Why Is My MongoDB Go Server Showing a 'Too Many Open Files' Error?

Too Many Open Files in MongoDB Go Server

Many organizations using MongoDB have encountered the "too many open files" error in their logs, particularly for servers written in Go using the MGO library. This error indicates that the maximum number of open files allowed by the system has been reached.

Root Cause

The root cause of this issue lies in the incorrect handling of MongoDB connections in the Go code. The error indicates that open file descriptors are not being properly closed after use, leading to an accumulation of unclosed files.

Restructuring Database Access

To resolve this issue, it is crucial to restructure the code to use proper connection handling. Instead of storing an "mgo.Database" instance, it is recommended to store an "mgo.Session." When interacting with MongoDB, a copy or clone of the session should be acquired and closed promptly using a "defer" statement. This practice ensures that connections are not leaked.

Code Example

Here's an example of the recommended code structure:

var session *mgo.Session

func init() {
    var err error
    if session, err = mgo.Dial("localhost"); err != nil {
        log.Fatal(err)
    }
}

func someHandler(w http.ResponseWriter, r *http.Request) {
    sess := session.Copy()
    defer sess.Close() // Must close!

    c := sess.DB("mapdb").C("tiles")
    // Do something with the collection, e.g.:
    var tile bson.M
    if err := c.FindId("someTileID").One(&result); err != nil {
        // Tile does not exist, send back error, e.g.:
        log.Printf("Tile with ID not found: %v, err: %v", "someTileID", err)
        http.NotFound(w, r)
        return
    }
    // Do something with tile
}

Proper Error Handling

It is also essential to handle errors correctly throughout the code. Each function that returns an error should be checked and acted upon appropriately. Neglecting error handling can lead to unexpected behavior and hinder troubleshooting.

Additional Resources

For further guidance, consider these related resources:

• [mgo - query performance seems consistently slow (500-650ms)](https://github.com/mongodb/mongo-go-driver/issues/232)
• [Concurrency in gopkg.in/mgo.v2 (Mongo, Go)](https://groups.google.com/g/golang-nuts/c/b18D9y1L3qY/m/8bvFYnoA2u8J)

By implementing these recommendations, organizations can effectively address the "too many open files" error in their MongoDB Go servers, ensuring optimal performance and stability.

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