Sunday, July 26, 2020

Backpressure in Project Reactor

Project Reactor implements the Reactive Streams specification, which is a standard for asynchronously processing a stream of data while respecting the processing capabilities of a consumer. 

At a very broad level, there are two entities involved, a Producer that produces the stream of data and a Consumer that consumes data. If the rate at which a Consumer consumes data is less than the rate at which a Producer produces data (referred to as a Fast Producer/Slow Consumer), then signals from the consumer can constrain the rate of production, this is referred to as Backpressure and in this post, I will be demonstrating a few backpressure examples using Project Reactor

Before I go ahead, I have to acknowledge that these examples are loosely based on what I learned from the "Reactive Programming with RxJava" book.

Producer

Flux in Project Reactor represents an asynchronous stream of 0..N data, where N can potentially be infinite. 

Consider a simple example, generating a sequence of numbers. There are built-in ways in Flux to do this, but for the example, I will be using an operator called Flux.generate. Sample code looks like this:
fun produce(targetRate: Int, upto: Long): Flux<Long> {
    val delayBetweenEmits: Long = 1000L / targetRate

    return Flux.generate(
        { 1L },
        { state: Long, sink: SynchronousSink<Long> ->
            sleep(delayBetweenEmits)
            val nextState: Long = state + 1
            if (state > upto) {
                sink.complete()
                nextState
            } else {
                LOGGER.info("Emitted {}", state)
                sink.next(state)
                nextState
            }
        }
    )
}

Here "targetRate" is the rate per second at which the Producer is expected to produce a sequence of numbers and "upto" represents the range for which the sequence is to be generated. "Thread.sleep" is used for introducing the delay between emissions.


Consumer

A consumer for this stream of data just consumes the sequence of numbers and to simulate processing while consuming the data, delays are again introduced just before reading the information, along these lines:
val delayBetweenConsumes: Long = 1000L / consumerRate
producer.produce(producerRate, count)
    .subscribe { value: Long ->
        sleep(delayBetweenConsumes)
        logger.info("Consumed {}", value)
    }

Just like with rate at the Producer side, there is a rate of consuming on the consumer side which drives the delay before consuming the data.


Scenario 1: Fast Producer, Slow Consumer without Threading

Now that I have a stream of data for which I can control the rate of production and rate of consumption, the first test that I ran was with the producer and the consumer chained together. 

The Producer produces at the rate of 100 requests a second and the consumer consuming it at 3 per second. 

If there were no backpressure mechanisms in place you would expect that Producer would merrily go along and produce all the records at its own pace of 100 per second and Consumer would slowly catch up at the rate of 3 per second.  This is NOT what happens though. 

The reason is not that intuitive I feel, it is not really backpressure coming into play either. The Producer is constrained to 10 requests per second merely because the entire flow from the Producer to the Consumer is synchronous by default and since the production and the consumption are happening on the same thread, the behavior is automatically constrained to what the Consumer is comfortable in consuming. 

Here is a graph which simply plots the rate of production and consumption over time and captures clearly the exact same rate of Production and Consumption throughout:


This behavior is borne out from the logs also, which show that the consumer and producer remain in sync:
2020-07-26 17:51:58.712  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 84
2020-07-26 17:51:59.048  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 84
2020-07-26 17:51:59.059  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 85
2020-07-26 17:51:59.393  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 85
2020-07-26 17:51:59.404  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 86
2020-07-26 17:51:59.740  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 86
2020-07-26 17:51:59.751  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 87
2020-07-26 17:52:00.084  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 87
2020-07-26 17:52:00.095  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 88
2020-07-26 17:52:00.430  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 88
2020-07-26 17:52:00.441  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 89
2020-07-26 17:52:00.777  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 89
2020-07-26 17:52:00.788  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 90
2020-07-26 17:52:01.087  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 90
2020-07-26 17:52:01.097  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 91
2020-07-26 17:52:01.432  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 91
2020-07-26 17:52:01.442  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 92
2020-07-26 17:52:01.777  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 92
2020-07-26 17:52:01.788  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 93
2020-07-26 17:52:02.123  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 93
2020-07-26 17:52:02.133  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 94
2020-07-26 17:52:02.467  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 94
2020-07-26 17:52:02.478  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 95
2020-07-26 17:52:02.813  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 95
2020-07-26 17:52:02.824  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 96
2020-07-26 17:52:03.157  INFO 1 --- [pool-1-thread-1] sample.meter.Consumer                    : Consumed 96
2020-07-26 17:52:03.168  INFO 1 --- [pool-1-thread-1] sample.meter.Producer                    : Emitted 97


Scenario 2: Fast Producer, Slow Consumer with Threading

The second scenario that I considered was with the Producer and the Consumer being produced independently in different threads. 

