We have a flow which we would like to implement with Reactive programming using Spring Boot 2 WebFlux. Currently we have no experience with Reactive programming.
As part of this flow we are going to create on or more HTTP requests (I guess using WebClient) and also read some data from DB.
We are considering to use AWS DynamoDB but as far as I understand the Java SDK does not support reactive API. This read will be a blocking I/O operation, my question is whether there is a benefit for implementing part of this flow with WebFlux? More generally, does a single blocking I/O operation in the flow eliminates all the benefit that we get from implementing with reactive programming?
Based on your question reactive is the idle way to deal with blocking operation especially IO (network, file and etc...)
you can use a library that implements this api in a reactive way or wrap a blocking request with a reactive api, this usually done by placing the blocking op on anther thread pool
in spring webflux you can achieve something similar like
#GetMapping
public Mono<Response> getResponse() {
return Mono.fromCallable(() -> blockingOp())
.publishOn(Schedulers.elastic());
}
publishOn in that case will cause all this flow to happen on another thread, you can choose dedicated thread pool as your choice
from the docs, elastic is a
Scheduler that dynamically creates ExecutorService-based Workers and caches the thread pools, reusing them once the Workers have been shut down.
The following may not answer your question fully, but might be a little helpful. There is a question mentioned in the FAQ for the Spring Framework 5, which is,
What if there is no reactive library for my database?
The answer to this is:
One suggestion for handling a mix of blocking and non-blocking code
would be to use the power of a microservice boundary to separate the
blocking backend datastore code from the non blocking front-end API.
Alternatively, you may also go with a worker thread pool for blocking
operations, keeping the main event loop non-blocking that way.
I think someone from Pivotal might be the right person to give more insights on this.
Related
Is it possible for a RestTemplate to consume an endpoint which is reactive based (Spring WebFlux)? I understand that the main idea of reactive programming is to avoid blocking and make better use of threads (eliminate thread per connection model) so what happens if my Client is non reactive?
Will I still be able to call the service even if it is in a blocking
manner?
To achieve full reactiveness (non blocking) both Client and Server
must be reactive?
Yes, that is not relevant to the clients of Reactive applications. The reason is that this is a regular HTTP call.
Each may be fully reactive on its own. Having said that, if you use WebFlux in both Client and Server you will have a system that is as a whole reactive. But there is nothing forcing you to do this. You can have only one of the services as a Reactive application. It comes down to your needs and context.
I'm a newbie to Spring Reactive Modules. What I got is basically, at its core it enable reactive programming and we can develop end to end reactive service.
But, suppose I just want to make my controller as Async, so that I can do work on multiple threads and send a reply like "Task Started" (not particularly this) and have my work going on and close the HTTP link.
I also got to know about #EnableAsync and #Async to make a method Async.
What if I just use #Async above my controller method that I want to make async. It worked but, is this a good practice? And can we use this in production codes?
I do not see any problem using #Asyncas this will release the request thread. But this is a simple approach and it has a lot of limitations. Note that if you want to deal with reactive streams, you do not have an API capable of that. For instance, if an #Async method calls another, the second will not be async.
The Webflux instead will bring the most complete API (in Java) to deal with things the reactive way. What you cannot do with only #Async. With Flux, for instance, you can process or access with multiple layers reactively and this you cannot reach the way you doing.
Nevertheless, it will bring a new universe for you, so if you just want to release the thread of the request, your approach is just fine, but if you need more you will have to deal with it in a more complex way.
Now, if you want to answer the HTTP request and then do the work asyncly, this is not what you want. I recommend that you have a JMS provider (like ActiveMQ), where your controller sends the message to be processed by a job and answers the request.
Hope it helps!
I'm trying to understand Spring WebFlux. The things I've found so far are reactive at the core, no Servlet API, no thread per request, HTTP 2, server pushes, application/stream+json.
But what is the difference between asynchronous calls in Spring MVC? I mean in Spring MVC when you return Future, DefferedResult and etc you get logic in the request handler (controller method) executed in a separate thread, so you can benefit from saving thread pool resources for dispatching requests as well.
So could you please highlight differences related to that? Why WebFlux is better here?
Thank you for your time very much!
The Servlet async model introduces an async boundary between the container threads (1 Servlet request/thread model) and the processing of the request in your application. Processing can happen on a different thread or wait. In the end, you have to dispatch back to a container thread and read/write in a blocking way (InputStream and OutputStream are inherently blocking APIs).
With that model, you need many threads to achieve concurrency (because many of those can be blocked waiting for I/O). This costs resources and it can be a tradeoff, depending on your use case.
With non-blocking code, you only need a few threads to process a lot of requests concurrently. This is a different concurrency model; like any model, there are benefits and tradeoffs coming with it.
For more information about that comparison, this Servlet vs. Reactive stacks talk should be of interest.
Servlet API is blocking I/O which requires 1 thread per HTTP request. Spring MVC async relies on Servlet APIs which only provides async behavior between container threads and request processing threads but not end to end.
Spring WebFlux on the other hand achieves concurrency by a fixed number of threads by using HTTP sockets and pushing chunks of data at a time through the sockets. This mechanism is called event loop, an idea made popular by Node.js. Such an approach is scalable and resilient. Spring 5's spring-webflux uses the event loop approach to provide async behavior.
