I am trying to define a monitor in which I receieve events and then handle them on multiple contexts (roughly equating to threads if I understand correctly) I know I can write
spawn myAction() to myNewContext;
and this will run that action in the new context.
However I want to have an action which will respond to an event when it comes into my monitor:
on all trigger() as t {
doMyThing()
}
on all otherTrigger() as ot {
doMyOtherThing()
}
Can I define my on all in a way that uses a specific context? Something like
on all trigger() as t in myContext {
doMyThing()
}
on all otherTrigger() as t in myOtherContext {
doMyOtherThing()
}
If not what is the best way to define this in Apama EPL? Also could I have multiple contexts handling the same events when they arrive, round robin style?
Apama events from external receivers (ie the outside world) are delivered only to public contexts, including the 'main' context. So depending on your architecture, you can either spawn your action to a public context
// set the receivesInput parameter to true to make this context public
spawn myAction() to context("myContext", true);
...
action myAction() {
on all trigger() as t {
doMyThing();
}
}
or, spawn your action to a private context and set up an event forwarder in a public context, usually the main context (which will always exist)
spawn myAction() to context("myNewContext");
on all trigger() as t {
send t to "myChannel"; // forward all trigger events to the "myChannel" channel
}
...
action myAction() {
monitor.subscribe("myChannel"); // receive all events delivered to the "myChannel" channel
on all trigger() as t {
doMyThing();
}
}
Spawning to a private context and leveraging the channels system is generally the better design as it only sends events to contexts that care about them
To extend a bit on Madden's answer (I don't have enough rep to comment yet), the private context and forwarders is also the only way to achieve true round-robin: otherwise all contexts will receive all events. The easiest approach is to use a partitioning strategy (e.g. IDs ending in 0 go to context-0, or you have one context per machine you're monitoring, etc.), because then each concern is tracked in the same context and you don't have to share state.
Also could I have multiple contexts handling the same events when they arrive, round robin style?
This isn't entirely clear to me. What benefit are you aiming for here? If you're looking to reduce latency by having the "next available" context pick up the event, this probably isn't the right way to achieve it - the deciding which context processes the event means you'd need inter-context communications and coordination, which will increase latency. If you want multiple contexts to process the same events (e.g. one context runs your temperature spike rule, and another runs your long-term temperature average rule, but both take temperature readings as inputs), then that's a good approach but it's not what I'd have called round-robin.
I profiled my kafka producer spring boot application and found many "kafka-producer-network-thread"s running (47 in total). Which would never stop running, even when no data is sending.
My application looks a bit like this:
var kafkaSender = KafkaSender(kafkaTemplate, applicationProperties)
kafkaSender.sendToKafka(json, rs.getString("KEY"))
with the KafkaSender:
#Service
class KafkaSender(val kafkaTemplate: KafkaTemplate<String, String>, val applicationProperties: ApplicationProperties) {
#Transactional(transactionManager = "kafkaTransactionManager")
fun sendToKafka(message: String, stringKey: String) {
kafkaTemplate.executeInTransaction { kt ->
kt.send(applicationProperties.kafka.topic, System.currentTimeMillis().mod(10).toInt(), System.currentTimeMillis().rem(10).toString(),
message)
}
}
companion object {
val log = LoggerFactory.getLogger(KafkaSender::class.java)!!
}
}
Since each time I want to send a message to Kafka I instantiate a new KafkaSender, I thought a new thread would be created which then sends the message to the kafka queue.
Currently it looks like a pool of producers is generated, but never cleaned up, even when none of them has anything to do.
Is this behaviour intended?
In my opinion the behaviour should be nearly the same as datasource pooling, keep the thread alive for some time, but when there is nothing to do, clear it up.
When using transactions, the producer cache grows on demand and is not reduced.
If you are producing messages on a listener container (consumer) thread; there is a producer for each topic/partition/consumer group. This is required to solve the zombie fencing problem, so that if a rebalance occurs and the partition moves to a different instance, the transaction id will remain the same so the broker can properly handle the situation.
If you don't care about the zombie fencing problem (and you can handle duplicate deliveries), set the producerPerConsumerPartition property to false on the DefaultKafkaProducerFactory and the number of producers will be much smaller.
EDIT
Starting with version 2.8 the default EOSMode is now V2 (aka BETA); which means it is no longer necessary to have a producer per topic/partition/group - as long as the broker version is 2.5 or later.
