Package com.hazelcast.ringbuffer

Interface Ringbuffer<E>

Type Parameters:
E - The type of the elements that the Ringbuffer contains
All Superinterfaces:
DistributedObject
public interface Ringbuffer<E> extends DistributedObject
A Ringbuffer is a data structure where the content is stored in a ring-like structure. A ringbuffer has a fixed capacity, so it won't grow beyond that capacity and endanger the stability of the system. If that capacity is exceeded, the oldest item in the ringbuffer is overwritten.

The ringbuffer has 2 always incrementing sequences:

  1. tailSequence(): this is the side where the youngest item is found. So the tail is the side of the ringbuffer where items are added to.
  2. headSequence(): this is the side where the oldest items are found. So the head is the side where items get discarded.
The items in the ringbuffer can be found by a sequence that is in between (inclusive) the head and tail sequence.

If data is read from a ringbuffer with a sequence that is smaller than the headSequence, it means that the data is not available anymore and a StaleSequenceException is thrown.

A Ringbuffer currently is a replicated, but not partitioned data structure. So all data is stored in a single partition, similarly to the IQueue implementation.

A Ringbuffer can be used in a way similar to the IQueue, but one of the key differences is that a queue.take is destructive, meaning that only 1 thread is able to take an item. A ringbuffer.read is not destructive, so you can have multiple threads reading the same item multiple times.

The Ringbuffer is the backing data structure for the reliable ITopic implementation. See ReliableTopicConfig.

A Ringbuffer can be configured to be backed by a RingbufferStore. All write methods will delegate to the store to persist the items, while reader methods will try to read items from the store if not found in the in-memory Ringbuffer.

When a Ringbuffer is constructed with a backing store, head and tail sequences are set to the following

  • tailSequence: lastStoreSequence
  • headSequence: lastStoreSequence + 1
where lastStoreSequence is the sequence of the previously last stored item.

Supports split brain protection SplitBrainProtectionConfig since 3.10 in cluster versions 3.10 and higher.

Asynchronous methods

Asynchronous methods return a CompletionStage that can be used to chain further computation stages. Alternatively, a CompletableFuture can be obtained via CompletionStage.toCompletableFuture() to wait for the operation to complete in a blocking way.

Actions supplied for dependent completions of default non-async methods and async methods without an explicit Executor argument are performed by the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least 2, in which case a new Thread is created per task).

Since:
3.5

  • Method Details

    • capacity

      long capacity()
      Returns the capacity of this Ringbuffer.
      Returns:
      the capacity.

    • size

      long size()
      Returns number of items in the Ringbuffer.

      If no TTL is set, the size will always be equal to capacity after the head completed the first loop around the ring. This is because no items are being removed.

      Returns:
      the size.

    • tailSequence

      long tailSequence()
      Returns the sequence of the tail. The tail is the side of the Ringbuffer where the items are added to.

      The initial value of the tail is -1 if the Ringbuffer is not backed by a store, otherwise tail sequence will be set to the sequence of the previously last stored item.

      Returns:
      the sequence of the tail.

    • headSequence

      long headSequence()
      Returns the sequence of the head. The head is the side of the Ringbuffer where the oldest items in the Ringbuffer are found.

      If the RingBuffer is empty, the head will be one more than the tail.

      The initial value of the head is 0 if the Ringbuffer is not backed by a store, otherwise head sequence will be set to the sequence of the previously last stored item + 1. In both cases head sequence is 1 more than the tail sequence.

      Returns:
      the sequence of the head.

    • remainingCapacity

      long remainingCapacity()
      Returns the remaining capacity of the ringbuffer. If TTL is enabled, then the returned capacity is equal to the total capacity of the ringbuffer minus the number of used slots in the ringbuffer which have not yet been marked as expired and cleaned up. Keep in mind that some slots could have expired items that have not yet been cleaned up and that the returned value could be stale as soon as it is returned.

      If TTL is disabled, the remaining capacity is equal to the total ringbuffer capacity.

