Websockets: Notify other clients of changes

9 files changed, 373 insertions, 322 deletions

diff --git a/Makefile b/Makefile
index cb5ef3e..658e764 100755
--- a/Makefile
+++ b/Makefile
@@ -13,7 +13,7 @@ VERSION=$(shell dpkg-parsechangelog --show-field Version)
 TGZNAME=$(PROJECTNAME)-$(VERSION).tar.xz
 
 INCLUDES=-I.
-HEADERS=config.h qrcode.h storage.h whiteboard.h compiledsql.h
+HEADERS=config.h qrcode.h storage.h whiteboard.h compiledsql.h connectionregistry.h
 SOURCES=$(HEADERS:.h=.cpp)
 OBJECTS=$(HEADERS:.h=.o)
 TARGETS=whiteboard
@@ -21,7 +21,7 @@ TARGETS=whiteboard
 build: $(TARGETS)
 
 all: build
-	./start.sh
+	./whiteboard -c whiteboard.conf
 
 install:
 	mkdir -p $(DESTDIR)/usr/bin
diff --git a/connectionregistry.cpp b/connectionregistry.cpp
new file mode 100644
index 0000000..1e48a96
--- /dev/null
+++ b/connectionregistry.cpp
@@ -0,0 +1,95 @@
+#include "connectionregistry.h"
+
+#include <iostream>
+#include <utility>
+
+void ConnectionRegistry::setId(ConnectionRegistry::connection c, const std::string& id)
+{
+ if (!m_connections.at(c).empty()) {
+  auto& connections_set{m_ids.at(id)};
+  connections_set.erase(c);
+  if (connections_set.empty())
+   m_ids.erase(id);
+ }
+ 
+ m_connections.at(c) = id;
+
+ if (!id.empty()) {
+  if (!m_ids.contains(id))
+   m_ids[id] = {};
+  m_ids.at(id).insert(c);
+ }
+
+}
+
+void ConnectionRegistry::addConnection(ConnectionRegistry::connection c)
+{
+ if (m_connections.contains(c))
+  throw std::runtime_error("ConnectionRegistry::addConnection: connection already exists");
+
+ m_connections.emplace(c, "");
+
+ std::cout << "Info: Added connection, now " << m_connections.size() << std::endl;
+}
+
+void ConnectionRegistry::delConnection(ConnectionRegistry::connection c)
+{
+ if (!m_connections.contains(c))
+  throw std::runtime_error("ConnectionRegistry::delConnection: connection doesn't exist");
+
+ std::string id {m_connections.at(c)};
+
+ m_connections.erase(c);
+
+ if (!id.empty()) {
+  auto& connections_set{m_ids.at(id)};
+  connections_set.erase(c);
+  if (connections_set.empty())
+   m_ids.erase(id);
+ }
+ 
+ std::cout << "Info: Deleted connection, now " << m_connections.size() << std::endl;
+}
+
+std::unordered_set<ConnectionRegistry::connection>::iterator ConnectionRegistry::begin(const std::string& id)
+{
+ return m_ids.at(id).begin();
+}
+
+std::unordered_set<ConnectionRegistry::connection>::iterator ConnectionRegistry::end(const std::string& id)
+{
+ return m_ids.at(id).end();
+}
+
+void ConnectionRegistry::dump()
+{
+ std::cout << "Connection Registry:" << std::endl;
+ 
+ std::cout << "Connections: " << std::endl;
+ for (auto& i: m_connections) {
+  std::cout << "    " << i.first << ": " << i.second << std::endl;
+ }
+ 
+ std::cout << "IDs: " << std::endl;
+ for (auto& i: m_ids) {
+  std::cout << "    " << i.first << ":";
+
+  for (auto& j: i.second) {
+   std::cout << " " << j;
+  }
+  std::cout << std::endl;
+ }
+}
+
+ConnectionRegistry::RegistryGuard::RegistryGuard(ConnectionRegistry& registry, ConnectionRegistry::connection c):
+ m_registry{registry},
+ m_connection{c}
+{
+ m_registry.addConnection(m_connection);
+}
+
+ConnectionRegistry::RegistryGuard::~RegistryGuard()
+{
+ m_registry.delConnection(m_connection);
+}
+
diff --git a/connectionregistry.h b/connectionregistry.h
new file mode 100644
index 0000000..2b14553
--- /dev/null
+++ b/connectionregistry.h
@@ -0,0 +1,46 @@
+#pragma once
+
+#include <algorithm>
+#include <memory>
+#include <unordered_map>
+#include <unordered_set>
+
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/beast/websocket.hpp>
+
+class ConnectionRegistry
+{
+public:
+ using connection = std::shared_ptr<boost::beast::websocket::stream<boost::asio::ip::tcp::socket>>;
+
+ ConnectionRegistry() = default;
+ ~ConnectionRegistry() = default;
+
+ void setId(connection c, const std::string& id);
+
+ // used via RegistryGuard
+ void addConnection(connection c);
+ void delConnection(connection c);
+
+ // map connection to id
+ std::unordered_map<connection, std::string> m_connections;
+ // map id to list of related connections, used for iteration over connections to notify about changes
+ std::unordered_map<std::string, std::unordered_set<connection>> m_ids;
+
+ std::unordered_set<connection>::iterator begin(const std::string& id);
+ std::unordered_set<connection>::iterator end(const std::string& id);
+
+ void dump();
+
+ class RegistryGuard
+ {
+ public:
+  RegistryGuard(ConnectionRegistry& registry, connection c);
+  ~RegistryGuard();
+ private:
+  ConnectionRegistry& m_registry;
+  connection m_connection;
+ };
+
+};
+
diff --git a/debian/whiteboard.conf b/debian/whiteboard.conf
new file mode 100644
index 0000000..126bef5
--- /dev/null
+++ b/debian/whiteboard.conf
@@ -0,0 +1,26 @@
+<config>
+ <!--
+  datapath: location in filesystem to store whiteboard data
+  Example: /var/lib/whiteboard
+ -->
+ <datapath>/var/lib/whiteboard</datapath>
+
+ <!--
+  port: socket to listen on for websocket
+  Example: ::1:8765
+ -->
+ <port>::1:8765</port>
+
+ <!--
+  Maximum age of individual whiteboard pages in seconds.
+  Older pages will be removed by whiteboard-cleanup and crond.
+  Example: 2592000 (~30 days)
+ -->
+ <maxage>2592000</maxage>
+ 
+ <!--
+  Number of threads to use
+  Example: 4
+ -->
+ <threads>4</threads>
+</config>
diff --git a/html/whiteboard.js b/html/whiteboard.js
index 7777d4e..9610468 100644
--- a/html/whiteboard.js
+++ b/html/whiteboard.js
@@ -3,145 +3,126 @@ function init() {
 	init_board();
 }
 