Even after reading plenty of SO questions (1,2) and articles, It is unclear on which is the better option to set for consumers. Multiple consumers or a higher prefetch value?
From what I understand, when it comes to SimpleRabbitListenerContainerFactory, as it was designed initially to have only one thread per connection it was designed to address a limitation that the amqp-client only had one thread per connection, does that mean that setting multiple consumers won't make much difference as there is only one thread that actually consumes from rabbit and than hands it off to the multiple consumers (threads)?
Or there are actually several consumers consuming at the same time?
So what is the best practice when it comes to spring implementation of rabbit concerning prefetch/consumers? When should one be used over the other? And should I switch to this new DirectRabbitListenerContainerFactory? Is it 'better' or just depends on the use case?
Some downsides I see when it comes to high prefetch is that maybe it can cause memory issues if an app consumes more messages that it can hold in the buffer? (haven't actually tested this yet, tbh)
And when it comes to multiple consumers, I see the downside of having more file descriptors opened on OS level and I saw this article about that each consumer actually pings rabbit for each ack and this making it slower.
FYI, if it is relevant, I usually have my config set up like this:
#Bean
public ConnectionFactory connectionFactory() {
final CachingConnectionFactory connectionFactory = new CachingConnectionFactory(server);
connectionFactory.setUsername(username);
connectionFactory.setPassword(password);
connectionFactory.setVirtualHost(virtualHost);
connectionFactory.setRequestedHeartBeat(requestedHeartBeat);
return connectionFactory;
}
#Bean
public AmqpAdmin amqpAdmin() {
AmqpAdmin admin = new RabbitAdmin(connectionFactory());
admin.declareQueue(getRabbitQueue());
return admin;
}
#Bean
public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory() {
final SimpleRabbitListenerContainerFactory factory = new SimpleRabbitListenerContainerFactory();
factory.setConnectionFactory(connectionFactory());
factory.setConcurrentConsumers(concurrency);
factory.setMaxConcurrentConsumers(maxConcurrency);
factory.setPrefetchCount(prefetch);
factory.setMissingQueuesFatal(false);
return factory;
}
#Bean
public Queue getRabbitQueue() {
final Map<String, Object> p = new HashMap<String, Object>();
p.put("x-max-priority", 10);
return new Queue(queueName, true, false, false, p);
}
No; the SMLC wasn't "designed for one thread per connection" it was designed to address a limitation that the amqp-client only had one thread per connection so that thread hands off to consumer threads via an in-memory queue; that is no longer the case. The client is multi-threaded and there is one dedicated thread per consumer.
Having multiple consumers (increasing the concurrency) is completely effective (and was, even with the older client).
Prefetch is really to reduce network chatter and improve overall throughput. Whether you need to increase concurrency really is orthogonal to prefetch. You would typically increase concurrency if (a) your listener is relatively slow to process each message and (b) strict message ordering is not important.
The DirectListenerContainer was introduced to provide a different threading model, where the listener is invoked directly on the amqp-client thread.
The reasons for choosing one container over the other is described in Choosing a Container.
The following features are available with the SMLC, but not the DMLC:
txSize - with the SMLC, you can set this to control how many messages are delivered in a transaction and/or to reduce the number of acks, but it may cause the number of duplicate deliveries to increase after a failure. (The DMLC does have mesagesPerAck which can be used to reduce the acks, the same as with txSize and the SMLC, but it can’t be used with transactions - each message is delivered and ack’d in a separate transaction).
maxConcurrentConsumers and consumer scaling intervals/triggers - there is no auto-scaling in the DMLC; it does, however, allow you to programmatically change the consumersPerQueue property and the consumers will be adjusted accordingly.
However, the DMLC has the following benefits over the SMLC:
Adding and removing queues at runtime is more efficient; with the SMLC, the entire consumer thread is restarted (all consumers canceled and re-created); with the DMLC, unaffected consumers are not canceled.
The context switch between the RabbitMQ Client thread and the consumer thread is avoided.
Threads are shared across consumers rather than having a dedicated thread for each consumer in the SMLC. However, see the IMPORTANT note about the connection factory configuration in the section called “Threading and Asynchronous Consumers”.
Consider a scenario in which I am implementing a system that processes incoming tasks using Akka. I have a primary actor that receives tasks and dispatches them to some worker actors that process the tasks.
My first instinct is to implement this by having the dispatcher create an actor for each incoming task. After the worker actor processes the task it is stopped.
