org.apache.kafka.streams.state

Interface ReadOnlyKeyValueStore<K,V>

Type Parameters:

K - the key type

V - the value type

All Known Subinterfaces:

KeyValueStore<K,V>, TimestampedKeyValueStore<K,V>

public interface ReadOnlyKeyValueStore<K,V>

A key-value store that only supports read operations. Implementations should be thread-safe as concurrent reads and writes are expected.

Please note that this contract defines the thread-safe read functionality only; it does not guarantee anything about whether the actual instance is writable by another thread, or whether it uses some locking mechanism under the hood. For this reason, making dependencies between the read and write operations on different StateStore instances can cause concurrency problems like deadlock.

Method Summary

Modifier and Type	Method and Description
KeyValueIterator<K,V>	all() Return an iterator over all keys in this store.
long	approximateNumEntries() Return an approximate count of key-value mappings in this store.
V	get(K key) Get the value corresponding to this key.
KeyValueIterator<K,V>	range(K from, K to) Get an iterator over a given range of keys.
default KeyValueIterator<K,V>	reverseAll() Return a reverse iterator over all keys in this store.
default KeyValueIterator<K,V>	reverseRange(K from, K to) Get a reverse iterator over a given range of keys.

Method Detail

get

V get(K key)

Get the value corresponding to this key.

Parameters:

key - The key to fetch

Returns:

The value or null if no value is found.

Throws:

NullPointerException - If null is used for key.

InvalidStateStoreException - if the store is not initialized

range

KeyValueIterator<K,V> range(K from,
                            K to)

Get an iterator over a given range of keys. This iterator must be closed after use. The returned iterator must be safe from ConcurrentModificationExceptions and must not return null values. Order is not guaranteed as bytes lexicographical ordering might not represent key order.

Parameters:

from - The first key that could be in the range, where iteration starts from.

to - The last key that could be in the range, where iteration ends.

Returns:

The iterator for this range, from smallest to largest bytes.

Throws:

NullPointerException - If null is used for from or to.

InvalidStateStoreException - if the store is not initialized

reverseRange

default KeyValueIterator<K,V> reverseRange(K from,
                                           K to)

Get a reverse iterator over a given range of keys. This iterator must be closed after use. The returned iterator must be safe from ConcurrentModificationExceptions and must not return null values. Order is not guaranteed as bytes lexicographical ordering might not represent key order.

Parameters:

from - The first key that could be in the range, where iteration ends.

to - The last key that could be in the range, where iteration starts from.

Returns:

The reverse iterator for this range, from largest to smallest key bytes.

Throws:

NullPointerException - If null is used for from or to.

InvalidStateStoreException - if the store is not initialized

all

KeyValueIterator<K,V> all()

Return an iterator over all keys in this store. This iterator must be closed after use. The returned iterator must be safe from ConcurrentModificationExceptions and must not return null values. Order is not guaranteed as bytes lexicographical ordering might not represent key order.

Returns:

An iterator of all key/value pairs in the store, from smallest to largest bytes.

Throws:

InvalidStateStoreException - if the store is not initialized

reverseAll

default KeyValueIterator<K,V> reverseAll()

Return a reverse iterator over all keys in this store. This iterator must be closed after use. The returned iterator must be safe from ConcurrentModificationExceptions and must not return null values. Order is not guaranteed as bytes lexicographical ordering might not represent key order.

Returns:

An reverse iterator of all key/value pairs in the store, from largest to smallest key bytes.

Throws:

InvalidStateStoreException - if the store is not initialized

approximateNumEntries

long approximateNumEntries()

Return an approximate count of key-value mappings in this store.

The count is not guaranteed to be exact in order to accommodate stores where an exact count is expensive to calculate.

Returns:

an approximate count of key-value mappings in the store.

Throws:

InvalidStateStoreException - if the store is not initialized