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TIBCO Rendezvous® 8.8.0

# Persistent Correspondents

We introduced the concept of persistent correspondents in the section CM Correspondent Name. A reusable name and a file-based ledger allow a persistent correspondent to continue certified delivery beyond the invalidation of a CM transport or the restart of a process.

## Example

Consider an example application system, in which program `JOE` generates important information, and sends it to program `SUE` in certified messages on the subject `REMEMBER.THIS`. Upon receipt, `SUE` stores the information in a database.

If either `JOE` or `SUE` terminate unexpectedly, it is crucial that certified messages still arrive for entry into the database. To ensure this result, both programs must represent persistent correspondents—that is, both programs create CM transports with reusable names (`JOE_PER` and `SUE_PER`), and each of these CM transports keeps a file-based ledger. In addition, `SUE` requires old messages by setting a parameter when creating the CM transport `SUE_PER`.

During operation, `JOE` has sent message number `57` on the subject `REMEMBER.THIS`, but has not yet received delivery confirmation for messages `53–56`. `SUE` is processing message `53`, when a sudden hardware failure causes `SUE` to terminate. Meanwhile, `JOE` continues to send messages `58–77`.

When the computer restarts, `SUE` restarts and recreates `SUE_PER`. The ledger file for `SUE_PER` indicates that message `52` was received and confirmed for the subject `REMEMBER.THIS`. As soon as `SUE_PER` discovers that `JOE_PER` sends labeled messages on the required subject, `SUE_PER` requests a certified delivery agreement for the subject `REMEMBER.THIS`. When `JOE_PER` accepts, `JOE_PER` retransmits the stored messages `53–77` on that subject.

Notice these details:

| • | `SUE` does not miss any `REMEMBER.THIS` messages. However, the new `SUE_PER` must gracefully fix any difficulties caused by partial processing of message `53` by the old `SUE_PER`.

| • | `JOE` and `SUE` communicate using a public subject name—not an inbox. Inbox names are unique, so they cannot continue beyond transport invalidation.

| • | `SUE` explicitly requires old messages by setting a parameter when creating the persistent correspondent named `SUE_PER`; this parameter ensures that `JOE_PER` retransmits certified messages that the previous instance of `SUE_PER` had not confirmed. If the value of the `requireOldMsgs` argument were false, `JOE_PER` would delete stored outbound messages for `SUE_PER` from its ledger, instead of retransmitting them.

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