实体框架添加更多行会降低性能

Question

I'll begin this post by noting that I'm entirely new to the .NET world. ASP, EntityFramework, Linq, etc. are all mostly unknown magic at this point.

Having said that, I've built myself a neat Web API chat-like application with SignalR support for real-time events. It works quite well, but I'm having some performance problems with the Add function.

In my chat application, there are "Pads" (chat rooms) which contain a number of "Mates" and "Messages". Here's my Pad model for reference:

public class Pad
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public Guid PadId { get; set; }
    public string StreetAddress { get; set; }
    public int ZipCode { get; set; }
    public virtual ICollection<Mate> Mates { get; set; }
    public virtual ICollection<Message> Messages { get; set; }
}

My problem lies in my SignalR hub that processes a new Message sent to a particular pad. These two lines take about half a second to process.

pad.Messages.Add(msg); // pad is the Pad entity already fetched from the db context
db.Messages.Add(msg);

But they only take that long when Pad.Messages contains a large number of messages. Thousands. If I am sending to a pad with few to no messages, it executes almost instantly.

My initial 'trick' to improve the perceived performance here is to move the adding functions to after I send the notification back to the clients, but I realize something like this could present a potential problem later when there are tens or hundreds of thousands of messages in one pad.

Any advice here would be greatly appreciated!

Here is the entire message send method for reference:

public void SendMessage(string pad_id, string body)
{
    var user_id = IdentityExtensions.GetUserId(Context.User.Identity);
    body = body.Trim();
    if (body.Length <= 0)
    {
        return;
    }
    // Check that the user belongs in this pad...
    var user = (from u in db.Users
                where u.Id == user_id
                select u).First();
    var pad = (from p in user.Pads where p.PadId == new Guid(pad_id) select p).FirstOrDefault();
    if (pad != null) {
        // Save the message to the database
        var msg = new Message()
        {
            MessageId = new Guid(),
            Author = user,
            Body = body,
            SendTime = DateTimeOffset.UtcNow,
            Pad = pad
        };

        pad.Messages.Add(msg); // These two lines
        db.Messages.Add(msg); // Are the culprit.
        db.SaveChangesAsync();
        Clients.Group(pad_id).messageReceived(user.Id, pad_id, body, DateTimeOffset.UtcNow); // Send message to clients
    }
}

EDIT : I'm on EF 6.0.0

Answer 1

From your code, all you want to do is add 1 row (Message) to the table.

While constructing the Message entity, just use the padid instead of the Pad object. That way you don't need to deal with all the pad objects. From a DB perspective, you just need to add a Message row with a PadId key.

    var msg = new Message()
    {
        MessageId = new Guid(),
        Author = user,
        Body = body,
        SendTime = DateTimeOffset.UtcNow,
        PadId = new Guid(pad_id)
    };

    // pad.Messages.Add(msg); // don't need this.
    db.Messages.Add(msg);

The above should always insert one row and not depend on the number of messages in the pad. If it still gives a performance problem, then adding a single row is causing a lot of index updates to your table.