160 lines
6.3 KiB
Markdown
160 lines
6.3 KiB
Markdown
# Duplicate Message Detection
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Apache ActiveMQ Artemis includes powerful automatic duplicate message detection,
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filtering out duplicate messages without you having to code your own
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fiddly duplicate detection logic at the application level. This chapter
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will explain what duplicate detection is, how Apache ActiveMQ Artemis uses it and how
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and where to configure it.
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When sending messages from a client to a server, or indeed from a server
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to another server, if the target server or connection fails sometime
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after sending the message, but before the sender receives a response
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that the send (or commit) was processed successfully then the sender
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cannot know for sure if the message was sent successfully to the
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address.
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If the target server or connection failed after the send was received
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and processed but before the response was sent back then the message
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will have been sent to the address successfully, but if the target
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server or connection failed before the send was received and finished
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processing then it will not have been sent to the address successfully.
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From the senders point of view it's not possible to distinguish these
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two cases.
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When the server recovers this leaves the client in a difficult
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situation. It knows the target server failed, but it does not know if
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the last message reached its destination ok. If it decides to resend the
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last message, then that could result in a duplicate message being sent
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to the address. If each message was an order or a trade then this could
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result in the order being fulfilled twice or the trade being double
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booked. This is clearly not a desirable situation.
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Sending the message(s) in a transaction does not help out either. If the
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server or connection fails while the transaction commit is being
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processed it is also indeterminate whether the transaction was
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successfully committed or not!
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To solve these issues Apache ActiveMQ Artemis provides automatic duplicate messages
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detection for messages sent to addresses.
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## Using Duplicate Detection for Message Sending
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Enabling duplicate message detection for sent messages is simple: you
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just need to set a special property on the message to a unique value.
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You can create the value however you like, as long as it is unique. When
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the target server receives the message it will check if that property is
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set, if it is, then it will check in its in memory cache if it has
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already received a message with that value of the header. If it has
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received a message with the same value before then it will ignore the
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message.
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> **Note:**
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>
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> Using duplicate detection to move messages between nodes can give you
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> the same *once and only once* delivery guarantees as if you were using
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> an XA transaction to consume messages from source and send them to the
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> target, but with less overhead and much easier configuration than
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> using XA.
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If you're sending messages in a transaction then you don't have to set
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the property for *every* message you send in that transaction, you only
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need to set it once in the transaction. If the server detects a
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duplicate message for any message in the transaction, then it will
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ignore the entire transaction.
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The name of the property that you set is given by the value of
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`org.apache.activemq.artemis.api.core.Message.HDR_DUPLICATE_DETECTION_ID`, which
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is `_AMQ_DUPL_ID`
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The value of the property can be of type `byte[]` or `SimpleString` if
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you're using the core API. If you're using JMS it must be a `String`,
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and its value should be unique. An easy way of generating a unique id is
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by generating a UUID.
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Here's an example of setting the property using the core API:
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```java
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...
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ClientMessage message = session.createMessage(true);
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SimpleString myUniqueID = "This is my unique id"; // Could use a UUID for this
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message.setStringProperty(HDR_DUPLICATE_DETECTION_ID, myUniqueID);
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```
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And here's an example using the JMS API:
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```java
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...
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Message jmsMessage = session.createMessage();
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String myUniqueID = "This is my unique id"; // Could use a UUID for this
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message.setStringProperty(HDR_DUPLICATE_DETECTION_ID.toString(), myUniqueID);
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...
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```
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## Configuring the Duplicate ID Cache
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The server maintains caches of received values of the
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`org.apache.activemq.artemis.core.message.impl.HDR_DUPLICATE_DETECTION_ID`
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property sent to each address. Each address has its own distinct cache.
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The cache is a circular fixed size cache. If the cache has a maximum
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size of `n` elements, then the `n + 1`th id stored will overwrite the
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`0`th element in the cache.
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The maximum size of the cache is configured by the parameter
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`id-cache-size` in `broker.xml`, the default value is
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`2000` elements.
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The caches can also be configured to persist to disk or not. This is
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configured by the parameter `persist-id-cache`, also in
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`broker.xml`. If this is set to `true` then each id will
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be persisted to permanent storage as they are received. The default
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value for this parameter is `true`.
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> **Note:**
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>
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> When choosing a size of the duplicate id cache be sure to set it to a
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> larger enough size so if you resend messages all the previously sent
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> ones are in the cache not having been overwritten.
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## Duplicate Detection and Bridges
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Core bridges can be configured to automatically add a unique duplicate
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id value (if there isn't already one in the message) before forwarding
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the message to it's target. This ensures that if the target server
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crashes or the connection is interrupted and the bridge resends the
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message, then if it has already been received by the target server, it
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will be ignored.
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To configure a core bridge to add the duplicate id header, simply set
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the `use-duplicate-detection` to `true` when configuring a bridge in
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`broker.xml`.
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The default value for this parameter is `true`.
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For more information on core bridges and how to configure them, please
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see [Core Bridges](core-bridges.md).
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## Duplicate Detection and Cluster Connections
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Cluster connections internally use core bridges to move messages
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reliable between nodes of the cluster. Consequently they can also be
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configured to insert the duplicate id header for each message they move
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using their internal bridges.
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To configure a cluster connection to add the duplicate id header, simply
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set the `use-duplicate-detection` to `true` when configuring a cluster
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connection in `broker.xml`.
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The default value for this parameter is `true`.
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For more information on cluster connections and how to configure them,
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please see [Clusters](clusters.md).
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