This seems to be the cleanest solution for me since it adheres to the principle of "one task, one actor". The other solution would be to reuse actors - but this involves the extra-complexity of cleanup and some pool management.
I know that actors in Akka are cheap. But I am wondering if there is an inherent cost associated with repeated creation and deletion of actors. Is there any hidden cost associated with the data structures Akka uses for the bookkeeping of actors ?
The load should be of the order of tens or hundreds of tasks per second - think of it as a production webserver that creates one actor per request.
Of course, the right answer lies in the profiling and fine tuning of the system based on the type of the incoming load.
But I wondered if anyone could tell me something from their own experience ?
LATER EDIT:
I should given more details about the task at hand:
Only N active tasks can run at some point. As #drexin pointed out - this would be easily solvable using routers. However, the execution of tasks isn't a simple run and be done type of thing.
Tasks may require information from other actors or services and thus may have to wait and become asleep. By doing so they release an execution slot. The slot can be taken by another waiting actor which now has the opportunity to run. You could make an analogy with the way processes are scheduled on one CPU.
Each worker actor needs to keep some state regarding the execution of the task.
Note: I appreciate alternative solutions to my problem, and I will certainly take them into consideration. However, I would also like an answer to the main question regarding the intensive creation and deletion of actors in Akka.
You should not create an actor for every request, you should rather use a router to dispatch the messages to a dynamic amount of actors. That's what routers are for. Read this part of the docs for more information: http://doc.akka.io/docs/akka/2.0.4/scala/routing.html
edit:
Creating top-level actors (system.actorOf) is expensive, because every top-level actor will initialize an error kernel as well and those are expensive. Creating child actors (inside an actor context.actorOf) is way cheaper.
But still I suggest you to rethink this, because depending on the frequency of the creation and deletion of actors you will also put afditional pressure on the GC.
edit2:
And most important, actors are not threads! So even if you create 1M actors, they will only run on as many threads as the pool has. So depending on the throughput setting in the config every actor will process n messages before the thread gets released to the pool again.
Note that blocking a thread (includes sleeping) will NOT return it to the pool!
An actor which will receive one message right after its creation and die right after sending the result can be replaced by a future. Futures are more lightweight than actors.
You can use pipeTo to receive the future result when its done. For instance in your actor launching the computations:
def receive = {
case t: Task => future { executeTask( t ) }.pipeTo(self)
case r: Result => processTheResult(r)
}
where executeTask is your function taking a Task to return a Result.
However, I would reuse actors from a pool through a router as explained in #drexin answer.
I've tested with 10000 remote actors created from some main context by a root actor, same scheme as in prod module a single actor was created. MBP 2.5GHz x2:
in main: main ? root // main asks root to create an actor
in main: actorOf(child) // create a child
in root: watch(child) // watch lifecycle messages
in root: root ? child // wait for response (connection check)
in child: child ! root // response (connection ok)
in root: root ! main // notify created
Code:
def start(userName: String) = {
logger.error("HELLOOOOOOOO ")
val n: Int = 10000
var t0, t1: Long = 0
t0 = System.nanoTime
for (i <- 0 to n) {
val msg = StartClient(userName + i)
Await.result(rootActor ? msg, timeout.duration).asInstanceOf[ClientStarted] match {
case succ # ClientStarted(userName) =>
// logger.info("[C][SUCC] Client started: " + succ)
case _ =>
logger.error("Terminated on waiting for response from " + i + "-th actor")
throw new RuntimeException("[C][FAIL] Could not start client: " + msg)
}
}
t1 = System.nanoTime
logger.error("Starting of a single actor of " + n + ": " + ((t1 - t0) / 1000000.0 / n.toDouble) + " ms")
}
The result:
Starting of a single actor of 10000: 0.3642917 ms
There was a message stating that "Slf4jEventHandler started" between "HELOOOOOOOO" and "Starting of a single", so the experiment seems even more realistic (?)
Dispatchers was a default (a PinnedDispatcher starting a new thread each and every time), and it seemed like all that stuff is the same as Thread.start() was, for a long long time since Java 1 - 500K-1M cycles or so ^)
That's why I've changed all code inside loop, to a new java.lang.Thread().start()
The result:
Starting of a single actor of 10000: 0.1355219 ms
Actors make great finite state machines so let that help drive your design here. If your request handling state is greatly simplified by having one actor per request then do that. I find that actors are particularly good at managing more than two states as a rule of thumb.
