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Subscribe to Publisher in the NullChannel (#3448)

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* Subscribe to Publisher in the NullChannel

If a payload of the message in the flow is a `org.reactivestreams.Publisher`,
it must be subscribed somewhere downstream to initiate reactive processing.
The `NullChannel` just ignores the message altogether and therefore `Publisher`
is lost

* Check the payload of the message in the `NullChannel` for `Publisher` type
and subscribe to it
* Verify in the test and mention this logic in the docs

**Cherry-pick to 5.4.x**

* Apply suggestions from code review

Co-authored-by: Gary Russell <grussell@vmware.com>

Co-authored-by: Gary Russell <grussell@vmware.com>
This commit is contained in:
Artem Bilan
2020-12-22 12:20:12 -05:00
committed by GitHub
parent ad57c6187a
commit c38da2f4ee
3 changed files with 50 additions and 5 deletions
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6
src/reference/asciidoc/channel.adoc
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@@ -1142,9 +1142,11 @@ For example, you can use this technique to configure a test case to verify messa
[[channel-special-channels]]
==== Special Channels
If namespace support is enabled, two special channels are defined within the application context by default: `errorChannel` and `nullChannel`.
The 'nullChannel' acts like `/dev/null`, logging any message sent to it at the `DEBUG` level and returning immediately.
Two special channels are defined within the application context by default: `errorChannel` and `nullChannel`.
The 'nullChannel' (an instance of `NullChannel`) acts like `/dev/null`, logging any message sent to it at the `DEBUG` level and returning immediately.
The special treatment is applied for an `org.reactivestreams.Publisher` payload of a sent message: it is subscribed to in this channel immediately, to initiate reactive stream processing, although the data is discarded.
Any time you face channel resolution errors for a reply that you do not care about, you can set the affected component's `output-channel` attribute to 'nullChannel' (the name, 'nullChannel', is reserved within the application context).
The 'errorChannel' is used internally for sending error messages and may be overridden with a custom configuration.
This is discussed in greater detail in <<./error-handling.adoc#error-handling,Error Handling>>.