More can be read from
Servlet vs. Reactive
Spring Boot performance battle
Comparing WebFlux with Spring Web MVC
Currently experimenting reactive programming with Spring 5.0.0.RC2, Reactor 3.1.0.M2 and Spring Boot 2.0.0.M2.
Wondering about the concurrency and threading model used by WebFlux and Reactor to properly code the application and handle the mutable state.
The Reactor doc states that the library is considered concurrency agnostic and mentions the Scheduler abstraction. The WebFlux doc does not give information.
Yet when using WebFlux through Spring Boot, a threading model is defined.
From my experimentations here is what I got:
The model is neither 1 event thread, nor 1 event thread + workers
Several thread pools are used
"reactor-http-nio-3" threads: probably one per core, handle the incoming HTTP requests
"Thread-7" threads: used by async requests to MongoDB or HTTP resources
"parallel-1" threads: one per core, created by Schedulers.parallel() from Reactor, used by delay operators and such
Shared mutable state must be synchronized by the application
ThreadLocal (for application state, MDC logging, etc) are not request scoped, so are not very interesting
Is this correct ? What is the concurrency and threading model of WebFlux: for example what are the default thread pools?
Thank you for the information
After the question, the present documentation does provide some clues about the concurrency model and the threads one could expect (but I still think that clearer/better descriptions of what happens under-the-scene from a multi-threading perspective would be highly appreciated by Spring newcomers).
It discusses the difference between Spring MVC and Spring WebFlux (1-thread-per-request model vs. event-loop):
In Spring MVC, and servlet applications in general, it is assumed that applications may block the current thread, e.g. for remote calls, and for this reason servlet containers use a large thread pool, to absorb potential blocking during request handling.
In Spring WebFlux, and non-blocking servers in general, it is assumed that applications will not block, and therefore non-blocking servers use a small, fixed-size thread pool (event loop workers) to handle requests.
Invoking a Blocking API
But notice that Spring MVC apps can also introduce some asynchronicity (cf., Servlet 3 Async). And I suggest this presentation for a discussion wrt Servlet 3.1 NIO and WebFlux.
Back to the docs: it also suggests that, when working with reactive streams, you have some control:
What if you do need to use a blocking library?
Both Reactor and RxJava provide the publishOn operator to continue
processing on a different thread.
(For more details on this, refer to scheduling in Reactor)
It also discusses the threads you may expect in WebFlux applications (bold is mine):
Threading Model
What threads should you expect to see on a server running with Spring WebFlux?
On a "vanilla" Spring WebFlux server (e.g. no data access, nor other optional dependencies), you can expect one thread for the server, and several others for request processing (typically as many as the number of CPU cores). Servlet containers, however, may start with more threads (e.g. 10 on Tomcat), in support of both servlet, blocking I/O and servlet 3.1, non-blocking I/O usage.
The reactive WebClient operates in event loop style. So you’ll see a small, fixed number of processing threads related to that, e.g. "reactor-http-nio-" with the Reactor Netty connector. However if Reactor Netty is used for both client and server, the two will share event loop resources by default.
Reactor and RxJava provide thread pool abstractions, called Schedulers, to use with the publishOn operator that is used to switch processing to a different thread pool. The schedulers have names that suggest a specific concurrency strategy, e.g. "parallel" for CPU-bound work with a limited number of threads, or "elastic" for I/O-bound work with a large number of threads. If you see such threads it means some code is using a specific thread pool Scheduler strategy.
Data access libraries and other 3rd party dependencies may also create and use threads of their own.
In part, you can configure the details of the threading model via configuration
To configure the threading model for a server, you’ll need to use server-specific config APIs, or if using Spring Boot, check the Spring Boot configuration options for each server. The WebClient can be configured directly. For all other libraries, refer to their respective documentation.
Moreover, as e.g. the discussion Default number of threads in Spring boot 2.0 reactive webflux configuration
highlights,
The default number of threads for request handling is determined by the underlying web server; by default, Spring Boot 2.0 is using Reactor Netty, which is using Netty's defaults
it is a matter of default components and their defaults (and overall configuration, including that injected transparently through annotations) -- which may also change across versions of Spring/Boot and corresponding dependencies.
Said that, your guesses seem correct.
I have written an application using embedded Jetty that makes network calls to other services.
I presume that the serving threads are idle whilst waiting for the network calls to complete.
Is there any way to have a worker thread that switches between requests to perform work that can be done at the current time and then when the network calls return also handle that? A request would be returned when all work has been completed for it.
I know this is a common paradigm, and I have used it for non-blocking TCP networking, but I'm unsure as to how to achieve this on a Java HTTP server whilst also waiting on external results.
Any links or explanations are appreciated.
Thanks
Update:
I'm using Membase and ElasticSearch (the only network calls). Membase returns "Future" objects and ElasticSearch returns "ListenableActionFuture". I'd like to be able to continue processing on a thread in response to these objects being returned.
You may take a look at Deft, which is single threaded, asynchronous, event driven web server.
Netty is a java library that allows you to do asynchronous networking.
http://www.jboss.org/netty
Netty supports http, but it is a fairly low level library.
A higher level library is finangle by twitter,
http://twitter.github.com/finagle/
Finangle is built on top of netty, but supports connection pooling, load balancing, and has a lot of other features. Finangle supports http.
If you want to do work at the same time as IO, I suggest you add a thread pool to perform the work. It is possible to re-use the existing threads but its a lot of extra work for possibly too little benefit.