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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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- SMA Undeserializable Messages - Messages that are received from an underlying SMA MessageBusBinding that cannot be deserialized into a message.
- SMA Unsolicited Messages - These are messages Messages that can be successfully deserialized into a message but are on a MessageChannel that is not joined or not known to the receiving application.
- AEP Unhandled Messages - These are Valid messages valid messages intentionally attracted to the application on a joined MessageChannel, but for which the application has no declared EventHandler.
The first two situation situations 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.
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Below are some typical causes for UnhandledMessageEvents.
- Receiving a message on 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 to 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.
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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 where 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 by tools which allow the unhandled message to be acknowledged upstream. Note, however, that UnhandledMessageEvents are emitted in exceptional cases, and even if the serialized payload is provided, there is not guaranteed no guarantee 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 the 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. |
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For unhandled Guaranteed messages, determining whether or not to acnowledge acknowledge unhandled message is an important consideration:.
If the message is not acknowledged:
- The message will be redelivered if the application is restarted or fails over to a backup, which can be undesirable.
- If there are a large number of unacknowledged message messages, it puts a burden on the messaging provider, which could hit resource limits.
- If the SMA provider doesn't support individual message acknowledgements, then failing to acknowledge the unhandled message can block subsequent acknowledgements.
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Prior to the 3.2 release, whether or not the triggering message was acknowledged was left to the SMA bus binding, which in most cases would not acknowledge the message.
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:discussed below.
Default Acknowledgement Behavior
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Auto acknowledgement for unhandled messages can be globally set to enabled by setting the configuration environment property:.
nv.sma.unhandledmessageevent.autoAck=true
As of 3.2, setting the above property will cause UnhandledMessageEvents to be acknowledged regardless of whether or not there is a handler for the messages, unless a handler specifically calls setAutoAck(false) on the event. Acknowledgement is done after the event is dispatched in the Aep Engine transaction thread.
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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. |
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An application can explicitly override the default acknowledgement behavior of unhandled messages by declaring an EventHandler for UnhandledMessageEvent and and setting the event's autoAck behavior:.
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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);
} |
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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.
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- 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 publishing.
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 attracted by the applications subscriptions.
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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 is acknowledged.
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