Implemented private channels for N-person direct messaging.

client/src/main/java/nl/andrewl/concord_client/ConcordClient.java

@@ -10,9 +10,9 @@ import com.googlecode.lanterna.terminal.Terminal;
 import lombok.Getter;
 import nl.andrewl.concord_client.event.EventManager;
 import nl.andrewl.concord_client.event.handlers.ChannelMovedHandler;
-import nl.andrewl.concord_client.event.handlers.ChannelUsersResponseHandler;
 import nl.andrewl.concord_client.event.handlers.ChatHistoryResponseHandler;
 import nl.andrewl.concord_client.event.handlers.ServerMetaDataHandler;
+import nl.andrewl.concord_client.event.handlers.ServerUsersHandler;
 import nl.andrewl.concord_client.gui.MainWindow;
 import nl.andrewl.concord_client.model.ClientModel;
 import nl.andrewl.concord_core.msg.Message;
@@ -48,7 +48,7 @@ public class ConcordClient implements Runnable {
 
 		// Add event listeners.
 		this.eventManager.addHandler(MoveToChannel.class, new ChannelMovedHandler());
-		this.eventManager.addHandler(ChannelUsersResponse.class, new ChannelUsersResponseHandler());
+		this.eventManager.addHandler(ServerUsers.class, new ServerUsersHandler());
 		this.eventManager.addHandler(ChatHistoryResponse.class, new ChatHistoryResponseHandler());
 		this.eventManager.addHandler(Chat.class, (msg, client) -> client.getModel().getChatHistory().addChat(msg));
 		this.eventManager.addHandler(ServerMetaData.class, new ServerMetaDataHandler());
@@ -70,10 +70,9 @@ public class ConcordClient implements Runnable {
 		this.serializer.writeMessage(new Identification(nickname), this.out);
 		Message reply = this.serializer.readMessage(this.in);
 		if (reply instanceof ServerWelcome welcome) {
-			var model = new ClientModel(welcome.getClientId(), nickname, welcome.getCurrentChannelId(), welcome.getMetaData());
+			var model = new ClientModel(welcome.getClientId(), nickname, welcome.getCurrentChannelId(), welcome.getCurrentChannelName(), welcome.getMetaData());
 			// Start fetching initial data for the channel we were initially put into.
-			this.sendMessage(new ChannelUsersRequest(this.model.getCurrentChannelId()));
+			this.sendMessage(new ChatHistoryRequest(model.getCurrentChannelId(), ""));
-			this.sendMessage(new ChatHistoryRequest(this.model.getCurrentChannelId(), ""));
 			return model;
 		} else {
 			throw new IOException("Unexpected response from the server after sending identification message.");

client/src/main/java/nl/andrewl/concord_client/event/ClientModelListener.java

@@ -7,7 +7,7 @@ import java.util.List;
 import java.util.UUID;
 
 public interface ClientModelListener {
-	default void channelMoved(UUID oldChannelId, UUID newChannelId) {}
+	default void channelMoved(UUID oldChannelId, String oldChannelName, UUID newChannelId, String newChannelName) {}
 
 	default void usersUpdated(List<UserData> users) {}
 

client/src/main/java/nl/andrewl/concord_client/event/handlers/ChannelMovedHandler.java

@@ -2,7 +2,6 @@ package nl.andrewl.concord_client.event.handlers;
 
 import nl.andrewl.concord_client.ConcordClient;
 import nl.andrewl.concord_client.event.MessageHandler;
-import nl.andrewl.concord_core.msg.types.ChannelUsersRequest;
 import nl.andrewl.concord_core.msg.types.ChatHistoryRequest;
 import nl.andrewl.concord_core.msg.types.MoveToChannel;
 
@@ -15,8 +14,7 @@ import nl.andrewl.concord_core.msg.types.MoveToChannel;
 public class ChannelMovedHandler implements MessageHandler<MoveToChannel> {
 	@Override
 	public void handle(MoveToChannel msg, ConcordClient client) throws Exception {
-		client.getModel().setCurrentChannelId(msg.getChannelId());
+		client.getModel().setCurrentChannel(msg.getId(), msg.getChannelName());
-		client.sendMessage(new ChatHistoryRequest(msg.getChannelId(), ""));
+		client.sendMessage(new ChatHistoryRequest(msg.getId(), ""));
-		client.sendMessage(new ChannelUsersRequest(msg.getChannelId()));
 	}
 }

client/src/main/java/nl/andrewl/concord_client/event/handlers/ChatHistoryResponseHandler.java

@@ -6,7 +6,7 @@ import nl.andrewl.concord_core.msg.types.ChatHistoryResponse;
 
 public class ChatHistoryResponseHandler implements MessageHandler<ChatHistoryResponse> {
 	@Override
-	public void handle(ChatHistoryResponse msg, ConcordClient client) throws Exception {
+	public void handle(ChatHistoryResponse msg, ConcordClient client) {
 		client.getModel().getChatHistory().setChats(msg.getMessages());
 	}
 }

client/src/main/java/nl/andrewl/concord_client/event/handlers/ServerUsersHandler.java

@@ -0,0 +1,12 @@
+package nl.andrewl.concord_client.event.handlers;
+
+import nl.andrewl.concord_client.ConcordClient;
+import nl.andrewl.concord_client.event.MessageHandler;
+import nl.andrewl.concord_core.msg.types.ServerUsers;
+
+public class ServerUsersHandler implements MessageHandler<ServerUsers> {
+	@Override
+	public void handle(ServerUsers msg, ConcordClient client) {
+		client.getModel().setKnownUsers(msg.getUsers());
+	}
+}

client/src/main/java/nl/andrewl/concord_client/gui/ChannelChatBox.java

@@ -53,9 +53,7 @@ public class ChannelChatBox extends Panel {
 	}
 
 	public void refreshBorder() {
-		String name = client.getModel().getServerMetaData().getChannels().stream()
+		String name = client.getModel().getCurrentChannelName();
-				.filter(channelData -> channelData.getId().equals(client.getModel().getCurrentChannelId()))
-				.findAny().orElseThrow().getName();
 		if (this.chatBorder != null) this.removeComponent(this.chatBorder);
 		this.chatBorder = Borders.doubleLine("#" + name);
 		this.chatBorder.setComponent(this.chatList);

client/src/main/java/nl/andrewl/concord_client/gui/ServerPanel.java

@@ -43,7 +43,7 @@ public class ServerPanel extends Panel implements ClientModelListener {
 	}
 
 	@Override
-	public void channelMoved(UUID oldChannelId, UUID newChannelId) {
+	public void channelMoved(UUID oldChannelId, String oldChannelName, UUID newChannelId, String newChannelName) {
 		this.getTextGUI().getGUIThread().invokeLater(() -> {
 			this.channelList.setChannels();
 			this.channelChatBox.getChatList().clearItems();

client/src/main/java/nl/andrewl/concord_client/gui/UserList.java

@@ -5,9 +5,10 @@ import com.googlecode.lanterna.gui2.Direction;
 import com.googlecode.lanterna.gui2.LinearLayout;
 import com.googlecode.lanterna.gui2.Panel;
 import nl.andrewl.concord_client.ConcordClient;
-import nl.andrewl.concord_core.msg.types.ChannelUsersResponse;
+import nl.andrewl.concord_core.msg.types.MoveToChannel;
 import nl.andrewl.concord_core.msg.types.UserData;
 
+import java.io.IOException;
 import java.util.List;
 
 public class UserList extends Panel {
@@ -22,7 +23,14 @@ public class UserList extends Panel {
 		this.removeAllComponents();
 		for (var user : usersResponse) {
 			Button b = new Button(user.getName(), () -> {
-				System.out.println("Opening DM channel with user " + user.getName() + ", id: " + user.getId());
+				if (!client.getModel().getId().equals(user.getId())) {
+					System.out.println("Opening DM channel with user " + user.getName() + ", id: " + user.getId());
+					try {
+						client.sendMessage(new MoveToChannel(user.getId()));
+					} catch (IOException e) {
+						e.printStackTrace();
+					}
+				}
 			});
 			this.addComponent(b);
 		}

client/src/main/java/nl/andrewl/concord_client/model/ClientModel.java

@@ -17,25 +17,29 @@ public class ClientModel {
 	private ServerMetaData serverMetaData;
 
 	private UUID currentChannelId;
+	private String currentChannelName;
 	private List<UserData> knownUsers;
 	private final ChatHistory chatHistory;
 
 	private final List<ClientModelListener> modelListeners;
 
