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PageBox Coordinator

Goal

This document has a pre-requisite you should read before: the PageBox Control document.

The Coordinator supports cross-control interactions for Web Application controls.

It aims to support the most common needs:

The Coordinator is a set of low-weight components designed to address Web Application control needs but it doesn’t use Web application objects. Therefore it can be used in other kinds of applications.

The Coordinator is packaged with the PageBox Grid API. The Grid API can be used by any kind of components and is actually designed to allow Web applications to communicate. This flexibility comes with a cost in term of infrastructure (repository, PageBoxes), configuration and programming. The Coordinator is a simpler environment limited to communications inside a single address space.

Principle

A Coordinator user is identified by its ID.

A Coordinator user can subscribe to events triggered by another user. The subscription creates a channel identified by its source ID, its destination ID and its session ID. A Coordinator user provides its session ID and its ID when it notifies an event.

In the same way, a Coordinator user provides its session ID and its ID when it puts an object on the heap.

A Coordinator user provides its session ID and the ID of the object source when it gets an object from the heap.

The Coordinator environment supports one-to-many communication: more than one Coordinator user can subscribe to an event or get data from a source. The Coordinator environment also supports many-to-many communication because many sources can notify an event or put an object on the heap with the same ID.

A Coordinator user can use different Ids either when it acts as a source or when it acts as a destination.

Notes:

Heap management

The Heap object is used to pass objects from one control to another on the same page or on different pages

The diagram shows a typical use of the Heap management:

Event management

To get notified about events occurring in other Controls, a Control uses a Subscribe method.

It provides an object (typically itself) implementing a Callback interface.

The Callback interface has a single method, Notify, which is called by the Coordinator when the event source calls the Notify method of the Coordinator.

The Coordinator implements veto-able events: If one of the event destinations answers false in its Notify method, then the Coordinator answers false to the source Notify call.

The Coordinator allows a Control to notify an Event to another Control on the same page

The diagram shows a typical use of the Event management:

Atomic transactions

To participate to transactions involving more than one control, a Control uses a Subscribe method.

It provides an object (typically itself) implementing a Transaction interface.

The Transaction interface has two methods, Prepare and Commit, which are called by the Coordinator when the Transaction controller (another Control) calls the Commit method of the Coordinator.

A Transaction participant typically checks that everything is ready to allow the subsequent Commit to succeed, for instance that the Web services invoked in the Commit are available and when it is not the case returns false. Then the Coordinator doesn’t call their Commit method and returns false to the source Commit call.

The Coordinator allows coordinating transactions that involve many Controls using two-phase commit

The diagram shows a typical use of the Event management:

  1. A Control Ctrl 2 and a Control Ctrl 3 subscribe to participate to a transaction driven by Ctrl 1

  2. The Control Ctrl 1 commits the transaction

  3. The Coordinator calls the Prepare method of Ctrl 1 and Ctrl 2. If the Prepare of either Ctrl 1 or Ctrl 2 fails, the Commit call issued by Ctrl 1 returns false

  4. Otherwise the Coordinator calls the Commit method of Ctrl 1 and Ctrl 2. If the Commit of either Ctrl 1 or Ctrl 2 fails, the Commit call issued by Ctrl 1 returns false

Transactions

Many documents have been written about transaction databases and two-phase commit.

In this discussion we focus on their internal design to explain the problem that they solve. Then we show that the same approach can also help to coordinate Web service invocations.

Global transaction coordinators were invented to allow synchronizing updates on two or more database instances, not necessarily from the same vendor.

Each database implements a SQL API with verbs such as select, update, insert, delete.

Global transaction management requires also using a small API with three verbs, begin, commit and rollback.

Two-phase commit performed by a Transaction coordinator for a global Transaction involving two Database instances

The diagram shows a typical global transaction processing.

