The producer-consumer botheration is a archetypal archetype of a multi-process synchronization problem. For best of us, this botheration is maybe the aboriginal synchronization botheration that we advised aback in academy and were adverse alongside algorithms for the aboriginal time. Simple as it is, it resumes the bigger claiming in alongside accretion — the administration of a distinct ability by assorted processes.
There are two processes, a ambassador and a consumer, that allotment a accepted absorber with a bound size. The ambassador “produces” abstracts and food it in the buffer, and the customer “consumes” the data, removing it from the buffer. Accepting two processes that run in parallel, we charge to accomplish abiding that the ambassador will not put new abstracts in the absorber back the absorber is abounding and the customer won’t try to aish abstracts from the absorber if the absorber is empty.
For analytic this accommodation problem, the ambassador and the customer will accept to acquaint with anniversary other. If the absorber is full, the ambassador will go to beddy-bye and will delay to be notified. After the customer will aish some abstracts from the buffer, it will acquaint the producer, and then, the ambassador will alpha refilling the absorber again. The aforementioned action will appear if the absorber is empty, but in this case, the customer will delay to be notified by the producer.
If this advice is not done properly, it can advance to a deadlock area both processes will delay for anniversary other.
Let’s see a archetypal Java band-aid to this problem.
Here we accept two threads, a producer and a customer thread, which allotment a accepted buffer. The ambassador cilia starts bearing new elements and food them in the buffer. If the absorber is full, it goes to beddy-bye and will delay to be notified. Otherwise, it will put a new aspect in the absorber and acquaint the consumer. Like I said before, the aforementioned action applies to the consumer. If the absorber is empty, the customer will delay to be notified by the producer. Otherwise, it will aish an aspect from the buffer and it will acquaint the consumer.
As you can see, in the antecedent example, both jobs are managed by the buffer object. The accoutrement are aloof calling buffer.produce() and buffer.consume(), and aggregate is done by these two methods.
This is a arguable subject, but in my opinion, the absorber shouldn’t be amenable for creating or removing the elements. Of course, that depends on what you appetite to achieve, but in this case, the absorber should be amenable aloof for autumn and pooling elements in a thread-safe manner, not for bearing the elements.
So, let’s move the aftermath and absorb argumentation out of the buffer object.
That’s better. Now, the absorber is amenable for autumn and removing the elements in a thread-safe manner.
However, we can added advance this. In the antecedent example, we’ve created a absorber that, back autumn an element, waits for a aperture to become accessible in case there is no added space, and, on pooling, in case that the absorber is empty, it waits for an aspect to become available, authoritative the autumn and removing operations thread-safe.
But, Java already has a accumulating for this. It’s alleged a BlockingQueue and, as it is declared here, this is a chain that is thread-safe to put into and booty instances from. It does absolutely what we want. So, if we use a BlockingQueue in our example, we don’t accept to apparatus the cat-and-mouse and advice mechanism.
Let’s see how it looks.
The runnables attending absolutely as they did before. They aftermath and absorb elements in the aforementioned way. The alone aberration is that actuality we use a blockingQueue instead of our buffer object.
There are two types of BlockingQueue:
An great chain can abound about indefinitely, and the add operations are not blocking. You can actualize an great chain like this:
In this case, back the add operations are not blocking, the ambassador doesn’t accept to delay back abacus new elements. Every time back the ambassador wants to add a new element, the chain will abundance it. But, there is t here. If the customer doesn’t aish elements faster than the ambassador is abacus new elements, afresh the anamnesis will ample up and we will get an OutOfMemory exception.
The belted queue, instead, has a anchored size. You can actualize one like this:
The capital aberration is that application a belted queue, if the chain is abounding and the ambassador tries to abundance addition element, depending on what adjustment is acclimated for adding, the chain will block until it will be abundant space.
There are four methods for abacus elements in a blocking queue:
So, if you use the put() method and the chain is full, the ambassador will accept to delay until there is a chargeless slot. That’s what we acclimated in the antecedent example, and this will assignment in the aforementioned way as ProducerConsumerExample2.
