ASP.NET Core – Log messages to the database in the background

I was reading about logging in ASP.NET when I came across this statement about logging to the database:

When logging to SQL Server, don’t do so directly. Instead, add log messages to an in-memory queue and have a background worker dequeue and insert data to SQL Server.

Paraphrased from

In this article, I’ll show how to implement this background database logger idea. First, I’ll start by showing the design, and then I’ll show the code.


We can implement Microsoft’s background database logger idea by using the consumer/producer pattern with a background service. To make sure this scales well, we’ll bulk insert log messages into the database.

I’ll go into details about this design, but first here’s the design diagram:

Design diagram - ASP.NET Service with controllers and a background service, which contains a log queue. The controllers enqueue messages to the log queue, and the messages are dequeued in batches and bulk inserted into the database.

With the consumer/producer pattern, you have one or more producers enqueuing messages to a shared queue. You have one or more consumers dequeuing messages from the shared queue and processing them. In our case, we’ll have multiple producers (anything that logs, mostly controllers), and a single consumer.

In ASP.NET, you can add hosted services that run in the background. These are referred to as background services. We’ll use a background service for two purposes: it’ll contain the shared queue, and it’ll act as the consumer.

The producers only need to be exposed to a Log() method. They don’t need to know they’re using a background service, or that it’s logging to the database. This is why we’re using the ILoggerService interface.

Since there can be multiple producers, there can be multiple log messages coming in at the same time. Executing lots of individual INSERT statements can degrade system performance. Instead, we’ll bulk insert the messages. In order to bulk insert, the consumer will need to be able to batch read from the queue.


In this section, I’ll show the code for the design shown above. I’ll be building this from the outside-in, and building the background service last.

Throughout this code, I’ll be using Console.WriteLine(). I’m running the service with a console interface so I can easily see what’s going on.

ILoggerService and a controller that uses it

First things first, we need to add ILoggerService. It’s best practice to code against interfaces instead of implementations. The producers only need to have access to the Log() method. They don’t need to know anything about the concrete implementation.

public interface ILoggerService { void Log(LogLevel logLevel, string message); }
Code language: C# (cs)

Note: I’m not using the built-in ILogger. It has a huge amount of methods, and I didn’t want to have to implement them in the background service.

Here’s an example of a controller that logs messages. It needs ILoggerService dependency injected.

[Route("[controller]")] [ApiController] public class RecipesController : ControllerBase { private readonly ILoggerService Logger; public RecipesController(ILoggerService logger) { Logger = logger; } [HttpGet("{id}")] public string Get(int id) { Logger.Log(LogLevel.Debug, $"GET /Recipes/{id}"); return "recipe"; } }
Code language: C# (cs)

Log repository for bulk inserting

We want to bulk insert the log messages. Any time you’re interacting with a database, it’s a good idea to implement the repository pattern. With this pattern, you encapsulate the database interaction logic in a repository class.

First, we need to add the LogMessage model class:

public class LogMessage { public int ThreadId { get; set; } public string Message { get; set; } public DateTimeOffset Timestamp { get; set; } }
Code language: C# (cs)

Next, since we want to use SqlBulkCopy, and we’re using .NET Core, we’ll need to install the System.Data.SqlClient nuget package. Do that by executing the following command (Note: this is using View > Other Windows > Package Manager Console):

Install-Package System.Data.SqlClient
Code language: PowerShell (powershell)

Now we can implement the LogRepository class. It will do a bulk insertion using SqlBulkCopy.

Whenever you’re interacting with an external dependency, such as a database, it’s a good idea to make it fault tolerant. In this case, we’ll try to make this resilient by catching a few transient SQL exceptions and retrying the bulk insertion a few times.