  1. The User program calls the Begin verb to create a global transaction. The most important task is not displayed on the diagram. The SQL APIs are notified that a global transaction has started. The auto-commit is disabled. The first request creates a local transaction on the DB instance and records its Local transaction ID (LCID) in the transaction coordinator.

  2. The User program runs SQL requests. These requests are processed by the Database instances as usual

  3. The User program calls the Commit verb. Then the Transaction coordinator first sends a Prepare request to each Database instance involved in the transaction. When a Database instance answers YES it means that the instance should be able to commit its local transaction. When all Database instances answer YES the Transaction coordinator sends a Commit request to each Database instance involved in the transaction. Then and only then the Database instances commit the changes.

This prepare-commit mechanism is also called two-phase commit. This mechanism is not 100% safe. If a Database host fails between the Prepare and the Commit then something called heuristic commit takes place: some changes can be committed and some are not. The user program cannot know because it has no way to differentiate a failure occurred just after the Commit (then the change is committed) and a failure occurred just before.

However the two-phase commit turned to be quite effective for three reasons:

  1. RDBMSs are the main causes of failure at commit time

  2. RDBMSs can accurately predict if they will be able to commit a set of changes

  3. RDBMSs can make their prediction at a small resource cost

The latter points are worth an additional explanation.

Let’s assume that you have to predict that you will be able to append 100 bytes to a file. The only safe way is to write 100 bytes, perhaps binary zeroes. If you succeed then you probably will be able to replace these bytes by the real data. The implementation is not simple and you need to write on the disk two times instead of one. With RDBMSs the performance penalty is negligible because of their design.

A RDBMS is a pagination system. An uncommitted change can be written on disk whereas a committed change stays in memory. It doesn’t matters because the RDBMS also maintains change logs. What happens at commit is nothing more than logging.

Let’s consider the case of a Web page that uses two Web Application Controls. These Controls call Web services. The page also contains a Commit button. When the user clicks on this button we must invoke the Controls’ Web services hosted on different sites. Both or none of these Web services should be called. On Internet the main risk is that one host is not running. A two-phase commit can address this problem:

The main weakness of this initial approach is that the availability check has a significant impact on the response time. This overhead is due to the network latency and is perhaps 100ms on average. PageBox hosted Web Services can reduce this overhead to 20ms. The Coordinator reduces it further with caching. On an active Web server many users query the same pages at about the same time. When a Control answered true to a Prepare request, the Coordinator can safely assume that a Prepare request on the same Control issued 100ms later would also succeed.

API

The API is implemented though two interfaces, Callback and Transaction, and three classes, Heap, CallbackCoordinator, TransactionCoordinator.

Heap

The Heap class implements three static methods, Get, Put and Remove.

Get

Get has the following prototype:

object Get(string sourceID, string sessionID);

Put has this prototype:

void Put(string sourceID, string sessionID, object toStore);

Remove

Remove has this prototype:

void Remove(string sourceID, string sessionID);

sourceID is the ID of the object source. It should be retrieved from a parameter set by the page designer.

sessionID should be unique for a user. It is typically the Web server session ID.

toStore is an object stores in the heap by Put and returned by Get

Notes:

CallbackCoordinator

The CallbackCoordinator class implements two static methods, Subscribe and Notify.

Subscribe

Subscribe has the following prototype:

Void Subscribe(string sourceID, string sessionID, string ID, Callback cb);

Notify

Notify has the following prototype:

bool Notify(string sourceID, string sessionID, object req, ref Hashtable resp, HttpRequest request);

Notify is often used in one to many scenarios where a source notifies an event to many destinations, the destination list being unknown of the source. Each destination can return a response and the source can be interested by all responses. Therefore the Notify method populates a resp Hashtable passed by the caller (the source) with entries whose key is the destination ID and the value is an object, which can be

Note:

If resp is null it means that the source is not interested by the destination responses

Coding style:

You must check the type of the resp elements.