-class AdjustingTimer {
-	constructor() {
-		this.update_counter = 0; // counting seconds since last counter reset
-	}
+var revision;
 
-	fast_mode() {
-		if (this.update_counter < 5*60)
-			return true;
-		return false;
-	}
+// helper for breaking feedback loop
+var caretpos = 0;
 
-	// private method
-	// returns current interval in ms
-	current_update_interval()
-	{
-	 if (this.fast_mode())
-	  return 2000; // 2s
-	 else
-	  return 5 * 60000; // 5min
-	};
+function showQRWindow()
+{
+ document.getElementById("qrwindow").style.display = 'block';
+}
 
-	// private
-	count() {
-		this.update_counter += this.current_update_interval() / 1000;
-	};
+function hideQRWindow()
+{
+ document.getElementById("qrwindow").style.display = 'none';
+}
 
-	// private
-	on_timeout() {
-		this.m_fn();
-		this.count();
-		var _this = this;
-		this.update_timer = setTimeout(function(){_this.on_timeout();}, this.current_update_interval());
-	};
+var websocket;
 
-	// to be called once on startup
-	start(fn) {
-		this.m_fn = fn;
-		var _this = this;
-		this.update_timer = setTimeout(function(){_this.on_timeout();}, this.current_update_interval());
-	};
+//
+// Callbacks for websocket data of different types
+//
 