-	public ClientModel(UUID id, String nickname, UUID currentChannelId, ServerMetaData serverMetaData) {
+	public ClientModel(UUID id, String nickname, UUID currentChannelId, String currentChannelName, ServerMetaData serverMetaData) {
 		this.modelListeners = new CopyOnWriteArrayList<>();
 		this.id = id;
 		this.nickname = nickname;
 		this.currentChannelId = currentChannelId;
+		this.currentChannelName = currentChannelName;
 		this.serverMetaData = serverMetaData;
 		this.knownUsers = new ArrayList<>();
 		this.chatHistory = new ChatHistory();
 	}
 
-	public void setCurrentChannelId(UUID newChannelId) {
+	public void setCurrentChannel(UUID channelId, String channelName) {
 		UUID oldId = this.currentChannelId;
-		this.currentChannelId = newChannelId;
+		String oldName = this.currentChannelName;
-		this.modelListeners.forEach(listener -> listener.channelMoved(oldId, newChannelId));
+		this.currentChannelId = channelId;
+		this.currentChannelName = channelName;
+		this.modelListeners.forEach(listener -> listener.channelMoved(oldId, oldName, channelId, channelName));
 	}
 
 	public void setKnownUsers(List<UserData> users) {

core/src/main/java/nl/andrewl/concord_core/msg/MessageUtils.java

@@ -18,11 +18,11 @@ public class MessageUtils {
 	/**
 	 * Gets the number of bytes that the given string will occupy when it is
 	 * serialized.
-	 * @param s The string.
+	 * @param s The string. This may be null.
 	 * @return The number of bytes used to serialize the string.
 	 */
 	public static int getByteSize(String s) {
-		return Integer.BYTES + s.getBytes(StandardCharsets.UTF_8).length;
+		return Integer.BYTES + (s == null ? 0 : s.getBytes(StandardCharsets.UTF_8).length);
 	}
 
 	/**
@@ -61,22 +61,35 @@ public class MessageUtils {
 	}
 
 	public static void writeEnum(Enum<?> value, DataOutputStream o) throws IOException {
-		o.writeInt(value.ordinal());
+		if (value == null) {
+			o.writeInt(-1);
+		} else {
+			o.writeInt(value.ordinal());
+		}
 	}
 
 	public static <T extends Enum<?>> T readEnum(Class<T> e, DataInputStream i) throws IOException {
 		int ordinal = i.readInt();
+		if (ordinal == -1) return null;
 		return e.getEnumConstants()[ordinal];
 	}
 
 	public static void writeUUID(UUID value, DataOutputStream o) throws IOException {
-		o.writeLong(value.getMostSignificantBits());
+		if (value == null) {
-		o.writeLong(value.getLeastSignificantBits());
+			o.writeLong(-1);
+			o.writeLong(-1);
+		} else {
+			o.writeLong(value.getMostSignificantBits());
+			o.writeLong(value.getLeastSignificantBits());
+		}
 	}
 
 	public static UUID readUUID(DataInputStream i) throws IOException {
 		long a = i.readLong();
 		long b = i.readLong();
+		if (a == -1 && b == -1) {
+			return null;
+		}
 		return new UUID(a, b);
 	}
 
@@ -95,15 +108,19 @@ public class MessageUtils {
 		}
 	}
 
-	public static <T extends Message> List<T> readList(Class<T> type, DataInputStream i) throws IOException, ReflectiveOperationException {
+	public static <T extends Message> List<T> readList(Class<T> type, DataInputStream i) throws IOException {
 		int size = i.readInt();
-		var constructor = type.getConstructor();
+		try {
-		List<T> items = new ArrayList<>(size);
+			var constructor = type.getConstructor();
-		for (int k = 0; k < size; k++) {
+			List<T> items = new ArrayList<>(size);
-			var item = constructor.newInstance();
+			for (int k = 0; k < size; k++) {
-			item.read(i);
+				var item = constructor.newInstance();
-			items.add(item);
+				item.read(i);
+				items.add(item);
+			}
+			return items;
+		} catch (ReflectiveOperationException e) {
+			throw new IOException(e);
 		}
-		return items;
 	}
 }

core/src/main/java/nl/andrewl/concord_core/msg/Serializer.java

@@ -35,9 +35,10 @@ public class Serializer {
 		registerType(4, ChatHistoryRequest.class);
 		registerType(5, ChatHistoryResponse.class);
 		registerType(6, ChannelUsersRequest.class);
-		registerType(7, ChannelUsersResponse.class);
+		registerType(7, ServerUsers.class);
 		registerType(8, ServerMetaData.class);
 		registerType(9, Error.class);
+		registerType(10, CreateThread.class);
 	}
 
 	/**

core/src/main/java/nl/andrewl/concord_core/msg/types/ChannelUsersRequest.java

@@ -15,6 +15,7 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
 @Data
 @NoArgsConstructor
 @AllArgsConstructor
+@Deprecated
 public class ChannelUsersRequest implements Message {
 	private UUID channelId;
 

core/src/main/java/nl/andrewl/concord_core/msg/types/ChannelUsersResponse.java

@@ -17,10 +17,12 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
  * the users in the channel that a client is in has changed. For example, when
  * a user leaves a channel, all others in that channel will be sent this message
  * to indicate that update.
+ * @deprecated Clients will be updated via a {@link ServerUsers} message.
  */
 @Data
 @NoArgsConstructor
 @AllArgsConstructor
+@Deprecated
 public class ChannelUsersResponse implements Message {
 	private List<UserData> users;
 
@@ -36,10 +38,6 @@ public class ChannelUsersResponse implements Message {
 
 	@Override
 	public void read(DataInputStream i) throws IOException {
-		try {
+		this.users = readList(UserData.class, i);
-			this.users = readList(UserData.class, i);
-		} catch (ReflectiveOperationException e) {
-			throw new IOException(e);
-		}
 	}
 }

core/src/main/java/nl/andrewl/concord_core/msg/types/Chat.java

@@ -1,5 +1,6 @@
 package nl.andrewl.concord_core.msg.types;
 
+import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
 import nl.andrewl.concord_core.msg.Message;
@@ -7,6 +8,7 @@ import nl.andrewl.concord_core.msg.Message;
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
+import java.util.Objects;
 import java.util.UUID;
 
 import static nl.andrewl.concord_core.msg.MessageUtils.*;
@@ -16,13 +18,18 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
  */
 @Data
 @NoArgsConstructor
+@AllArgsConstructor
 public class Chat implements Message {
+	private static final long ID_NONE = 0;
+
+	private UUID id;
 	private UUID senderId;
 	private String senderNickname;
 	private long timestamp;
 	private String message;
 
 	public Chat(UUID senderId, String senderNickname, long timestamp, String message) {
+		this.id = null;
 		this.senderId = senderId;
 		this.senderNickname = senderNickname;
 		this.timestamp = timestamp;
@@ -40,6 +47,7 @@ public class Chat implements Message {
 
 	@Override
 	public void write(DataOutputStream o) throws IOException {
+		writeUUID(this.id, o);
 		writeUUID(this.senderId, o);
 		writeString(this.senderNickname, o);
 		o.writeLong(this.timestamp);
@@ -48,6 +56,7 @@ public class Chat implements Message {
 
