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Overview
From the platform point of view an unhandled message is a message that is received but not processed by a message handler. This section discusses how such receipt is handled.
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The first two situation are trapped by SMA and are reported via an UnhandledMessageEvent, the latter situation is trapped by the AepEngine and reported to the application as an AepUnhandledMessageEvent. These events are discussed below.
SMA Undeserializable & Unsolicited Messages
Undeserializable and unsolicited messages are trapped by SMA and are reported to the AepEngine via an UnhandledMessageEvent which the AepEngine will pass through to any application EventHandlers.
Causes
Below are some typical causes for UnhandledMessageEvents
- Receiving a message a channel that has not been joined: this usually indicates a configuration or design error in topics or subscriptions. The intent behind the SMA's message channel abstraction is define named logical conduits between peers. When a message is received on an unjoined channel it means that messages are being received by the application on a channel that the application designer wasn't expecting to receive traffic on.
- Receiving a message on an unknown channel: This typically usually indicates that there is a problem with topic subscriptions in which two channels are overlapping and causing an application to attract messages from a sender on another channel.
- Receiving a message that has no channel or metadata: Messages sent to X applications are typically expected to have SMA MessageMetadata that can be used identify the message's transmission channel, type and factory. Receipt of a message without metadata for a bus that expects will thus result in an error.
- Deserialization Errors: The SMA contract is to pass the message in deserialized MessageView (POJO) form to the AepEngine. If the message's view factory cannot be found or the data is corrupted, it will result in an error.
UnhandledMessageEvent Fields
The following fields are available on a the UnhandledMessageEvent.
| Field | Description | ||||||
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| BackingMessage | When running in a Talon Server, this will contain a SrvMonUnhandledMessageMessage. This is a monitoring alert message that contains this event in a SMA serializable form which allows applications to serialize the contents of this event for auditing and administrative purposes. | ||||||
| MessageBusBinding | Get the message bus binding that UnhandledMessageEvent originated from or null if not available. | ||||||
| MessageKey | The unhandled message's key (e.g. bus destination) on which the message was received (if available and the source binding supports transport of the key). | ||||||
| MessageMetadata | The unhandled message's metadata. See the MessageMetadata javadoc for a description of these fields. | ||||||
| MessageSMATransportMessageId | The unhandled message's SMA transport specific message id (if available). The SMA transport specific message ID may be supplied by bindings for which there is a notion of a unique identifier for a message. A null value means that either the binding doesn't support the notion of such an id or that that the id wasn't available.
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| SerializedPayloadBlob | The serialized payload blob representing the received serialized message that was unhandled. The serialized payload is provided with the UnhandledMessageEvent to allow handlers to quarantine (save) the message for subsequent handling by administrators or tools which allow the unhandled message to acknowledged upstream. Note, however, that UnhandledMessageEvents are emitted in exceptional cases, and even if the serialized payload is provided there is not guaranteed that it was not corrupted during receipt. This field is not guaranteed to be available. Reasons why it may not be available include:
The encoding type of these bytes is determined by the corresponding metadata, so it is also generally necessary to ensure that that metadata is persisted along with the serialized payload to allow this serialized form to be deserialized.
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| SerializedMetadataBlob | The serialized metadata blob representing the metadata as received in serialized form. The serialized metadata is provided with the UnhandledMessageEvent to allow handlers to quarantine (save) the message for subsequent diagnostic handling in cases where received metadata was corrupted on the wire or during receipt in the binding. This field is not guaranteed to be available. Reasons why it may not be available include:
The encoding type of these bytes is determined by the corresponding metadata, so it is also generally necessary to ensure that that metadata is persisted along with the serialized payload to allow this serialized form to be deserialized.
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| UnwrappedMessage | The unwrapped (un-deserialized) message. This is the raw transported form of the message which is dependent on the source binding. For example for a JMS binding this could be a javax.jms.Message, or a buffer when using the native Solace binding. | ||||||
| Reason | A Throwable object describing the reason why the message was unhandled. |
Acknowledgement
For unhandled Guaranteed messages determining whether or not to acnowledge unhandled message is an important consideration:
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As of the 3.2 release of the platform an application that declares an EventHandler for the UnhandledMessageEvent can control whether or not the triggering message is acknowledged. By default the platform leans towards ensuring that the message is not discarded and will not acknowledge the message unless instructed to do so by the application. An application can control acknowledgement as follows:
Default Acknowledgement Behavior.
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| Tip |
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In the 3.1.3 patch release, the Solace binding was patched allowing this property to control whether the receiving thread will acknowledge the message. When set the message may be acknowledged prior to the event being dispatched to the application by the aep engine transaction thread. |
Auto Acknowledge
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| Code Block | ||||
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@EventHandler
public void onUnhandledMessage(UnhandledMessageEvent event) {
byte [] toQuarantine = event.getBackingMessage().serializeToByteArray();
// ... quarantine the above message in some fashion.
// Mark the event for auto acknowledgement, the
// engine will acknowledge it.
event.setAutoAck(true);
} |
Explicit Acknowledgement
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| Code Block | ||||
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@EventHandler
public void onUnhandledMessage(UnhandledMessageEvent event) {
// Disable auto acknowledgement:
event.setAutoAck(false);
// Acquire the event so the platform doesn't return it
// to a pool:
event.acquire();
// Asynchronously handle the event:
Thread thread = new Thread(new Runnable() {
public void run() {
byte [] toQuarantine = event.getBackingMessage().serializeToByteArray();
// ... quarantine the above message in some fashion that might block.
try {
event.acknowledge();
}
catch (SmaException e) {
e.printStackTrace();
}
finally {
event.dispose();
}
}
}
} |
Aep Unhandled Message Events
An AepUnhandledMessageEvent is emitted when a message is successfully dispatched to an AepEngine from an SMA binding but no application EventHandler is found by the AepEngine.
Causes
- The application is incorrectly configured to join a Message Channel that it shouldn't have joined,
- The application's channel filter is not narrow enough and some message types are coming though that shouldn't be.
- A publisher is publishing messages to the channel that it shouldn't be.
An AepUnhandledMessageEvent doesn't necessarily indicate a serious problem, but the presence of such events at the very least indicates inefficiencies in the type of messages that are being attraced by the applications subscriptions.
Acknowledgement
AEP unhandled messages are acknowledged like a normally handled message except they result in an empty transaction with external effects. The message is acknowledged when the transaction is stabilized. For an application using EventSourcing that is using a disk based persister such messages will end up in the application's recovery transaction log, and consequently will not be lost in the event of acknowledgement. For a StateReplication application or an EventSourcing application without a Store persister, an application should ensure that the message is appropriately quarantined prior to returning from the AepUnhandledMessageEvent event handler and acknowledged.
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