> For the complete documentation index, see [llms.txt](https://docs.rumi.systems/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rumi.systems/rumi-core/concepts/messaging-model/loopback-binding.md).

# Loopback Binding

The Loopback binding enables in-process message exchange between Rumi microservices for testing and development.

## Overview

The platform's Loopback binding can be used to send messages to other applications running in the **same process**, not across different processes. This binding is often used for unit testing in which several applications may be launched in the same process for easy debugging.

When you configure a bus binding to use a loopback bus, a LoopbackBus instance is statically created on demand using the address provided by the binding. Two applications that connect to a loopback bus with the same name may exchange messages with one another.

For configuration details, see [Loopback Binding Configuration](/rumi-core/guides/developing-applications/configuring-messaging/configuring-bus-connections/loopback-binding.md).

{% hint style="warning" %}
Loopback buses work **only within a single process**. They cannot be used for communication between different processes.
{% endhint %}

## Loopback Bus Topic Format

Loopback bus topics can be specified hierarchically with each level in the topic separated by a `/` character. A valid loopback topic may start with a `/`, but it is not required.

**Example topics**:

* `orders/events`
* `/market-data/trades`
* `alerts/critical/region1`

## Wildcard Topics

The loopback bus supports 2 wildcard character sequences that can be used in [channel filters](/rumi-core/guides/developing-applications/configuring-messaging/registering-message-interest.md) to match multiple sending topics. Wildcards are **not** applied to sent topics (treated as literals).

The Loopback binding preserves these wildcards when `nv.sma.cleanchannelfilter=true`:

| Wildcard | Description                                                                             | Example                                                                                                                                                                                                                                                                   |
| -------- | --------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `*`      | Matches 0 or more characters within a topic level                                       | <p><code>orders/</code><em><code>gin</code></em> matches:<br>- <code>orders/gin</code><br>- <code>orders/begin</code><br>- <code>orders/ginseng</code><br>- <code>orders/beginning</code><br><br>Does not match:<br>- <code>orders</code><br>- <code>orders/in</code></p> |
| `...`    | Matches multiple topic levels. Must be used as the last level in a topic filter string. | <p><code>orders/...</code> matches:<br>- <code>orders/events/MSFT</code><br>- <code>orders/updates/APPL</code><br>- <code>orders/cancels/US/IBM</code><br><br>Does not match:<br>- <code>orders</code><br>- <code>order-updates</code></p>                                |

## Use Cases

The Loopback binding is primarily used for:

* **Unit Testing** - Test multiple microservices in a single process without requiring external message infrastructure
* **Development** - Simplify debugging by running multiple applications in the same JVM
* **Integration Testing** - Test microservice interactions without network dependencies
* **Configuration Flexibility** - Use property substitution to default to loopback for testing but switch to production message buses at runtime

## See Also

* [Loopback Binding Configuration](/rumi-core/guides/developing-applications/configuring-messaging/configuring-bus-connections/loopback-binding.md) - Configuration reference
* [Messaging Model](/rumi-core/concepts/messaging-model.md) - Overview of Rumi messaging
* [Configuring Bus Connections](/rumi-core/guides/developing-applications/configuring-messaging/configuring-bus-connections.md) - General bus configuration