-	// to be called on activity:
-	// * changes from remote
-	// * changes by ourselves
-	// * local activity, e.g. mouse move, or key presses
-	reset() {
-		if (!this.fast_mode()) {
-			this.update_counter = 0;
-			clearTimeout(this.update_timer);
-			var _this = this;
-			this.update_timer = setTimeout(function(){_this.on_timeout();}, this.current_update_interval());
-		} else {
-			this.update_counter = 0;
-		}
-	};
+function on_getfile(data, rev, pos)
+{
+	var board = document.getElementById("board");
+	if (board.value != data) {
+		board.value = data;
+	}
+	textAreaSetPos("board", pos);
+	revision = rev;
 }
 
-var timer = new AdjustingTimer();
+function on_newid(id)
+{
+	var new_location = document.location.origin + document.location.pathname + '?id=' + id;
+	window.location.href = new_location;
+}
 
-function showQRWindow()
+function on_qrcode(png)
 {
- document.getElementById("qrwindow").style.display = 'block';
+	var blob = new Blob([png], {type: 'image/png'});
+	var url = URL.createObjectURL(blob);
+	var img = document.getElementById("qrcode");
+	img.src = url;
+	showQRWindow();
 }
 
-function hideQRWindow()
+function on_modify_ack(rev)
 {
- document.getElementById("qrwindow").style.display = 'none';
+	revision = rev;
 }
 
-function init_board() {
-	var xhr = new XMLHttpRequest();
+function on_message(e) {
+	var parser = new DOMParser();
+	var xmlDocument = parser.parseFromString(e.data, "text/xml");
 	
-        const searchParams = (new URL(document.location)).searchParams;
-        if (!searchParams.has('id')) {
-                redirect_to_new_page();
-                return;
-        }
-
-	// run on data received back
-	xhr.onreadystatechange = function() {
-				if (this.readyState == 3) {
-					//set_status("Please wait while downloading " + filename + " ...");
-					return;
-				}
-                                if (this.readyState != 4) {
-					return;
-				}
-				if (this.status != 200) {
-					//set_status("Server Error while retrieving " + filename + ", status: " + this.status + " " + this.statusText);
-                                        return;
-                                }
-
-                                var file = new Blob([this.response]);
-				reader = new FileReader();
-				reader.onload = function() {
-					var board = document.getElementById("board");
-					var pos = reader.result.indexOf('\x01');
-					if (pos == -1) { // not found
-					 board.value = reader.result;
-					} else {
-					 board.value = reader.result.substr(0, pos) + reader.result.substr(pos + 1);
-					}
-					textAreaSetPos("board", pos);
-
-					// Initialization done. Now we can start modifying.
-					board.addEventListener("input", function() {on_modify(); });
-					board.addEventListener("selectionchange", function() {on_modify(); });
-
-					// Initialization done. Now we can start modifying.
-					document.addEventListener("mousemove", function() {timer.reset(); });
-					
-					timer.start(checkupdate);
-				}
-				
-			        reader.readAsBinaryString(file);
-
-				//set_status(""); // OK
+	var type = xmlDocument.getElementsByTagName("type")[0].textContent;
+
+	if (type == "getfile") {
+		on_getfile(xmlDocument.getElementsByTagName("data")[0].textContent,
+			   parseInt(xmlDocument.getElementsByTagName("revision")[0].textContent),
+			   parseInt(xmlDocument.getElementsByTagName("cursorpos")[0].textContent));
+	} else if (type == "modify") {
+		on_modify_ack(parseInt(xmlDocument.getElementsByTagName("revision")[0].textContent));
+	} else if (type == "newid") {
+		on_newid(xmlDocument.getElementsByTagName("id")[0].textContent);
+	} else if (type == "qrcode") {
+		on_qrcode(xmlDocument.getElementsByTagName("png")[0].textContent);
+	} else if (type == "error") {
+		alert(xmlDocument.getElementsByTagName("message")[0].textContent);
+	} else {
+		alert("Unhandled message type: " + e.data + "|" + type);
 	}
+}
 