Commonly though, one request handling actor that references request state from within a collection that it maintains as part of its own state is a common approach. Note that this can also be achieved with an Akka reactive stream and the use of the scan stage.
Recently I tried to Access a textbox from a thread (other than the UI thread) and an exception was thrown. It said something about the "code not being thread safe" and so I ended up writing a delegate (sample from MSDN helped) and calling it instead.
But even so I didn't quite understand why all the extra code was necessary.
Update:
Will I run into any serious problems if I check
Controls.CheckForIllegalCrossThread..blah =true
Eric Lippert has a nice blog post entitled What is this thing you call "thread safe"? about the definition of thread safety as found of Wikipedia.
3 important things extracted from the links :
“A piece of code is thread-safe if it functions correctly during
simultaneous execution by multiple threads.”
“In particular, it must satisfy the need for multiple threads to
access the same shared data, …”
“…and the need for a shared piece of data to be accessed by only one
thread at any given time.”
Definitely worth a read!
In the simplest of terms threadsafe means that it is safe to be accessed from multiple threads. When you are using multiple threads in a program and they are each attempting to access a common data structure or location in memory several bad things can happen. So, you add some extra code to prevent those bad things. For example, if two people were writing the same document at the same time, the second person to save will overwrite the work of the first person. To make it thread safe then, you have to force person 2 to wait for person 1 to complete their task before allowing person 2 to edit the document.
Wikipedia has an article on Thread Safety.
This definitions page (you have to skip an ad - sorry) defines it thus:
In computer programming, thread-safe describes a program portion or routine that can be called from multiple programming threads without unwanted interaction between the threads.
A thread is an execution path of a program. A single threaded program will only have one thread and so this problem doesn't arise. Virtually all GUI programs have multiple execution paths and hence threads - there are at least two, one for processing the display of the GUI and handing user input, and at least one other for actually performing the operations of the program.
This is done so that the UI is still responsive while the program is working by offloading any long running process to any non-UI threads. These threads may be created once and exist for the lifetime of the program, or just get created when needed and destroyed when they've finished.
As these threads will often need to perform common actions - disk i/o, outputting results to the screen etc. - these parts of the code will need to be written in such a way that they can handle being called from multiple threads, often at the same time. This will involve things like:
Working on copies of data
Adding locks around the critical code
Opening files in the appropriate mode - so if reading, don't open the file for write as well.
Coping with not having access to resources because they're locked by other threads/processes.
Simply, thread-safe means that a method or class instance can be used by multiple threads at the same time without any problems occurring.
Consider the following method:
private int myInt = 0;
public int AddOne()
{
int tmp = myInt;
tmp = tmp + 1;
myInt = tmp;
return tmp;
}
Now thread A and thread B both would like to execute AddOne(). but A starts first and reads the value of myInt (0) into tmp. Now for some reason, the scheduler decides to halt thread A and defer execution to thread B. Thread B now also reads the value of myInt (still 0) into it's own variable tmp. Thread B finishes the entire method so in the end myInt = 1. And 1 is returned. Now it's Thread A's turn again. Thread A continues. And adds 1 to tmp (tmp was 0 for thread A). And then saves this value in myInt. myInt is again 1.
So in this case the method AddOne() was called two times, but because the method was not implemented in a thread-safe way the value of myInt is not 2, as expected, but 1 because the second thread read the variable myInt before the first thread finished updating it.
Creating thread-safe methods is very hard in non-trivial cases. And there are quite a few techniques. In Java you can mark a method as synchronized, this means that only one thread can execute that method at a given time. The other threads wait in line. This makes a method thread-safe, but if there is a lot of work to be done in a method, then this wastes a lot of space. Another technique is to 'mark only a small part of a method as synchronized' by creating a lock or semaphore, and locking this small part (usually called the critical section). There are even some methods that are implemented as lock-less thread-safe, which means that they are built in such a way that multiple threads can race through them at the same time without ever causing problems, this can be the case when a method only executes one atomic call. Atomic calls are calls that can't be interrupted and can only be done by one thread at a time.
In real world example for the layman is
Let's suppose you have a bank account with the internet and mobile banking and your account have only $10.
You performed transfer balance to another account using mobile banking, and the meantime, you did online shopping using the same bank account.
