[Backport 2.x] Introduced writing layer, getting rid of writing logic that uses an absolute path in the filesystem.

[0ctopus13prime]

Description

Backporting #2241 to 2.x branch.

[0ctopus13prime]

Will check the failure in Windows

[0ctopus13prime]

The failure does not seem like related to the change.
It failed to get a value from mocking instance.

We set the mocking here, but for some reasons, the mocked value is not being returned in Windows.
I'm suspecting this is a concurrent issue in testing.

Model mockModel = new Model(modelMetadata1, modelBlob, modelId);
when(modelDao.get(modelId)).thenReturn(mockModel); <---------------- This should return 'test-model'
when(modelDao.getMetadata(modelId)).thenReturn(modelMetadata1);

Error :

  1> java.lang.IllegalArgumentException: Model ID 'test-model' is not created.
  1> 	at org.opensearch.knn.indices.ModelUtil.getModelMetadata(ModelUtil.java:54)
  1> 	at org.opensearch.knn.common.FieldInfoExtractor.extractKNNEngine(FieldInfoExtractor.java:42)
  1> 	at org.opensearch.knn.index.codec.util.KNNCodecUtil.getNativeKNNEngine(KNNCodecUtil.java:126)

[0ctopus13prime]

Reran the test.
If it is still failing, then will fix the code in Windows and reraise the PR!

[0ctopus13prime]

All tests passed 😅
Will merge the PR.
Also will create a new issue regarding the mocking failure.