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| |
| <div class="section"> |
| <h2>Events System Configuration<a name="Events_System_Configuration"></a></h2> |
| |
| |
| <p>Consult <a class="externalLink" href="https://github.com/apache/james-project/tree/master/server/app/src/main/resources/events-template.xml">events-template.xml</a> in GIT to get some examples and hints.</p> |
| |
| |
| <p>Use this configuration to define the type of Event System you want.</p> |
| |
| |
| <p> |
| James relies on an event system. Each operations performed on the mailbox will trigger related events. Some software |
| components (MailboxListeners) can register themselves on this event system to be called when an event is fired. |
| </p> |
| |
| |
| <p> |
| Here are typical use cases for Mailbox Listeners (non exhaustive list) : |
| </p> |
| <ul> |
| |
| <li>Message search indexation, for instance in Lucene or ElasticSearch</li> |
| |
| <li>Local cache invalidation (caching mailbox project)</li> |
| |
| <li>Quota calculation</li> |
| |
| <li>IMAP IDLE feature : live notification of actions performed on a mailbox, allowing publish subscribe on mailboxes events</li> |
| |
| <li>Message Sequence Number consistence</li> |
| </ul> |
| The Mailbox Listeners can be classified in two categories : |
| |
| <ul> |
| |
| <li>Mailbox registered : The mailbox listener is only notified on events affecting this mailbox. IDLE is a good example of this.</li> |
| |
| <li>Global Listeners : This event listener is triggered upon each events.</li> |
| </ul> |
| Note that Global Listeners can also be classified in two categories : |
| |
| <ul> |
| |
| <li>Those which needs to be triggered only once in your cluster. For instance ElasticSearch indexing is an example of this.</li> |
| |
| <li>Those which needs to be triggered on each servers. For instance, each Lucene indexer needs to be triggered on each server |
| for the search feature to stay consistent.</li> |
| </ul> |
| |
| |
| |
| <p> |
| The default implementation is a synchronous in memory event system. The performance are really good, as their is no need to serialize |
| events, and no network overhead. However, this event system is limited to one computer and you might want a distributed systems. |
| </p> |
| |
| |
| <p> |
| Other implementations, distributed environment friendly are available. |
| </p> |
| |
| |
| <p> |
| The simplest one is broadcast based. Each James servers listen the same message queue, and each James server will be notified upon events. |
| Here are the pros and cons of this implementations : |
| </p> |
| <ul> |
| Pros: |
| |
| <li>It supports every type of listener described above</li> |
| |
| <li>It allows you to scale your James infrastructure without changing your middlewares. You just need a message queue</li> |
| Cons : |
| |
| <li>Your scalability is limited as each servers is notified on all events</li> |
| |
| <li>Network overhead on event transmissions</li> |
| |
| <li>Event serialization and deserialization</li> |
| </ul> |
| To use this implementation, you need two other components (that will be discussed) : a publishing system and an event serializer |
| |
| |
| |
| <p> |
| The other mode is based on registrations. |
| Each server reads messages from a dedicated message queue, and other servers send messages addressed to this sever on this message queue. |
| Registrations are performed on an eternal data-store supporting document deletion after a fixed amount of time. |
| These registrations are periodically refreshed. This data-store is then triggered on event generation, that, if needed are serialized and |
| send to the given queues. |
| The pros and cons of this implementations are : |
| </p> |
| <ul> |
| Pros : |
| |
| <li>Linear scalability</li> |
| |
| <li>A server receives only events concerning him</li> |
| Cons : |
| |
| <li>Possible event serialization costs</li> |
| |
| <li>Registration and registration refresh costs</li> |
| |
| <li>Need to find interested servers on event delivery</li> |
| |
| <li>Network overhead on event transmissions</li> |
| </ul> |
| |
| |
| |
| <div class="section"> |
| <h2>Failure modes<a name="Failure_modes"></a></h2> |
| |
| |
| <ul> |
| Default implementation : |
| |
| <li>The default implementation might not deliver some events on server stop.</li> |
| </ul> |
| |
| |
| <ul> |
| Broadcast implementation : |
| |
| <li>The broadcast implementation might not deliver some events on server stop.</li> |
| |
| <li>The broadcast implementation is tight to limitation of the underlying publisher.</li> |
| </ul> |
| |
| |
| <ul> |
| Registered implementation : |
| |
| <li>The registered implementation might not deliver some events on server stop.</li> |
| |
| <li>The registered implementation is tight to limitation of the underlying publisher, and underlying registration system.</li> |
| </ul> |
| |
| |
| <div class="section"> |
| <h2>Publisher<a name="Publisher"></a></h2> |
| |
| |
| <p> |
| Available implementation is Kafka based. Kafka ensure at least one delivery. This means some messages might be |
| delivered two times. You need to compile and run James using Java 8 in order to use the Kafka messaging system. |
| </p> |
| |
| |
| <div class="section"> |
| <h2>Event serializer<a name="Event_serializer"></a></h2> |
| |
| |
| <p>There are two types of event serialization systems : |
| </p> |
| <ul> |
| |
| <li>Json : events are converted to JSON</li> |
| |
| <li>Message Pack : a binary representation of JSON. 2 times smaller in average but two times longer to compute. It allows you to trade |
| bandwidth and data readability against CPU time.</li> |
| </ul> |
| |
| |
| |
| <div class="section"> |
| <h2>Registration systems<a name="Registration_systems"></a></h2> |
| |
| |
| <p> |
| Available implementation is based on Cassandra. It is used on an AP fashion, enforcing availability instead of consistency. Some |
| messages might get delivered to no more registered servers. This is just extra work. Worst, messages might not be delivered to |
| recently registered servers. But we make sure that we have the more up to date version of the registrations we can, and will not time out in |
| the face of network partitions, nor enforce some default behaviour. |
| </p> |
| |
| </div> |
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