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# Run the Distributed Action Handler in Mirror Mode

In mirror mode, the DAH distributes ACI actions to identical IDOL Servers (that is, all the IDOL Servers have the same configuration and contain the same data).

DAH can distribute all actions in mirror mode. For details of the available actions in IDOL Server, refer to the IDOL Server Reference.

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To run the DAH in mirror mode

1. Open the DAH configuration file in a text editor.

2. Find the `MirrorMode` setting in the `[Server]` section, and set it to `True`.

3. In the `[Server]` section, set the `DistributionMethod` setting to one of the following options. This setting determines how the DAH distributes actions to the connected IDOL Servers:

   -     DistributionMethod=0

     Failover distribution method. The DAH forwards incoming actions to the first IDOL server that you list in the DAH configuration file `[DistributedEngineN]` section. This server is the primary server.

     If this IDOL Server stops responding for any reason (for example, because of a hardware failure or network outage), the DAH marks it as down. It switches to the next IDOL Server (the second one listed in the `[DistributedEngineN]` section), which becomes the primary server, and so on, and seamlessly continues to service client actions. You can also manually set the primary server by using the `EngineManagement` action.

     If the IDOL Server comes online again (for example, because the hardware has been rebooted or the network connection repaired), the DAH adds it to the list of active IDOL Servers. However, it continues to use the new primary server unless that server stops responding, or unless you manually change the primary server.

   -     DistributionMethod=1

     Load balancing distribution method. The DAH assigns each incoming action to just one of the connected IDOL Servers (using a cumulative predictive algorithm that spreads the action load efficiently). When this IDOL Server responds with a result, the DAH forwards it to the client software.

     If an IDOL Server stops responding for any reason (for example, because of a hardware failure or network outage), the DAH marks it as down. It assigns incoming actions only to the IDOL Servers that are running, saving the time that it takes to attempt to communicate with the failed IDOL Server.

     If the IDOL Server comes online again (for example, because the hardware has been rebooted or the network connection repaired), the DAH adds it to the list of active IDOL Servers. The IDOL Server again becomes a valid choice for actions.

   -     DistributionMethod=2

     View documents by reference distribution method. The DAH uses the distribute by reference distribution method. You can use this method only to distribute `View`, `GetLink`, and `ViewGetDocInfo` actions in mirror mode. The DAH distributes `View` and `GetLink` actions between View servers based on the reference of the document to view. Actions that request the same document are then always sent to the same View server.

     When View server first receives a request to view a document, it caches any embedded images in the result document. When you distribute between View servers, using distribute by reference ensures that each document is cached in only one View server. The cached image is then used in subsequent requests for the same document.

4. Save and close the configuration file.

5. Restart the DAH for your changes to take effect.

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