Adds documentation for Job Scheduler plugin

_config.yml

@@ -70,6 +70,9 @@ collections:
   notifications-plugin:
     permalink: /:collection/:path/
     output: true
+  job-scheduler-plugin:
+    permalink: /:collection/:path/
+    output: true
   clients:
     permalink: /:collection/:path/
     output: true
@@ -136,6 +139,9 @@ just_the_docs:
     notifications-plugin:
       name: Notifications plugin
       nav_fold: true
+    job-scheduler-plugin:
+      name: Job Scheduler plugin
+      nav_fold: true
     clients:
       name: Clients
       nav_fold: true

_job-scheduler-plugin/index.md

@@ -0,0 +1,130 @@
+---
+layout: default
+title: Job Scheduler
+nav_order: 1
+has_children: false
+has_toc: false
+---
+
+# Job Scheduler
+
+The OpenSearch Job Scheduler plugin provides a framework that can be used to build schedules for common tasks performed on your cluster. You use the job scheduler’s Service Provider Interface (SPI) to define schedules for cluster management tasks such as taking snapshots, managing your data’s lifecycle, and running periodic jobs. Job Scheduler has a sweeper that listens for updated events on the OpenSearch cluster, and a scheduler that manages when jobs run.
+
+You can install the Job Scheduler plugin by following the standard [OpenSearch plugin installation]({{site.url}}{{site.baseurl}}/install-and-configure/install-opensearch/plugins/) process. The sample-extension-plugin example provided in the [Job Scheduler Github repo](https://github.com/opensearch-project/job-scheduler) provides a complete example of utilizing Job Scheduler when building a plugin. To define schedules, you build a plugin that implements the interfaces provided in the job scheduler library. You can schedule jobs by specifying an interval, or you can use a Unix cron expression such as `0 12 * * ?` which runs at noon every day, to define a more flexible schedule for which to execute a job.
+
+## Building a plugin for Job Scheduler
+
+OpenSearch plugin developers can extend the Job Scheduler plugin to schedule jobs to perform on the cluster. Jobs you can schedule include running aggregation queries against raw data and saving the aggregated data into a new index every hour, or continuing to monitor the shard allocation by calling the OpenSearch API and then posting the output to a webhook.
+
+For examples of building a plugin that utilizes the Job Scheduler plugin, see the Job Scheduler [README](https://github.com/opensearch-project/job-scheduler/blob/main/README.md).
+
+## Defining an endpoint
+
+You can configure your plugin's API endpoint by referencing the [example](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleExtensionRestHandler.java) `SampleExtensionRestHandler.java` file. Set the endpoint URL that your plugin will expose with `WATCH_INDEX_URI`:
+
+```java
+public class SampleExtensionRestHandler extends BaseRestHandler {
+    public static final String WATCH_INDEX_URI = "/_plugins/scheduler_sample/watch";
+```
+
+You can define the job configuration by [extending](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobParameter.java) `ScheduledJobParameter`. You can also define the fields used by your plugin like `indexToWatch` as shown in the [example](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobParameter.java) `SampleJobParameter` file. This job configuration will be saved as a document in an index defined by you as shown in this [example](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleExtensionPlugin.java#L54).
+
+## Parameters configuration
+
+You can configure your plugin's parameters by referencing and modifying the [example](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobParameter.java) `SampleJobParameter.java` file to fit your needs:
+
+```java
+/**
+ * A sample job parameter.
+ * <p>
+ * It adds an additional "indexToWatch" field to {@link ScheduledJobParameter}, which stores the index
+ * the job runner will watch.
+ */
+public class SampleJobParameter implements ScheduledJobParameter {
+    public static final String NAME_FIELD = "name";
+    public static final String ENABLED_FILED = "enabled";
+    public static final String LAST_UPDATE_TIME_FIELD = "last_update_time";
+    public static final String LAST_UPDATE_TIME_FIELD_READABLE = "last_update_time_field";
+    public static final String SCHEDULE_FIELD = "schedule";
+    public static final String ENABLED_TIME_FILED = "enabled_time";
+    public static final String ENABLED_TIME_FILED_READABLE = "enabled_time_field";
+    public static final String INDEX_NAME_FIELD = "index_name_to_watch";
+    public static final String LOCK_DURATION_SECONDS = "lock_duration_seconds";
+    public static final String JITTER = "jitter";
+
+    private String jobName;
+    private Instant lastUpdateTime;
+    private Instant enabledTime;
+    private boolean isEnabled;
+    private Schedule schedule;
+    private String indexToWatch;
+    private Long lockDurationSeconds;
+    private Double jitter;
+```
+
+Next, configure the request parameters you would like your plugin to use with Job Scheduler. These will be based on the variables that are declared when configuring your plugin. The sample below shows the request parameters you set when building your plugin:
+
+```java
+public SampleJobParameter(String id, String name, String indexToWatch, Schedule schedule, Long lockDurationSeconds, Double jitter) {
+        this.jobName = name;
+        this.indexToWatch = indexToWatch;
+        this.schedule = schedule;
+
+        Instant now = Instant.now();
+        this.isEnabled = true;
+        this.enabledTime = now;
+        this.lastUpdateTime = now;
+        this.lockDurationSeconds = lockDurationSeconds;
+        this.jitter = jitter;
+    }
+
+    @Override
+    public String getName() {
+        return this.jobName;
+    }
+
+    @Override
+    public Instant getLastUpdateTime() {
+        return this.lastUpdateTime;
+    }
+
+    @Override
+    public Instant getEnabledTime() {
+        return this.enabledTime;
+    }
+
+    @Override
+    public Schedule getSchedule() {
+        return this.schedule;
+    }
+
+    @Override
+    public boolean isEnabled() {
+        return this.isEnabled;
+    }
+
+    @Override
+    public Long getLockDurationSeconds() {
+        return this.lockDurationSeconds;
+    }
+
+    @Override public Double getJitter() {
+        return jitter;
+    }
+```
+
+The following table describes the request parameters configured in the previous sample. All the request parameters shown are required.
+
+| Field | Data type | Description |
+:--- | :--- | :---
+| getName | String | Returns the name of the job. |
+| getLastUpdateTime | Time unit | Returns the last time the job ran. |
+| getEnabledTime | Time unit | Returns the time that the job was enabled. |
+| getSchedule | Unix cron | Returns the schedule that the job runs on formatted in Unix cron syntax. |
+| isEnabled | Boolean | Whether or not the job is enabled. |
+| getLockDurationSeconds | Integer | Returns the duration of time that the job is locked. |
+| getJitter | Integer | Returns the defined jitter value. |
+
+The logic used by your job should be defined by a class extended from `ScheduledJobRunner` in the `SampleJobParameter.java` sample file, such as `SampleJobRunner`. While the job is running, there is a locking mechanism you can use to prevent other nodes from running the same job. [Acquire](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobRunner.java#L96) the lock. Then, make sure to release the lock before the [job finishes](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobRunner.java#L116).
+
+For more information, see the Job Scheduler [sample extention](https://github.com/opensearch-project/job-scheduler/blob/main/sample-extension-plugin/src/main/java/org/opensearch/jobscheduler/sampleextension/SampleJobParameter.java) directory in the [Job Scheduler Github repository](https://github.com/opensearch-project/job-scheduler).