THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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Proposed Changes
Introduce a enum type ReadDirection.BACKWARD|FORWARD to ReadOnlyKeyValueStore#range|all and ReadOnlyWindowStore#fetch|fetchAll|all:
public enum ReadDirection {
FORWARD, BACKWARD
}
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new StreamsConfig configuration to flag support for backwards iteration:
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public class StreamsConfig extends AbstractConfig { public static final |
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String ENABLE_BACKWARD_ITERATION_CONFIG = "enable.backward.iteration"; |
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private static final |
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String ENABLE_BACKWARD_ITERATION_DOC = "If true any range operations will accept (from, to) arguments to be from > to, returning recent records first";
} |
If true, ReadOnlyKeyValueStore will support (from, to) argument pairs to be: from > to, returning iteration in reverse order.
To complete support for backwards iteration, `all` operations will be companioned by a `reverseAll`:
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public interface ReadOnlyKeyValueStore<K, V> { KeyValueIterator<K, V> reverseAll(); }
public interface ReadOnlyWindowStore<K, V> { KeyValueIterator<Windowed<K>, V> reverseAll(); } |
StreamConfig flag will be passed to Stores via `ProcessorContext`.
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Internally, both implementations: persistent (RocksDB), and in-memory (TreeMap) support reverse/descending iteration:
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final RocksIterator iter = db.newIterator(); |
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iter.seekToFirst(); |
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iter.next(); // |
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final RocksIterator reverse = db.newIterator(); |
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reverse.seekToLast(); |
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reverse.prev(); |
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// final |
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TreeMap<String, |
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String> map = new |
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TreeMap<>(); final |
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NavigableSet<String> nav = map.navigableKeySet(); final |
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NavigableSet<String> rev = map.descendingKeySet(); |
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Compatibility, Deprecation, and Migration Plan
Default methods would be in-place avoid affecting previous versions.
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StreamsConfig will mitigate affecting users that are relying on current behaviour: if from>to, then return empty iterator. Only when users are enabling flag from>to will return a reversed iterator.
Therefore this change is backwards compatible.
Rejected Alternatives
- Initially it was considered to have additional parameter on all readOnlyStore methods e.g. Store#fetch(keyFrom, keyTo, timeFrom, timeTo, ReadDirection.FORWARD|BACKWARD), but has been declines as passing arguments in inverse is more intuitive. As this could cause unexpected effects in future versions, a flag has been added to overcome this.