      Returns:
      the remaining capacity
      See Also:

    • add

      long add(@Nonnull E item)
      Adds an item to the tail of the Ringbuffer. If there is no space in the Ringbuffer, the add will overwrite the oldest item in the ringbuffer no matter what the TTL is. For more control on this behavior, check the addAsync(Object, OverflowPolicy) and the OverflowPolicy.

      The returned value is the sequence of the added item. Using this sequence you can read the added item.

      Using the sequence as ID

      This sequence will always be unique for this Ringbuffer instance, so it can be used as a unique ID generator if you are publishing items on this Ringbuffer. However, you need to take care of correctly determining an initial ID when any node uses the Ringbuffer for the first time. The most reliable way to do that is to write a dummy item into the Ringbuffer and use the returned sequence as initial ID. On the reading side, this dummy item should be discarded. Please keep in mind that this ID is not the sequence of the item you are about to publish but from a previously published item. So it can't be used to find that item.

      If the Ringbuffer is backed by a RingbufferStore, the item gets persisted by the underlying store via RingbufferStore.store(long, Object). Note that in case an exception is thrown by the store, it prevents the item from being added to the Ringbuffer, keeping the store, primary and the backups consistent.

      Parameters:
      item - the item to add.
      Returns:
      the sequence of the added item.
      Throws:
      NullPointerException - if item is null.
      See Also:

    • addAsync

      CompletionStage<Long> addAsync(@Nonnull E item, @Nonnull OverflowPolicy overflowPolicy)
      Asynchronously writes an item with a configurable OverflowPolicy.

      If there is space in the Ringbuffer, the call will return the sequence of the written item. If there is no space, it depends on the overflow policy what happens:

      1. OverflowPolicy.OVERWRITE: we just overwrite the oldest item in the Ringbuffer and we violate the TTL
      2. OverflowPolicy.FAIL: we return -1

      The reason that FAIL exist is to give the opportunity to obey the TTL. If blocking behavior is required, this can be implemented using retrying in combination with an exponential backoff. Example: 

      
 long sleepMs = 100;
 for (; ; ) {
   long result = ringbuffer.addAsync(item, FAIL).toCompletableFuture().get();
   if (result != -1) {
     break;
   }
   TimeUnit.MILLISECONDS.sleep(sleepMs);
   sleepMs = min(5000, sleepMs * 2);
 }

      If the Ringbuffer is backed by a RingbufferStore, the item gets persisted by the underlying store via RingbufferStore.store(long, Object). Note that in case an exception is thrown by the store, it prevents the item from being added to the Ringbuffer, keeping the store, primary and the backups consistent.

      Parameters:
      item - the item to add
      overflowPolicy - the OverflowPolicy to use.
      Returns:
      the sequenceId of the added item, or -1 if the add failed.
      Throws:
      NullPointerException - if item or overflowPolicy is null.

    • readOne

      E readOne(long sequence) throws InterruptedException
      Reads one item from the Ringbuffer.

      If the sequence is one beyond the current tail, this call blocks until an item is added. This means that the ringbuffer can be processed using the following idiom: 

      
 Ringbuffer<String> ringbuffer = hz.getRingbuffer("rb");
 long seq = ringbuffer.headSequence();
 while(true){
   String item = ringbuffer.readOne(seq);
   seq++;
   ... process item
 }

      This method is not destructive unlike e.g. a BaseQueue.take(). So the same item can be read by multiple readers, or it can be read multiple times by the same reader.

      Currently, it isn't possible to control how long this call is going to block. In the future we could add e.g. tryReadOne(long sequence, long timeout, TimeUnit unit).

      If the item is not in the Ringbuffer an attempt is made to read it from the underlying RingbufferStore via RingbufferStore.load(long) if store is configured for the Ringbuffer. These cases may increase the execution time significantly depending on the implementation of the store. Note that exceptions thrown by the store are propagated to the caller.

