Hi, The markers are necessary to offer file system semantics on top of object stores. You will get a ton of subtle bugs otherwise.
If instead of arrow::FileSystem, Arrow offered an arrow::ObjectStore interface that wraps local filesystems and object stores with object-store semantics (i.e. no concept of empty directory or atomic directory deletion), then application developers would have more control of the actions performed on the object store they are using. Cons would be slower operations when working with a local filesystem and no concept of directory. > 1. **Add an Option**: Introduce an option in *S3Options* to control whether empty directory markers are created, giving users the choice. Then it wouldn't be an honest implementation of arrow::FileSystem for the reasons listed above. > **Change Default Behavior**: Modify the default behavior to avoid creating empty directory markers when a file is deleted. That would bring in the bugs because an arrow::FileSystem instance would behave differently depending on what is backing it. > 3. **Smarter Directory Creation**: Improve the implementation to check for other objects in the same path before creating an empty directory marker. This might be a problem when more than one client or thread is mutating the object store through the arrow::FileSystem. You can check now and once you're done deleting all the other files you thought existed are deleted as well. Very likely if clients decide to implement parallel deletion. The existing solution of always creating a marker when done is not perfect either, but less likely to break. ## Suggested Workaround Avoiding file by file operations so that internal functions can batch as much as possible. -- Felipe On Fri, Jul 12, 2024 at 7:22 AM Hyunseok Seo <hsseo0...@gmail.com> wrote: > Hello. community! > > I am currently working on addressing the issue described in [[C++] Add > option to not create parent directory with S3 delete_file]( > https://github.com/apache/arrow/issues/36275). In this process, I have > found it necessary to gather feedback on how to best resolve this issue. > Below is a summary and some questions I have for the community. > > > *### Background* > Currently, the *S3FileSystem* generates an empty directory marker (by > calling the *EnsureParentExists* function) when a file is deleted and the > directory becomes empty. This behavior maintains the appearance of the > directory structure. However, there have been issues raised by users > regarding this behavior in issues [1][2]. > > > *### Why Maintain Empty Directory Markers?* > From what I understand, object stores like S3 do not have a concept of > directories. The motivation behind maintaining these markers could be to > manage the object store as if it were a traditional file system. If anyone > knows the context behind the implementation of S3FileSystem, it would be > great if you could share it. > > > *### Issues with Marker Creation* > Users who have raised concerns about the creation of empty directory > markers cite the following reasons: > > - **Increase in Unnecessary Requests [2]**: Creating empty directory > markers leads to additional S3 requests, which can increase costs and > affect performance. > - **File System Consistency Issues [1]**: S3 is designed as an object > store, and creating empty directory markers can break the inherent > consistency of the file system. > > > *### Proposed Solutions* > Issue [1] suggests the following approaches: > > 1. **Add an Option**: Introduce an option in *S3Options* to control whether > empty directory markers are created, giving users the choice. > 2. **Change Default Behavior**: Modify the default behavior to avoid > creating empty directory markers when a file is deleted. > 3. **Smarter Directory Creation**: Improve the implementation to check for > other objects in the same path before creating an empty directory marker. > > *Here is my personal thought (*approach* 1 + 3)*: > > (*approach 1*) I believe it would be best to add the Marker as an option > (as some users might not want this enhancement). > > (*approach 3*) When the option is enabled, if there are no files (objects) > in the path (prefix) corresponding to a directory based on the file system > concept, we should maintain the Marker. Otherwise, we should check the > number of files in the same path and avoid calling EnsureParentExists if > there are two or more files. > > On the other hand, I also feel that this approach might make the logic more > complicated. > > > *### We Would Like Your Feedback* > - What are your thoughts on the creation of empty directory markers? > - Which of the proposed solutions do you prefer? > - Do you have any additional suggestions or comments? > > We appreciate your valuable feedback and aim to find the best solution > based on your input. > > Thank you. > > [1]: https://github.com/apache/arrow/issues/36275 > [2]: https://github.com/apache/arrow/issues/40589 >