If this bank account is not threadsafe, then the bank allows you to perform two transactions at the same time and then the bank will become bankrupt.
Threadsafe means that an object's state doesn't change if simultaneously multiple threads try to access the object.
You can get more explanation from the book "Java Concurrency in Practice":
A class is thread‐safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.
A module is thread-safe if it guarantees it can maintain its invariants in the face of multi-threaded and concurrence use.
Here, a module can be a data-structure, class, object, method/procedure or function. Basically scoped piece of code and related data.
The guarantee can potentially be limited to certain environments such as a specific CPU architecture, but must hold for those environments. If there is no explicit delimitation of environments, then it is usually taken to imply that it holds for all environments that the code can be compiled and executed.
Thread-unsafe modules may function correctly under mutli-threaded and concurrent use, but this is often more down to luck and coincidence, than careful design. Even if some module does not break for you under, it may break when moved to other environments.
Multi-threading bugs are often hard to debug. Some of them only happen occasionally, while others manifest aggressively - this too, can be environment specific. They can manifest as subtly wrong results, or deadlocks. They can mess up data-structures in unpredictable ways, and cause other seemingly impossible bugs to appear in other remote parts of the code. It can be very application specific, so it is hard to give a general description.
Thread safety: A thread safe program protects it's data from memory consistency errors. In a highly multi-threaded program, a thread safe program does not cause any side effects with multiple read/write operations from multiple threads on same objects. Different threads can share and modify object data without consistency errors.
You can achieve thread safety by using advanced concurrency API. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Executors define a high-level API for launching and managing threads. Executor implementations provided by java.util.concurrent provide thread pool management suitable for large-scale applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
Producing Thread-safe code is all about managing access to shared mutable states. When mutable states are published or shared between threads, they need to be synchronized to avoid bugs like race conditions and memory consistency errors.
I recently wrote a blog about thread safety. You can read it for more information.
You are clearly working in a WinForms environment. WinForms controls exhibit thread affinity, which means that the thread in which they are created is the only thread that can be used to access and update them. That is why you will find examples on MSDN and elsewhere demonstrating how to marshall the call back onto the main thread.
Normal WinForms practice is to have a single thread that is dedicated to all your UI work.
I find the concept of http://en.wikipedia.org/wiki/Reentrancy_%28computing%29 to be what I usually think of as unsafe threading which is when a method has and relies on a side effect such as a global variable.
For example I have seen code that formatted floating point numbers to string, if two of these are run in different threads the global value of decimalSeparator can be permanently changed to '.'
//built in global set to locale specific value (here a comma)
decimalSeparator = ','
function FormatDot(value : real):
//save the current decimal character
temp = decimalSeparator
//set the global value to be
decimalSeparator = '.'
//format() uses decimalSeparator behind the scenes
result = format(value)
//Put the original value back
decimalSeparator = temp
To understand thread safety, read below sections:
4.3.1. Example: Vehicle Tracker Using Delegation
As a more substantial example of delegation, let's construct a version of the vehicle tracker that delegates to a thread-safe class. We store the locations in a Map, so we start with a thread-safe Map implementation, ConcurrentHashMap. We also store the location using an immutable Point class instead of MutablePoint, shown in Listing 4.6.
Listing 4.6. Immutable Point class used by DelegatingVehicleTracker.
class Point{
public final int x, y;
public Point() {
this.x=0; this.y=0;
}
public Point(int x, int y) {
this.x = x;
this.y = y;
}
}
Point is thread-safe because it is immutable. Immutable values can be freely shared and published, so we no longer need to copy the locations when returning them.
DelegatingVehicleTracker in Listing 4.7 does not use any explicit synchronization; all access to state is managed by ConcurrentHashMap, and all the keys and values of the Map are immutable.
Listing 4.7. Delegating Thread Safety to a ConcurrentHashMap.
public class DelegatingVehicleTracker {
private final ConcurrentMap<String, Point> locations;
private final Map<String, Point> unmodifiableMap;
public DelegatingVehicleTracker(Map<String, Point> points) {
this.locations = new ConcurrentHashMap<String, Point>(points);
this.unmodifiableMap = Collections.unmodifiableMap(locations);
}
public Map<String, Point> getLocations(){
return this.unmodifiableMap; // User cannot update point(x,y) as Point is immutable
}
public Point getLocation(String id) {
return locations.get(id);
}
public void setLocation(String id, int x, int y) {
if(locations.replace(id, new Point(x, y)) == null) {
throw new IllegalArgumentException("invalid vehicle name: " + id);
}
}
}
If we had used the original MutablePoint class instead of Point, we would be breaking encapsulation by letting getLocations publish a reference to mutable state that is not thread-safe. Notice that we've changed the behavior of the vehicle tracker class slightly; while the monitor version returned a snapshot of the locations, the delegating version returns an unmodifiable but “live” view of the vehicle locations. This means that if thread A calls getLocations and thread B later modifies the location of some of the points, those changes are reflected in the Map returned to thread A.
