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>, VersionedBytesStore

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.
default <PS extends Serializer<P>,P> KeyValueIterator<K,V>	prefixScan(P prefix, PS prefixKeySerializer) Return an iterator over all keys with the specified prefix.
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 based on the serialized byte[] of the keys, not the 'logical' key order.

Parameters:

from - The first key that could be in the range, where iteration starts from. A null value indicates that the range starts with the first element in the store.

to - The last key that could be in the range, where iteration ends. A null value indicates that the range ends with the last element in the store.

Returns:

The iterator for this range, from key with the smallest bytes to the key with the largest bytes of keys.

Throws:

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 based on the serialized byte[] of the keys, not the 'logical' key order.

Parameters:

from - The first key that could be in the range, where iteration ends. A null value indicates that the range starts with the first element in the store.

to - The last key that could be in the range, where iteration starts from. A null value indicates that the range ends with the last element in the store.

Returns:

The iterator for this range, from key with the smallest bytes to the key with the largest bytes of keys.

Throws:

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

prefixScan

default <PS extends Serializer<P>,P> KeyValueIterator<K,V> prefixScan(P prefix,
                                                                      PS prefixKeySerializer)

Return an iterator over all keys with the specified prefix. Since the type of the prefix can be different from that of the key, a serializer to convert the prefix into the format in which the keys are stored in the stores needs to be passed to this method. The returned iterator must be safe from ConcurrentModificationExceptions and must not return null values. Since prefixScan() relies on byte lexicographical ordering and not on the ordering of the key type, results for some types might be unexpected. For example, if the key type is Integer, and the store contains keys [1, 2, 11, 13], then running store.prefixScan(1, new IntegerSerializer()) will return [1] and not [1,11,13]. In contrast, if the key type is String the keys will be sorted [1, 11, 13, 2] in the store and store.prefixScan(1, new StringSerializer()) will return [1,11,13]. In both cases prefixScan() starts the scan at 1 and stops at 2.

Type Parameters:

PS - Prefix Serializer type

P - Prefix Type.

Parameters:

prefix - The prefix.

prefixKeySerializer - Serializer for the Prefix key type

Returns:

The iterator for keys having the specified prefix.

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