Project reactor makes this possible through two operators subscribeOn() which changes the thread where in my case the Producer produces the sequence and a publishOn() which shifts the consumption to a different thread. 

With these in place, the code looks like this:
producer.produce(producerRate, count)
    .subscribeOn(subscribeOnScheduler)
    .publishOn(publishOnScheduler)
    .subscribe { value: Long ->
        sleep(delayBetweenConsumes)
        logger.info("Consumed {}", value)
    }
The results were a little surprising, this is what I saw in the logs:
...
2020-07-26 18:42:41.774  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 252
2020-07-26 18:42:41.786  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 253
2020-07-26 18:42:41.797  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 254
2020-07-26 18:42:41.809  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 255
2020-07-26 18:42:41.819  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 256
2020-07-26 18:42:42.019  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 9
2020-07-26 18:42:42.354  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 10
2020-07-26 18:42:42.689  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 11
2020-07-26 18:42:43.024  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 12
2020-07-26 18:42:43.358  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 13
2020-07-26 18:42:43.691  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 14
2020-07-26 18:42:44.027  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 15
2020-07-26 18:42:44.363  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 16
.....
2020-07-26 18:43:43.724  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 299
2020-07-26 18:43:43.735  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 300
2020-07-26 18:43:43.913  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 194
2020-07-26 18:43:44.248  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 195
2020-07-26 18:43:44.581  INFO 1 --- [      publish-2] sample.meter.Consumer                    : Consumed 196
...
A sequence of numbers upto 256 was produced immediately and then the Producer waited for the Consumer to catch up, once the consumer caught up, the remaining emissions happened. This is how the graph for this looks: 



Clearly, backpressure is acting on this stream of data. The surprising aspect for me was the backpressure appeared to be triggering at a large value of 256 records from upstream.

Analyzing this is a little, the reason I realized is that an intermediate operation is buffering the requests. The intermediate operation in this instance happens to be the "publishOn()" operator that I am using, a variant of "publishOn()" which additionally takes in a prefetch parameter fixes the size of the buffer. 

In my case setting it to 10 felt reasonable, the code looks like this now:
producer.produce(producerRate, count)
    .subscribeOn(subscribeOnScheduler)
    .publishOn(publishOnScheduler, 10)
    .subscribe { value: Long ->
        sleep(delayBetweenConsumes)
        logger.info("Consumed {}", value)
    }
and the graph with the Producer and Consumer remains closely in sync:

Producer In Green, Consumer in Red


Scenario 3: Fast Producer, Multi-threaded Consumer

If you look closely at the name of the threads in logs from the first two scenarios then you would notice that the names of the thread at the point of production and at the point of consumption are always the same. The operators "publishOn()" and "subscribeOn()" don't parallelize the operation, they only switch the execution context of the operations. To really parallelize the operations, two approaches can be taken: 

  1. Using the parallel operator 
  2. Using flatMap flavors with their own "subscribeOn" operators 

For the 3rd scenario, I went for the second option of using flatMap and it looks something like this:
producer.produce(producerRate, count)
    .subscribeOn(subscribeOnScheduler)
    .publishOn(publishOnScheduler, 10)
    .flatMap({ value: Long ->
        Mono.fromSupplier {
            sleep(delayBetweenConsumes)
            logger.info("Consumed {}", value)
            null
        }.subscribeOn(flatMapScheduler)
    }, concurrency)
    .subscribe()
The work of consuming the produced sequence of numbers is being done inside the flatMap operation, the number of concurrent consumption is set to 5 by default. Running this scenario produces the following logs, the consumers are now running 5 at a time on multiple threads:
2020-07-26 23:26:27.212  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 1
2020-07-26 23:26:27.321  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 2
2020-07-26 23:26:27.423  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 3
...
2020-07-26 23:26:28.040  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 9
2020-07-26 23:26:28.143  INFO 1 --- [    subscribe-3] sample.meter.Producer                    : Emitted 10
2020-07-26 23:26:28.222  INFO 1 --- [      flatMap-4] sample.meter.Consumer                    : Consumed 1
2020-07-26 23:26:28.328  INFO 1 --- [      flatMap-5] sample.meter.Consumer                    : Consumed 2
2020-07-26 23:26:28.428  INFO 1 --- [      flatMap-6] sample.meter.Consumer                    : Consumed 3
2020-07-26 23:26:28.527  INFO 1 --- [      flatMap-7] sample.meter.Consumer                    : Consumed 4
...
The rate of production lines up with the rate of consumption

