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docs: improve fan-out/fan-in

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Ilkka Seppälä
2024-04-20 15:21:02 +03:00
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fanout-fanin/README.md
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---
title: Fan-Out/Fan-In
category: Integration
category: Concurrency
language: en
tag:
- Microservices
- Asynchronous
- Data processing
- Decoupling
- Scalability
---
## Also known as
* Scatter-Gather
## Intent
The pattern is used when a source system needs to run one or more long-running processes that will fetch some data.
The source will not block itself waiting for the reply. <br> The pattern will run the same function in multiple
services or machines to fetch the data. This is equivalent to invoking the function multiple times on different chunks of data.
The Fan-Out/Fan-In pattern aims to improve concurrency and optimize processing time by dividing a task into multiple sub-tasks that can be processed in parallel (fan-out) and then combining the results of these sub-tasks into a single outcome (fan-in).
## Explanation
The FanOut/FanIn service will take in a list of requests and a consumer. Each request might complete at a different time.
FanOut/FanIn service will accept the input params and returns the initial system an ID to acknowledge that the pattern
service has received the requests. Now the caller will not wait or expect the result in the same connection.
Meanwhile, the pattern service will invoke the requests that have come. The requests might complete at different time.
These requests will be processed in different instances of the same function in different machines or services. As the
requests get completed, a callback service every time is called that transforms the result into a common single object format
that gets pushed to a consumer. The caller will be at the other end of the consumer receiving the result.
The Fan-Out/Fan-In service will take in a list of requests and a consumer. Each request might complete at a different time. Fan-Out/Fan-In service will accept the input params and returns the initial system an ID to acknowledge that the pattern service has received the requests. Now the caller will not wait or expect the result in the same connection.
Meanwhile, the pattern service will invoke the requests that have come. The requests might complete at different time. These requests will be processed in different instances of the same function in different machines or services. As the
requests get completed, a callback service every time is called that transforms the result into a common single object format that gets pushed to a consumer. The caller will be at the other end of the consumer receiving the result.
**Programmatic Example**
The implementation provided has a list of numbers and end goal is to square the numbers and add them to a single result.
`FanOutFanIn` class receives the list of numbers in the form of list of `SquareNumberRequest` and a `Consumer` instance
that collects the results as the requests get over. `SquareNumberRequest` will square the number with a random delay
to give the impression of a long-running process that can complete at any time. `Consumer` instance will add the results from
different `SquareNumberRequest` that will come random time instances.
The implementation provided has a list of numbers and end goal is to square the numbers and add them to a single result. `FanOutFanIn` class receives the list of numbers in the form of list of `SquareNumberRequest` and a `Consumer` instance
that collects the results as the requests get over. `SquareNumberRequest` will square the number with a random delay to give the impression of a long-running process that can complete at any time. `Consumer` instance will add the results from different `SquareNumberRequest` that will come random time instances.
Let's look at `FanOutFanIn` class that fans out the requests in async processes.
Let's look at `FanOutFanIn` class that fans out the requests in async processes.
```java
public class FanOutFanIn {
public static Long fanOutFanIn(
final List<SquareNumberRequest> requests, final Consumer consumer) {
public static Long fanOutFanIn(final List<SquareNumberRequest> requests, final Consumer consumer) {
ExecutorService service = Executors.newFixedThreadPool(requests.size());
ExecutorService service = Executors.newFixedThreadPool(requests.size());
// fanning out
List<CompletableFuture<Void>> futures =
requests.stream()
.map(
request ->
CompletableFuture.runAsync(() -> request.delayedSquaring(consumer), service))
.collect(Collectors.toList());
// fanning out
List<CompletableFuture<Void>> futures = requests
.stream()
.map(request -> CompletableFuture.runAsync(() -> request.delayedSquaring(consumer), service))
.collect(Collectors.toList());
CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
CompletableFuture.allOf(futures.toArray(new CompletableFuture[0])).join();
return consumer.getSumOfSquaredNumbers().get();
}
return consumer.getSumOfSquaredNumbers().get();
}
}
```
`Consumer` is used a callback class that will be called when a request is completed. This will aggregate
the result from all requests.
`Consumer` is used a callback class that will be called when a request is completed. This will aggregate the result from all requests.
```java
public class Consumer {
private final AtomicLong sumOfSquaredNumbers;
private final AtomicLong sumOfSquaredNumbers;
Consumer(Long init) {
sumOfSquaredNumbers = new AtomicLong(init);
}
Consumer(Long init) {
sumOfSquaredNumbers = new AtomicLong(init);
}
public Long add(final Long num) {
return sumOfSquaredNumbers.addAndGet(num);
}
public Long add(final Long num) {
return sumOfSquaredNumbers.addAndGet(num);
}
}
```
Request is represented as a `SquareNumberRequest` that squares the number with random delay and calls the
`Consumer` once it is squared.
