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A library used at otto.de to implement Spring Boot based event-sourcing microservices.
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package de.otto.synapse.state;

import com.fasterxml.jackson.databind.ObjectMapper;
import net.openhft.chronicle.hash.ChronicleHashClosedException;
import net.openhft.chronicle.map.ChronicleMap;
import net.openhft.chronicle.map.ChronicleMapBuilder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Optional;

import static de.otto.synapse.translator.ObjectMappers.currentObjectMapper;

/**
 * A {@code StateRepository} that is using a {@code ChronicleMap} to store the event-sourced entities off the heap.
 *
 * Suitable for larger amounts of data, without problems because of garbage-collection issues because of the
 * usage of off-heap memory.
 *
 * In order to be able to access the {@link #keySet()} of the stored entities, this implementation is storing
 * a copy of the keys in a separate {@link java.util.concurrent.ConcurrentSkipListSet}
 *
 * @param  The type of the event-sourced entities stored in the {@code StateRepository}
 */
public class ChronicleMapStateRepository extends ConcurrentMapStateRepository {

    private static final Logger LOG = LoggerFactory.getLogger(ChronicleMapStateRepository.class);

    private static final int DEFAULT_KEY_SIZE_BYTES = 128;
    private static final double DEFAULT_VALUE_SIZE_BYTES = 512;
    private static final long DEFAULT_ENTRY_COUNT = 100_000;

    private ChronicleMapStateRepository(final String name,
                                        final ChronicleMap chronicleMap) {
        super(name, chronicleMap);
    }

    @Override
    public Optional put(final String key, final V value) {
        try {
            return super.put(key, value);
        } catch (ChronicleHashClosedException e) {
            LOG.warn("could not put on closed state repository", e);
            return Optional.empty();
        }
    }

    @Override
    public Optional get(final String key) {
        try {
            return super.get(key);
        } catch (ChronicleHashClosedException e) {
            LOG.warn("could not get on closed state repository", e);
            return Optional.empty();
        }
    }

    @Override
    public long size() {
        try {
            return super.size();
        } catch (final ChronicleHashClosedException e) {
            LOG.warn("could not get size on closed state repository", e);
            return 0;
        }
    }
    public static  Builder builder(Class clazz) {
        return new Builder<>(clazz);
    }

    public static final class Builder {

        private ObjectMapper objectMapper = currentObjectMapper();

        private final Class clazz;
        private String name;
        private ChronicleMapBuilder chronicleMapBuilder;
        private boolean customValueMarshaller = false;


        private Builder(Class clazz) {
            this.clazz = clazz;
            this.name = clazz.getSimpleName();
        }

        public Builder withObjectMapper(ObjectMapper val) {
            objectMapper = val;
            return this;
        }

        public Builder withName(final String val) {
            name = val;
            return this;
        }

        public Builder withMapBuilder(ChronicleMapBuilder val) {
            chronicleMapBuilder = val;
            return this;
        }

        public Builder withCustomValueMarshaller(boolean val) {
            this.customValueMarshaller = val;
            return this;
        }

        public ChronicleMapStateRepository build() {

            if (chronicleMapBuilder == null) {
                chronicleMapBuilder = ChronicleMapBuilder.of(String.class, clazz)
                        .averageKeySize(DEFAULT_KEY_SIZE_BYTES)
                        .averageValueSize(DEFAULT_VALUE_SIZE_BYTES)
                        .entries(DEFAULT_ENTRY_COUNT);
            }

            boolean doesClassNeedToBeSerialized = clazz != String.class;
            if (!customValueMarshaller && doesClassNeedToBeSerialized) {
                chronicleMapBuilder.valueMarshaller(new ChronicleMapBytesMarshaller<>(objectMapper, clazz));
            }

            return new ChronicleMapStateRepository<>(name, chronicleMapBuilder.create());
        }
    }
}