#813 WIP: Extract DbConn, Searcher & Worker into Riker Actors

Can you compile this branch? I couldn’t...

Note: SearcherActor cannot commit indices due to EnterError(https://github.com/tantivy-search/tantivy/issues/898). This issue might be related: https://github.com/rust-lang/futures-rs/issues/2090.

This occurs because perhaps both Riker and Tantivy use futures::excutor::LocalPool and Tantivy’s block_on is called in Riker’s LocalPool.

Possible options (based on my poor understanding of async) are:

• SearcherActor doesn’t commit indices. Raw Searcher does periodically as current implementation does.
• Write patches to Riker so that we can choose other async runtime like async-std and use it.
• Do the same to Tantivy.

Note that we cannot search new posts until their documents are commited not just added.

I respect your great research and implementation, and completely agree with introducing Riker. In addition, I make a suggestion: use Riker’s Channels as a global variable. There are two points:

• The primary is to make it global, and
• The secondary is use of pub/sub model using channels

What is solved?

At current attempting (its strategy and current implementation: 8aa4ae4302), we need touch searcher more than ever before. Also, we need flow, some agents (like searcher actor or post ids) through multiple structs.

See updating search document procedure. When a user updates a post, a request handler does two (or more) things: it

• updates posts and related records in database
• sends a UpdateDocument message to SearcherActor

Developers need always do them together. This is needed for not only request handlers but AsObject and FromId implementations for models. As such, developers need write more code than now. And they might have forgotten sending message to the actor.

To make matters worse, this introduces more complexity. Assume you’re deleting a user. routes::user::delete() request handler is called. It needs to know all blogs and posts that the deleting user owns in order to send DeleteDocument messeges corresponding to deleted posts to searcher actor. How?

One way is: Post::delete() returns itself to Blog, Blog::delete() returns deleted Posts to User, User::delete() returns deleted Blogs’ posts to the handler, and fanally the handler sends messeges to the actor. If some posts could not be deleted, the handler need make sure to send DeleteDocument for each only deleted post. That’s complex.

The another is: the handler passes the searcher actor to User::delete() as an argument, User::delete() passes it to Blog::delete(), Blog::delete() calls Post::delete() and send a DeleteDocument message to the actor several times. This is essentially same to above.

Introducing a global variable solves this. Using it, Post::insert(), update() and delete() need to neither accept a searcher as an agument nor return back itself (or its ID) to the caller. What it needs to do is getting global searcher (or so) and sending a messege that it is deleted. That’s it.

There are options what struct should be global like a searcher actor, an actor system or a channel. I suggest Riker’s channel because...

More benefit

Use of channel introduces pub/sub model. In the case above, Post publishes “delete” event to a channel and SearcherActor subscribes to the event and deletes the search index corresponding to the post. More than that, we can add other actors which concern post’s deletion without touching any code of request handlers and models. Examples are:

• an actor (say, “outbox”) which subscribes to post’s publishing, updating, deleting event and sends ActivityPub requests to other instances
• an actor (“notifier”) which subscribes to post’s events and updates notification page of followers and mentioned users
• similar to the above but sends notification mails
• and so on.

A proof of concept is here: https://git.joinplu.me/KitaitiMakoto/Plume/compare/igalic/refactor/extract-actors...channel. Pay your attention to

// a `SearcherActor` subscribes to any event (`"*"`)

POST_CHAN.get().unwrap().tell(Subscribe { actor: Box::new(searcher_actor), topic: "*".into() }, None);

// `Post` publishes an event without the need knowing who accepts it

        if post.published {
            crate::POST_CHAN.get().unwrap().tell(Publish { msg: PostUpdated(post.clone()), topic: "post.updated".into() }, None);
        }

Rebasing

What do you think? I you aggree with me, I’m thiking it might be better to restart from current 0.6.0+ code (of course, I will do it). Any questions and discussions are welcome!

that’s a good idea, i hope I’ll have time to do that soon.
but i do feel like every time i attempt this i have learned something in between, and so it becomes a little bit easier

Thanks! I will start later.