如何在NLog MongoDB配置中动态指定集合名与运行时修改数据库名

Alright, let's break down how to solve both of your requirements for NLog's MongoDB target. I've worked through similar scenarios before, so here's a practical approach:

1. Using a Variable for collectionName #

NLog lets you use layout renderers or configuration variables to make collectionName dynamic. This is straightforward since the Mongo target supports layout values for this property.

Option 1: Use a Configuration Variable #

Define a variable in your NLog config, then reference it in the target:

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.nlog-project.org/schemas/NLog.xsd NLog.xsd" autoReload="true" throwExceptions="true">
  <!-- Define your collection name variable -->
  <variable name="LogCollection" value="DefaultLogDocument" />

  <extensions>
    <add assembly="NLog.Mongo" />
  </extensions>

  <targets async="true">
    <target xsi:type="Mongo" 
            name="mongoCustom" 
            includeDefaults="false" 
            connectionString="mongodb://mongodb0.example.com:27017" 
            databaseName="logDb" 
            <!-- Reference the variable here -->
            collectionName="${var:LogCollection}" 
            cappedCollectionSize="26214400">
      <field name="Date" layout="${longdate}" bsonType="DateTime" />
      <field name="Level" layout="${level}"/>
      <field name="Message" layout="${message}" />
      <field name="Logger" layout="${logger}"/>
      <field name="Location" layout="${callsite:className=true:methodName=true}" />
      <field name="Exception" layout="${exception:format=tostring}" />
      <field name="ThreadId" layout="${threadid}" bsonType="Int32" />
      <field name="ThreadName" layout="${threadname}" />
      <field name="ProcessId" layout="${processid}" bsonType="Int32" />
      <field name="ProcessName" layout="${processname:fullName=true}" />
    </target>
  </targets>
  <rules>
    <logger name="*" minlevel="Warn" writeTo="mongoCustom" />
  </rules>
</nlog>

Option 2: Dynamic Collection Name (e.g., Date-Based) #

For more flexibility, you can use a layout renderer directly to generate collection names on the fly (like daily collections):

collectionName="Log_${date:format=yyyyMMdd}"

This creates a new collection each day (e.g., Log_20240520).

2. Dynamically Change databaseName at Runtime #

Unlike collectionName, databaseName isn't always layout-compatible in older NLog.Mongo versions. The most reliable way is to update the target's property directly in code and reconfigure NLog.

Step 1: Update the Target in Code #

Since your target is wrapped in an AsyncTargetWrapper, you'll need to access the underlying MongoTarget first:

// Get the async wrapper target
var asyncWrapper = LogManager.Configuration.FindTargetByName<AsyncTargetWrapper>("mongoCustom");
// Extract the actual Mongo target from the wrapper
var mongoTarget = asyncWrapper.WrappedTarget as MongoTarget;

if (mongoTarget != null)
{
    // Update to your desired database name
    mongoTarget.DatabaseName = "newLogDb";
    // Apply changes to all existing loggers
    LogManager.ReconfigExistingLoggers();
}

If your target isn't async, skip the wrapper step:

var mongoTarget = LogManager.Configuration.FindTargetByName<MongoTarget>("mongoCustom");
if (mongoTarget != null)
{
    mongoTarget.DatabaseName = "newLogDb";
    LogManager.ReconfigExistingLoggers();
}

Option 2: Per-Context Database Name (Advanced) #

If you need to switch databases based on request or logical context, use NLog's Mapped Diagnostics Logical Context (MDLC) (requires a recent NLog.Mongo version that supports layout for databaseName):

1. Update the config to use MDLC:

databaseName="${mdlc:item=CurrentDatabase:default=logDb}"

2. Set the database name in your code when needed (e.g., at the start of a request):

// Assign database name for the current logical context
MappedDiagnosticsLogicalContext.Set("CurrentDatabase", "userSpecificDb");

This routes logs to the specified database for all events in that context (works seamlessly with async/await).

Key Notes #

• Ensure you're using a recent version of NLog.Mongo to support these features.
• If autoReload="true" is enabled, config file changes will override code-based updates—adjust this setting if needed.
• Always call LogManager.ReconfigExistingLoggers() after modifying target properties to apply changes immediately.

内容的提问来源于stack exchange，提问作者Mico