ctubbsii commented on code in PR #5451:
URL: https://github.com/apache/accumulo/pull/5451#discussion_r2033742089


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core/src/main/java/org/apache/accumulo/core/util/tables/TableMapping.java:
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@@ -0,0 +1,201 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.accumulo.core.util.tables;
+
+import static java.util.Collections.emptySortedMap;
+import static java.util.Objects.requireNonNull;
+import static 
org.apache.accumulo.core.clientImpl.NamespaceMapping.deserializeMap;
+import static 
org.apache.accumulo.core.clientImpl.NamespaceMapping.serializeMap;
+
+import java.util.Map;
+import java.util.Objects;
+import java.util.SortedMap;
+import java.util.stream.Stream;
+
+import org.apache.accumulo.core.Constants;
+import 
org.apache.accumulo.core.clientImpl.AcceptableThriftTableOperationException;
+import org.apache.accumulo.core.clientImpl.ClientContext;
+import org.apache.accumulo.core.clientImpl.Namespace;
+import org.apache.accumulo.core.clientImpl.thrift.TableOperation;
+import org.apache.accumulo.core.clientImpl.thrift.TableOperationExceptionType;
+import org.apache.accumulo.core.data.NamespaceId;
+import org.apache.accumulo.core.data.TableId;
+import org.apache.accumulo.core.fate.zookeeper.ZooReaderWriter;
+import org.apache.accumulo.core.metadata.AccumuloTable;
+import org.apache.accumulo.core.zookeeper.ZcStat;
+import org.apache.accumulo.core.zookeeper.ZooCache;
+import org.apache.zookeeper.KeeperException;
+
+import com.google.common.collect.ImmutableSortedMap;
+
+public class TableMapping {
+
+  private final ClientContext context;
+  private final NamespaceId namespaceId;
+  private volatile SortedMap<TableId,String> currentTableMap = 
emptySortedMap();
+  private volatile SortedMap<String,TableId> currentTableReverseMap = 
emptySortedMap();
+  private volatile long lastMzxid;
+
+  public TableMapping(ClientContext context, NamespaceId namespaceId) {
+    this.context = context;
+    this.namespaceId = namespaceId;
+  }
+
+  public void put(final ClientContext context, TableId tableId, String 
tableName,
+      TableOperation operation)
+      throws InterruptedException, KeeperException, 
AcceptableThriftTableOperationException {
+    var zoo = context.getZooSession().asReaderWriter();
+    Stream.of(zoo, tableId, namespaceId, 
tableName).forEach(Objects::requireNonNull);
+    String zTableMapPath = getZTableMapPath(namespaceId);
+    if (!zoo.exists(zTableMapPath)) {
+      throw new KeeperException.NoNodeException(zTableMapPath + " does not 
exist in ZooKeeper");
+    }
+    if (isBuiltInZKTable(tableId)) {
+      throw new AssertionError("Putting built-in tables in map should not be 
possible after init");
+    }
+    zoo.mutateExisting(zTableMapPath, data -> {
+      var tables = deserializeMap(data);
+      final String currentName = tables.get(tableId.canonical());
+      if (tableName.equals(currentName)) {
+        return null; // mapping already exists; operation is idempotent, so no 
change needed
+      }
+      if (currentName != null) {
+        throw new AcceptableThriftTableOperationException(null, 
tableId.canonical(), operation,
+            TableOperationExceptionType.EXISTS, "Table Id already exists");
+      }
+      if (tables.containsValue(tableName)) {
+        throw new AcceptableThriftTableOperationException(null, 
tableId.canonical(), operation,
+            TableOperationExceptionType.EXISTS, "Table name already exists");
+      }
+      tables.put(tableId.canonical(), tableName);
+      return serializeMap(tables);
+    });
+  }
+
+  public void remove(final ClientContext context, final TableId tableId)
+      throws InterruptedException, KeeperException, 
AcceptableThriftTableOperationException {
+    var zoo = context.getZooSession().asReaderWriter();
+    Stream.of(zoo, tableId).forEach(Objects::requireNonNull);
+    if (isBuiltInZKTable(tableId)) {
+      throw new AssertionError("Removing built-in tables in map should not be 
possible");
+    }
+    zoo.mutateExisting(getZTableMapPath(getNamespaceOfTableId(zoo, tableId)), 
data -> {
+      var tables = deserializeMap(data);
+      if (!tables.containsKey(tableId.canonical())) {
+        throw new AcceptableThriftTableOperationException(null, 
tableId.canonical(),

Review Comment:
   I think the point of this code is to return to the caller a meaningful 
"table not found" exception when there's a race condition between two 
concurrent requests to delete a table. Normally, when you delete something that 
doesn't exist, you get an exception saying it doesn't exist. This code 
preserves that behavior even in cases where two requests occur to delete the 
table at the same time. It would be weird to get a TableNotFoundException in 
one case, but not in other cases, depending on timing. This same check was done 
in the NamespaceMapping code.



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