# Topics

Topics can both export and import messages. Accordingly, you can configure topic definitions (in the configuration file `topics.conf`) with `import` and `export` properties that specify one or more external transports:

## import

`import` instructs `tibemsd` to import messages that arrive on those EMS transports from TIBCO FTL, and deliver them to the EMS destination. Each named `tibftl` transport can be named on only one EMS destination. That is, if the transport FTL01 is included on `import` property for destination `myTopics.Fiction`, it cannot also be added to the destination `myTopics.Nonfiction`.

Warning: An EMS transport for TIBCO FTL may be specified as an import transport by only one destination. If the `topics.conf` configuration has a transport for TIBCO FTL included as an import transport by more than one destination, the server handles this as a configuration error.

## export

`export` instructs `tibemsd` to take messages that arrive on the EMS destination, and export them to TIBCO FTL using the specified EMS transport for TIBCO FTL.

Note: The EMS server never re-exports an imported message on the same topic.

(For general information about topics.conf syntax and semantics, see topics.conf. You can also configure topics using the administration tool command `addprop topic`.)

## Example

For example, the following `tibemsadmin` commands configure the topic `myTopics.news` to `import` messages on the transports `FTL01` and `FTL02`, and to `export` messages on the transport `FTL02`.

    addprop topic myTopics.news import="FTL01,FTL02"
    addprop topic myTopics.news export="FTL02"

TIBCO FTL messages with subject `myTopics.news` arrive at `tibemsd` over the transports `FTL01` and `FTL02`. EMS clients can receive those messages by subscribing to `myTopics.news`.

EMS messages sent to `myTopics.news` are exported to TIBCO FTL over transport `FTL02`. TIBCO FTL clients of the corresponding daemons can receive those messages by subscribing to the endpoint associated with `myTopics.news` in the `FTL02` transport definition.

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