int-http:inbound-gateway with pollable request channel

Question

I'm looking through some code that is based on spring integration and I've noticed that all int-http:inbound-gateways use pollable request channels:

<int-http:inbound-gateway id="someId"
                                  request-channel="queue-channel"
                                  reply-channel="reply-channel"
                                  request-payload-type="java.lang.String"
                                  supported-methods="POST"
                                  path="/rest/notifications"
                                  auto-startup="true" />

<int:channel id="queue-channel" datatype="java.lang.String">
    <int:queue capacity="100" />
</int:channel>

An explicit poller is specified in the configuration:

<int:poller id="mainSystemPoller" default="true" fixed-delay="500" max-messages-per-poll="1">
    <int:transactional transaction-manager="transactionManager" propagation="REQUIRES_NEW" isolation="DEFAULT"/>
</int:poller>

So the very first channel up the stream is pollable. What are the benefits of using this approach? Does it just give us more flexibility over the business flow(transactional configurations, queue capacity, etc)?

Answer 1

Without the transaction configuration, there is little, if any, value in using a poller there because the http thread will wait in the gateway for the reply anyway.

However, in your case, it will start a transaction and cause everything downstream of the gateway to run in a transaction. But, as configured, this will single-thread your requests; you would need a task executor to handle multiple concurrent requests.

There are other ways to run concurrent web requests in transactions; you can use a transactional gateway instead and then the flow will run on on the web container thread, with the concurrency managed by your web container.