Request is represented as a `SquareNumberRequest` that squares the number with random delay and calls the `Consumer` once it is squared.
```java
public class SquareNumberRequest {
private final Long number;
public void delayedSquaring(final Consumer consumer) {
private final Long number;
var minTimeOut = 5000L;
public void delayedSquaring(final Consumer consumer) {
SecureRandom secureRandom = new SecureRandom();
var randomTimeOut = secureRandom.nextInt(2000);
var minTimeOut = 5000L;
try {
// this will make the thread sleep from 5-7s.
Thread.sleep(minTimeOut + randomTimeOut);
} catch (InterruptedException e) {
LOGGER.error("Exception while sleep ", e);
Thread.currentThread().interrupt();
} finally {
consumer.add(number * number);
SecureRandom secureRandom = new SecureRandom();
var randomTimeOut = secureRandom.nextInt(2000);
try {
// this will make the thread sleep from 5-7s.
Thread.sleep(minTimeOut + randomTimeOut);
} catch (InterruptedException e) {
LOGGER.error("Exception while sleep ", e);
Thread.currentThread().interrupt();
} finally {
consumer.add(number * number);
}
}
}
}
```
## Class diagram
![alt-text](./etc/fanout-fanin.png)
![Fan-Out/Fan-In](./etc/fanout-fanin.png)
## Applicability
Use this pattern when you can divide the workload into multiple chunks that can be dealt with separately.
Appropriate in scenarios where tasks can be broken down and executed in parallel, especially suitable for data processing, batch processing, and situations requiring aggregation of results from various sources.
## Known Uses
* Large-scale data processing applications.
* Services requiring aggregation from multiple sources before delivering a response, such as in distributed caching or load balancing systems.
## Consequences
Benefits:
* Enhances performance by parallel processing.
* Increases responsiveness of systems.
* Efficient utilization of multi-core processor architectures.
Trade-offs:
* Increased complexity in error handling.
* Potential for increased overhead due to task synchronization and result aggregation.
* Dependency on the underlying infrastructure's ability to support concurrent execution.
## Related Patterns
* MapReduce: Similar to Fan-Out/Fan-In, MapReduce also involves distributing tasks across a number of workers (map) and aggregating the results (reduce), which is particularly useful for processing large data sets.
* [Command](https://java-design-patterns.com/patterns/command/): Command Pattern facilitates the decoupling of the sender and the receiver, akin to how Fan-Out/Fan-In decouples task submission from task processing.
* [Producer-Consumer](https://java-design-patterns.com/patterns/producer-consumer/): Works synergistically with Fan-Out/Fan-In by organizing task execution where producers distribute tasks that are processed by multiple consumers, and results are then combined, enhancing throughput and efficiency in data processing.
## Related patterns
@@ -114,4 +136,6 @@ Use this pattern when you can divide the workload into multiple chunks that can
* [Understanding Azure Durable Functions - Part 8: The Fan Out/Fan In Pattern](http://dontcodetired.com/blog/post/Understanding-Azure-Durable-Functions-Part-8-The-Fan-OutFan-In-Pattern)
* [Fan-out/fan-in scenario in Durable Functions - Cloud backup example](https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-cloud-backup)
* [Understanding the Fan-Out/Fan-In API Integration Pattern](https://dzone.com/articles/understanding-the-fan-out-fan-in-api-integration-p)
* [Understanding the Fan-Out/Fan-In API Integration Pattern](https://dzone.com/articles/understanding-the-fan-out-fan-in-api-integration-p)
* [Java Concurrency in Practice](https://amzn.to/3vXytsb)
* [Patterns of Enterprise Application Architecture](https://amzn.to/49QQcPD)
fanout-fanin/src/main/java/com/iluwatar/fanout/fanin/FanOutFanIn.java
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import java.util.concurrent.Executors;
/**
* FanOutFanIn class processes long running requests, when any of the processes gets over, result is
* FanOutFanIn class processes long-running requests, when any of the processes gets over, result is
* passed over to the consumer or the callback function. Consumer will aggregate the results as they
* keep on completing.
*/
fanout-fanin/src/main/java/com/iluwatar/fanout/fanin/SquareNumberRequest.java
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@@ -29,7 +29,7 @@ import lombok.AllArgsConstructor;
import lombok.extern.slf4j.Slf4j;
/**
* Squares the number with a little timeout to give impression of long running process that return
* Squares the number with a little timeout to give impression of long-running process that return
* at different times.
*/
@Slf4j
@@ -39,7 +39,7 @@ public class SquareNumberRequest {
private final Long number;
/**
* Squares the number with a little timeout to give impression of long running process that return
* Squares the number with a little timeout to give impression of long-running process that return
* at different times.
* @param consumer callback class that takes the result after the delay.
* */