> ## Documentation Index
> Fetch the complete documentation index at: https://www.orionjs.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Dogs adapts its MongoDB acquisition index

> Let each scheduler choose the fastest job acquisition index for its own workload

`@orion-js/dogs@4.6.2` can now choose between two MongoDB indexes when claiming jobs. Each
`startWorkers()` instance measures both alternatives against its own configured job names and keeps
the faster hint in memory.

This matters because the best acquisition plan depends on the scheduler's workload. An application
that accepts a small subset of job names can benefit from leading the index with `jobName`, while a
server that accepts most jobs may scan less work when the index starts with `priority`.

## The two acquisition indexes

Dogs declares both indexes on `orionjs.jobs_dogs_records`:

```javascript theme={null}
{jobName: 1, priority: -1, nextRunAt: 1}
{priority: -1, nextRunAt: 1}
```

The first remains the initial hint. The second is the alternative evaluated by the scheduler. The
real atomic `findOneAndUpdate` claim always receives the hint currently selected by that
`startWorkers()` instance.

## Measure the real claim shape

The comparison runs once at startup in the background and then every 30 minutes. It uses the same
eligibility filter and sort as the real claim, including every job configured for the scheduler:

```javascript theme={null}
find(eligibilitySelector)
  .sort({priority: -1, nextRunAt: 1})
  .limit(1)
  .explain('executionStats')
```

Each probe runs three explains per hint, alternating their order to reduce ordering bias. Queries
run on the primary with `maxTimeMS: 1000`, and Dogs compares the median
`executionTimeMillis` reported by MongoDB.

The first successful probe applies its winner immediately. Later probes require the alternative to
win twice consecutively before changing the active hint. Ties, failures, and timeouts keep the
current hint and reset the pending win streak.

## Operational behavior

The feature requires no configuration and stores no state outside the running process. Different
servers can therefore select different hints for the same collection when their configured jobs or
data distributions differ.

Dogs logs the initial selection and later changes at `info`, retained hints at `debug`, and probe
failures at `warn`. A scheduler with zero configured jobs does not run probes.

Calling `stop()` cancels the next scheduled probe and waits for an explain already in flight. Once
that explain returns, Dogs discards the rest of the incomplete comparison.

## Upgrade notes

* There are no public API or job-document changes.
* Allow Orion's normal collection index setup to create both declared indexes before starting work.
* Each scheduler adds six lightweight explain queries at startup and every 30 minutes.
* Both indexes remain present so the scheduler can adapt again when the workload changes.

See the [Jobs guide](/overview/controllers/jobs) for worker configuration and shutdown behavior.