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Serilog.Sinks.MongoDB 7.2.0

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Serilog.Sinks.MongoDB

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A Serilog sink that writes events as documents to MongoDB.

Package - Serilog.Sinks.MongoDB Platforms - .NET 4.7.2, .NET 6.0+, .NET Standard 2.1

Whats New

See for complete version history.

New in v7.2

• Fixed MongoDB v7.x compatibility - Error handling now uses error codes instead of string matching (#98, #99)
• Added comprehensive unit tests for MongoDB error handling
• Improved CI performance

Installation

Install the sink via NuGet Package Manager Console:

Install-Package Serilog.Sinks.MongoDB

or via the .NET CLI:

dotnet add package Serilog.Sinks.MongoDB

Usage Examples

In the examples below, the sink is writing to the database logs with structured Bson. The default collection name is log, but a custom collection can be supplied with the optional CollectionName parameter. The database and collection will be created if they do not exist.

Basic:

using Serilog;

// use BSON structured logs
var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson("mongodb://mymongodb/logs")
 .CreateLogger();

log.Information("This is a test log message");

Capped Collection:

// capped collection using BSON structured logs
var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson("mongodb://mymongodb/logs", cfg =>
 {
 // optional configuration options:
 cfg.SetCollectionName("log");
 cfg.SetBatchPeriod(TimeSpan.FromSeconds(1));

 // create capped collection that is max 100mb
 cfg.SetCreateCappedCollection(100);
 })
 .CreateLogger();

Custom Mongodb Settings:

// create sink instance with custom mongodb settings.
var log = new LoggerConfiguration()
	.WriteTo.MongoDBBson(cfg =>
 {
		// custom MongoDb configuration
		var mongoDbSettings = new MongoClientSettings
		{
			UseTls = true,			
			AllowInsecureTls = true,
			Credential = MongoCredential.CreateCredential("databaseName", "username", "password"),
			Server = new MongoServerAddress("127.0.0.1")
		};

		var mongoDbInstance = new MongoClient(mongoDbSettings).GetDatabase("serilog");
		
		// sink will use the IMongoDatabase instance provided
		cfg.SetMongoDatabase(mongoDbInstance);
		cfg.SetRollingInterval(RollingInterval.Month);
 })
	.CreateLogger();

JSON (Microsoft.Extensions.Configuration)

Keys and values are not case-sensitive. This is an example of configuring the MongoDB sink arguments from Appsettings.json:

{
 "Serilog": {
 "MinimumLevel": {
 "Default": "Information",
 "Override": {
 "Microsoft": "Error",
 "System": "Warning"
 }
 },
 "WriteTo": [
 {
 	"Name": "MongoDBBson",
 "Args": {
 "databaseUrl": "mongodb://username:password@ip:port/dbName?authSource=admin",
 "collectionName": "logs",
 "cappedMaxSizeMb": "1024",
 "cappedMaxDocuments": "50000",
 "rollingInterval": "Month"
 }
 }
 ]
 }
}

Advanced Configuration

Authentication & Secure Connections

For password-protected MongoDB instances, Azure Cosmos DB, or SSL/TLS connections:

var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson(cfg =>
 {
 var mongoDbSettings = new MongoClientSettings
 {
 UseTls = true,
 AllowInsecureTls = false, // set true only for dev/testing
 Credential = MongoCredential.CreateCredential("databaseName", "username", "password"),
 Server = new MongoServerAddress("your-server.com", 27017)
 };

 var mongoDbInstance = new MongoClient(mongoDbSettings).GetDatabase("logs");
 cfg.SetMongoDatabase(mongoDbInstance);
 })
 .CreateLogger();

Azure Cosmos DB (MongoDB API):

var connectionString = "mongodb://cosmosdb-account:key@cosmosdb-account.mongo.cosmos.azure.com:10255/?ssl=true&replicaSet=globaldb&retrywrites=false";
var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson(connectionString)
 .CreateLogger();

TTL / Auto-Expiration

Automatically delete old logs using MongoDB's TTL feature:

var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson(cfg =>
 {
 cfg.SetMongoUrl("mongodb://localhost/logs");
 cfg.SetExpireTTL(TimeSpan.FromDays(30)); // logs expire after 30 days
 })
 .CreateLogger();

Exclude Redundant Fields

Reduce storage costs by excluding the MessageTemplate field (the rendered message is still stored):

var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson(cfg =>
 {
 cfg.SetMongoUrl("mongodb://localhost/logs");
 cfg.SetExcludeMessageTemplate(true); // saves storage space
 })
 .CreateLogger();

Rolling Collections

Create time-based collections (e.g., one per day/month):

var log = new LoggerConfiguration()
 .WriteTo.MongoDBBson(cfg =>
 {
 cfg.SetMongoUrl("mongodb://localhost/logs");
 cfg.SetCollectionName("log");
 cfg.SetRollingInterval(RollingInterval.Day); // creates: log-20251004, log-20251005, etc.
 })
 .CreateLogger();

Collection naming patterns:

• RollingInterval.Day → log-yyyyMMdd (e.g., log-20251004)
• RollingInterval.Month → log-yyyyMM (e.g., log-202510)
• RollingInterval.Year → log-yyyy (e.g., log-2025)

Querying rolling collections:

// Query specific date range - you need to target the correct collection(s)
var collectionName = $"log-{DateTime.UtcNow:yyyyMMdd}";
var collection = database.GetCollection<BsonDocument>(collectionName);
var todayLogs = collection.Find(Builders<BsonDocument>.Filter.Empty).ToList();

Migration Guide

From Legacy .MongoDB() to .MongoDBBson()

The legacy .MongoDB() sink converts logs to JSON then to BSON. The newer .MongoDBBson() writes structured BSON directly for better performance and features.

Legacy (still supported):

.WriteTo.MongoDB("mongodb://localhost/logs") // converts to JSON first

New Bson sink (recommended):

.WriteTo.MongoDBBson("mongodb://localhost/logs") // native BSON

MongoDBBson exclusive features:

• TTL/Expiration (SetExpireTTL)
• Exclude message template (SetExcludeMessageTemplate)
• Rolling collections (SetRollingInterval)
• Better type mapping and performance

Troubleshooting

MongoDB Connection Issues

Problem: Application hangs or fails when MongoDB is unavailable Solution: Ensure your MongoDB connection string includes appropriate timeouts:

"mongodb://localhost/logs?connectTimeoutMS=3000&serverSelectionTimeoutMS=3000"

Type Mapping Errors

Problem: System.Guid cannot be mapped to BsonValue Solution: Ensure you're using the latest version (v7.1+) which includes Guid mapping fixes.

Capped Collections Not Created

Problem: Capped collection configuration not working Solution: Ensure the collection doesn't already exist. Drop existing collection first:

database.DropCollection("logs");
// Then configure with capped settings

Rolling Collections Not Named Correctly

Problem: Collection created without time format suffix Solution: Ensure you're using MongoDBBson sink (not legacy MongoDB) and v7.1+.

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