chronicle-flux/README.md

chronicle-flux

A reactive driver for chronicle-queue

This project builds on Chronicle Queue (see also their product page) to offer a simple reactive data store.

Chronicle Queue is a low latency library that persists everything on disk with memory mapped files (off-heap).

Functionalities

With the provided ChronicleStore, you can:

• Store a reactive stream.
• Subscribe to the history + live updates of the store with a reactive stream (the store returns a Flux from project reactor-core).
• Replay the history with the original timing, with a time acceleration (e.g. replay at twice the speed).
• Replay the history in a loop.

Use cases

This project can be useful for different use cases:

• event sourcing, to store all events entering an application.
• you can use it as an off-heap buffer in a reactive stream if you need to handle data bursts.
• testing, to replay the history of an application.
• performance testing, to replay the events at higher speeds.
• a simple alternative to Kafka for basic uses cases: no need to install anything, it's all persisted on disk 😃

Getting started

For Gradle users:

compile 'ch.streamly:chronicle-flux:1.0.0'

For Maven users:

<dependency>
    <groupId>ch.streamly</groupId> 
    <artifactId>chronicle-flux</artifactId> 
    <version>1.0.0</version> 
    <type>pom</type> 
</dependency>

Example usage

Create a Chronicle Store:

Please note that you must provide a way to serialize the data as binary.
The easiest solution is to use a binary serialization protocol such as Protobuf, Avro, etc. In this case it will be supported out of the box.

The first argument is the directory path used to persist on disk.
Since Chronicle is using memory mapped file, you should not use a path mounted on a network file system, for more details, have a look at the Chronicle documentation
The 2nd and 3rd arguments are the serializer and deserializer for your data.

ChronicleStore<DummyObject> chronicleStore = new ChronicleStore<>(PATH, DummyObject::toBinary, DummyObject::fromBinary);

Store a stream of data

The store method will return a handle that can be used to stop the storage.
Otherwise the data stream will be stored until it completes or an error is received on the stream.

Flux<DummyObject> source = ... 
Disposable handle = chronicleStore.store(source);

Subscribe to the store

We can subscribe to the store and print old values, as well as new values being persisted in the store.

Flux<DummyObject> allValues = chronicleStore.retrieveAll();
        allValues.doOnNext(System.out::println)
                .blockLast();

We can also replay the history with the same timing as the original stream, and in an infinite loop:

chronicleStore.replayHistory(DummyObject::timestamp)
                .withOriginalTiming()
                .inLoop()
                .doOnNext(i -> System.out.println(Instant.now() + " " + i))
                .blockLast();

In order to replay data with the original timing, we need to provide a function to extract the epoch time (in milliseconds) from the data.

Chronicle Store vs Chronicle Journal

A Chronicle Journal adds a timestamp to every value saved in the journal, and gives you a stream of Timed values.

This means that you can replay the values with the timestamps assigned by the journal without providing a custom timestamp extractor.

ChronicleJournal<DummyObject> chronicleJournal = new ChronicleJournal<>(PATH, DummyObject::toBinary, DummyObject::fromBinary);

chronicleJournal.replayHistory()
                .withOriginalTiming()
                .inLoop()
                .doOnNext(i -> System.out.println(Instant.now() + " " + i))
                .blockLast();

Although this is convenient, it is usually a better idea to time the values as soon as they enter the application. When the journal adds the timestamp, your values might have gone through several queues and delays, resulting in a meaningless timestamp.

Runnable demo

if you want to run some code samples, have a look at the demo folder in the test directory that contains several runnable classes.