-	var parser = new DOMParser();
-	var xmlDocument = parser.parseFromString("<request></request>", "text/xml");
-	
-	var requestElement = xmlDocument.getElementsByTagName("request")[0];
+function handleSelection() {
+  const activeElement = document.activeElement
 
-	var commandElement = xmlDocument.createElement("command");
-	commandElement.appendChild(document.createTextNode("getfile"));
-	requestElement.appendChild(commandElement);
+  if (activeElement && activeElement.id === 'board') {
+    if (caretpos != activeElement.selectionStart) {
+      on_selectionchange(activeElement.selectionStart);
+      caretpos = activeElement.selectionStart;
+    }
+  }
+}
 
-	var idElement = xmlDocument.createElement("id");
-	idElement.appendChild(document.createTextNode(get_id()));
-	requestElement.appendChild(idElement);
+function init_board() {
+	var newlocation = location.origin + location.pathname;
+	newlocation = newlocation.replace(/^http/, 'ws');
+	if (newlocation.slice(-1) != "/")
+		newlocation += "/";
+	newlocation += "websocket";
+	websocket = new WebSocket(newlocation);
+
+	websocket.onmessage = function(e) { on_message(e); };
+
+	websocket.onopen = function(e) {
+		const searchParams = (new URL(document.location)).searchParams;
+		if (!searchParams.has('id')) {
+			redirect_to_new_page();
+			return;
+		}
 
-	xhr.open("POST", "whiteboard.fcgi", true);
-	xhr.setRequestHeader("Content-type", "text/xml");
-	xhr.responseType = 'blob';
-	xhr.send(xmlDocument);
+		var board = document.getElementById("board");
+		board.addEventListener("input", function() {on_input(); });
+                // Need this workaround (different from direct on_selectionchange) for Chrome.
+		// Otherwise, callback will not be called on Chrome.
+		document.addEventListener("selectionchange", handleSelection);
+		//board.addEventListener("selectionchange", function() {on_selectionchange(); });
+
+		websocket.send("<request><command>getfile</command><id>" + get_id() + "</id></request>");
+	};
+	
+	websocket.onclose = function(e) {
+		alert("Server connection closed.");
+	};
+
+	websocket.onerror = function(e) {
+		alert("Error: Server connection closed.");
+	};
 
-	//set_status("Please wait while server prepares " + filename + " ...");
-        
         document.getElementById("qrwindow").onclick = function() {
          hideQRWindow();
         }
@@ -161,81 +142,20 @@ function get_id()
         return searchParams.get('id');
 }
 
+// from html
 function on_new_page()
 {
-        redirect_to_new_page();
+	redirect_to_new_page();
 }
 
 function redirect_to_new_page()
 {
-	var xhr = new XMLHttpRequest();
-	
-	// run on data received back
-	xhr.onreadystatechange = function() {
-				if (this.readyState == 3) {
-					//set_status("Please wait while downloading " + filename + " ...");
-					return;
-				}
-                                if (this.readyState != 4) {
-					return;
-				}
-				if (this.status != 200) {
-					//set_status("Server Error while retrieving " + filename + ", status: " + this.status + " " + this.statusText);
-                                        return;
-                                }
-
-                                var id = this.responseText;
-                                //alert("location=" + document.location.href);
-                                var new_location = document.location.href;
-                                var pos = new_location.search("\\?");
-                                if (pos >= 0)
-                                        new_location = new_location.substring(0, pos);
-                                new_location += '?id=' + id;
-
-                                window.location.href = new_location;
-				//set_status(""); // OK
-	}
-
-	var parser = new DOMParser();
-	var xmlDocument = parser.parseFromString("<request></request>", "text/xml");
-	
-	var requestElement = xmlDocument.getElementsByTagName("request")[0];
-
-	var commandElement = xmlDocument.createElement("command");
-	commandElement.appendChild(document.createTextNode("newid"));
-	requestElement.appendChild(commandElement);
-
-	xhr.open("POST", "whiteboard.fcgi", true);
-	xhr.setRequestHeader("Content-type", "text/xml");
-	xhr.send(xmlDocument);
-
-	//set_status("Please wait while server prepares " + filename + " ...");
+	websocket.send("<request><command>newid</command></request>");
 }
 
