A Serilog sink that writes to Azure CosmosDB and supports PartitionKey for better performance. This code is based on serilog-sinks-azuredocumentdb and adapted to use the latest Microsof.Azure.Cosmos SDK version 3.12.0 and uses a custom JsonSerializationSettings which helps to keep the sink alive when serialization fails sometimes specially for Exceptions!

You can start by installing the NuGet package.

Configure logger by calling WriteTo.AzureCosmosDB(<uri>, <key>)

var logger = new LoggerConfiguration()
    .WriteTo.AzureCosmosDB(<uri>, <secure-key>)
    .CreateLogger();

Default partition key name is /UtcDate althought it can be overrided using parameter like below

Log.Logger = new LoggerConfiguration()
                .WriteTo.AzureCosmosDB(
                    endpointUri: <uri>,
                    authorizationKey: <secure-key>,
                    partitionKey: "MyCustomKeyName"
                )
                .CreateLogger();

The DefaultPartitionkeyProvide will generate a utc date string with the format "dd.MM.yyyy". If you want to override it, you need to define a class and implement IPartitionKeyProvider interface and pass an instance of it in the arguments list.

Azure CosmosDB is making easier to prune old data with support of Time To Live (TTL) so does Sink. AzureCosmosDB Sink offers TTL at two levels.

Sink supports TTL at collection level, if collection does not already exist.

To enable TTL at collection level, set timeToLive parameter in code.

.WriteTo.AzureCosmosDB(<uri>, <secure-key>, timeToLive: TimeSpan.FromDays(7))

If collection in CosmosDB doesn't exists, it will create one and set TTL on collection level causing all logs messages purge older than 7 days.

Sink do support TTL at individual message level. This allows developer to retian log message of high importance longer than of lesser importance.

logger.Information("This message will expire and purge automatically after {@_ttl} seconds", 60);

logger.Information("Log message will be retained for 30 days {@_ttl}", 2592000); // 30*24*60*60

logger.Information("Messages of high importance will never expire {@_ttl}", -1); 

See TTL behavior in CosmosDB documentation for in depth explianation.

Note: {@_ttl} is a reserved expression for TTL.

To use the AzureCosmosDB sink with the Serilog.Settings.AppSettings package, first install that package if you haven't already done so:

Install-Package Serilog.Settings.AppSettings

In your code, call ReadFrom.AppSettings()

var logger = new LoggerConfiguration()
    .ReadFrom.AppSettings()
    .CreateLogger();

In your application's App.config or Web.config file, specify the CosmosDB sink assembly and required endpointUrl and authorizationKey parameters under the <appSettings>

<appSettings>
  <add key="serilog:using:AzureCosmosDB" value="Serilog.Sinks.AzureCosmosDB" />
  <add key="serilog:write-to:AzureCosmosDB.endpointUrl" value="https://****.cosmos.azure.com:443" />
  <add key="serilog:write-to:AzureCosmosDB.authorizationKey" value="****" />
    
  <!-- Liefspan of log messages in CosmosDB in seconds, leave empty to disable expiration. -->
  <add key="serilog:write-to:AzureCosmosDB.timeToLive" value="60" />
</appSettings>

Sink buffers log internally and flush to Azure CosmosDB in batches using dedicated thread. However, it highly depends on type of Azure CosmosDB subscription you have.