While stream types (like StreamSource, InputStream and Reader) are commonly used in messaging for performance reasons, they also have an important drawback: they can only be read once. In order to be able to work with message content multiple times, the stream needs to be cached.
Streams are caching in memory. In Camel 2.0, large stream messages (over 64 Kb) will be cached in a temporary file instead – Camel itself will handle deleting the temporary file once the cached stream is no longer necessary.
In Camel 2.0 stream cache is default disabled out of the box.
In Camel 1.x stream cache is default enabled out of the box.
|StreamCache Affects your payload object|
The StreamCache will affect your payload object as it will replace the Stream payload with a org.apache.camel.StreamCache object.
This StreamCache is capable of being re-readable and thus possible to better be routed within Camel using redelivery or Content Based Router or the likes.
However to not change the payload under the covers without the end user really knowing we changed the default in Camel 2.0 to disabled. So in Camel 2.0 you have to explicit enable it if you want to use it.
Enabling stream caching
In Apache Camel, you can explicitly enable stream caching for a single route with the streamCaching DSL method:
In Spring XML you enable it by setting the streamCache="true" attribute on the route tag.
StreamCache supports the global and per route scope. So by setting the streamCache attribute on camelContext you can enable/disable it globally.
The route scope is configured by the streamCache attribute on the <route> tag such as:
You can mix and match for instance you can enable it globally and disable it on a particular route such as:
Enabling from Java DSL
In Camel 2.0 you can enable stream cache by setting the property on CamelContext, for instance in a RouteBuilder class:
Implicitly enabled for multicast and dead letter channel (Camel 1.x)
Some EIPs require that the message content can be read multiple times. Stream caching will be automatically enabled when using these EIPs in your routes:
- Multicast will implicitly cache streams to ensure that all the endpoints can access the message content
- Dead Letter Channel uses stream caching to ensure that the message content can actually be read again when redelivering a message
Streaming cache to files
When stream cache is enabled it will by default spool big streams to files instead of keeping them in memory. The default threshold is 64kb but you can configure it with the following properties:
| Property || Default || Description |
| CamelCachedOutputStreamThreshold || 64kb || Size in bytes when the stream should be spooled to disk instead of keeping in memory. Use a value of 0 or negative to disable it all together so streams is always kept in memory regardless of their size. |
| CamelCachedOutputStreamOutputDirectory || java.io.tmpdir || Base directory where temporary files for spooled streams should be stored. |
You set these properties on the CamelContext as shown below:
Disabling spooling to disk
Available as of *Camel 1.6.2/2.0
You can disable spooling to disk by setting a threshold of 0 or a negative value.
How it works?
In order to determine if a type requires caching, we leverage the type converter feature. Any type that requires stream caching can be converted into an org.apache.camel.StreamCache instance.