# Using Gremlin with the Neptune DFE query engine
<a name="gremlin-with-dfe"></a>

You can enable Neptune's [alternative query engine](neptune-dfe-engine.md), known as the DFE, in two ways:
+ Set the [`neptune_dfe_query_engine`](parameters.md#parameters-instance-parameters-neptune_dfe_query_engine) instance parameter to `enabled` for all queries on the instance.
+ Use the [`useDFE` query hint](gremlin-query-hints-useDFE.md) on individual queries (requires `neptune_dfe_query_engine` to be set to its default value of `viaQueryHint`).

When DFE is enabled, Neptune translates read-only Gremlin queries and traversals into an intermediate logical representation and runs them on the DFE engine whenever possible.

However, the DFE does not yet support all Gremlin steps. When a step can't be run natively on the DFE, Neptune falls back on TinkerPop to run the step. The `explain` and `profile` reports include warnings when this happens.

## Query planning interleaving
<a name="gremlin-with-dfe-interleaving"></a>

When the translation process encounters a Gremlin step that does not have a corresponding native DFE operator, before falling back to using Tinkerpop it tries to find other intermediate query parts that can be run natively on the DFE engine. It does this by applying interleaving logic to the top level traversal. The result is that supported steps are used wherever possible.

Any such intermediate, non-prefix query translation is represented using `NeptuneInterleavingStep` in the `explain` and `profile` outputs.

For performance comparison, you might want to turn off interleaving in a query, while still using the DFE engine to run the prefix part. Or, you might want to use only the TinkerPop engine for non-prefix query execution. You can do this by using `disableInterleaving` query hint.

Just as the [useDFE](gremlin-query-hints-useDFE.md) query hint with a value of `false` prevents a query from being run on the DFE at all, the `disableInterleaving` query hint with a value of `true` turns off DFE interleaving for translation of a query. For example:

```
g.with('Neptune#disableInterleaving', true)
 .V().has('genre','drama').in('likes')
```

## Updated Gremlin `explain` and `profile` output
<a name="gremlin-with-dfe-explain-update"></a>

Gremlin [explain](gremlin-explain.md) provides details about the optimized traversal that Neptune uses to run a query. See the [sample DFE `explain` output](gremlin-explain-api.md#gremlin-explain-dfe) for an example of what `explain` output looks like when the DFE engine is enabled.

The [Gremlin `profile` API](gremlin-profile-api.md) runs a specified Gremlin traversal, collects various metrics about the run, and produces a profile report that contains details about the optimized query plan and the runtime statistics of various operators. See [sample DFE `profile` output](gremlin-profile-api.md#gremlin-profile-sample-dfe-output) for an example of what `profile` output looks like when the DFE engine is enabled.

**Note**  
Because DFE support for Gremlin is an experimental feature, the exact format of the `explain` and `profile` output is subject to change.