), lookupFlushID(extent, tabletsKeyValues), lookupCompactID(extent, tabletsKeyValues));
  }

lookupDataFiles reaches back out to the metadata table to fetch this column. I'm not sure if there's a reason why we can't do this all at once.

Issue Links

• relates to: ACCUMULO-3339 Replace SortedMap<Key,Value> from Tablet constructor