 // local change done
-function on_modify()
+function on_input()
 {
-	timer.reset();
-
-	var xhr = new XMLHttpRequest();
-	
-	// run on data received back
-	xhr.onreadystatechange = function() {
-				if (this.readyState == 3) {
-					//set_status("Please wait while downloading " + filename + " ...");
-					return;
-				}
-                                if (this.readyState != 4) {
-					return;
-				}
-				if (this.status != 200) {
-					//set_status("Server Error while retrieving " + filename + ", status: " + this.status + " " + this.statusText);
-                                        return;
-                                }
-
-				//set_status(""); // OK
-	}
-
 	var parser = new DOMParser();
 	var xmlDocument = parser.parseFromString("<request></request>", "text/xml");
 	
@@ -250,131 +170,47 @@ function on_modify()
 	requestElement.appendChild(idElement);
 
 	var dataElement = xmlDocument.createElement("data");
-	dataElement.appendChild(document.createTextNode(addPos(document.getElementById("board").value, document.getElementById("board").selectionStart)));
+	dataElement.appendChild(document.createTextNode(document.getElementById("board").value));
 	requestElement.appendChild(dataElement);
 
-	xhr.open("POST", "whiteboard.fcgi", true);
-	xhr.setRequestHeader("Content-type", "text/xml");
-	xhr.responseType = 'blob';
-	xhr.send(xmlDocument);
-
-	//set_status("Please wait while server prepares " + filename + " ...");
-}
-
-// checksum of string
-function checksum32(s) {
-	var result = 0;
-	for (var i = 0; i < s.length; i++) {
-		result = ((((result >>> 1) | ((result & 1) << 31)) | 0) ^ (s.charCodeAt(i) & 0xFF)) | 0;
-	}
-	return (result & 0x7FFFFFFF) | 0;
+	websocket.send(new XMLSerializer().serializeToString(xmlDocument));
 }
 
-function textAreaSetPos(id, pos)
+// for cursor position
+function on_selectionchange(pos)
 {
-	document.getElementById(id).selectionStart = pos;
-	document.getElementById(id).selectionEnd = pos;
-}
-
-function addPos(s, pos)
-{
-	return s.substr(0, pos) + '\x01' + s.substr(pos);
-}
-
-// gets called by regular polling
-function checkupdate() {
-	var xhr = new XMLHttpRequest();
-	
-	// run on data received back
-	xhr.onreadystatechange = function() {
-				if (this.readyState == 3) {
-					//set_status("Please wait while downloading " + filename + " ...");
-					return;
-				}
-                                if (this.readyState != 4) {
-					return;
-				}
-				if (this.status != 200) {
-					//set_status("Server Error while retrieving " + filename + ", status: " + this.status + " " + this.statusText);
-                                        return;
-                                }
-
-				// no change if response is text/plain
-				if (this.getResponseHeader("Content-Type") == "application/octet-stream") {
-					timer.reset();
-					var file = new Blob([this.response]);
-					reader = new FileReader();
-					reader.onload = function() {
-						var board = document.getElementById("board");
-						var pos = reader.result.indexOf('\x01');
-						if (pos == -1) { // not found
-						 board.value = reader.result;
-						} else {
-						 board.value = reader.result.substr(0, pos) + reader.result.substr(pos + 1);
-						}
-						textAreaSetPos("board", pos);
-					}
-					
-					reader.readAsBinaryString(file);
-				}
-
-				//set_status(""); // OK
-	}
-
 	var parser = new DOMParser();
 	var xmlDocument = parser.parseFromString("<request></request>", "text/xml");
 	
 	var requestElement = xmlDocument.getElementsByTagName("request")[0];
 
 	var commandElement = xmlDocument.createElement("command");
-	commandElement.appendChild(document.createTextNode("checkupdate"));
+	commandElement.appendChild(document.createTextNode("cursorpos"));
 	requestElement.appendChild(commandElement);
 
 	var idElement = xmlDocument.createElement("id");
 	idElement.appendChild(document.createTextNode(get_id()));
 	requestElement.appendChild(idElement);
 
