pjs/grendel/storage/intertwingle/BGDB.java

212 строки
6.2 KiB
Java
Исходник Обычный вид История

1998-09-09 04:52:38 +04:00
/* -*- Mode: java; indent-tabs-mode: nil; c-basic-offset: 2 -*-
*
* The contents of this file are subject to the Mozilla Public License
* Version 1.0 (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.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS"
* basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
* the License for the specific language governing rights and limitations
* under the License.
*
* The Original Code is the Grendel mail/news client.
*
* The Initial Developer of the Original Code is Netscape Communications
* Corporation. Portions created by Netscape are Copyright (C) 1997
* Netscape Communications Corporation. All Rights Reserved.
*
* Created: Terry Weissman <terry@netscape.com>, 26 Sep 1997.
*/
package grendel.storage.intertwingle;
import calypso.util.Assert;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Enumeration;
import java.util.Vector;
/** This implements a RDF-ish database, where changes take very little time.
It works by queueing up those changes, and having a low-priority
background thread process that queue into real changes. The pending changes
also get reflected into a disk file, so that if we crash, we'll recover
unfinished stuff when the app next starts.
<p>
Queries on the database will search the queue for pending changes that
could affect that query, and will commit those changes immediately. */
public final class BGDB extends BaseDB implements Runnable {
static final boolean DEBUG = false;
static void Spew(String s) {
if (DEBUG) System.err.println("BGDB: " + s);
}
class Command {
public String command;
public String name;
public String slot;
public String value;
Command(String c, String n, String s, String v) {
command = c;
name = n;
slot = s;
value = v;
}
public String toString() {
return command + ": '" + name + "','" + slot + "','" + value + "'";
}
};
DB base;
File log;
RandomAccessFile logfid;
Vector queue = new Vector();
Thread thread;
BGDB(DB d, File t) throws IOException {
log = t;
base = d;
logfid = new RandomAccessFile(log, "rw");
long length = logfid.length();
while (logfid.getFilePointer() < length) {
try {
queue.addElement(new Command(logfid.readUTF(),
logfid.readUTF(),
logfid.readUTF(),
logfid.readUTF()));
} catch (IOException e) {
break;
}
}
thread = new Thread(this);
thread.setDaemon(true);
thread.setPriority(Thread.NORM_PRIORITY - 1);
thread.start();
}
public synchronized void assert(String name, String slot, String value)
throws IOException
{
logfid.seek(logfid.length());
logfid.writeUTF("assert");
logfid.writeUTF(name);
logfid.writeUTF(slot);
logfid.writeUTF(value);
queue.addElement(new Command("assert", name, slot, value));
notifyAll(); // Inform bg thread it has work to do.
}
public synchronized void unassert(String name, String slot, String value)
throws IOException
{
logfid.seek(logfid.length());
logfid.writeUTF("unassert");
logfid.writeUTF(name);
logfid.writeUTF(slot);
logfid.writeUTF(value);
queue.addElement(new Command("unassert", name, slot, value));
notifyAll(); // Inform bg thread it has work to do.
}
public synchronized String findFirst(String name, String slot,
boolean reverse) throws IOException
{
flushChanges(name, slot, reverse);
return base.findFirst(name, slot, reverse);
}
public synchronized Enumeration findAll(String name, String slot,
boolean reverse)
throws IOException
{
flushChanges(name, slot, reverse);
return base.findAll(name, slot, reverse);
}
private void flushChanges(String name, String slot, boolean reverse)
throws IOException
{
for (int i=0 ; i<queue.size() ; i++) {
Command c = (Command) queue.elementAt(i);
while (slot.equals(c.slot) && name.equals(reverse ? c.value : c.name)) {
flushCommand(c);
queue.removeElementAt(i);
if (i >= queue.size()) break;
c = (Command) queue.elementAt(i);
}
}
}
private void truncateLog() {
try {
logfid.close();
} catch (IOException e) {
// ### What to do...
System.out.println("logfid.close() failed in BGDB.truncateLog: " + e);
}
logfid = null;
log.delete();
try {
logfid = new RandomAccessFile(log, "rw");
} catch (IOException e) {
// ### What to do...
System.out.println("opening logfid failed in BGDB.truncateLog: " + e);
}
}
public synchronized void flushChanges() throws IOException {
while (queue.size() > 0) {
flushCommand((Command) queue.elementAt(0));
queue.removeElementAt(0);
}
truncateLog();
}
private void flushCommand(Command c) throws IOException {
if (DEBUG) {
String who;
Spew(((Thread.currentThread() == thread) ? "bg" : "fg") + " executing " +
c);
}
if (c.command.equals("assert")) {
base.assert(c.name, c.slot, c.value);
} else if (c.command.equals("unassert")) {
base.unassert(c.name, c.slot, c.value);
} else {
Assert.NotReached("Unknown command type in BGDB.flushCommand");
}
}
public void run() {
Spew("bg Startup.");
for (;;) {
synchronized(this) {
if (queue.size() > 0) {
try {
flushCommand((Command) queue.elementAt(0));
} catch (IOException e) {
// ### What to do...
System.out.println("flushCommand failed in BGDB.run: " + e);
}
queue.removeElementAt(0);
} else {
truncateLog();
try {
Spew("bg waiting...");
wait();
Spew("bg awake.");
} catch (InterruptedException e) {
return; // ### Is this right???
}
}
}
}
}
}