are spring integration channels single threaded?

Question

If I have a rest service then, I know for sure that each request is treated by a separate thread and the threads can run in parallel.

What happens if I have a rest(http) service as inbound channel in spring integration. Will each request still be treated in parallel or the requests will be placed in queues... and it will be more like single threaded

Answer 1

Normal channels (DirectChannel) use the same execution thread as the object that put something into the channel (they are basically a way of abstracting a method call), so they are multi threaded.

From the docs:

In addition to being the simplest point-to-point channel option, one of its most important features is that it enables a single thread to perform the operations on "both sides" of the channel. For example, if a handler is subscribed to a DirectChannel, then sending a Message to that channel will trigger invocation of that handler's handleMessage(Message) method directly in the sender's thread, before the send() method invocation can return.

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Edit

You have a very good point in your question. When you set a Queue element in a channel, spring automatically converts it to a QueueChannel (documentation), and as far as I can remember only one thread will be able to consume from the queue at at time. If you want "real" queue semantics (several producer and consumer threads) you can use an ExecutorChannel

Answer 2

When using Rest (http), the threading is managed by the servlet container; containers support multiple concurrent requests but setting the concurrency is done in the container, not Spring Integration.

With default Direct channels, the container threads will invoke the Spring Integration flow concurrently on the container threads.