# 24. Semaphores

db4o Semaphores are named flags that can only be owned by one client/transaction at one time. They are supplied to be used as locks for exclusive access to code blocks in applications and to signal states from one client to the server and to all other clients.
The naming of semaphores is up to the application. Any string can be used.
Semaphores are freed automatically when a client disconnects correctly or when a clients presence is no longer detected by the server, that constantly monitors all client connections.
Semaphores can be set and released with the following two methods:

`ExtObjectContainer#setSemaphore(String name, int waitMilliSeconds); ExtObjectContainer#releaseSemaphore(String name);`

The concept of db4o semaphores is very similar to the concept of synchronization in OO programming languages:
Java

`synchronized(monitorObject){ // exclusive code block here }`

C#

`lock(monitorObject){ // exclusive code block here }`

db4o semaphore

`if(objectContainer.ext().setSemaphore(semaphoreName, 1000){ // exclusive code block here objectContainer.ext().releaseSemaphore(semaphoreName); }`

Although the principle of semaphores is very simple they are powerful enough for a wide range of usecases:

## 24.1. Locking objects

`import com.db4o.*;
import com.db4o.ext.*;
/**
* This class demonstrates a very rudimentary implementation
* of virtual "locks" on objects with db4o. All code that is
* intended to obey these locks will have to call lock() and
* unlock().
*/
public class LockManager {
private final String SEMAPHORE_NAME = "locked: ";
private final int WAIT_FOR_AVAILABILITY = 300; // 300 milliseconds
private final ExtObjectContainer _objectContainer;
public LockManager(ObjectContainer objectContainer){
_objectContainer = objectContainer.ext();
}
public boolean lock(Object obj){
long id = _objectContainer.getID(obj);
return _objectContainer.setSemaphore(SEMAPHORE_NAME + id, WAIT_FOR_AVAILABILITY);
}
public void unlock(Object obj){
long id = _objectContainer.getID(obj);
_objectContainer.releaseSemaphore(SEMAPHORE_NAME + id);
}
}`

## 24.2. Ensuring Singletons

`import com.db4o.*;import com.db4o.query.*;
/**
* This class demonstrates the use of a semaphore to ensure that only
* one instance of a certain class is stored to an ObjectContainer.
*
* Caution !!! The getSingleton method contains a commit() call.
*/
public class Singleton {
/**
* returns a singleton object of one class for an ObjectContainer.
* <br><b>Caution !!! This method contains a commit() call.</b>
*/
public static Object getSingleton(ObjectContainer objectContainer, Class clazz) {
Object obj = queryForSingletonClass(objectContainer, clazz);
if (obj != null) {
return obj;
}
String semaphore = "Singleton#getSingleton_" + clazz.getName();
if (!objectContainer.ext().setSemaphore(semaphore, 10000)) {
throw new RuntimeException("Blocked semaphore " + semaphore);
}
obj = queryForSingletonClass(objectContainer, clazz);
if (obj == null) {
try {
obj = clazz.newInstance();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
objectContainer.set(obj);
/* !!! CAUTION !!!
* There is a commit call here.
*
* The commit call is necessary, so other transactions
* can see the new inserted object.
*/
objectContainer.commit();
}
objectContainer.ext().releaseSemaphore(semaphore);
return obj;
}
private static Object queryForSingletonClass(ObjectContainer objectContainer, Class clazz) {
Query q = objectContainer.query();
q.constrain(clazz);
ObjectSet objectSet = q.execute();
if (objectSet.size() == 1) {
return objectSet.next();
}
if (objectSet.size() > 1) {
throw new RuntimeException(
"Singleton problem. Multiple instances of: " + clazz.getName());
}
return null;
}
}`

