{"id":926,"date":"2020-07-31T15:52:46","date_gmt":"2020-07-31T15:52:46","guid":{"rendered":"http:\/\/spiderwiz.org\/project\/?page_id=926"},"modified":"2025-04-09T06:15:58","modified_gmt":"2025-04-09T06:15:58","slug":"lesson-9","status":"publish","type":"page","link":"https:\/\/spiderwiz.org\/project\/tutorial\/lesson-9\/","title":{"rendered":"Lesson 9: Manual Data Object Reset"},"content":{"rendered":"\n

In the previous lessons we led you through the Spiderwiz paradigm of the Shared Data Object Tree<\/em>. You exercised with application code that accessed data rather than communicating with services. One of the major concepts of this paradigm is that as soon as the application starts, it gets all the data that it needs in its space and can access and manipulate it with no extra mechanism.<\/p>\n\n\n\n

Under the hood, this concept is implemented with a mechanism called Data Object Reset<\/em>. When an application starts (or when data integrity is compromised, see below<\/a>), it broadcasts a Reset<\/em> request for all the object types it consumes. Producers of these types respond by sending the objects that they have created over the channel through which the Reset request arrived.<\/p>\n\n\n\n

Normally the entire process is automatic and programmers need not intervene. However there are cases when the programmer wants to control the process programmatically, for instance for resetting disposable<\/a> objects that are fetched from an external source and not kept in the shared data object tree. This technique is demonstrated in this lesson.<\/p>\n\n\n\n

We stay with the chat service of the previous lessons and add a new service \u2013 Room Producer<\/em> \u2013 that creates chat rooms by reading a list of room names (possibly with the \u2018+\u2019 adult indicator) from a file, one name per line. Room Producer<\/em> does not keep the rooms in memory (it overrides isDisposable()<\/a> of ChatRoom<\/code> to return true<\/code>), so every time a consumer of this object type asks for reset it needs to open the file, read it, create ChatRoom<\/code> objects from it and send them to the requester. This is done in RoomProducerMain<\/code>:<\/p>\n\n\n\n

