In Iguana 4.0 or later, a filter can be configured for any channel that you create. This filter can define a transformation that is to be performed on incoming messages before they are sent to the destination component of the channel. The filter can also specify that messages of certain types are to be ignored.

When you configure a filter for a channel, you can specify that the filter is to be applied before or after the message is logged:

• If the filter is applied before the message is logged, the transformed message is added to the log. Messages that are ignored are not logged.

• If the filter is applied after the message is logged, all messages are logged right away. Messages that are ignored are included in the log, but are skipped.

For best results, applying the filter after logging the message is recommended. Here are the reasons why this approach is best:

• If a filter is applied after the original message is logged, any future log entries for this message can be related to the original message. This is helpful when you are debugging your channel.

• Because the original message is logged, it can be resubmitted to the filter and destination if necessary.

• If an error occurs during filtering, and the filter is configured to stop the channel when an error occurs, the message that caused the error will still be in the message queue. If the filter is applied before the message, the original message is lost.

• If the filter is performing a transformation, and the transformation script in the VMD file takes a long time to execute, other messages can still be added to the message queue. If the filter is applied before the message, the transformation must be completed before other messages can be added to the message queue.

However, applying the filter before logging the message has its advantages:

• If an incoming message contains confidential data, the message can be ignored, which ensures that it does not appear in the Iguana logs. You can also transform a message to eliminate the confidential data before the message is logged.

• If a message is too large to be logged, a transformation script can eliminate unneeded information and make the message smaller.

• If your incoming messages are in XML format, and you only want HL7 messages to appear in your logs or in your message queue, you can apply the filter's transformation before logging the message.

	If your source component is From Channel, you cannot apply the filter before logging the message, since your channel is receiving messages that have been queued by another channel. To filter messages before logging, define a message filter in the channel that is sending you the messages. If your destination component is To Channel, you cannot apply the filter after logging the message, as the destination channel is responsible for removing messages from the message queue. If you want to transform messages after they are logged, you can use the destination channel to do this.