 	@Override
 	public void read(DataInputStream i) throws IOException {
+		this.id = readUUID(i);
 		this.senderId = readUUID(i);
 		this.senderNickname = readString(i);
 		this.timestamp = i.readLong();
@@ -63,6 +72,7 @@ public class Chat implements Message {
 	public boolean equals(Object o) {
 		if (o.getClass().equals(this.getClass())) {
 			Chat other = (Chat) o;
+			if (Objects.equals(this.getId(), other.getId())) return true;
 			return this.getSenderId().equals(other.getSenderId()) &&
 					this.getTimestamp() == other.getTimestamp() &&
 					this.getSenderNickname().equals(other.getSenderNickname()) &&

core/src/main/java/nl/andrewl/concord_core/msg/types/ChatHistoryRequest.java

@@ -41,6 +41,9 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
  *         <li><code>to</code> - ISO-8601 timestamp indicating the timestamp
  *         before which messages should be fetched. Only messages before this
  *         point in time are returned.</li>
+ *         <li><code>id</code> - A single message id to fetch. If this parameter
+ *         is present, all others are ignored, and a list containing the single
+ *         message is returned, if it could be found, otherwise an empty list.</li>
  *     </ul>
  * </p>
  * <p>

core/src/main/java/nl/andrewl/concord_core/msg/types/CreateThread.java

@@ -0,0 +1,56 @@
+package nl.andrewl.concord_core.msg.types;
+
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import nl.andrewl.concord_core.msg.Message;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.util.UUID;
+
+import static nl.andrewl.concord_core.msg.MessageUtils.*;
+
+/**
+ * This message is sent by clients when they indicate that they would like to
+ * create a new thread in their current channel.
+ * <p>
+ *     Conversely, this message is also sent by the server when a thread has
+ *     been created by someone, and all clients need to be notified so that they
+ *     can properly display to the user that a message has been turned into a
+ *     thread.
+ * </p>
+ */
+@Data
+@NoArgsConstructor
+public class CreateThread implements Message {
+	/**
+	 * The id of the message from which the thread will be created. This will
+	 * serve as the entry point of the thread, and the unique identifier for the
+	 * thread.
+	 */
+	private UUID messageId;
+
+	/**
+	 * The title for the thread. This may be null, in which case the thread does
+	 * not have any title.
+	 */
+	private String title;
+
+	@Override
+	public int getByteCount() {
+		return UUID_BYTES + getByteSize(title);
+	}
+
+	@Override
+	public void write(DataOutputStream o) throws IOException {
+		writeUUID(this.messageId, o);
+		writeString(this.title, o);
+	}
+
+	@Override
+	public void read(DataInputStream i) throws IOException {
+		this.messageId = readUUID(i);
+		this.title = readString(i);
+	}
+}

core/src/main/java/nl/andrewl/concord_core/msg/types/MoveToChannel.java

@@ -1,5 +1,6 @@
 package nl.andrewl.concord_core.msg.types;
 
+import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
 import nl.andrewl.concord_core.msg.Message;
@@ -19,28 +20,48 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
  *     Conversely, a client can send this request to the server to indicate that
  *     they would like to switch to the specified channel.
  * </p>
+ * <p>
+ *     Clients can also send this message and provide the id of another client
+ *     to request that they enter a private message channel with the referenced
+ *     client.
+ * </p>
  */
 @Data
+@AllArgsConstructor
 @NoArgsConstructor
 public class MoveToChannel implements Message {
-	private UUID channelId;
+	/**
+	 * The id of the channel that the client is requesting or being moved to, or
+	 * the id of another client that the user wishes to begin private messaging
+	 * with.
+	 */
+	private UUID id;
+
+	/**
+	 * The name of the channel that the client is moved to. This is null in
+	 * cases where the client is requesting to move to a channel, and is only
+	 * provided by the server when it moves a client.
+	 */
+	private String channelName;
 
 	public MoveToChannel(UUID channelId) {
-		this.channelId = channelId;
+		this.id = channelId;
 	}
 
 	@Override
 	public int getByteCount() {
-		return UUID_BYTES;
+		return UUID_BYTES + getByteSize(this.channelName);
 	}
 
 	@Override
 	public void write(DataOutputStream o) throws IOException {
-		writeUUID(this.channelId, o);
+		writeUUID(this.id, o);
+		writeString(this.channelName, o);
 	}
 
 	@Override
 	public void read(DataInputStream i) throws IOException {
-		this.channelId = readUUID(i);
+		this.id = readUUID(i);
+		this.channelName = readString(i);
 	}
 }

core/src/main/java/nl/andrewl/concord_core/msg/types/ServerMetaData.java

@@ -13,6 +13,11 @@ import java.util.UUID;
 
 import static nl.andrewl.concord_core.msg.MessageUtils.*;
 
+/**
+ * Metadata is sent by the server to clients to inform them of the structure of
+ * the server. This includes basic information about the server's own properties
+ * as well as information about all top-level channels.
+ */
 @Data
 @NoArgsConstructor
 @AllArgsConstructor
@@ -34,13 +39,13 @@ public class ServerMetaData implements Message {
 	@Override
 	public void read(DataInputStream i) throws IOException {
 		this.name = readString(i);
-		try {
+		this.channels = readList(ChannelData.class, i);
-			this.channels = readList(ChannelData.class, i);
-		} catch (ReflectiveOperationException e) {
-			throw new IOException("Reflection exception", e);
-		}
 	}
 
+	/**
+	 * Metadata about a top-level channel in the server which is visible and
+	 * joinable for a user.
+	 */
 	@Data
 	@NoArgsConstructor
 	@AllArgsConstructor

core/src/main/java/nl/andrewl/concord_core/msg/types/ServerUsers.java

@@ -0,0 +1,40 @@
+package nl.andrewl.concord_core.msg.types;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import nl.andrewl.concord_core.msg.Message;
+
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import java.util.List;
+
+import static nl.andrewl.concord_core.msg.MessageUtils.*;
+
+/**
+ * This message is sent from the server to the client whenever a change happens
+ * which requires the server to notify clients about a change of the list of
+ * global users.
+ */
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+public class ServerUsers implements Message {
+	private List<UserData> users;
+
+	@Override
+	public int getByteCount() {
+		return getByteSize(this.users);
+	}
+
+	@Override
+	public void write(DataOutputStream o) throws IOException {
+		writeList(this.users, o);
+	}
+
+	@Override
+	public void read(DataInputStream i) throws IOException {
+		this.users = readList(UserData.class, i);
+	}
+}

core/src/main/java/nl/andrewl/concord_core/msg/types/ServerWelcome.java

@@ -22,17 +22,19 @@ import static nl.andrewl.concord_core.msg.MessageUtils.*;
 public class ServerWelcome implements Message {
 	private UUID clientId;
 	private UUID currentChannelId;
+	private String currentChannelName;
 	private ServerMetaData metaData;
 
 	@Override
 	public int getByteCount() {
-		return 2 * UUID_BYTES + this.metaData.getByteCount();
+		return 2 * UUID_BYTES + getByteSize(this.currentChannelName) + this.metaData.getByteCount();
 	}
 
 	@Override
 	public void write(DataOutputStream o) throws IOException {
 		writeUUID(this.clientId, o);
 		writeUUID(this.currentChannelId, o);
+		writeString(this.currentChannelName, o);
 		this.metaData.write(o);
 	}
 
@@ -41,6 +43,7 @@ public class ServerWelcome implements Message {
 		this.clientId = readUUID(i);
 		this.currentChannelId = readUUID(i);
 		this.metaData = new ServerMetaData();
+		this.currentChannelName = readString(i);
 		this.metaData.read(i);
 	}
 }

server/src/main/java/nl/andrewl/concord_server/ChannelManager.java

@@ -1,74 +0,0 @@
-package nl.andrewl.concord_server;
-
-import nl.andrewl.concord_core.msg.types.MoveToChannel;
-
-import java.util.Map;
-import java.util.Optional;
-import java.util.Set;
-import java.util.UUID;
-import java.util.concurrent.ConcurrentHashMap;
-
-public class ChannelManager {
-	private final ConcordServer server;
-	private final Map<String, Channel> channelNameMap;
-	private final Map<UUID, Channel> channelIdMap;
-
-	public ChannelManager(ConcordServer server) {
-		this.server = server;
-		this.channelNameMap = new ConcurrentHashMap<>();
-		this.channelIdMap = new ConcurrentHashMap<>();
-		// Initialize the channels according to what's defined in the server's config.
-		for (var channelConfig : server.getConfig().getChannels()) {
-			this.addChannel(new Channel(
-					server,
-					UUID.fromString(channelConfig.getId()),
-					channelConfig.getName(),
-					server.getDb().getCollection("channel-" + channelConfig.getId())
-			));
-		}
-	}
-
-	public Set<Channel> getChannels() {
-		return Set.copyOf(this.channelIdMap.values());
-	}
-
-	public Optional<Channel> getDefaultChannel() {
-		var optionalGeneral = this.getChannelByName("general");
-		if (optionalGeneral.isPresent()) {
-			return optionalGeneral;
-		}
-		for (var channel : this.getChannels()) {
-			return Optional.of(channel);
-		}
-		return Optional.empty();
-	}
-
-	public void addChannel(Channel channel) {
-		this.channelNameMap.put(channel.getName(), channel);
-		this.channelIdMap.put(channel.getId(), channel);
-	}
-
-	public void removeChannel(Channel channel) {
-		this.channelNameMap.remove(channel.getName());
-		this.channelIdMap.remove(channel.getId());
-	}
-
-	public Optional<Channel> getChannelByName(String name) {
-		return Optional.ofNullable(this.channelNameMap.get(name));
-	}
-
-	public Optional<Channel> getChannelById(UUID id) {
-		return Optional.ofNullable(this.channelIdMap.get(id));
-	}
-
-	public void moveToChannel(ClientThread client, Channel channel) {
-		if (client.getCurrentChannel() != null) {
-			var previousChannel = client.getCurrentChannel();
-			previousChannel.removeClient(client);
-		}
-		channel.addClient(client);
-		client.setCurrentChannel(channel);
-		client.sendToClient(new MoveToChannel(channel.getId()));
-		System.out.println("Moved client " + client + " to channel " + channel);
-	}
-}

