127 lines
3.4 KiB
Java
127 lines
3.4 KiB
Java
/*
|
|
* MicroQueue.java
|
|
*
|
|
* This source file is part of the FoundationDB open source project
|
|
*
|
|
* Copyright 2013-2018 Apple Inc. and the FoundationDB project authors
|
|
*
|
|
* Licensed 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.
|
|
*/
|
|
|
|
import com.foundationdb.*;
|
|
import com.foundationdb.async.Function;
|
|
import com.foundationdb.subspace.Subspace;
|
|
import com.foundationdb.tuple.Tuple;
|
|
|
|
import java.util.Random;
|
|
|
|
public class MicroQueue {
|
|
|
|
private static final FDB fdb;
|
|
private static final Database db;
|
|
private static final Subspace queue;
|
|
private static final Random randno;
|
|
|
|
static{
|
|
fdb = FDB.selectAPIVersion(300);
|
|
db = fdb.open();
|
|
queue = new Subspace(Tuple.from("Q"));
|
|
randno = new Random();
|
|
}
|
|
|
|
// TODO The next four elements are in the recipe book:
|
|
// dequeue, enqueue, firstItem, and lastIndex.
|
|
|
|
// Remove the top element from the queue.
|
|
public static Object dequeue(TransactionContext tcx){
|
|
// Remove from the top of the queue.
|
|
return tcx.run(new Function<Transaction,Void>(){
|
|
public Void apply(Transaction tr){
|
|
final KeyValue item = firstItem(tr);
|
|
if(item == null){
|
|
return null;
|
|
}
|
|
|
|
tr.clear(item.getKey());
|
|
// Return the old value.
|
|
return Tuple.fromBytes(item.getValue()).get(0);
|
|
}
|
|
});
|
|
|
|
}
|
|
|
|
// Add an element to the queue.
|
|
public static void enqueue(TransactionContext tcx, final Object value){
|
|
tcx.run(new Function<Transaction,Void>() {
|
|
public Void apply(Transaction tr){
|
|
byte[] rands = new byte[20];
|
|
randno.nextBytes(rands); // Create random seed to avoid conflicts.
|
|
tr.set(queue.subspace(Tuple.from(lastIndex(tr)+1, rands)).pack(),
|
|
Tuple.from(value).pack());
|
|
|
|
return null;
|
|
}
|
|
});
|
|
}
|
|
|
|
// Get the top element of the queue.
|
|
private static KeyValue firstItem(TransactionContext tcx){
|
|
return tcx.run(new Function<Transaction, KeyValue>() {
|
|
public KeyValue apply(Transaction tr){
|
|
for(KeyValue kv : tr.getRange(queue.range(), 1)){
|
|
return kv;
|
|
}
|
|
|
|
return null; // Empty queue. Should never be reached.
|
|
}
|
|
});
|
|
}
|
|
|
|
// Get the last index in the queue.
|
|
private static long lastIndex(TransactionContext tcx){
|
|
return tcx.run(new Function<Transaction, Long>() {
|
|
public Long apply(Transaction tr){
|
|
for(KeyValue kv : tr.snapshot().getRange(queue.range(), 1, true)){
|
|
return (long)queue.unpack(kv.getKey()).get(0);
|
|
}
|
|
return 0l;
|
|
}
|
|
});
|
|
}
|
|
|
|
// Clears subspaces of a database.
|
|
public static void clearSubspace(TransactionContext tcx, final Subspace s){
|
|
tcx.run(new Function<Transaction,Void>() {
|
|
public Void apply(Transaction tr){
|
|
tr.clear(Range.startsWith(s.getKey()));
|
|
return null;
|
|
}
|
|
});
|
|
}
|
|
|
|
public static void main(String[] args){
|
|
clearSubspace(db, queue);
|
|
String[] line = {"Alice", "Bob", "Carol", "Dave", "Eve", "Frank","George",
|
|
"Harry", "Ian","Jack","Liz","Mary","Nathan"};
|
|
|
|
for(int i = 0; i < line.length; i++){
|
|
enqueue(db, line[i]);
|
|
}
|
|
|
|
Object o;
|
|
while((o = dequeue(db)) != null){
|
|
System.out.println(o);
|
|
}
|
|
}
|
|
}
|