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A library used at otto.de to integrate synapse-core with edison-microservice.
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package de.otto.synapse.edison.trace;

import com.google.common.collect.EvictingQueue;
import de.otto.synapse.endpoint.EndpointType;
import de.otto.synapse.endpoint.MessageEndpoint;

import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.stream.Stream;

import static com.google.common.collect.ImmutableList.copyOf;
import static com.google.common.collect.Sets.newTreeSet;
import static java.lang.Boolean.TRUE;

/**
 * Thread-safe in-memory implementation of a circular MessageStore that is storing all traceEntries in insertion order
 * per channel with a configurable capacity per channel.
 *
 * Each time an element is added to a full message store, the message store automatically removes its head element.
 */

public class MessageTrace {

    private static final class RegisteredEndpoints {
        private final String channelName;
        private final EndpointType endpointType;

        RegisteredEndpoints(final MessageEndpoint messageEndpoint) {
            this.channelName = messageEndpoint.getChannelName();
            this.endpointType = messageEndpoint.getEndpointType();
        }

        String getChannelName() {
            return channelName;
        }

        EndpointType getEndpointType() {
            return endpointType;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            RegisteredEndpoints that = (RegisteredEndpoints) o;
            return Objects.equals(channelName, that.channelName) &&
                    endpointType == that.endpointType;
        }

        @Override
        public int hashCode() {
            return Objects.hash(channelName, endpointType);
        }

        @Override
        public String toString() {
            return "RegisteredEndpoints{" +
                    "channelName='" + channelName + '\'' +
                    ", endpointType=" + endpointType +
                    '}';
        }
    }

    private final Map> traceEntries;
    private final int capacityPerChannel;
    private final ConcurrentMap senders = new ConcurrentHashMap<>();
    private final ConcurrentMap receivers = new ConcurrentHashMap<>();

    /**
     * Creates a new instance with specified capacity per channel.
     *
     * @param capacityPerChannel the size of the underlying ring buffers.
     */
    public MessageTrace(final int capacityPerChannel) {
        traceEntries = new ConcurrentHashMap<>();
        this.capacityPerChannel = capacityPerChannel;
    }

    public SortedSet getSenderChannels() {
        return newTreeSet(senders.keySet());
    }

    public SortedSet getReceiverChannels() {
        return newTreeSet(receivers.keySet());
    }

    public int getCapacityPerChannel() {
        return capacityPerChannel;
    }

    /**
     * Adds a Message to the MessageStore.
     *
     * 
If the capacity of the ring buffer is reached, the oldest message is removed
     * @param traceEntry the message to add
     */
    public synchronized void add(final TraceEntry traceEntry) {
        traceEntries.putIfAbsent(traceEntry.getChannelName(), EvictingQueue.create(capacityPerChannel));

        traceEntries.get(traceEntry.getChannelName()).add(traceEntry);

        if (traceEntry.getEndpointType() == EndpointType.RECEIVER) {
            receivers.putIfAbsent(traceEntry.getChannelName(), TRUE);
        } else if (traceEntry.getEndpointType() == EndpointType.SENDER) {
            senders.putIfAbsent(traceEntry.getChannelName(), TRUE);
        }
    }

    /**
     * Returns a Stream of {@link TraceEntry traceEntries} contained in the RegisteredEndpoints.
     * 
     *     The stream will maintain the per-channel insertion order of the traceEntries, but not overall insertion order.
     * 
     *
     * @return Stream of traceEntries
     */
    public synchronized Stream stream() {
        return copyOf(traceEntries.values())
                .stream().flatMap(Collection::stream);
    }

    public synchronized Stream stream(final String channelName, final EndpointType endpointType) {
        if (!traceEntries.containsKey(channelName)) {
            return Stream.empty();
        }
        return copyOf(traceEntries.get(channelName))
                .stream()
                .filter(traceEntry -> traceEntry.getEndpointType().equals(endpointType));
    }

}