In C# you can use the is verb:

Hashtable ht = new Hashtable();

if (PageBoxGrid.CallbackCoordinator.Notify(ID, Session.SessionID, null, ref ht, Request))

labStatus.Text = "Event accepted";

else

labStatus.Text = "Event vetoed";

if (ht.Count > 0) {

IDictionaryEnumerator ide = ht.GetEnumerator();

ide.MoveNext();

if (ide.Value is Exception)

labInfo.Text = "[" + (string)ide.Key + "] " + ((Exception)ide.Value).Message;

else

if (ide.Value is string)

labInfo.Text = "[" + (string)ide.Key + "] " + (string)ide.Value;

}

In this snippet we enumerate the resp elements, we check their value and we display them.

TransactionCoordinator

The TransactionCoordinator class implements three static methods, Subscribe, Commit and Rollback.

Subscribe

Subscribe has the following prototype:

void Subscribe(string sourceID, string sessionID, string ID, Transaction trans, int timeout);

Commit

Commit has the following prototype:

bool Commit(string sourceID, string sessionID, object req, ref Hashtable resp, HttpRequest request);

Commit is normally used in one to many scenarios where a source commit a transaction involving many destinations, the destination list being unknown of the source. Each destination can return a response and the source can be interested by all responses. Therefore the Commit method populates a resp Hashtable passed by the caller (the source) with entries whose key is the destination ID and the value is an object, which can be

Commit returns true if all Subscriber Prepare and Commit have succeeded and false otherwise.

Note:

Coding style:

You must check the type of the resp elements.

In C# you can use the is verb:

Hashtable ht = new Hashtable();

if (!PageBoxGrid.TransactionCoordinator.Commit(ID, Session.SessionID, null, ref ht, Request)) {

if (ht.Count > 0) {

IDictionaryEnumerator ide = ht.GetEnumerator();

ide.MoveNext();

if (ide.Value is Exception)

labStatus.Text = "[" + (string)ide.Key + "] " + ((Exception)ide.Value).Message;

else

if (ide.Value is string)

labStatus.Text = "[" + (string)ide.Key + "] " + (string)ide.Value;

}

} else

labStatus.Text = "Transaction committed";

In this snippet we enumerate the resp elements, we check their value and we display them.

The developer of a PageBox control that register to a transaction should return in the Prepare and Commit resp parameter:

Rollback

Rollback has the following prototype:

bool Rollback(string sourceID, string sessionID, object req, ref Hashtable resp, HttpRequest request);

Rollback is normally used in one to many scenarios where a source rollbacks a transaction involving many destinations, the destination list being unknown of the source. Each destination can return a response and the source can be interested by all responses. Therefore the Rollback method populates a resp Hashtable passed by the caller (the source) with entries whose key is the destination ID and the value is an object, which can be

The Rollback function is to notify the transaction destinations that they must cleanup the changes they would have committed if Commit had been invoked. Rollback acts as a transaction delimiter. For a destination a transaction begins when

Therefore the implementer of a destination class must clean the changes in all Transaction interface methods:

Callback interface

The Callback interface has one method, Notify.

Notify has the following prototype:

bool Notify(string sourceID, object req, ref object resp, HttpRequest request);

Notify should return true in case of success and false otherwise.

When at least one subscriber returns false in the Notify method of its Callabck object the Notify method of the Event source returns false. It allows the subscribers to veto a change proposed by the Event source.

Transaction interface

The Transaction interface has three methods, Prepare, Commit and Rollback.

Prepare has the following prototype:

bool Prepare(string sourceID, object req, ref object resp, HttpRequest request);

Commit has this prototype:

bool Commit(string sourceID, object req, ref object resp, HttpRequest request);

Rollback has this prototype:

bool Rollback(string sourceID, object req, ref object resp, HttpRequest request);

These methods should return true in case of success and false otherwise.

Notes:

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2001-2004 Alexis Grandemange   Last modified