-	var checksumElement = xmlDocument.createElement("checksum");
-	checksumElement.appendChild(document.createTextNode(checksum32(addPos(document.getElementById("board").value, document.getElementById("board").selectionStart))));
-	requestElement.appendChild(checksumElement);
-
-	xhr.open("POST", "whiteboard.fcgi", true);
-	xhr.setRequestHeader("Content-type", "text/xml");
-	xhr.responseType = 'blob';
-	xhr.send(xmlDocument);
+	var dataElement = xmlDocument.createElement("pos");
+	dataElement.appendChild(document.createTextNode(pos));
+	requestElement.appendChild(dataElement);
 
-	//set_status("Please wait while server prepares " + filename + " ...");
+	websocket.send(new XMLSerializer().serializeToString(xmlDocument));
 }
 
-function on_qrcode()
+function textAreaSetPos(id, pos)
 {
-	var xhr = new XMLHttpRequest();
-	
-	// run on data received back
-	xhr.onreadystatechange = function() {
-				if (this.readyState == 3) {
-					//set_status("Please wait while downloading " + filename + " ...");
-					return;
-				}
-                                if (this.readyState != 4) {
-					return;
-				}
-				if (this.status != 200) {
-					//set_status("Server Error while retrieving " + filename + ", status: " + this.status + " " + this.statusText);
-                                        return;
-                                }
-
-				if (this.getResponseHeader("Content-Type") == "image/png") {
-					var blob = new Blob([this.response], {type: 'image/png'});
-                                        var url = URL.createObjectURL(blob);
-					var img = document.getElementById("qrcode");
-                                        img.src = url;
-                                        showQRWindow();
-				}
-
-				//set_status(""); // OK
+	if (document.getElementById(id).selectionStart != pos) {
+		document.getElementById(id).selectionStart = pos;
+		document.getElementById(id).selectionEnd = pos;
+		caretpos = pos;
 	}
+}
 
+// HTML button
+function on_qrcode()
+{
 	var parser = new DOMParser();
 	var xmlDocument = parser.parseFromString("<request></request>", "text/xml");
 	
@@ -388,9 +224,6 @@ function on_qrcode()
 	idElement.appendChild(document.createTextNode(document.location));
 	requestElement.appendChild(idElement);
 
-	xhr.open("POST", "whiteboard.fcgi", true);
-	xhr.setRequestHeader("Content-type", "text/xml");
-	xhr.responseType = 'blob';
-	xhr.send(xmlDocument);
+	websocket.send(new XMLSerializer().serializeToString(xmlDocument));
 }
 
diff --git a/tests/Makefile b/tests/Makefile
index 778fd37..15e4106 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -11,7 +11,7 @@ CXXFLAGS+=--coverage
 LDFLAGS+=--coverage
 endif
 
-UNITS=storage.cpp config.cpp compiledsql.cpp qrcode.cpp whiteboard.cpp
+UNITS=storage.cpp config.cpp compiledsql.cpp qrcode.cpp whiteboard.cpp connectionregistry.cpp
 
 UNITTESTS=test-config.cpp \
 	  test-storage.cpp \
@@ -49,6 +49,9 @@ unittests: libgmock.a $(UNITTESTS:.cpp=.o) $(UNITS:.cpp=.o)
 config.o: ../config.cpp
 	$(CXX) $(CXXFLAGS) -o $@ -c $<
 
+connectionregistry.o: ../connectionregistry.cpp
+	$(CXX) $(CXXFLAGS) -o $@ -c $<
+
 storage.o: ../storage.cpp
 	$(CXX) $(CXXFLAGS) -o $@ -c $<
 
diff --git a/whiteboard.conf b/whiteboard.conf
index 126bef5..055e7ba 100644
--- a/whiteboard.conf
+++ b/whiteboard.conf
@@ -9,7 +9,7 @@
   port: socket to listen on for websocket
   Example: ::1:8765
  -->
- <port>::1:8765</port>
+ <port>::1:9014</port>
 