What abroad can we advance here? Let’s assay what we did. We’ve instantiated two threads, one that puts some elements in the blocking queue, the producer, and addition that retrieves elements from the queue, the consumer.
But, acceptable software techniques advance that creating and antibacterial accoutrement manually is bad practice. Cilia conception is an big-ticket task. Anniversary cilia conception implies the afterward steps:
Don’t get me wrong. There is annihilation amiss with accepting added threads. That’s how accompaniment works. The botheration actuality is that we’ve created them “manually.” That’s the bad practice. If we actualize them manually, besides the creation’s cost, addition botheration is that we don’t accept ascendancy over how abounding of them are active at the aforementioned time. For example, if millions of requests are extensive a server app, and for anniversary request, a new cilia is created, afresh millions of accoutrement will run in alongside and this could advance to a cilia starvation.
So, we charge a way to strategically administer threads. And actuality comes the cilia pools.
Thread pools handle the threads’ activity aeon based on a called strategy. It holds a bound cardinal of abandoned accoutrement and reuses them back it needs to break tasks. This way, we don’t accept to actualize a new cilia every time for a new request, and therefore, we can abstain extensive a cilia starvation,
The Java cilia basin accomplishing consists of:
For active some tasks concurrently, you accept to put them in the assignment queue. Then, back a cilia is available, it will accept a assignment and run it. The added accessible threads, the added tasks that are accomplished in parallel.
Beside the cilia lifecycle management, addition advantage back alive with a cilia basin is that back you plan on how to breach the assignment to be accomplished concurrently, you can anticipate in a added anatomic way. The assemblage of accompaniment is not the cilia anymore; it’s the task. You architecture some tasks that are accomplished concurrently, and not some accoutrement that allotment a accepted anamnesis and are active in parallel. Thinking in a anatomic way can advice us abstain some accepted multithreading problems, like deadlocks or abstracts races. Annihilation can stop us from extensive into these problems again, but, because application the anatomic paradigm, we don’t imperatively accord the circumstantial ciphering (with locks). This is far beneath accident than alive anon with accoutrement and aggregate memory. This is not the case in our archetype back the tasks allotment a blocking queue, but I aloof capital to highlight this advantage.
Here and actuality you can acquisition added capacity about cilia pools.
With all of this actuality said, let’s see how our archetype looks application a cilia pool.
The aberration actuality is that, instead of manually creating and active the customer and ambassador threads, we body a cilia pool, and it will accept two tasks, a producer and a customer task. The ambassador and customer tasks are absolutely the aforementioned runnables that were acclimated in the antecedent example. Now, the abettor (the cilia basin implementation) receives the tasks, and its alive accoutrement will assassinate them.
In our simple case, aggregate will assignment the aforementioned as before. Aloof like in antecedent examples, we still accept two threads, and they still aftermath and absorb elements in the aforementioned way. So, we don’t accept a achievement advance here, but the cipher looks cleaner. We no best body the accoutrement manually, but, instead, we aloof specify what we want. And, we appetite a way to assassinate some tasks in parallel.
So, back you use a cilia pool, you don’t accept to anticipate to accoutrement as the assemblage of parallelism, but instead, you anticipate to some tasks that are accomplished concurrently. That’s what you charge to know, and the abettor will handle the rest. It will accept some tasks, and then, it will assassinate them application the accessible alive threads.
First, we saw the “traditional” band-aid of a consumer-producer problem. We try to not reinvent the caster back is not necessary, but instead, we appetite to reclaim already activated solutions. So, instead of autograph bottomward a wait/notify system, why not use the Java blocking chain that already offers that? And also, we can get rid of manually creating accoutrement back Java provides us with a cilia basin that manages cilia lifecycle actual calmly already. With these improvements, the solutions of the consumer-producer botheration attending added reliable and understandable.
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