Github user zznate commented on a diff in the pull request:

    https://github.com/apache/cassandra/pull/205#discussion_r173962979
  
    --- Diff: src/java/org/apache/cassandra/db/VirtualTable.java ---
    @@ -0,0 +1,148 @@
    +/*
    + * 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
    + *
    + *     http://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.cassandra.db;
    +
    +import static java.lang.String.format;
    +
    +import java.lang.reflect.InvocationTargetException;
    +import java.util.List;
    +
    +import org.apache.cassandra.cql3.Operation;
    +import org.apache.cassandra.cql3.QueryOptions;
    +import org.apache.cassandra.cql3.UpdateParameters;
    +import org.apache.cassandra.cql3.statements.SelectStatement;
    +import org.apache.cassandra.db.filter.DataLimits;
    +import org.apache.cassandra.db.partitions.PartitionUpdate;
    +import org.apache.cassandra.db.rows.Row;
    +import org.apache.cassandra.exceptions.CassandraException;
    +import org.apache.cassandra.exceptions.ConfigurationException;
    +import org.apache.cassandra.exceptions.InvalidRequestException;
    +import org.apache.cassandra.schema.TableMetadata;
    +import org.apache.cassandra.transport.messages.ResultMessage;
    +import org.apache.cassandra.utils.FBUtilities;
    +
    +import com.google.common.collect.Lists;
    +
    +/**
    + * Base requirements for a VirtualTable. This is required to provide 
metadata about the virtual table, such as the
    + * partition and clustering keys, and provide a ReadQuery for a 
SelectStatement.
    + */
    +public abstract class VirtualTable
    +{
    +    protected final TableMetadata metadata;
    +    protected String keyspace;
    +    protected String name;
    +
    +    public VirtualTable(TableMetadata metadata)
    +    {
    +        this.metadata = metadata;
    +    }
    +
    +    public String getTableName()
    +    {
    +        return this.metadata.name;
    +    }
    +
    +    /**
    +     * Is this table writable?
    +     *
    +     * @return True if UPDATE is supported
    +     */
    +    public boolean writable()
    +    {
    +        return false;
    +    }
    +
    +    /**
    +     * If the table allows unrestricted queries (ie filter on clustering 
key with no partition). Since These tables are
    +     * not backed by the C* data model, this restriction isnt always 
necessary.
    +     */
    +    public boolean allowFiltering()
    +    {
    +        return true;
    +    }
    +
    +    /**
    +     * Return some implementation of a ReadQuery for a given select 
statement and query options.
    +     * 
    +     * @param selectStatement
    +     * @param options
    +     * @param limits
    +     * @param nowInSec
    +     * @return ReadQuery
    +     */
    +    public abstract ReadQuery getQuery(SelectStatement selectStatement, 
QueryOptions options, DataLimits limits,
    +            int nowInSec);
    +
    +    /**
    +     * Execute an update operation.
    +     *
    +     * @param partitionKey
    +     *            partition key for the update.
    +     * @param params
    +     *            parameters of the update.
    +     */
    +    public void mutate(DecoratedKey partitionKey, Row row) throws 
CassandraException
    +    {
    +        // this should not be called unless writable is overridden
    +        throw new InvalidRequestException("Not Implemented");
    +    }
    +
    +    public static Class<? extends VirtualTable> classFromName(String name)
    +    {
    +        String className = name.contains(".")
    +                ? name
    +                : "org.apache.cassandra.db.virtual." + name;
    +        Class<VirtualTable> strategyClass = 
FBUtilities.classForName(className, "virtual table");
    +
    +        if (!VirtualTable.class.isAssignableFrom(strategyClass))
    +        {
    +            throw new ConfigurationException(format("Compaction strategy 
class %s is not derived from VirtualTable",
    +                    className));
    +        }
    +
    +        return strategyClass;
    +    }
    +
    +    public static Key createKey()
    +    {
    +        return new Key();
    --- End diff --
    
    Does it make sense to allow empty key creation as API? If not, switch it to 
a builder so's it can be validated. 


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