server/src/main/java/nl/andrewl/concord_server/ChatThread.java

@@ -1,4 +0,0 @@
-package nl.andrewl.concord_server;
-
-public class ChatThread {
-}

server/src/main/java/nl/andrewl/concord_server/ConcordServer.java

@@ -1,26 +1,27 @@
 package nl.andrewl.concord_server;
 
 import lombok.Getter;
-import nl.andrewl.concord_core.msg.Message;
 import nl.andrewl.concord_core.msg.Serializer;
-import nl.andrewl.concord_core.msg.types.Identification;
 import nl.andrewl.concord_core.msg.types.ServerMetaData;
-import nl.andrewl.concord_core.msg.types.ServerWelcome;
+import nl.andrewl.concord_server.channel.ChannelManager;
-import nl.andrewl.concord_core.msg.types.UserData;
 import nl.andrewl.concord_server.cli.ServerCli;
+import nl.andrewl.concord_server.client.ClientManager;
+import nl.andrewl.concord_server.client.ClientThread;
 import nl.andrewl.concord_server.config.ServerConfig;
+import nl.andrewl.concord_server.event.EventManager;
+import nl.andrewl.concord_server.util.IdProvider;
+import nl.andrewl.concord_server.util.UUIDProvider;
 import org.dizitart.no2.Nitrite;
 
-import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.net.ServerSocket;
 import java.net.Socket;
 import java.nio.file.Path;
 import java.util.Comparator;
-import java.util.List;
+import java.util.concurrent.ExecutorService;
-import java.util.Map;
+import java.util.concurrent.Executors;
-import java.util.UUID;
+import java.util.concurrent.ScheduledExecutorService;
-import java.util.concurrent.*;
+import java.util.concurrent.TimeUnit;
 import java.util.stream.Collectors;
 
 /**
@@ -30,7 +31,6 @@ public class ConcordServer implements Runnable {
 	private static final Path CONFIG_FILE = Path.of("server-config.json");
 	private static final Path DATABASE_FILE = Path.of("concord-server.db");
 
-	private final Map<UUID, ClientThread> clients;
 	private volatile boolean running;
 	private final ServerSocket serverSocket;
 
@@ -79,6 +79,12 @@ public class ConcordServer implements Runnable {
 	@Getter
 	private final ChannelManager channelManager;
 
+	/**
+	 * Manager that handles the collection of clients connected to this server.
+	 */
+	@Getter
+	private final ClientManager clientManager;
+
 	private final DiscoveryServerPublisher discoveryServerPublisher;
 	private final ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
 
@@ -87,51 +93,13 @@ public class ConcordServer implements Runnable {
 		this.config = ServerConfig.loadOrCreate(CONFIG_FILE, idProvider);
 		this.discoveryServerPublisher = new DiscoveryServerPublisher(this.config);
 		this.db = Nitrite.builder().filePath(DATABASE_FILE.toFile()).openOrCreate();
-		this.clients = new ConcurrentHashMap<>(32);
 		this.eventManager = new EventManager(this);
 		this.channelManager = new ChannelManager(this);
+		this.clientManager = new ClientManager(this);
 		this.serverSocket = new ServerSocket(this.config.getPort());
 		this.serializer = new Serializer();
 	}
 
-	/**
-	 * Registers a new client as connected to the server. This is done once the
-	 * client thread has received the correct identification information from
-	 * the client. The server will register the client in its global set of
-	 * connected clients, and it will immediately move the client to the default
-	 * channel.
-	 * @param identification The client's identification data.
-	 * @param clientThread The client manager thread.
-	 */
-	public void registerClient(Identification identification, ClientThread clientThread) {
-		var id = this.idProvider.newId();
-		System.out.printf("Client \"%s\" joined with id %s.\n", identification.getNickname(), id);
-		this.clients.put(id, clientThread);
-		clientThread.setClientId(id);
-		clientThread.setClientNickname(identification.getNickname());
-		// Immediately add the client to the default channel and send the initial welcome message.
-		var defaultChannel = this.channelManager.getDefaultChannel().orElseThrow();
-		clientThread.sendToClient(new ServerWelcome(id, defaultChannel.getId(), this.getMetaData()));
-		// It is important that we send the welcome message first. The client expects this as the initial response to their identification message.
-		defaultChannel.addClient(clientThread);
-		clientThread.setCurrentChannel(defaultChannel);
-		System.out.println("Moved client " + clientThread + " to " + defaultChannel);
-	}
-
-	/**
-	 * De-registers a client from the server, removing them from any channel
-	 * they're currently in.
-	 * @param clientId The id of the client to remove.
-	 */
-	public void deregisterClient(UUID clientId) {
-		var client = this.clients.remove(clientId);
-		if (client != null) {
-			client.getCurrentChannel().removeClient(client);
-			client.shutdown();
-			System.out.println("Client " + client + " has disconnected.");
-		}
-	}
-
 	/**
 	 * @return True if the server is currently running, meaning it is accepting
 	 * connections, or false otherwise.
@@ -155,13 +123,6 @@ public class ConcordServer implements Runnable {
 		}
 	}
 
-	public List<UserData> getClients() {
-		return this.clients.values().stream()
-				.sorted(Comparator.comparing(ClientThread::getClientNickname))
-				.map(ClientThread::toData)
-				.collect(Collectors.toList());
-	}
-
 	public ServerMetaData getMetaData() {
 		return new ServerMetaData(
 				this.config.getName(),
@@ -172,24 +133,6 @@ public class ConcordServer implements Runnable {
 		);
 	}
 
-	/**
-	 * Sends a message to every connected client, ignoring any channels. All
-	 * clients connected to this server will receive this message.
-	 * @param message The message to send.
-	 */
-	public void broadcast(Message message) {
-		ByteArrayOutputStream baos = new ByteArrayOutputStream(message.getByteCount());
-		try {
-			this.serializer.writeMessage(message, baos);
-			byte[] data = baos.toByteArray();
-			for (var client : this.clients.values()) {
-				client.sendToClient(data);
-			}
-		} catch (IOException e) {
-			e.printStackTrace();
-		}
-	}
-
 	/**
 	 * Shuts down the server cleanly by doing the following things:
 	 * <ol>
@@ -201,8 +144,8 @@ public class ConcordServer implements Runnable {
 	 */
 	private void shutdown() {
 		System.out.println("Shutting down the server.");
-		for (var clientId : this.clients.keySet()) {
+		for (var clientId : this.clientManager.getConnectedIds()) {
-			this.deregisterClient(clientId);
+			this.clientManager.deregisterClient(clientId);
 		}
 		this.scheduledExecutorService.shutdown();
 		this.executorService.shutdown();
@@ -231,12 +174,16 @@ public class ConcordServer implements Runnable {
 				ClientThread clientThread = new ClientThread(socket, this);
 				clientThread.start();
 			} catch (IOException e) {
-				System.err.println("Could not accept new client connection: " + e.getMessage());
+				if (!e.getMessage().equalsIgnoreCase("socket closed")) {
+					System.err.println("Could not accept new client connection: " + e.getMessage());
+				}
 			}
 		}
 		this.shutdown();
 	}
 
+
+
 	public static void main(String[] args) throws IOException {
 		var server = new ConcordServer();
 		new Thread(server).start();

server/src/main/java/nl/andrewl/concord_server/channel/Channel.java

@@ -1,11 +1,11 @@
-package nl.andrewl.concord_server;
+package nl.andrewl.concord_server.channel;
 
 import lombok.Getter;
 import nl.andrewl.concord_core.msg.Message;
-import nl.andrewl.concord_core.msg.Serializer;
-import nl.andrewl.concord_core.msg.types.ChannelUsersResponse;
 import nl.andrewl.concord_core.msg.types.UserData;
-import org.dizitart.no2.IndexOptions;
+import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.client.ClientThread;
+import nl.andrewl.concord_server.util.CollectionUtils;
 import org.dizitart.no2.IndexType;
 import org.dizitart.no2.NitriteCollection;
 