Producer In Green/Consumer in Red


Conclusion

These are different experiments that I was able to run to simulate backpressure scenarios with Project Reactor and the behavior should be true for most Reactive Streams based libraries. 

Project Reactor has sane defaults in managing the backpressure needs of a Consumer and provides ways to override the defaults. 

In all scenarios that I have run in this post, the Producer throttled the production to a rate that the Consumer was comfortable in consuming. 

 If you are interested in exploring the scenarios further, my codebase along with the grafana/prometheus set up for graphing the output is available in my github repository here - https://github.com/bijukunjummen/backpressure-demo

Saturday, July 4, 2020

Expressing a conditional expression using Json - Java Implementation

I had a need recently to express a conditional expression in a form that a front end Javascript application and a backend Java application could both create and read. Expressing the conditional expression as a Json felt logical and after a quick search, JsonLogic library appeared to fit exactly what I was looking for.

JsonLogic follows a prefix notation for its expressions, along these lines:

{"operator" : ["values" ... ]}
So for eg, given a JSON input data that looks like this:
{
  "a": "hello",
  "b": 1,
  "c": [
    "elem1",
    "elem2"
  ]
}
For equality, an expression using JsonLogic is the following:
{"==" : [ { "var" : "a" }, "hello" ] }
Here the data is being looked up using "var" expression and the equality is checked using the "==" operator. 

Though it is a good fit, I decided to go with an alternate way of expressing the conditional expression but heavily inspired by JsonLogic. So, in my implementation, equality with the sample JSON looks like this:
{
    "equals": [
        "/a", "hello"
    ]
}
Fairly similar, the location to the data is expressed as a Json Pointer and the operators are textual ("equals" vs "==") The full set of supported features are also much smaller than JsonLogic as that sufficed my needs for the project. So now I have a small Java-based library that supports these simplified conditional expressions and this post will go into the details of the operators and the usage of the library.



Sample Expressions

Once more to touch on the sample conditional expressions, everything takes the form of:
{"operator" : ["values" ... ]}
A check for equality looks like this:
{
    "equals": [
        "/a", "hello"
    ]
}
Not operator:
{
    "not": [
        {
            "equals": [
                "/a",
                "hello"
            ]
        }
    ]
}
And/Or operator:
{
    "and": [
        {
            "equal": [
                "/a", "hello"
            ]
        },
        {
            "equal": [
                "/b", 1
            ]
        }
    ]
}
There are a few operators that work on collections, for eg, to check if "c" in the sample JSON has elements "elem1", "elem2":
{
    "contains": [
        "/c", ["elem1", "elem2"]
    ]
}
or to check if the collection has any of the elements "elem1", "elem2":
{
    "containsAnyOf": [
        "/c", ["elem1", "elem2"]
    ]
}

Details of the Library

The Java-based library is built on top of the excellent Jackson JSON parser library and uses it to parse the expression which once parsed is interpreted by the library. A Gradle based project can pull in the dependency the following way(published currently to JCenter):
implementation 'com.github.bijukunjummen:json-conditional-expression:0.4.0'
and use the library along these lines, using a sample Kotlin code:
val jsonExpressionEvaluator: JsonExpressionEvaluator = JsonExpressionEvaluator(ObjectMapper())
jsonExpressionEvaluator.matches(expression, json) //returns true

Conclusion

The conditional expression and the corresponding Java-based interpreter are fairly simple and have sufficed my needs with the kind of operators that I needed for my project, however, I will be more than happy to extend and support more extensive operators if there is enough interest in using the library.


Reference

1. JsonLogic which provided the inspiration for using a prefix notation to represent a conditional expression