Append data to existing gridfs file
gridfs, java, mongodb, mongodb-java
Solution
GridFS is not a core feature of MongoDB but a convention of storing binary data with accompanying metadata. You should be able to modify any document in `fs.chunks` collection in a usual manner while keeping corresponding document in `fs.files` intact. The main problem will be recalculating MD5 checksum, but AFAIK it isn't used anywhere and is just a "free" bonus. Anyway it's still possible for appending-only modification (see MD5 digest of a resumed download).
So to append to an existing GridFS file you need to locate corresponding document in `fs.files`. Then depending on last chunk filling ratio (`length` % `chunkSize` == 0) you either rewrite the last chunk document in `fs.chunks` respecting the `chunkSize`, and/or simply add new chunk(s) with incrementing `n` field. Next update `length` in `fs.files` and possibly other metadata.
Problem
As i can see java mongo driver does not provide capability to get `OutputStream` from existing gridFS file `com.mongodb.gridfs.GridFSFile` I have to create `GridFSInputFile` directly or use `gridFs.createFile()` method. Is it a lack of java driver or a restriction of gridfs ? Could you suggest any workaround except create new file/delete old one ? Thank you