      Parameters:
      sequence - the sequence of the item to read.
      Returns:
      the read item
      Throws:
      StaleSequenceException - if the sequence is smaller than headSequence(). Because a Ringbuffer won't store all event indefinitely, it can be that the data for the given sequence doesn't exist anymore and the StaleSequenceException is thrown. It is up to the caller to deal with this particular situation, e.g. throw an Exception or restart from the last known head. That is why the StaleSequenceException contains the last known head.
      IllegalArgumentException - if sequence is smaller than 0 or larger than tailSequence()+1.
      InterruptedException - if the call is interrupted while blocking.

    • addAllAsync

      CompletionStage<Long> addAllAsync(@Nonnull Collection<? extends E> collection, @Nonnull OverflowPolicy overflowPolicy)
      Adds all the items of a collection to the tail of the Ringbuffer.

      An addAll is likely to outperform multiple calls to add(Object) due to better io utilization and a reduced number of executed operations. If the batch is empty, the call is ignored.

      When the collection is not empty, the content is copied into a different data-structure. This means that:

      1. after this call completes, the collection can be re-used.
      2. the collection doesn't need to be serializable

      If the collection is larger than the capacity of the Ringbuffer, then the items that were written first will be overwritten. Therefore, this call will not block.

      The items are inserted in the order of the Iterator of the collection. If an addAll is executed concurrently with an add or addAll, no guarantee is given that items are contiguous.

      The result of the future contains the sequenceId of the last written item.

      If the Ringbuffer is backed by a RingbufferStore, the items are persisted by the underlying store via RingbufferStore.storeAll(long, Object[]). Note that in case an exception is thrown by the store, it makes the Ringbuffer not adding any of the items to the primary and the backups. Keeping the store consistent with the primary and the backups is the responsibility of the store.

      Parameters:
      collection - the batch of items to add.
      Returns:
      the CompletionStage to synchronize on completion.
      Throws:
      NullPointerException - if batch is null, or if an item in this batch is null or if overflowPolicy is null
      IllegalArgumentException - if collection is empty

    • readManyAsync

      CompletionStage<ReadResultSet<E>> readManyAsync(long startSequence, int minCount, int maxCount, @Nullable IFunction<E,Boolean> filter)
      Reads a batch of items from the Ringbuffer. If the number of available items after the first read item is smaller than the maxCount, these items are returned. So it could be the number of items read is smaller than the maxCount.

      If there are fewer items available than minCount, then this call blocks.

      Warning:

      These blocking calls consume server memory and if there are many calls, it can be possible to see leaking memory or OOME.

      Reading a batch of items is likely to perform better because less overhead is involved.

      A filter can be provided to only select items that need to be read. If the filter is null, all items are read. If the filter is not null, only items where the filter function returns true are returned. Using filters is a good way to prevent getting items that are of no value to the receiver. This reduces the amount of IO and the number of operations being executed, and can result in a significant performance improvement.

      For each item not available in the Ringbuffer an attempt is made to read it from the underlying RingbufferStore via multiple invocations of RingbufferStore.load(long), if store is configured for the Ringbuffer. These cases may increase the execution time significantly depending on the implementation of the store. Note that exceptions thrown by the store are propagated to the caller.

      If the startSequence is smaller than the smallest sequence still available in the Ringbuffer (headSequence(), then the smallest available sequence will be used as the start sequence and the minimum/maximum number of items will be attempted to be read from there on.

      If the startSequence is bigger than the last available sequence in the Ringbuffer (tailSequence()), then the last available sequence plus one will be used as the start sequence and the call will block until further items become available, and it can read at least the minimum number of items.

      Parameters:
      startSequence - the startSequence of the first item to read.
      minCount - the minimum number of items to read.
      maxCount - the maximum number of items to read.
      filter - the filter. Filter is allowed to be null, indicating there is no filter.
      Returns:
      a future containing the items read.
      Throws:
      IllegalArgumentException - if startSequence is smaller than 0 or if minCount smaller than 0 or if minCount larger than maxCount, or if maxCount larger than the capacity of the ringbuffer or if maxCount larger than 1000 (to prevent overload)