  <!--
   Maximum age of individual whiteboard pages in seconds.
diff --git a/whiteboard.cpp b/whiteboard.cpp
index 6782385..b15ebbe 100644
--- a/whiteboard.cpp
+++ b/whiteboard.cpp
@@ -93,7 +93,30 @@ std::string make_xml(const std::initializer_list<std::pair<std::string, std::str
  return oss.str();
 }
 
-std::string Whiteboard::handle_request(const std::string& request)
+void Whiteboard::notify_other_connections(Whiteboard::connection& c, const std::string& id)
+{
+ std::for_each(m_registry.begin(id), m_registry.end(id), [&](const Whiteboard::connection& ci)
+               {
+                if (c != ci) {
+                 boost::beast::flat_buffer buffer;
+                 boost::beast::ostream(buffer) << make_xml({
+                   {"type", "getfile"},
+                   {"data", m_storage->getDocument(id)},
+                   {"revision", std::to_string(m_storage->getRevision(id)) },
+                   {"cursorpos", std::to_string(m_storage->getCursorPos(id)) }
+                   });
+                 std::lock_guard<std::mutex> lock(m_websocket_mutex);
+                 try {
+                  ci->write(buffer.data());
+                 } catch (const std::exception& ex) {
+                  std::cerr << "Warning: Notify write for " << ci << " not possible, id " << id << std::endl;
+                  m_registry.dump();
+                 }
+                }
+               });
+}
+
+std::string Whiteboard::handle_request(Whiteboard::connection& c, const std::string& request)
 {
  try {
   std::lock_guard<std::mutex> lock(m_storage_mutex);
@@ -109,27 +132,41 @@ std::string Whiteboard::handle_request(const std::string& request)
   if (command == "modify") {
    std::string id {xml.get<std::string>("request.id")};
    std::string data {xml.get<std::string>("request.data")};
-   m_storage->setDocument(id, data);
-   return make_xml({{"type", "modify"}, {"revision", std::to_string(m_storage->getRevision(id)) }}); 
+   if (m_storage->getDocument(id) != data) {
+    m_storage->setDocument(id, data);
+    m_registry.setId(c, id);
+    notify_other_connections(c, id);
+    return make_xml({{"type", "modify"}, {"revision", std::to_string(m_storage->getRevision(id)) }});
+   }
+   return {};
+  } else if (command == "cursorpos") {
+   std::string id {xml.get<std::string>("request.id")};
+   int pos {xml.get<int>("request.pos")};
+   if (m_storage->getCursorPos(id) != pos) {
+    m_storage->setCursorPos(id, pos);
+    notify_other_connections(c, id);
+   }
+   return {};
   } else if (command == "getfile") {
    std::string id {xml.get<std::string>("request.id")};
 
-   std::string filedata {m_storage->getDocument(id)};
+   std::string filedata;
+   try {
+    filedata = m_storage->getDocument(id);
+   } catch (const std::runtime_error&) {
+    m_storage->setDocument(id, filedata);
+   }
 
    if (filedata.size() > 30000000)
     throw std::runtime_error("File too big");
-
-   return make_xml({{"type", "getfile"}, {"data", filedata}, {"revision", std::to_string(m_storage->getRevision(id)) }}); 
-  } else if (command == "checkupdate") {
-   std::string id {xml.get<std::string>("request.id")};
-   int request_revision {xml.get<int>("request.revision")};
-
-   int revision {m_storage->getRevision(id)};
-   if (revision != request_revision) {
-    return make_xml({{"type", "update"}, {"data", m_storage->getDocument(id)}, {"revision", std::to_string(revision) }}); 
-   } else {
-    return {}; // no reply
-   }
+   m_registry.setId(c, id);
+
+   return make_xml({
+                   {"type", "getfile"},
+                   {"data", filedata},
+                   {"revision", std::to_string(m_storage->getRevision(id)) },
+                   {"cursorpos", std::to_string(m_storage->getCursorPos(id)) }
+                   }); 
   } else if (command == "newid") {
    return make_xml({{"type", "newid"}, {"id", m_storage->generate_id()}}); 
   } else if (command == "qrcode") {
@@ -146,7 +183,7 @@ std::string Whiteboard::handle_request(const std::string& request)
   }
 