@@ -16,51 +16,36 @@ import java.util.concurrent.ConcurrentHashMap;
 
 /**
  * Represents a single communication area in which messages are sent by clients
- * and received by all connected clients.
+ * and received by all connected clients. A channel is a top-level communication
+ * medium, and usually this is a server channel or private message between two
+ * clients in a server.
  */
 @Getter
 public class Channel {
 	private final ConcordServer server;
-	private UUID id;
+	private final UUID id;
 	private String name;
 
 	private final Set<ClientThread> connectedClients;
 
+	/**
+	 * A document collection which holds all messages created in this channel,
+	 * indexed on id, timestamp, message, and sender's nickname.
+	 */
 	private final NitriteCollection messageCollection;
 
-	public Channel(ConcordServer server, UUID id, String name, NitriteCollection messageCollection) {
+	public Channel(ConcordServer server, UUID id, String name) {
 		this.server = server;
 		this.id = id;
 		this.name = name;
 		this.connectedClients = ConcurrentHashMap.newKeySet();
-		this.messageCollection = messageCollection;
+		this.messageCollection = server.getDb().getCollection("channel-" + id);
-		this.initCollection();
+		CollectionUtils.ensureIndexes(this.messageCollection, Map.of(
-	}
+				"timestamp", IndexType.NonUnique,
-
+				"senderNickname", IndexType.Fulltext,
-	/**
+				"message", IndexType.Fulltext,
-	 * Initializes this channel's nitrite database collection, which involves
+				"id", IndexType.Unique
-	 * creating any indexes that don't yet exist.
+		));
-	 */
-	private void initCollection() {
-		if (!this.messageCollection.hasIndex("timestamp")) {
-			System.out.println("Adding index on \"timestamp\" field to collection " + this.messageCollection.getName());
-			this.messageCollection.createIndex("timestamp", IndexOptions.indexOptions(IndexType.NonUnique));
-		}
-		if (!this.messageCollection.hasIndex("senderNickname")) {
-			System.out.println("Adding index on \"senderNickname\" field to collection " + this.messageCollection.getName());
-			this.messageCollection.createIndex("senderNickname", IndexOptions.indexOptions(IndexType.Fulltext));
-		}
-		if (!this.messageCollection.hasIndex("message")) {
-			System.out.println("Adding index on \"message\" field to collection " + this.messageCollection.getName());
-			this.messageCollection.createIndex("message", IndexOptions.indexOptions(IndexType.Fulltext));
-		}
-		var fields = List.of("timestamp", "senderNickname", "message");
-		for (var index : this.messageCollection.listIndices()) {
-			if (!fields.contains(index.getField())) {
-				System.out.println("Dropping unknown index " + index.getField() + " from collection " + index.getCollectionName());
-				this.messageCollection.dropIndex(index.getField());
-			}
-		}
 	}
 
 	/**
@@ -70,11 +55,11 @@ public class Channel {
 	 */
 	public void addClient(ClientThread clientThread) {
 		this.connectedClients.add(clientThread);
-		try {
+//		try {
-			this.sendMessage(new ChannelUsersResponse(this.getUserData()));
+//			this.sendMessage(new ChannelUsersResponse(this.getUserData()));
-		} catch (IOException e) {
+//		} catch (IOException e) {
-			e.printStackTrace();
+//			e.printStackTrace();
-		}
+//		}
 	}
 
 	/**
@@ -84,11 +69,11 @@ public class Channel {
 	 */
 	public void removeClient(ClientThread clientThread) {
 		this.connectedClients.remove(clientThread);
-		try {
+//		try {
-			this.sendMessage(new ChannelUsersResponse(this.getUserData()));
+//			this.sendMessage(new ChannelUsersResponse(this.getUserData()));
-		} catch (IOException e) {
+//		} catch (IOException e) {
-			e.printStackTrace();
+//			e.printStackTrace();
-		}
+//		}
 	}
 
 	/**
@@ -125,12 +110,13 @@ public class Channel {
 	public boolean equals(Object o) {
 		if (this == o) return true;
 		if (!(o instanceof Channel channel)) return false;
-		return name.equals(channel.name);
+		if (Objects.equals(this.id, channel.getId())) return true;
+		return Objects.equals(this.name, channel.getName());
 	}
 
 	@Override
 	public int hashCode() {
-		return Objects.hash(name);
+		return Objects.hash(id, name);
 	}
 
 	@Override

server/src/main/java/nl/andrewl/concord_server/channel/ChannelManager.java

@@ -0,0 +1,174 @@
+package nl.andrewl.concord_server.channel;
+
+import nl.andrewl.concord_core.msg.types.MoveToChannel;
+import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.client.ClientThread;
+import nl.andrewl.concord_server.util.CollectionUtils;
+import org.dizitart.no2.Document;
+import org.dizitart.no2.IndexType;
+import org.dizitart.no2.NitriteCollection;
+import org.dizitart.no2.filters.Filters;
+
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+
+/**
+ * This manager is responsible for keeping track of all the channels in the
+ * server, and controlling modifications to them.
+ */
+public class ChannelManager {
+	private final ConcordServer server;
+	private final Map<String, Channel> channelNameMap;
+	private final Map<UUID, Channel> channelIdMap;
+
+	private final Map<Set<UUID>, Channel> privateChannels;
+	private final NitriteCollection privateChannelCollection;
+
+	public ChannelManager(ConcordServer server) {
+		this.server = server;
+		this.channelNameMap = new ConcurrentHashMap<>();
+		this.channelIdMap = new ConcurrentHashMap<>();
+		this.privateChannels = new ConcurrentHashMap<>();
+		this.privateChannelCollection = this.server.getDb().getCollection("private-channels");
+		CollectionUtils.ensureIndexes(this.privateChannelCollection, Map.of(
+				"idHash", IndexType.Unique,
+				"id", IndexType.Unique
+		));
+		// Initialize the channels according to what's defined in the server's config.
+		for (var channelConfig : server.getConfig().getChannels()) {
+			this.addChannel(new Channel(server, UUID.fromString(channelConfig.getId()), channelConfig.getName()));
+		}
+	}
+
+	public Set<Channel> getChannels() {
+		return Set.copyOf(this.channelIdMap.values());
+	}
+
+	public Optional<Channel> getDefaultChannel() {
+		var optionalDefault = this.getChannelByName(this.server.getConfig().getDefaultChannel());
+		if (optionalDefault.isPresent()) {
+			return optionalDefault;
+		}
+		System.err.println("Could not find a channel with the name \"" + this.server.getConfig().getDefaultChannel() + "\".");
+		for (var channel : this.getChannels()) {
+			return Optional.of(channel);
+		}
+		System.err.println("Could not find any channel to use as a default channel.");
+		return Optional.empty();
+	}
+
+	public void addChannel(Channel channel) {
+		this.channelNameMap.put(channel.getName(), channel);
+		this.channelIdMap.put(channel.getId(), channel);
+	}
+
+	public void removeChannel(Channel channel) {
+		this.channelNameMap.remove(channel.getName());
+		this.channelIdMap.remove(channel.getId());
+	}
+
+	public Optional<Channel> getChannelByName(String name) {
+		return Optional.ofNullable(this.channelNameMap.get(name));
+	}
+
+	public Optional<Channel> getChannelById(UUID id) {
+		return Optional.ofNullable(this.channelIdMap.get(id));
+	}
+
+	/**
+	 * Moves a client to the given channel. This involves removing the client
+	 * from whatever channel they're currently in, if any, moving them to the
+	 * new channel, and sending them a message to indicate that it has been done.
+	 * @param client The client to move.
+	 * @param channel The channel to move the client to.
+	 */
+	public void moveToChannel(ClientThread client, Channel channel) {
+		if (client.getCurrentChannel() != null) {
+			var previousChannel = client.getCurrentChannel();
+			previousChannel.removeClient(client);
+		}
+		channel.addClient(client);
+		client.setCurrentChannel(channel);
+		client.sendToClient(new MoveToChannel(channel.getId(), channel.getName()));
+		System.out.println("Moved client " + client + " to channel " + channel);
+	}
+
+	/**
+	 * Gets or creates a private channel for the given client ids to be able to
+	 * communicate together. No other clients are allowed to access the channel.
+	 * @param clientIds The id of each client which should have access to the
+	 *                  channel.
+	 * @return The private channel.
+	 */
+	public Channel getPrivateChannel(Set<UUID> clientIds) {
+		if (clientIds.size() < 2) {
+			throw new IllegalArgumentException("At least 2 client ids are required for a private channel.");
+		}
+		return this.privateChannels.computeIfAbsent(clientIds, this::getPrivateChannelFromDatabase);
+	}
+
+	/**
+	 * Gets a private channel, given the id of a client who is part of the
+	 * channel, and the id of the channel.
+	 * @param clientId The id of the client that's requesting the channel.
+	 * @param channelId The id of the private channel.
+	 * @return The private channel.
+	 */
+	public Optional<Channel> getPrivateChannel(UUID clientId, UUID channelId) {
+		Channel privateChannel = this.privateChannels.entrySet().stream()
+				.filter(entry -> entry.getKey().contains(clientId) && entry.getValue().getId().equals(channelId))
+				.findAny().map(Map.Entry::getValue).orElse(null);
+		if (privateChannel == null) {
+			var cursor = this.privateChannelCollection.find(Filters.and(Filters.eq("id", channelId), Filters.in("clientIds", clientId)));
+			Document channelInfo = cursor.firstOrDefault();
+			if (channelInfo != null) {
+				privateChannel = new Channel(
+						this.server,
+						channelInfo.get("id", UUID.class),
+						channelInfo.get("name", String.class)
+				);
+				Set<UUID> clientIds = Set.of(channelInfo.get("clientIds", UUID[].class));
+				this.privateChannels.put(clientIds, privateChannel);
+			}
+		}
+		return Optional.ofNullable(privateChannel);
+	}
+
+	/**
+	 * Gets and instantiates a private channel from information stored in the
+	 * "private-channels" collection of the database, or creates it if it does
+	 * not exist yet.
+	 * @param clientIds The set of client ids that the channel is for.
+	 * @return The private channel.
+	 */
+	private Channel getPrivateChannelFromDatabase(Set<UUID> clientIds) {
+		// First check if a private channel for these clients exists in the database.
+		String idHash = clientIds.stream().sorted().map(UUID::toString).collect(Collectors.joining());
+		var cursor = this.privateChannelCollection.find(Filters.eq("idHash", idHash));
+		Document channelInfo = cursor.firstOrDefault();
+		if (channelInfo != null) {
+			// If it does exist, instantiate a channel with its info.
+			return new Channel(
+					this.server,
+					channelInfo.get("id", UUID.class),
+					channelInfo.get("name", String.class)
+			);
+		} else {
+			// Otherwise, create the channel anew and save it in the collection.
+			var channel = new Channel(this.server, this.server.getIdProvider().newId(), "Private Channel");
+			channelInfo = new Document(Map.of(
+					"idHash", idHash,
+					"id", channel.getId(),
+					"name", channel.getName(),
+					"clientIds", clientIds.toArray(new UUID[0])
+			));
+			this.privateChannelCollection.insert(channelInfo);
+			System.out.println("Created new private channel for clients: " + clientIds);
+			return channel;
+		}
+	}
+}