Here’s the LogRepository class:

using System.Data; using System.Data.SqlClient; public class LogRepository : ILogRepository { private const string TABLE = "Log"; private readonly string ConnectionString; private readonly HashSet<int> transientSqlErrors = new HashSet<int>() { -2, 258, 4060 }; private const int MAX_RETRIES = 3; private const int RETRY_SECONDS = 5; public LogRepository(string connectionString) { ConnectionString = connectionString; } public async Task Insert(List<LogMessage> logMessages) { DataTable table = new DataTable(); table.TableName = TABLE; table.Columns.Add(nameof(LogMessage.Timestamp), typeof(DateTimeOffset)); table.Columns.Add(nameof(LogMessage.Message), typeof(string)); table.Columns.Add(nameof(LogMessage.ThreadId), typeof(int)); foreach (var logMessage in logMessages) { var row = table.NewRow(); row[nameof(LogMessage.Timestamp)] = logMessage.Timestamp; row[nameof(LogMessage.Message)] = logMessage.Message ?? (object)DBNull.Value; row[nameof(LogMessage.ThreadId)] = logMessage.ThreadId; table.Rows.Add(row); } await BulkInsertWithRetries(table); } private async Task BulkInsertWithRetries(DataTable table) { int attempts = 1; while (true) { try { using (var sqlBulkCopy = new SqlBulkCopy(ConnectionString)) { sqlBulkCopy.DestinationTableName = table.TableName; await sqlBulkCopy.WriteToServerAsync(table); return; } } catch (SqlException sqlEx) when (transientSqlErrors.Contains(sqlEx.Number) && attempts <= MAX_RETRIES) { Console.WriteLine($"Transient SQL error. Retrying in {RETRY_SECONDS} seconds"); await Task.Delay(TimeSpan.FromSeconds(RETRY_SECONDS)); attempts++; } } } }
Code language: C# (cs)

Note: We could use a table-valued parameter (TVP) approach in this scenario instead of doing a bulk insert. The main goal here is to do a set-based approach so we’re not spamming the system with lots of individual inserts. Both approaches (TVP and bulk insert) work fine for this.

Background service with a log queue

Finally we can add the background service class and call it DatabaseLoggerService. To make this run as a background service, we have to implement BackgroundService.

We’ll implement the consumer/producer pattern by adding an async queue from System.Threading.Channels. The ILoggerService.Log() method will allow producers to enqueue log messages. We’ll implement the consumer loop in ExecuteAsync() (a method from BackgroundService).

Here’s the DatabaseLoggerService class:

using Microsoft.Extensions.Hosting; using Microsoft.Extensions.Logging; using System.Threading.Channels; public class DatabaseLoggerService : BackgroundService, ILoggerService { private readonly Channel<LogMessage> logMessageQueue; private readonly IHostApplicationLifetime HostApplicationLifetime; private const int MAX_BATCH_SIZE = 10; private readonly ILogRepository LogRepository; public DatabaseLoggerService(ILogRepository logRepository, IHostApplicationLifetime hostApplicationLifetime) { logMessageQueue = Channel.CreateUnbounded<LogMessage>(); LogRepository = logRepository; HostApplicationLifetime = hostApplicationLifetime; } public async override Task StopAsync(CancellationToken cancellationToken) { await base.StopAsync(cancellationToken); } protected async override Task ExecuteAsync(CancellationToken stoppingToken) { while(!stoppingToken.IsCancellationRequested) { try { Console.WriteLine("Waiting for log messages"); var batch = await GetBatch(stoppingToken); Console.WriteLine($"Got a batch with {batch.Count}(s) log messages. Bulk inserting them now."); //Let non-retryable errors from this bubble up and crash the service await LogRepository.Insert(batch); } catch (TaskCanceledException) { Console.WriteLine("Stopping token was canceled, which means the service is shutting down."); return; } catch (Exception ex) { Console.WriteLine($"Fatal exception in database logger. Crashing service. Error={ex}"); HostApplicationLifetime.StopApplication(); return; } } } public void Log(LogLevel logLevel, string message) { //The reason to use Writer.TryWrite() is because it's synchronous. //We want the logging to be as fast as possible for the client, so //we don't want the overhead of async //Note: We're using an unbounded Channel, so TryWrite() *should* only fail //if we call writer.Complete(). //Guard against it anyway var logMessage = new LogMessage() { Message = message, ThreadId = System.Threading.Thread.CurrentThread.ManagedThreadId, Timestamp = DateTimeOffset.Now }; if (!logMessageQueue.Writer.TryWrite(logMessage)) { throw new InvalidOperationException("Failed to write the log message"); } } private async Task<List<LogMessage>> GetBatch(CancellationToken cancellationToken) { await logMessageQueue.Reader.WaitToReadAsync(cancellationToken); var batch = new List<LogMessage>(); while (batch.Count < MAX_BATCH_SIZE && logMessageQueue.Reader.TryRead(out LogMessage message)) { batch.Add(message); } return batch; } }
Code language: C# (cs)

The producers will call Log() synchronously. This is fast because all it’s doing is enqueuing the message.