4.3.2. Independent State Variables
We can also delegate thread safety to more than one underlying state variable as long as those underlying state variables are independent, meaning that the composite class does not impose any invariants involving the multiple state variables.
VisualComponent in Listing 4.9 is a graphical component that allows clients to register listeners for mouse and keystroke events. It maintains a list of registered listeners of each type, so that when an event occurs the appropriate listeners can be invoked. But there is no relationship between the set of mouse listeners and key listeners; the two are independent, and therefore VisualComponent can delegate its thread safety obligations to two underlying thread-safe lists.
Listing 4.9. Delegating Thread Safety to Multiple Underlying State Variables.
public class VisualComponent {
private final List<KeyListener> keyListeners
= new CopyOnWriteArrayList<KeyListener>();
private final List<MouseListener> mouseListeners
= new CopyOnWriteArrayList<MouseListener>();
public void addKeyListener(KeyListener listener) {
keyListeners.add(listener);
}
public void addMouseListener(MouseListener listener) {
mouseListeners.add(listener);
}
public void removeKeyListener(KeyListener listener) {
keyListeners.remove(listener);
}
public void removeMouseListener(MouseListener listener) {
mouseListeners.remove(listener);
}
}
VisualComponent uses a CopyOnWriteArrayList to store each listener list; this is a thread-safe List implementation particularly suited for managing listener lists (see Section 5.2.3). Each List is thread-safe, and because there are no constraints coupling the state of one to the state of the other, VisualComponent can delegate its thread safety responsibilities to the underlying mouseListeners and keyListeners objects.
4.3.3. When Delegation Fails
Most composite classes are not as simple as VisualComponent: they have invariants that relate their component state variables. NumberRange in Listing 4.10 uses two AtomicIntegers to manage its state, but imposes an additional constraint—that the first number be less than or equal to the second.
Listing 4.10. Number Range Class that does Not Sufficiently Protect Its Invariants. Don't do this.
public class NumberRange {
// INVARIANT: lower <= upper
private final AtomicInteger lower = new AtomicInteger(0);
private final AtomicInteger upper = new AtomicInteger(0);
public void setLower(int i) {
//Warning - unsafe check-then-act
if(i > upper.get()) {
throw new IllegalArgumentException(
"Can't set lower to " + i + " > upper ");
}
lower.set(i);
}
public void setUpper(int i) {
//Warning - unsafe check-then-act
if(i < lower.get()) {
throw new IllegalArgumentException(
"Can't set upper to " + i + " < lower ");
}
upper.set(i);
}
public boolean isInRange(int i){
return (i >= lower.get() && i <= upper.get());
}
}
NumberRange is not thread-safe; it does not preserve the invariant that constrains lower and upper. The setLower and setUpper methods attempt to respect this invariant, but do so poorly. Both setLower and setUpper are check-then-act sequences, but they do not use sufficient locking to make them atomic. If the number range holds (0, 10), and one thread calls setLower(5) while another thread calls setUpper(4), with some unlucky timing both will pass the checks in the setters and both modifications will be applied. The result is that the range now holds (5, 4)—an invalid state. So while the underlying AtomicIntegers are thread-safe, the composite class is not. Because the underlying state variables lower and upper are not independent, NumberRange cannot simply delegate thread safety to its thread-safe state variables.
NumberRange could be made thread-safe by using locking to maintain its invariants, such as guarding lower and upper with a common lock. It must also avoid publishing lower and upper to prevent clients from subverting its invariants.
If a class has compound actions, as NumberRange does, delegation alone is again not a suitable approach for thread safety. In these cases, the class must provide its own locking to ensure that compound actions are atomic, unless the entire compound action can also be delegated to the underlying state variables.
If a class is composed of multiple independent thread-safe state variables and has no operations that have any invalid state transitions, then it can delegate thread safety to the underlying state variables.