server/src/main/java/nl/andrewl/concord_server/cli/command/AddChannelCommand.java

@@ -1,12 +1,15 @@
 package nl.andrewl.concord_server.cli.command;
 
-import nl.andrewl.concord_server.Channel;
 import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.channel.Channel;
 import nl.andrewl.concord_server.cli.ServerCliCommand;
 import nl.andrewl.concord_server.config.ServerConfig;
 
 import java.util.UUID;
 
+/**
+ * This command adds a new channel to the server.
+ */
 public class AddChannelCommand implements ServerCliCommand {
 	@Override
 	public void handle(ConcordServer server, String[] args) throws Exception {
@@ -32,8 +35,7 @@ public class AddChannelCommand implements ServerCliCommand {
 		server.getConfig().getChannels().add(channelConfig);
 		server.getConfig().save();
 
-		var col = server.getDb().getCollection("channel-" + id);
+		server.getChannelManager().addChannel(new Channel(server, id, name));
-		server.getChannelManager().addChannel(new Channel(server, id, name, col));
+		server.getClientManager().broadcast(server.getMetaData());
-		server.broadcast(server.getMetaData());
 	}
 }

server/src/main/java/nl/andrewl/concord_server/cli/command/ListClientsCommand.java

@@ -3,10 +3,13 @@ package nl.andrewl.concord_server.cli.command;
 import nl.andrewl.concord_server.ConcordServer;
 import nl.andrewl.concord_server.cli.ServerCliCommand;
 
+/**
+ * This command shows a list of all clients that are currently connected to the server.
+ */
 public class ListClientsCommand implements ServerCliCommand {
 	@Override
 	public void handle(ConcordServer server, String[] args) throws Exception {
-		var users = server.getClients();
+		var users = server.getClientManager().getClients();
 		if (users.isEmpty()) {
 			System.out.println("There are no connected clients.");
 		} else {

server/src/main/java/nl/andrewl/concord_server/cli/command/RemoveChannelCommand.java

@@ -1,6 +1,6 @@
 package nl.andrewl.concord_server.cli.command;
 
-import nl.andrewl.concord_server.Channel;
+import nl.andrewl.concord_server.channel.Channel;
 import nl.andrewl.concord_server.ConcordServer;
 import nl.andrewl.concord_server.cli.ServerCliCommand;
 
@@ -38,7 +38,7 @@ public class RemoveChannelCommand implements ServerCliCommand {
 		server.getDb().getContext().dropCollection(channelToRemove.getMessageCollection().getName());
 		server.getConfig().getChannels().removeIf(channelConfig -> channelConfig.getName().equals(channelToRemove.getName()));
 		server.getConfig().save();
-		server.broadcast(server.getMetaData());
+		server.getClientManager().broadcast(server.getMetaData());
 		System.out.println("Removed the channel " + channelToRemove);
 	}
 }

server/src/main/java/nl/andrewl/concord_server/client/ClientManager.java

@@ -0,0 +1,101 @@
+package nl.andrewl.concord_server.client;
+
+import nl.andrewl.concord_core.msg.Message;
+import nl.andrewl.concord_core.msg.types.Identification;
+import nl.andrewl.concord_core.msg.types.ServerUsers;
+import nl.andrewl.concord_core.msg.types.ServerWelcome;
+import nl.andrewl.concord_core.msg.types.UserData;
+import nl.andrewl.concord_server.ConcordServer;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+
+/**
+ * The client manager is responsible for managing the set of clients connected
+ * to a server.
+ */
+public class ClientManager {
+	private final ConcordServer server;
+	private final Map<UUID, ClientThread> clients;
+
+	public ClientManager(ConcordServer server) {
+		this.server = server;
+		this.clients = new ConcurrentHashMap<>();
+	}
+
+	/**
+	 * Registers a new client as connected to the server. This is done once the
+	 * client thread has received the correct identification information from
+	 * the client. The server will register the client in its global set of
+	 * connected clients, and it will immediately move the client to the default
+	 * channel.
+	 * @param identification The client's identification data.
+	 * @param clientThread The client manager thread.
+	 */
+	public void registerClient(Identification identification, ClientThread clientThread) {
+		var id = this.server.getIdProvider().newId();
+		System.out.printf("Client \"%s\" joined with id %s.\n", identification.getNickname(), id);
+		this.clients.put(id, clientThread);
+		clientThread.setClientId(id);
+		clientThread.setClientNickname(identification.getNickname());
+		// Immediately add the client to the default channel and send the initial welcome message.
+		var defaultChannel = this.server.getChannelManager().getDefaultChannel().orElseThrow();
+		clientThread.sendToClient(new ServerWelcome(id, defaultChannel.getId(), defaultChannel.getName(), this.server.getMetaData()));
+		// It is important that we send the welcome message first. The client expects this as the initial response to their identification message.
+		defaultChannel.addClient(clientThread);
+		clientThread.setCurrentChannel(defaultChannel);
+		System.out.println("Moved client " + clientThread + " to " + defaultChannel);
+		this.broadcast(new ServerUsers(this.getClients()));
+	}
+
+	/**
+	 * De-registers a client from the server, removing them from any channel
+	 * they're currently in.
+	 * @param clientId The id of the client to remove.
+	 */
+	public void deregisterClient(UUID clientId) {
+		var client = this.clients.remove(clientId);
+		if (client != null) {
+			client.getCurrentChannel().removeClient(client);
+			client.shutdown();
+			System.out.println("Client " + client + " has disconnected.");
+			this.broadcast(new ServerUsers(this.getClients()));
+		}
+	}
+
+	/**
+	 * Sends a message to every connected client, ignoring any channels. All
+	 * clients connected to this server will receive this message.
+	 * @param message The message to send.
+	 */
+	public void broadcast(Message message) {
+		ByteArrayOutputStream baos = new ByteArrayOutputStream(message.getByteCount());
+		try {
+			this.server.getSerializer().writeMessage(message, baos);
+			byte[] data = baos.toByteArray();
+			for (var client : this.clients.values()) {
+				client.sendToClient(data);
+			}
+		} catch (IOException e) {
+			e.printStackTrace();
+		}
+	}
+
+	public List<UserData> getClients() {
+		return this.clients.values().stream()
+				.sorted(Comparator.comparing(ClientThread::getClientNickname))
+				.map(ClientThread::toData)
+				.collect(Collectors.toList());
+	}
+
+	public Set<UUID> getConnectedIds() {
+		return this.clients.keySet();
+	}
+
+	public Optional<ClientThread> getClientById(UUID id) {
+		return Optional.ofNullable(this.clients.get(id));
+	}
+}