The consumer loop reads a batch of messages from the queue and then awaits the bulk insertion. I wouldn’t suggest fire and forgetting the bulk insertion. For one, waiting for it to complete acts as a throttling mechanism. You’re only ever doing one bulk insertion at once. Second, it simplifies error handling.

Notice that this is is calling StopApplication() if an exception bubbles up from LogRepository. As noted in the LogRepository section, it’s retrying a few times if there are transient SQL exceptions. For any other type of error, or if it exceeds the max retries, it’ll throw. This will trigger a graceful shutdown of the entire service, not just crash the background service. There are definitely other possible ways to handle this – like logging to a fallback file – but I decided to do the simplest approach with the assumption that this logging is critical and the service should stop if it fails to log.

Note: TryWrite() could fail, but it’s highly unlikely (if not impossible) when using an unbounded channel (and nothing is putting the channel into a completed state).

Registering the services

You have to register DatabaseLoggerService as a singleton (so it can be dependency injected to the controllers and any other producers), and also as a hosted service so it’ll run as a background service.

public class Startup { //rest of class public void ConfigureServices(IServiceCollection services) { //rest of method services.AddSingleton<ILogRepository, LogRepository>(_ => new LogRepository(Configuration.GetConnectionString("Default"))); services.AddSingleton<ILoggerService, DatabaseLoggerService>(); services.AddHostedService(sp => sp.GetService<ILoggerService>() as DatabaseLoggerService); } }
Code language: C# (cs)

Default connection string in appsettings.json

We can add a placeholder for the connection string in appsettings.json, and then add the real connection string a user secret.

{ "Logging": { "LogLevel": { "Default": "Information", "Microsoft": "Warning", "Microsoft.Hosting.Lifetime": "Information" } }, "AllowedHosts": "*", "ConnectionStrings": { "Default": "The connection string is defined in the user secrets file" } }
Code language: JSON / JSON with Comments (json)

Run it

To test this out and see it in action, run the ASP.NET service and send concurrent requests. Check the Log table in the database to verify that it inserted the messages.

To see the bulk insertions work, use the following test client that sends tons of concurrent requests:

static async Task Main(string[] args) { var httpClient = new HttpClient(); var go = new ManualResetEventSlim(); for (int i = 1; i <= 77; i++) { var num = i; //capture for closure Task.Run(async () => { Console.WriteLine($"Num {num} waiting"); go.Wait(); Console.WriteLine($"Num {num} going"); var response = await httpClient.GetAsync($"https://localhost:12345/Recipes/{num}"); response.EnsureSuccessStatusCode(); Console.WriteLine($"Num {num} done"); }); } go.Set(); Console.ReadLine(); }
Code language: C# (cs)

Note: I tried sending concurrent requests with Postman, but it was too slow.

In addition to seeing the bulk insertions working, you can induce a transient SQL exception to see it do retries. The simplest way to do that is to set the database offline manually, wait for it to report the transient error, and then set the database back online manually.

Here’s an example of what it looks like when you run this:

CommandLoop starting Waiting for log messages Got a batch with 7(s) log messages. Bulk inserting them now. Transient SQL error. Retrying in 5 seconds Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages Got a batch with 10(s) log messages. Bulk inserting them now. Waiting for log messages
Code language: plaintext (plaintext)

The consumer reads between 1-10 messages from the queue. If there are 10 messages available, it’ll read all 10. Otherwise, it will read as many as it can. In the first instance, notice that it only read 7 messages. That’s because there were only 7 messages available on the queue at the time. After that, it was able to read 10 messages every time.

Also notice that it detected the transient SQL error, waited 5 seconds, and tried again. It succeeded when it retried.

Source code

The full source code for the background database logger shown in this article is available here:

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