Apache Camel: turn/replace synchronous direct into async - asynchronous

Question

My question is very close to this discussion: https://issues.apache.org/jira/browse/CAMEL-4863. I got caught by synchronous behavior of direct as well.
I designed an application like this:
Route1:
from(direct:step1-in).process(...).to(direct:step1-out)
Route2:
from(direct:step2-in).process(...).to(direct:step2-out)
CoreRoute (connects all steps):
from(direct:step1-out).to(direct:step2-in)
After a year I still like this approach. I can implement and test those steps separately and for example switch to seda:step1-out if I need to decouple them.
But seda is rather a heavy way to decouple: you create a queue and a thread pool with fixed number of threads. So even if you most of the time need 1 thread and sometimes 100, you will always have 100 in use.
Sometimes you just want to free the current thread, because it should be doing let's say unzip of the next file already, instead of processing the current.
From documentation I understood you would use threads(1,100) for this:
from(direct:input-list-processor).split(body()).threads(1,100).delay(1000).process(...).to(step-process-unzipped-out)
But this will block the thread routing the exchange to direct-input-list-processor until all threads finish processing.
Instead I would like that the exchange is copied to subsequent threads and current exchange is done.

Answer 1

Thanks to comment from Claus, I came up with the following which seems to do what I want using wireTap:
from("direct:step1-in").to("direct:step1-out");
from("direct:step2-in")
.log("receive async")
.delay(1000).to("log:output");
// core route connecting processing step1 and processing step2
from("direct:step1-out").log("send async").wireTap("direct:step2-in")
.executorService(getContext().getExecutorServiceManager().newThreadPool(this,
"step2-worker", 1, 100));
I think this is the way to go. Though I am confused because
Camel in Action book suggests: 10.4.3 Asynchronous caller using multiple
threads
from("seda:start")
.to("log:A")
.threads(5, 10)
.to("log:B");
This is the most obvious approach, but as stated here: http://camel.apache.org/seda.html it is not the way to go as it can wind up and the suggested alternative to use direct instead of seda does not work for me, hence the question. Also the seda way has an unlimited queue which is a potential "memory leak", so you would need to decide and set an appropriate size everytime you want to use this pattern.
#Claus thank you very much for your great work! Your answers guided me a lot through the project. May be you could clarify if you think it is a good solution or am I still missing something. The only "disadvantages" I see is that a new exchange will be created, also a transaction would not span multiple processing steps.
This log shows the solution with wireTap works asynchronously and scales up in a flexible way
2017-07-30T16:36:15.607 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.609 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.610 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.614 | -1 | Camel Thread #2 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.614 | -1 | Camel Thread #1 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.614 | -1 | Camel Thread #3 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.614 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.615 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.616 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.616 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.617 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.618 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.618 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.619 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.620 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.620 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.621 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.622 | -1 | Camel Thread #4 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.622 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.623 | -1 | Camel Thread #5 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.623 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.624 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.624 | -1 | Camel Thread #6 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.625 | -1 | Camel Thread #7 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.625 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.626 | -1 | Camel Thread #8 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.626 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.627 | -1 | Camel Thread #9 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.627 | -1 | main | INFO | --- --- --- route3 | route3 | - send async
2017-07-30T16:36:15.628 | -1 | Camel Thread #10 - step2-worker | INFO | --- --- --- route2 | route2 | - receive async
2017-07-30T16:36:15.629 | -1 | main | INFO | --- --- --- route3 | route3 | - send async