  } catch (const std::exception& ex) {
-  return "Message handling error: "s + ex.what();
+  return make_xml({{"type", "error"}, {"message", "Message handling error: "s + ex.what()}}); 
  }
 }
 
@@ -154,10 +191,11 @@ void Whiteboard::do_session(boost::asio::ip::tcp::socket socket)
 {
  try {
   // Construct the stream by moving in the socket
-  boost::beast::websocket::stream<boost::asio::ip::tcp::socket> ws{std::move(socket)};
+  std::shared_ptr ws{std::make_shared<boost::beast::websocket::stream<boost::asio::ip::tcp::socket>>(std::move(socket))};
+  ConnectionRegistry::RegistryGuard guard(m_registry, ws);
 
   // Set a decorator to change the Server of the handshake
-  ws.set_option(boost::beast::websocket::stream_base::decorator(
+  ws->set_option(boost::beast::websocket::stream_base::decorator(
       [](boost::beast::websocket::response_type& res)
       {
        res.set(boost::beast::http::field::server,
@@ -168,27 +206,31 @@ void Whiteboard::do_session(boost::asio::ip::tcp::socket socket)
   boost::beast::http::request<boost::beast::http::string_body> req;
   boost::beast::flat_buffer buffer;
 
-  boost::beast::http::read(ws.next_layer(), buffer, parser);
+  boost::beast::http::read(ws->next_layer(), buffer, parser);
   req = parser.get();
 
-  ws.accept(req);
+  ws->accept(req);
 
   while (true) {
    boost::beast::flat_buffer buffer;
 
-   ws.read(buffer);
+   ws->read(buffer);
 
-   ws.text(ws.got_text());
+   ws->text(ws->got_text());
    std::string data(boost::asio::buffers_begin(buffer.data()), boost::asio::buffers_end(buffer.data()));
-   data = handle_request(data);
-   buffer.consume(buffer.size());
-   boost::beast::ostream(buffer) << data;
-   if (buffer.data().size() > 0)
-    ws.write(buffer.data());
+   data = handle_request(ws, data);
+   if (buffer.data().size() > 0) {
+    buffer.consume(buffer.size());
+   }
+   if (data.size() > 0) {
+    boost::beast::ostream(buffer) << data;
+    std::lock_guard<std::mutex> lock(m_websocket_mutex);
+    ws->write(buffer.data());
+   }
   }
  } catch (boost::beast::system_error const& se) {
   // This indicates that the session was closed
-  if (se.code() != boost::beast::websocket::error::closed)
+  if (se.code() != boost::beast::websocket::error::closed && se.code() != boost::asio::error::eof)
    std::cerr << "Boost system_error in session: " << se.code().message() << std::endl;
  } catch (std::exception const& ex) {
   std::cerr << "Error in session: " << ex.what() << std::endl;
diff --git a/whiteboard.h b/whiteboard.h
index d645115..e39b94e 100644
--- a/whiteboard.h
+++ b/whiteboard.h
@@ -8,6 +8,7 @@
 #include <boost/asio/ip/tcp.hpp>
 
 #include "config.h"
+#include "connectionregistry.h"
 #include "storage.h"
 
 class Whiteboard
@@ -20,8 +21,13 @@ private:
  std::unique_ptr<Config> m_config;
  std::unique_ptr<Storage> m_storage;
  std::mutex m_storage_mutex;
+ std::mutex m_websocket_mutex;
+
+ ConnectionRegistry m_registry;
  
- std::string handle_request(const std::string& request);
+ using connection = std::shared_ptr<boost::beast::websocket::stream<boost::asio::ip::tcp::socket>>;
+ std::string handle_request(connection& c, const std::string& request);
+ void notify_other_connections(connection& c, const std::string& id); // notify all other id-related connections about changes
  void do_session(boost::asio::ip::tcp::socket socket);
  void storage_cleanup();
 };