How to Fix 'Too Many Open Files' Errors in MongoDB with MGO in Go?

Too Many Open Files Error in MongoDB with MGO

When running a MongoDB server written in Go using MGO, users may encounter the following error in the logs:

Accept error: accept tcp [::]:80: accept4: too many open files;

This error indicates that the system has reached its maximum number of open files.

Causes

The issue arises when connections to the MongoDB server are not properly closed. Each request to the server typically opens a connection, and if connections are not closed after use, they accumulate, eventually reaching the maximum limit.

Solutions

1. Close MongoDB Connections:

To resolve this issue, it's crucial to close MongoDB connections promptly after use. The recommended approach is:

• Store an MGO session (not database instance)
• Acquire a session copy or clone for each request
• Close the session copy or clone after completing the request (using defer)

2. Verify Session Type:

Ensure that the stored session is of type mgo.Session and not mgo.Database. This is important because the mgo.Session type handles connection management.

3. Check for Errors:

It's essential to check for errors whenever interacting with MongoDB. If an error occurs, handle it gracefully (e.g., log or print it), as neglecting errors can lead to inconsistent behavior.

4. Example Code:

Here's a corrected version of the code that includes proper session management:

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 result bson.M
    if err := c.FindId("someTileID").One(&result); err != nil {
        // Handle error, e.g.:
        log.Printf("Tile with ID not found: %v, err %v", "someTileID", err)
        http.NotFound(w, r)
        return
    }
    // Do something with result
}

Related Questions

• [mgo - query performance seems consistently slow (500-650ms)](https://stackoverflow.com/questions/33291016)
• [Concurrency in gopkg.in/mgo.v2 (Mongo, Go)](https://stackoverflow.com/questions/31175844)

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