Add README

Author: leventov

README.md

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+# CronScheduler: a reliable Java scheduler for external interactions
+
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.timeandspace/cron-scheduler/badge.svg)](
+https://maven-badges.herokuapp.com/maven-central/io.timeandspace/cron-scheduler)
+[![Build Status](https://travis-ci.org/TimeAndSpaceIO/CronScheduler.svg?branch=master)](
+https://travis-ci.org/TimeAndSpaceIO/CronScheduler)
+
+`CronScheduler` is an alternative to `ScheduledThreadPoolExecutor` and `Timer` with the following
+advantages:
+
+ - CronScheduler is proof against unbounded [clock drift](https://en.wikipedia.org/wiki/Clock_drift)
+ relative to UTC or system time for both one-shot or periodic tasks.
+ - It [takes into account machine suspension](https://bugs.openjdk.java.net/browse/JDK-8146527)
+ (like sleep or hibernation).
+ - It has convenient methods to prevent several tasks from piling up in case of blocks, long GC
+ pauses, or abrupt forward system time shifts (likely made by the user or the administrator of the
+ machine).
+ - It correctly handles significant system time setbacks (backward shifts), likely made by the user
+ or the administrator of the machine.
+ - It has convenient methods to schedule tasks at round times within a day in a given time zone
+ (for example, every midnight), transparently handling all time zone and [daylight saving time](
+ https://en.wikipedia.org/wiki/Daylight_saving_time) complexity.
+
+See [this blog post](
+https://medium.com/@leventov/cronscheduler-a-reliable-java-scheduler-for-external-interactions-cb7ce4a4f2cd)
+for more details and specific recommendations about when to use `ScheduledThreadPoolExecutor`,
+`CronScheduler`, or other scheduling facilities.
+
+## Usage
+
+Maven:
+```xml
+<dependency>
+ <groupId>io.timeandspace</groupId>
+ <artifactId>cron-scheduler</artifactId>
+ <version>0.1</version>
+</dependency>
+```
+
+Gradle:
+```xml
+dependencies {
+ compile 'io.timeandspace:cron-scheduler:0.1'
+}
+```
+
+Server-side usage example:
+```java
+Duration syncPeriod = Duration.ofMinutes(1);
+CronScheduler cron = CronScheduler.create(syncPeriod);
+cron.scheduleAtFixedRateSkippingToLatest(0, 1, TimeUnit.MINUTES, runTimeMillis -> {
+ // Collect and send summary metrics to a remote monitoring system
+});
+```
+
+Client-side usage example:
+```java
+Duration oneHour = Duration.ofHours(1);
+CronScheduler cron = CronScheduler.create(oneHour);
+cron.scheduleAtRoundTimesInDaySkippingToLatest(oneHour, ZoneId.systemDefault(), runTimeMillis -> {
+ notifyUser("It's time to get up and make a short break from work!");
+});
+```
+
+See [**Javadocs**](
+https://javadoc.io/doc/io.timeandspace/cron-scheduler/latest/io/timeandspace/cronscheduler/CronScheduler.html).