160 lines
6.3 KiB
Markdown
160 lines
6.3 KiB
Markdown
# Duplicate Message Detection
|
|
|
|
Apache ActiveMQ Artemis includes powerful automatic duplicate message detection,
|
|
filtering out duplicate messages without you having to code your own
|
|
fiddly duplicate detection logic at the application level. This chapter
|
|
will explain what duplicate detection is, how Apache ActiveMQ Artemis uses it and how
|
|
and where to configure it.
|
|
|
|
When sending messages from a client to a server, or indeed from a server
|
|
to another server, if the target server or connection fails sometime
|
|
after sending the message, but before the sender receives a response
|
|
that the send (or commit) was processed successfully then the sender
|
|
cannot know for sure if the message was sent successfully to the
|
|
address.
|
|
|
|
If the target server or connection failed after the send was received
|
|
and processed but before the response was sent back then the message
|
|
will have been sent to the address successfully, but if the target
|
|
server or connection failed before the send was received and finished
|
|
processing then it will not have been sent to the address successfully.
|
|
From the senders point of view it's not possible to distinguish these
|
|
two cases.
|
|
|
|
When the server recovers this leaves the client in a difficult
|
|
situation. It knows the target server failed, but it does not know if
|
|
the last message reached its destination ok. If it decides to resend the
|
|
last message, then that could result in a duplicate message being sent
|
|
to the address. If each message was an order or a trade then this could
|
|
result in the order being fulfilled twice or the trade being double
|
|
booked. This is clearly not a desirable situation.
|
|
|
|
Sending the message(s) in a transaction does not help out either. If the
|
|
server or connection fails while the transaction commit is being
|
|
processed it is also indeterminate whether the transaction was
|
|
successfully committed or not!
|
|
|
|
To solve these issues Apache ActiveMQ Artemis provides automatic duplicate messages
|
|
detection for messages sent to addresses.
|
|
|
|
## Using Duplicate Detection for Message Sending
|
|
|
|
Enabling duplicate message detection for sent messages is simple: you
|
|
just need to set a special property on the message to a unique value.
|
|
You can create the value however you like, as long as it is unique. When
|
|
the target server receives the message it will check if that property is
|
|
set, if it is, then it will check in its in memory cache if it has
|
|
already received a message with that value of the header. If it has
|
|
received a message with the same value before then it will ignore the
|
|
message.
|
|
|
|
> **Note:**
|
|
>
|
|
> Using duplicate detection to move messages between nodes can give you
|
|
> the same *once and only once* delivery guarantees as if you were using
|
|
> an XA transaction to consume messages from source and send them to the
|
|
> target, but with less overhead and much easier configuration than
|
|
> using XA.
|
|
|
|
If you're sending messages in a transaction then you don't have to set
|
|
the property for *every* message you send in that transaction, you only
|
|
need to set it once in the transaction. If the server detects a
|
|
duplicate message for any message in the transaction, then it will
|
|
ignore the entire transaction.
|
|
|
|
The name of the property that you set is given by the value of
|
|
`org.apache.activemq.artemis.api.core.Message.HDR_DUPLICATE_DETECTION_ID`, which
|
|
is `_AMQ_DUPL_ID`
|
|
|
|
The value of the property can be of type `byte[]` or `SimpleString` if
|
|
you're using the core API. If you're using JMS it must be a `String`,
|
|
and its value should be unique. An easy way of generating a unique id is
|
|
by generating a UUID.
|
|
|
|
Here's an example of setting the property using the core API:
|
|
|
|
```java
|
|
...
|
|
|
|
ClientMessage message = session.createMessage(true);
|
|
|
|
SimpleString myUniqueID = "This is my unique id"; // Could use a UUID for this
|
|
|
|
message.setStringProperty(HDR_DUPLICATE_DETECTION_ID, myUniqueID);
|
|
|
|
```
|
|
|
|
And here's an example using the JMS API:
|
|
|
|
```java
|
|
...
|
|
|
|
Message jmsMessage = session.createMessage();
|
|
|
|
String myUniqueID = "This is my unique id"; // Could use a UUID for this
|
|
|
|
message.setStringProperty(HDR_DUPLICATE_DETECTION_ID.toString(), myUniqueID);
|
|
|
|
...
|
|
```
|
|
|
|
## Configuring the Duplicate ID Cache
|
|
|
|
The server maintains caches of received values of the
|
|
`org.apache.activemq.artemis.core.message.impl.HDR_DUPLICATE_DETECTION_ID`
|
|
property sent to each address. Each address has its own distinct cache.
|
|
|
|
The cache is a circular fixed size cache. If the cache has a maximum
|
|
size of `n` elements, then the `n + 1`th id stored will overwrite the
|
|
`0`th element in the cache.
|
|
|
|
The maximum size of the cache is configured by the parameter
|
|
`id-cache-size` in `broker.xml`, the default value is
|
|
`20000` elements.
|
|
|
|
The caches can also be configured to persist to disk or not. This is
|
|
configured by the parameter `persist-id-cache`, also in
|
|
`broker.xml`. If this is set to `true` then each id will
|
|
be persisted to permanent storage as they are received. The default
|
|
value for this parameter is `true`.
|
|
|
|
> **Note:**
|
|
>
|
|
> When choosing a size of the duplicate id cache be sure to set it to a
|
|
> larger enough size so if you resend messages all the previously sent
|
|
> ones are in the cache not having been overwritten.
|
|
|
|
## Duplicate Detection and Bridges
|
|
|
|
Core bridges can be configured to automatically add a unique duplicate
|
|
id value (if there isn't already one in the message) before forwarding
|
|
the message to its target. This ensures that if the target server
|
|
crashes or the connection is interrupted and the bridge resends the
|
|
message, then if it has already been received by the target server, it
|
|
will be ignored.
|
|
|
|
To configure a core bridge to add the duplicate id header, simply set
|
|
the `use-duplicate-detection` to `true` when configuring a bridge in
|
|
`broker.xml`.
|
|
|
|
The default value for this parameter is `true`.
|
|
|
|
For more information on core bridges and how to configure them, please
|
|
see [Core Bridges](core-bridges.md).
|
|
|
|
## Duplicate Detection and Cluster Connections
|
|
|
|
Cluster connections internally use core bridges to move messages
|
|
reliable between nodes of the cluster. Consequently they can also be
|
|
configured to insert the duplicate id header for each message they move
|
|
using their internal bridges.
|
|
|
|
To configure a cluster connection to add the duplicate id header, simply
|
|
set the `use-duplicate-detection` to `true` when configuring a cluster
|
|
connection in `broker.xml`.
|
|
|
|
The default value for this parameter is `true`.
|
|
|
|
For more information on cluster connections and how to configure them,
|
|
please see [Clusters](clusters.md).
|