Haskell database connections

Question

Please look at this scotty app (it's taken directly from this old answer from 2014):

import Web.Scotty
import Database.MongoDB
import qualified Data.Text.Lazy as T
import Control.Monad.IO.Class

runQuery :: Pipe -> Query -> IO [Document]
runQuery pipe query = access pipe master "nutrition" (find query >>= rest) 

main = do
  pipe <- connect $ host "127.0.0.1"
  scotty 3000 $ do
    get "/" $ do
      res <- liftIO $ runQuery pipe (select [] "stock_foods")
      text $ T.pack $ show res

You see how the the database connection (pipe) is created only once when the web app launches. Subsequently, thousands if not millions of visitors will hit the "/" route simultaneously and read from the database using the same connection (pipe).

I have questions about how to properly use Database.MongoDB:

1. Is this the proper way of setting things up? As opposed to creating a database connection for every visit to "/". In this latter case, we could have millions of connections at once. Is that discouraged? What are the advantages and drawbacks of such an approach?
2. In the app above, what happens if the database connection is lost for some reason and needs to be created again? How would you recover from that?
3. What about authentication with the auth function? Should the auth function only be called once after creating the pipe, or should it be called on every hit to "/"?
4. Some say that I'm supposed to use a pool (Data.Pool). It looks like that would only help limit the number of visitors using the same database connection simultaneously. But why would I want to do that? Doesn't the MongoDB connection have a built-in support for simultaneous usages?

Answer 1

1. Even if you create connection per client you won't be able to create too many of them. You will hit ulimit. Once you hit that ulimit the client that hit this ulimit will get a runtime error. The reason it doesn't make sense is because mongodb server will be spending too much time polling all those connections and it will have only as many meaningful workers as many CPUs your db server has. One connection is not a bad idea, because mongodb is designed to send several requests and wait for responses. So, it will utilize as much resources as your mongodb can have with only one limitation - you have only one pipe for writing, and if it closes accidentally you will need to recreate this pipe yourself. So, it makes more sense to have a pool of connections. It doesn't need to be big. I had an app which authenticates users and gives them tokens. With 2500 concurrent users per second it only had 3-4 concurrent connections to the database.

Here are the benefits connection pool gives you:

• If you hit pool connection limit you will be waiting for the next available connection and will not get runtime error. So, you app will wait a little bit instead of rejecting your client.

• Pool will be recreating connections for you. You can configure pool to close excess of connections and create more up until certain limit as you need them. If you connection breaks while you read from it or write to it, then you just take another connection from the pool. If you don't return that broken connection to the pool pool will create another connection for you.

  2. If the database connection is closed then: mongodb listener on this connection will exit printing a error message on your terminal, your app will receive an IO error. In order to handle this error you will need to create another connection and try again. When it comes to handling this situation you understand that it's easier to use a db pool. Because eventually you solution to this will resemble connection pool very much.

  3. I do auth once as part of opening a connection. If you need to auth another user later you can always do it.

  4. Yes, mongodb handles simultaneous usage, but like I said it gives only one pipe to write and it soon becomes a bottle neck. If you create at least as many connections as your mongodb server can afford threads for handling them(CPU count), then they will be going at full speed.

If I missed something feel free to ask for clarifications. Thank you for your question.

Answer 2

What you really want is a database connection pool. Take a look at the code from this other answer.

Instead of auth, you can use withMongoDBPool to if your MongoDB server is in secure mode.

Answer 3

Is this the proper way of setting things up? As opposed to creating a database connection for every visit to "/". In this latter case, we could have millions of connections at once. Is that discouraged? What are the advantages and drawbacks of such an approach?

You do not want to open one connection and then use it. The HTTP server you are using, which underpins Scotty, is called Warp. Warp has a multi-core, multi-green-thread design. You are allowed to share the same connection across all threads, since Database.MongoDB says outright that connections are thread-safe, but what will happen is that when one thread is blocked waiting for a response (the MongoDB protocol follows a simple request-response design) all threads in your web service will block. This is unfortunate.

We can instead create a connection on every request. This trivially solves the problem of one thread's blocking another but leads to its own share of problems. The overhead of setting up a TCP connection, while not substantial, is also not zero. Recall that every time we want to open or close a socket we have to jump from the user to the kernel, wait for the kernel to update its internal data structures, and then jump back (a context switch). We also have to deal with the TCP handshake and goodbyes. We would also, under high load, run out file descriptors or memory.

It would be nice if we had a solution somewhere in between. The solution should be

• Thread-safe
• Let us max-bound the number of connections so we don't exhaust the finite resources of the operating system
• Quick
• Share connections across threads under normal load
• Create new connections as we experience increased load
• Allow us to clean up resources (like closing a handle) as connections are deleted under reduced load
• Hopefully already written and battle-tested by other production systems

It is this exactly problem that resource-pool tackles.

Some say that I'm supposed to use a pool (Data.Pool). It looks like that would only help limit the number of visitors using the same database connection simultaneously. But why would I want to do that? Doesn't the MongoDB connection have a built-in support for simultaneous usages?

It is unclear what you mean by simultaneous usages. There is one interpretation I can guess at: you mean something like HTTP/2, which has pipelining built into the protocol.

standard picture of pipelining http://research.worksap.com/wp-content/uploads/2015/08/pipeline.png

Above we see the client making multiple requests to the server, without waiting for a response, and then the client can receive responses back in some order. (Time flows from the top to the bottom.) This MongoDB does not have. This is a fairly complicated protocol design that is not that much better than just asking your clients to use connection pools. And MongoDB is not alone here: the simple request-and-response design is something that Postgres, MySQL, SQL Server, and most other databases have settled on.

And: it is true that connection pool limits the load you can take as a web service before all threads are blocked and your user just sees a loading bar. But this problem would exist in any of the three scenarios (connection pooling, one shared connection, one connection per request)! The computer has finite resources, and at some point something will collapse under sufficient load. Connection pooling's advantages are that it scales gracefully right up until the point it cannot. The correct solution to handling more traffic is to increase the number of computers; we should not avoid pooling simply due to this problem. 

In the app above, what happens if the database connection is lost for some reason and needs to be created again? How would you recover from that?

I believe these kinds of what-if's are outside the scope of Stack Overflow and deserve no better answer than "try it and see." Buuuuuuut given that the server terminates the connection, I can take a stab at what might happen: assuming Warp forks a green thread for each request (which I think it does), each thread will experience an unchecked IOException as it tries to write to the closed TCP connection. Warp would catch this exception and serve it as an HTTP 500, hopefully writing something useful to the logs also. Assuming a single-connection model like you have now, you could either do something clever (but high in lines of code) where you "reboot" your main function and set up a second connection. Something I do for hobby projects: should anything odd occur, like a dropped connection, I ask my supervisor process (like systemd) to watch the logs and restart the web service. Though clearly not a great solution for a production, money-makin' website, it works well enough for small apps.

What about authentication with the auth function? Should the auth function only be called once after creating the pipe, or should it be called on every hit to "/"?

It should be called once after creating the connection. MongoDB authentication is per-connection. You can see an example here of how the db.auth() command mutates the MongoDB server's data structures corresponding to the current client connection.