The Persistence API provides an easy and fast way to store data. It supports two main storages, namely PersistentSequence and PersistentQueue.

PersistentSequence

PersistentSequence provides functionality for storing sequential data (for example, logs). Each record should have a unique index. Indexer implementation is used to provide an index for a certain storing record. Also, PersistentSequence requires custom Serializer implementation to serialize objects to byte buffer and restore it back.

PersistenceFactory factory = ...

Indexer<MyRecord> indexer = new Indexer<MyRecord>() {
    @Override
    public long getIndex(MyRecord item) {
        return item.getId();
    }
};

Serializer<MyRecord> serializer = new Serializer<MyRecord>() {
    @Override
    public void serialize(MyRecord item, ElasticByteBuffer buffer) {
        // serialize instance ...
    }

    @Override
    public MyRecord deserialize(ElasticByteBuffer buffer, int length) {
        MyRecord record = new MyRecord;
        // load data to instance...
        return record;
    }
};

final PersistentSequence<MyRecord> sequence = factory.buildSequence("seq_sample", indexer, serializer);

//store record to sequence
sequence.append(record);

In addition to storing data, PersistentSequence provides methods for retrieving records from storage. It supports reading a single item by index or iterating through items:

//get single item with index 100
final MyRecord myRecord = sequence.get(100);

//iterate items from index 0 till 100
sequence.retrieveItems(0, 100, new RetrieveSequenceItemsListener<MyRecord>() {
    @Override
    public void onItem(long id, MyRecord record) {
        //...
    }
}, true);

There is a possibility to review stored records and remove some of them from storage:

sequence.cleanUp(new CleanupItemListener<MyRecord>() {
    @Override
    public boolean checkForClean(long id, MyRecord item) {
        //return true to removed this record from storage
        return false;
    }
});

Or remove all and reset the sequence:

//remove all items and reset index
sequence.reset();

The default PersistentSequence implementation is optimized for writing data, so reading operations may take a bit more time.

PersistentQueue

PersistentQueue works like a queue but persists all items to the disk. Thus, it can restore its state after application restart. PersistentQueue has methods similar to java.util.Queue:

final PersistentQueue<MyRecord> queue = factory.buildQueue("queue_sample", serializer);

//push item to the tail of queue
queue.add(record);

// read iem from head but doesn't remove
record = queue.peek();

// extract item from head and remove it from queue
record = queue.poll();

Also PersistentQueue allows iterating all its items:

queue.iterate(new RetrieveQueueItemsListener<MyRecord>() {
    @Override
    public void onItem(MyRecord record) {
        //....
    }
});

To remove all the items from the queue, you can use the clean method:

//remove all items and clean file
queue.clear();