JevonCowell opened a new pull request, #53558:
URL: https://github.com/apache/spark/pull/53558
### What changes were proposed in this pull request?
- Added a new streaming query event called `onQueryExecutionStart`. This
query event is published every time a streaming query is triggered.
- Added the necessary methods to publish this new event to all
`StreamingQueryListener`s with backwards compatibility.
### Why are the changes needed?
Currently, `StreamingQueryListener`s does not notify users when a query is
triggered; it only provides updates when a query starts, progresses, becomes
idle, or terminates. By introducing an event that is emitted whenever a query
is triggered, users will be informed of this occurrence. This new event allows
`StreamingQueryListener`s to function as a "state machine."
In my use case, I plan to use this new event along with `QueryProgress` and
`QueryIdle` to develop a Maintenance Job Orchestration System. This system will
programmatically submit and execute Spark commands, determining the eligibility
of maintenance job executions based on the activity status of streaming queries.
Bare bone example of Maintenance Job Orchestration System:
```mermaid
sequenceDiagram
participant Scheduler as MaintenanceScheduler
participant Orchestrator as MaintenanceJobOrchestrator
participant Job as MaintenanceJob
participant BackgroundExecutor as BackgroundTaskExecutor
participant Listener as StreamingQueryListener
Orchestrator->>Listener: Register consumer to update query status
Scheduler->>Scheduler: scheduleJob(Job)
alt MaintenanceJob is null
Scheduler->>Scheduler: throw IllegalArgumentException
else MaintenanceJob is not enabled
Scheduler->>Scheduler: return
else
loop Submit to Orchestrator Queue on a schedule
Scheduler->>Orchestrator: enqueue(MaintenanceJobRequest)
end
end
loop Continuous Operation
BackgroundExecutor->>Orchestrator: attempt to run jobs in queue
Listener->>Orchestrator: isStreamingQueryActivelyRunning()
Orchestrator->>Orchestrator: checkIfSafeToRunJob()
alt A Spark job is currently executing
Orchestrator->>Orchestrator: return
else No Spark jobs are currently executing
Orchestrator->>BackgroundExecutor:
submit(job.execute(sparkSession))
BackgroundExecutor->>Job: execute()
Job->>Orchestrator: return result
Orchestrator->>Orchestrator: log job
end
end
Listener->>Listener: shutdown()
BackgroundExecutor->>BackgroundExecutor: shutdown()
Orchestrator->>Orchestrator: shutdown()
Scheduler->>Scheduler: shutdown()
```
I envision several other valuable use cases, such as enhanced metrics,
improved alerting capabilities, and potential optimizations within Spark itself.
Initially, I considered introducing a `QueryAwaitEvent` to complement the
`onQueryExecutionStart`. This could provide a cleaner approach, avoiding the
need to manage both `onQueryProgress` and `onQueryIdle`. However, I believe
addressing the current gap with `onQueryExecutionStart` alone is more effective.
### Does this PR introduce _any_ user-facing change?
Yes/No. I tried my best to match how `QueryIdle` was added to maintain
backwards compatibility & `onQueryExecutionStart` egress is disabled by
default. Users will have to use spark conf to opt into this feature. Existing
implementations should **not** break as a result of this.
Users should do the following to opt-in:
```java
import org.apache.spark.sql.SparkSession
val spark = SparkSession.builder
.appName("ExampleApp")
.config("spark.sql.streaming.query.trigger.start.event.enabled", true)
.config("spark.sql.streaming.query.trigger.start.event.minInterval", 2000)
.getOrCreate()
```
To utilize the new method they just need to override it like so:
Scala:
```scala
/**
* Called when a query's microbatch trigger is started.
* @since 4.2.0
*/
def onQueryTriggerStart(event: onQueryExecutionStart): Unit = {
println(s"Query trigger started: ${event.id}")
}
```
Java:
```java
@Override
public void onQueryTriggerStart(onQueryExecutionStart event) {
System.out.println("Query trigger started: " + event.getId());
}
```
Python:
```python
def onQueryTriggerStart(self, event):
print(f"Query trigger started: {event.id}")
```
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