server/src/main/java/nl/andrewl/concord_server/client/ClientThread.java

@@ -1,10 +1,12 @@
-package nl.andrewl.concord_server;
+package nl.andrewl.concord_server.client;
 
 import lombok.Getter;
 import lombok.Setter;
 import nl.andrewl.concord_core.msg.Message;
 import nl.andrewl.concord_core.msg.types.Identification;
 import nl.andrewl.concord_core.msg.types.UserData;
+import nl.andrewl.concord_server.channel.Channel;
+import nl.andrewl.concord_server.ConcordServer;
 
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
@@ -109,7 +111,7 @@ public class ClientThread extends Thread {
 		}
 
 		if (this.clientId != null) {
-			this.server.deregisterClient(this.clientId);
+			this.server.getClientManager().deregisterClient(this.clientId);
 		}
 		try {
 			if (!this.socket.isClosed()) {
@@ -133,7 +135,7 @@ public class ClientThread extends Thread {
 			try {
 				var msg = this.server.getSerializer().readMessage(this.in);
 				if (msg instanceof Identification id) {
-					this.server.registerClient(id, this);
+					this.server.getClientManager().registerClient(id, this);
 					return true;
 				}
 			} catch (IOException e) {

server/src/main/java/nl/andrewl/concord_server/config/ServerConfig.java

@@ -5,7 +5,7 @@ import com.fasterxml.jackson.databind.ObjectMapper;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
-import nl.andrewl.concord_server.IdProvider;
+import nl.andrewl.concord_server.util.IdProvider;
 
 import java.io.IOException;
 import java.io.UncheckedIOException;
@@ -23,6 +23,7 @@ public final class ServerConfig {
 	private int chatHistoryMaxCount;
 	private int chatHistoryDefaultCount;
 	private int maxMessageLength;
+	private String defaultChannel;
 	private List<ChannelConfig> channels;
 
 	private List<String> discoveryServers;
@@ -53,6 +54,7 @@ public final class ServerConfig {
 					100,
 					50,
 					8192,
+					"general",
 					List.of(new ChannelConfig(idProvider.newId().toString(), "general", "Default channel for general discussion.")),
 					List.of(),
 					filePath

server/src/main/java/nl/andrewl/concord_server/event/ChannelMoveHandler.java

@@ -1,16 +1,36 @@
 package nl.andrewl.concord_server.event;
 
+import nl.andrewl.concord_core.msg.types.Error;
 import nl.andrewl.concord_core.msg.types.MoveToChannel;
-import nl.andrewl.concord_server.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.client.ClientThread;
+
+import java.util.List;
+import java.util.Set;
 
 /**
- * Handles client requests to move to another channel.
+ * Handles client requests to move to another channel. We first check if the id
+ * which the client sent refers to a channel, in which case we move them to that
+ * channel. Otherwise, we look for a client with that id, and try to move the
+ * requester into a private channel with them.
  */
 public class ChannelMoveHandler implements MessageHandler<MoveToChannel> {
 	@Override
 	public void handle(MoveToChannel msg, ClientThread client, ConcordServer server) {
-		var optionalChannel = server.getChannelManager().getChannelById(msg.getChannelId());
+		var optionalChannel = server.getChannelManager().getChannelById(msg.getId());
-		optionalChannel.ifPresent(channel -> server.getChannelManager().moveToChannel(client, channel));
+		if (optionalChannel.isPresent()) {
+			server.getChannelManager().moveToChannel(client, optionalChannel.get());
+		} else {
+			var optionalClient = server.getClientManager().getClientById(msg.getId());
+			if (optionalClient.isPresent()) {
+				var privateChannel = server.getChannelManager().getPrivateChannel(Set.of(
+						client.getClientId(),
+						optionalClient.get().getClientId()
+				));
+				server.getChannelManager().moveToChannel(client, privateChannel);
+			} else {
+				client.sendToClient(Error.warning("Unknown channel or client id."));
+			}
+		}
 	}
 }

server/src/main/java/nl/andrewl/concord_server/event/ChannelUsersRequestHandler.java

@@ -2,7 +2,7 @@ package nl.andrewl.concord_server.event;
 
 import nl.andrewl.concord_core.msg.types.ChannelUsersRequest;
 import nl.andrewl.concord_core.msg.types.ChannelUsersResponse;
-import nl.andrewl.concord_server.ClientThread;
+import nl.andrewl.concord_server.client.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
 
 public class ChannelUsersRequestHandler implements MessageHandler<ChannelUsersRequest> {

server/src/main/java/nl/andrewl/concord_server/event/ChatHandler.java

@@ -2,13 +2,18 @@ package nl.andrewl.concord_server.event;
 
 import nl.andrewl.concord_core.msg.types.Chat;
 import nl.andrewl.concord_core.msg.types.Error;
-import nl.andrewl.concord_server.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.client.ClientThread;
 import org.dizitart.no2.Document;
 
 import java.io.IOException;
 import java.util.Map;
 
+/**
+ * This handler is responsible for taking incoming chat messages and saving them
+ * to the channel's message collection, and then relaying the new message to all
+ * clients in the channel.
+ */
 public class ChatHandler implements MessageHandler<Chat> {
 	@Override
 	public void handle(Chat msg, ClientThread client, ConcordServer server) throws IOException {
@@ -16,16 +21,22 @@ public class ChatHandler implements MessageHandler<Chat> {
 			client.getCurrentChannel().sendMessage(Error.warning("Message is too long."));
 			return;
 		}
-		server.getExecutorService().submit(() -> {
+		/*
-			var collection = client.getCurrentChannel().getMessageCollection();
+		When we receive a message from the client, it will have a random UUID.
-			Document doc = new Document(Map.of(
+		A compromised client could try and send a duplicate or otherwise
-					"senderId", msg.getSenderId(),
+		malicious UUID, so we overwrite it with a server-generated id which we
-					"senderNickname", msg.getSenderNickname(),
+		know is safe.
-					"timestamp", msg.getTimestamp(),
+		 */
-					"message", msg.getMessage()
+		msg.setId(server.getIdProvider().newId());
-			));
+		var collection = client.getCurrentChannel().getMessageCollection();
-			collection.insert(doc);
+		Document doc = new Document(Map.of(
-		});
+				"id", msg.getId(),
+				"senderId", msg.getSenderId(),
+				"senderNickname", msg.getSenderNickname(),
+				"timestamp", msg.getTimestamp(),
+				"message", msg.getMessage()
+		));
+		collection.insert(doc);
 		System.out.printf("#%s | %s: %s\n", client.getCurrentChannel(), client.getClientNickname(), msg.getMessage());
 		client.getCurrentChannel().sendMessage(msg);
 	}

server/src/main/java/nl/andrewl/concord_server/event/ChatHistoryRequestHandler.java

