Milvus integration

[lalitpagaria]

This PR to integrate Milvus as document store with Haystack.

Task list -

• Fix code (Currently it have lot of copied code from fiass)
• Parameterise index search and creation parameters using kwargs dict
• Add test cases
• Fix Doc string

Not handling these task in this PR, because it require discussion as milvus server need DB folder mounting for persistent storage -

• Add tutorial
• Integrate milvus in docker compose instead of ES

[lalitpagaria]

@tholor and @tanaysoni

Please find WIP PR for Milvus integration. Currently I just merged my old PR, and latest master changes. There are many things require changes (inside the methods), I will do it in coming days.

From design perspective I using same design as FaissDocumentStore, but my old PR had different design. With my old PR we would use any of storage ie RDBMS, ES or Memory to store documents and it's metadata. But I thought now we want to get rid of ES so not used that design. Please let me know if you want to make this document store more generic. Also review task list and add/remove/update accordingly.

[lalitpagaria]

@tholor and @tanaysoni PR is ready for review.
I am handling following tasks with this PR, because it require discussion as milvus server need DB folder mounting for persistent storage -

• Add tutorial
• Integrate milvus in docker compose instead of ES

[tholor]

Great job @lalitpagaria !

I think the overall design is great. I left a few minor comments around naming and exceptions.
I will also adjust a few docstrings tomorrow.

I am handling following tasks with this PR, because it require discussion as milvus server need DB folder mounting for persistent storage -

I am not sure what you meant here. Are you talking about the integration into the docker-compose requiring mounted volumes for persisting the data?

Open TODOs (we can handle them in a separate PR if preferred):

• Tutorial
• Docker integrations
• Support filters
• Documentation why/when to use milvus FYI @brandenchan
• Benchmarks FYI @brandenchan

[lalitpagaria]

Sorry I mean "I am not handling".
Yes need to adjust docker compose and adding these changes will increase PR size. Better to create another PR to address the tasks your already added in your above comment.

[tanaysoni]

Thank you for working on this @lalitpagaria!

[lalitpagaria]

@tholor and @tanaysoni can you please check see if anything is pending.

[xpatronum]

@lalitpagaria Have you tried benchmarking MILVUS vs FAISS on the number of passages retrieved per sec.?

[lalitpagaria]

No it will be done after merging this PR.
Please refer this ticket to track progress #778

[lalitpagaria]

And for latest Faiss benchmark please refer this #773