package org.spiderwiz.tutorial.lesson9.roomProducer;\n\nimport java.io.BufferedReader;\nimport java.io.FileInputStream;\nimport java.io.IOException;\nimport java.io.InputStreamReader;\nimport java.util.List;\nimport org.spiderwiz.core.DataObject;\nimport org.spiderwiz.core.Resetter;\nimport org.spiderwiz.tutorial.objectLib.ChatRoom;\nimport org.spiderwiz.tutorial.objectLib.ConsoleMain;\n\n\/**\n * Provides the entry point of the application. Initializes and executes the Spiderwiz framework.\n *\/\npublic class RoomProducerMain extends ConsoleMain {\n    private static final String ROOT_DIRECTORY = "";\n    private static final String CONF_FILENAME = "room-producer.conf";\n    private static final String APP_NAME = "Room Producer";\n    private static final String APP_VERSION = "Z1.01";  \/\/ Version Z1.01: Initial version\n    private static final String ROOM_FILE = "room file";\n\n    \/**\n     * Class constructor with constant parameters.\n     *\/\n    public RoomProducerMain() {\n        super(ROOT_DIRECTORY, CONF_FILENAME, APP_NAME, APP_VERSION);\n    }\n    \n    \/**\n     * Application entry point. Instantiate the class and initialize the instance.\n     * \n     * @param args the command line arguments. Not used in this application.\n     *\/\n    public static void main(String[] args) {\n        new RoomProducerMain().init();\n    }\n\n    \/**\n     * @return the list of produced objects, in this case ChatRoom is the only one.\n     *\/\n    @Override\n    protected String[] getProducedObjects() {\n        return new String[]{ChatRoom.ObjectCode};\n    }\n\n    \/**\n     * @return the list of consumed objects, in this case we do not consume any so we return an empty list.\n     *\/\n    @Override\n    protected String[] getConsumedObjects() {\n        return new String[]{};\n    }\n\n    \/**\n     * Add ChatRoom class to the object factory list of this application.\n     * @param factoryList\n     *\/\n    @Override\n    protected void populateObjectFactory(List<Class<? extends DataObject>> factoryList) {\n        super.populateObjectFactory(factoryList);\n        factoryList.add(ChatRoomProducer.class);\n    }\n\n    \/**\n     * If room file is not defined in the configuration file, log an error message in the log file and in stdout and quit\n     * the application.\n     * @return true if and only if a configuration file is defined.\n     *\/\n    @Override\n    protected boolean preStart() {\n        if (getConfig().getProperty(ROOM_FILE) == null) {\n            getLogger().logEvent("Room file is not defined.");\n            return false;\n        }\n        return true;\n    }\n\n    \/**\n     * If the resetter object is for ChatRoom reset all chat rooms from the file defined in the configuration file and return true;\n     * @param resetter\n     * @return true if reset is done here.\n     *\/\n    @Override\n    protected boolean onObjectReset(Resetter resetter) {\n        \/\/ Check resetter type\n        if (!resetter.getObjectCode().equals(ChatRoom.ObjectCode))\n            return false;\n        \n        \/\/ Set reset rate to zero in order to eliminate moderation so that every object is sent as soon as possible\n        resetter.setResetRate(0);\n        \n        \/\/ Get configured file name\n        String filename = getConfig().getProperty(ROOM_FILE);\n        \n        \/\/ Read lines from the file\n        try (BufferedReader in = new BufferedReader(\n                new InputStreamReader(new FileInputStream(filename))))\n        {\n            String line;\n            while ((line = in.readLine()) != null) {\n                \/\/ Check if the resetter has not been aborted by an overriding reset request.\n                if (resetter.isAborted())\n                    return true;\n                \n                \/\/ Parse the line for room name and adult symbol\n                String name[] = line.split("+", -1);\n                String roomName = name[0];\n                boolean adult = name.length > 1;\n                \n                \/\/ Create a ChatRoom object, set its properties and reset it.\n                ChatRoom obj = createTopLevelObject(ChatRoom.class, roomName);\n                obj.setAdult(adult);\n                resetter.resetObject(obj);\n            }\n            \n            \/\/ Mark end of data\n            resetter.endOfData();\n        } catch (IllegalAccessException | NoSuchFieldException ex) {\n            sendExceptionMail(ex, "When creating a ChatRoom object", null, false);\n        } catch (IOException ex) {\n            getLogger().logEvent("Open file failure: %1$s.", ex.getMessage());\n            sendNotificationMail("Open file failure", ex.getMessage(), null, true);\n        }\n        return true;\n    }\n\n    \/**\n     * Called after delivery of all items. Print item count.\n     * @param resetter  the Resetter object used for delivery.\n     *\/\n    @Override\n    protected void onResetCompleted(Resetter resetter) {\n        System.out.printf("Reset done. %d items have been delivered.n", resetter.getResetCount());\n    }\n\n    \/**\n     * @param line\n     * @return true since we do not do any line processing\n     *\/\n    @Override\n    protected boolean processConsoleLine(String line) {\n        return true;\n    }\n}<\/pre><\/div>\n\n\n\n
As expected, the application produces ChatRoom<\/code> (line 43) and does not consume anything (line 51). It extends ChatRoom<\/code> by ChatRoomProducer<\/code> and registers it (line 62).<\/p>\n\n\n\n
The file that contains the room names should be specified in the configuration file. In preStart()<\/a> (line 71) we verify the existence of the room file<\/code> property and terminate the program by returning false<\/code> if it does not.<\/p>\n\n\n\n
The interesting method that performs the reset is the overriding onObjectReset()<\/a> (line 85). The method is called every time a reset for a specific object type that is produced by this application is requested by any peer application. Its parameter is a Resetter<\/a> object that specifies the requested object type and also serves as a carrier that delivers objects of this type back to the requester, as we are going to see now.<\/p>\n\n\n\n
The first thing onObjectReset()<\/code> does is to check whether the requested type is what we care about. This is done by calling getObjectCode()<\/a> of the resetter<\/code> parameter and comparing it to ChatRoom.ObjectCode<\/code>.<\/p>\n\n\n\n
We call the object\u2019s setResetRate()<\/a> to eliminate its default moderation effect, because in our test we expect to deliver a relatively small amount of objects that should better be delivered as quickly as possible. If the test file contained a very large number of items and the network speed were constrained then we could set an appropriate rate (or leave the default of 30,000 items per minute) to avoid network congestion.<\/p>\n\n\n\n
We then open the configured input file and loop on reading its lines. Within each iteration we first verify that the resetter is still active by calling isAborted()<\/a>. It might be aborted if, during its operation, another reset request for the same object type arrives on the same channel. In this case, in order to avoid unnecessary duplicate data, the framework aborts the operation of the former resetter assuming that the data would be completely delivered by the latter.<\/p>\n\n\n\n
If everything is OK, we parse the input line, create one object by calling createTopLevelObject()<\/a>, set its values and call the resetter\u2019s resetObject()<\/a> method to deliver the object.<\/p>\n\n\n\n
When we reach the file end we call the reseter\u2019s optional endOfData()<\/a> method. We will see its use in a moment.<\/p>\n\n\n\n
We will use the occasion to dwell upon the two catch<\/code> clauses that enclose the code. The first catches IllegalAccessException<\/code> and NoSuchFieldException<\/code> exceptions that might happen if ChatRoom<\/code> class does not define an ObjectCode<\/code> static field or the field is not declared public<\/code>. In these cases we call sendExceptionMail()<\/a> to report the exception and its stack trace to the following:<\/p>\n\n\n\n