@@ -3,9 +3,10 @@ package nl.andrewl.concord_server.event;
 import nl.andrewl.concord_core.msg.types.Chat;
 import nl.andrewl.concord_core.msg.types.ChatHistoryRequest;
 import nl.andrewl.concord_core.msg.types.ChatHistoryResponse;
-import nl.andrewl.concord_server.Channel;
+import nl.andrewl.concord_core.msg.types.Error;
-import nl.andrewl.concord_server.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.channel.Channel;
+import nl.andrewl.concord_server.client.ClientThread;
 import org.dizitart.no2.*;
 import org.dizitart.no2.filters.Filters;
 
@@ -17,10 +18,21 @@ import java.util.*;
 public class ChatHistoryRequestHandler implements MessageHandler<ChatHistoryRequest> {
 	@Override
 	public void handle(ChatHistoryRequest msg, ClientThread client, ConcordServer server) {
-		var optionalChannel = server.getChannelManager().getChannelById(msg.getChannelId());
+		// First try and find a public channel with the given id.
-		if (optionalChannel.isPresent()) {
+		var channel = server.getChannelManager().getChannelById(msg.getChannelId()).orElse(null);
-			var channel = optionalChannel.get();
+		if (channel == null) {
-			var params = msg.getQueryAsMap();
+			// Couldn't find a public channel, so look for a private channel this client is involved in.
+			channel = server.getChannelManager().getPrivateChannel(client.getClientId(), msg.getChannelId()).orElse(null);
+		}
+		// If we couldn't find a public or private channel, give up.
+		if (channel == null) {
+			client.sendToClient(Error.warning("Unknown channel id."));
+			return;
+		}
+		var params = msg.getQueryAsMap();
+		if (params.containsKey("id")) {
+			this.handleIdRequest(client, channel, params.get("id"));
+		} else {
 			Long count = this.getOrDefault(params, "count", (long) server.getConfig().getChatHistoryDefaultCount());
 			if (count > server.getConfig().getChatHistoryMaxCount()) {
 				return;
@@ -31,14 +43,19 @@ public class ChatHistoryRequestHandler implements MessageHandler<ChatHistoryRequ
 		}
 	}
 
-	private Long getOrDefault(Map<String, String> params, String key, Long defaultValue) {
+	/**
-		String value = params.get(key);
+	 * Handles a request for a single message from a channel.
-		if (value == null) return defaultValue;
+	 * @param client The client who's requesting the data.
-		try {
+	 * @param channel The channel in which to search for the message.
-			return Long.parseLong(value);
+	 * @param id The id of the message.
-		} catch (NumberFormatException e) {
+	 */
-			return defaultValue;
+	private void handleIdRequest(ClientThread client, Channel channel, String id) {
+		var cursor = channel.getMessageCollection().find(Filters.eq("id", id));
+		List<Chat> chats = new ArrayList<>(1);
+		for (var doc : cursor) {
+			chats.add(this.read(doc));
 		}
+		client.sendToClient(new ChatHistoryResponse(channel.getId(), chats));
 	}
 
 	private ChatHistoryResponse getResponse(Channel channel, long count, Long from, Long to) {
@@ -60,15 +77,43 @@ public class ChatHistoryRequestHandler implements MessageHandler<ChatHistoryRequ
 
 		List<Chat> chats = new ArrayList<>((int) count);
 		for (Document doc : cursor) {
-			chats.add(new Chat(
+			chats.add(this.read(doc));
-					doc.get("senderId", UUID.class),
-					doc.get("senderNickname", String.class),
-					doc.get("timestamp", Long.class),
-					doc.get("message", String.class)
-			));
 		}
 		col.close();
 		chats.sort(Comparator.comparingLong(Chat::getTimestamp));
 		return new ChatHistoryResponse(channel.getId(), chats);
 	}
+
+	/**
+	 * Helper method to read a {@link Chat} from a document retrieved from a
+	 * collection.
+	 * @param doc The document to read.
+	 * @return The chat that was read.
+	 */
+	private Chat read(Document doc) {
+		return new Chat(
+				doc.get("id", UUID.class),
+				doc.get("senderId", UUID.class),
+				doc.get("senderNickname", String.class),
+				doc.get("timestamp", Long.class),
+				doc.get("message", String.class)
+		);
+	}
+
+	/**
+	 * Helper method to get a long value or fall back to a default.
+	 * @param params The parameters to check.
+	 * @param key The key to get the value for.
+	 * @param defaultValue The default value to return if no value exists.
+	 * @return The value that was found, or the default value.
+	 */
+	private Long getOrDefault(Map<String, String> params, String key, Long defaultValue) {
+		String value = params.get(key);
+		if (value == null) return defaultValue;
+		try {
+			return Long.parseLong(value);
+		} catch (NumberFormatException e) {
+			return defaultValue;
+		}
+	}
 }

server/src/main/java/nl/andrewl/concord_server/event/EventListener.java

@@ -1,7 +1,7 @@
 package nl.andrewl.concord_server.event;
 
 import nl.andrewl.concord_core.msg.types.Chat;
-import nl.andrewl.concord_server.ClientThread;
+import nl.andrewl.concord_server.client.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
 
 public interface EventListener {

server/src/main/java/nl/andrewl/concord_server/event/EventManager.java

@@ -1,12 +1,12 @@
-package nl.andrewl.concord_server;
+package nl.andrewl.concord_server.event;
 
 import lombok.extern.java.Log;
 import nl.andrewl.concord_core.msg.Message;
-import nl.andrewl.concord_core.msg.types.ChannelUsersRequest;
 import nl.andrewl.concord_core.msg.types.Chat;
 import nl.andrewl.concord_core.msg.types.ChatHistoryRequest;
 import nl.andrewl.concord_core.msg.types.MoveToChannel;
-import nl.andrewl.concord_server.event.*;
+import nl.andrewl.concord_server.ConcordServer;
+import nl.andrewl.concord_server.client.ClientThread;
 
 import java.util.HashMap;
 import java.util.Map;
@@ -25,7 +25,6 @@ public class EventManager {
 		this.messageHandlers = new HashMap<>();
 		this.messageHandlers.put(Chat.class, new ChatHandler());
 		this.messageHandlers.put(MoveToChannel.class, new ChannelMoveHandler());
-		this.messageHandlers.put(ChannelUsersRequest.class, new ChannelUsersRequestHandler());
 		this.messageHandlers.put(ChatHistoryRequest.class, new ChatHistoryRequestHandler());
 	}
 

server/src/main/java/nl/andrewl/concord_server/event/MessageHandler.java

@@ -1,7 +1,7 @@
 package nl.andrewl.concord_server.event;
 
 import nl.andrewl.concord_core.msg.Message;
-import nl.andrewl.concord_server.ClientThread;
+import nl.andrewl.concord_server.client.ClientThread;
 import nl.andrewl.concord_server.ConcordServer;
 
 public interface MessageHandler<T extends Message> {

server/src/main/java/nl/andrewl/concord_server/util/CollectionUtils.java

@@ -0,0 +1,32 @@
+package nl.andrewl.concord_server.util;
+
+import org.dizitart.no2.IndexOptions;
+import org.dizitart.no2.IndexType;
+import org.dizitart.no2.NitriteCollection;
+
+import java.util.Map;
+
+public class CollectionUtils {
+	/**
+	 * Ensures that the given nitrite collection has exactly the given set of
+	 * indexes. It will remove any non-conforming indexes, and create new ones
+	 * as necessary.
+	 * @param collection The collection to operate on.
+	 * @param indexMap A mapping containing keys referring to fields, and values
+	 *                 that represent the type of index that should be on that
+	 *                 field.
+	 */
+	public static void ensureIndexes(NitriteCollection collection, Map<String, IndexType> indexMap) {
+		for (var index : collection.listIndices()) {
+			var entry = indexMap.get(index.getField());
+			if (entry == null || !index.getIndexType().equals(entry)) {
+				collection.dropIndex(index.getField());
+			}
+		}
+		for (var entry : indexMap.entrySet()) {
+			if (!collection.hasIndex(entry.getKey())) {
+				collection.createIndex(entry.getKey(), IndexOptions.indexOptions(entry.getValue()));
+			}
+		}
+	}
+}

server/src/main/java/nl/andrewl/concord_server/util/IdProvider.java

@@ -1,4 +1,4 @@
-package nl.andrewl.concord_server;
+package nl.andrewl.concord_server.util;
 
 import java.util.UUID;
 

server/src/main/java/nl/andrewl/concord_server/util/UUIDProvider.java

@@ -1,4 +1,4 @@
-package nl.andrewl.concord_server;
+package nl.andrewl.concord_server.util;
 
 import java.util.UUID;