## 24.3. Limiting the number of users

`import java.io.*;import com.db4o.*;
/**
* This class demonstrates the use of semaphores to limit the
* number of logins to a server.
*/
public class LimitLogins {
static final String HOST = "localhost";
static final int PORT = 4455;
static final String USER = "db4o";
static final String PASSWORD = "db4o";
static final int MAXIMUM_USERS = 10;
public static ObjectContainer login(){
ObjectContainer objectContainer;
try {
objectContainer = Db4o.openClient(HOST, PORT, USER, PASSWORD);
} catch (IOException e) {
return null;
}
boolean allowedToLogin = false;
for (int i = 0; i < MAXIMUM_USERS; i++) {
if(objectContainer.ext().setSemaphore("max_user_check_" + i, 0)){
allowedToLogin = true;
break;
}
}
if(! allowedToLogin){
objectContainer.close();
return null;
}
return objectContainer;
}
}`

## 24.4. Controlling log-in information

`import java.util.*;import com.db4o.*;
import com.db4o.config.*;
import com.db4o.ext.*;
import com.db4o.query.*;
/**
* This class demonstrates how semaphores can be used
* to rule out race conditions when providing exact and
* up-to-date information about all connected clients
* on a server. The class also can be used to make sure
* that only one login is possible with a give user name
* and ipAddress combination.
*/
public class ConnectedUser {
static final String SEMAPHORE_CONNECTED = "ConnectedUser_";
static final String SEMAPHORE_LOCK_ACCESS = "ConnectedUser_Lock_";
static final int TIMEOUT = 10000; // concurrent access timeout 10 seconds
String userName;
String ipAddress;
public ConnectedUser(String userName, String ipAddress){
this.userName = userName;
this.ipAddress = ipAddress;
}
// make sure to call this on the server before opening the database
// to improve querying speed
public static void configure(){
ObjectClass objectClass = Db4o.configure().objectClass(ConnectedUser.class);
objectClass.objectField("userName").indexed(true);
objectClass.objectField("ipAddress").indexed(true);
}
// call this on the client to ensure to have a ConnectedUser record
// in the database file and the semaphore set
public static void login(ObjectContainer client, String userName, String ipAddress){
if(! client.ext().setSemaphore(SEMAPHORE_LOCK_ACCESS, TIMEOUT)){
throw new RuntimeException("Timeout trying to get access to ConnectedUser lock");
}
Query q = client.query();
q.constrain(ConnectedUser.class);
q.descend("userName").constrain(userName);
q.descend("ipAddress").constrain(ipAddress);
if(q.execute().size() == 0){
client.set(new ConnectedUser(userName, ipAddress));
client.commit();
}
String connectedSemaphoreName = SEMAPHORE_CONNECTED + userName + ipAddress;
boolean unique = client.ext().setSemaphore(connectedSemaphoreName, 0);
client.ext().releaseSemaphore(SEMAPHORE_LOCK_ACCESS);
if(! unique){
throw new RuntimeException("Two clients with same userName and ipAddress");
}
}
// here is your list of all connected users, callable on the server
public static List connectedUsers(ObjectServer server){
ExtObjectContainer serverObjectContainer = server.ext().objectContainer().ext();
if(serverObjectContainer.setSemaphore(SEMAPHORE_LOCK_ACCESS, TIMEOUT)){
throw new RuntimeException("Timeout trying to get access to ConnectedUser lock");
}
List list = new ArrayList();
Query q = serverObjectContainer.query();
q.constrain(ConnectedUser.class);
ObjectSet objectSet = q.execute();
while(objectSet.hasNext()){
ConnectedUser connectedUser = (ConnectedUser)objectSet.next();
String connectedSemaphoreName =
SEMAPHORE_CONNECTED +
connectedUser.userName +
connectedUser.ipAddress;
if(serverObjectContainer.setSemaphore(connectedSemaphoreName, TIMEOUT)){
serverObjectContainer.delete(connectedUser);
}else{
list.add(connectedUser);
}
}
serverObjectContainer.commit();
serverObjectContainer.releaseSemaphore(SEMAPHORE_LOCK_ACCESS);
return list;
}
}`

--
generated by Doctor courtesy of db4objects Inc.