---
alias: user-guide-message-bus
description: "The Message Bus facilitates sending and subscribing to messages via a subject-based system"
---

# Message Bus

## Overview

:lock: Administration.**MessageBus**.

Critical Manufacturing MES comes with a high-performance publish/subscribe message bus that implements a subject-based address system.

The *Message Bus* page within the *Administration* page group provides access to the message bus functionality, allowing the sending of custom messages, normally used for testing purposes as well as the subscription to multiple subjects.

![messagebus00][messagebus00]

## Subject Names

The Message Bus uses a hierarchical subject name structure with the possibility of using wildcards. Each subject is formed by a sequence of tokens delimited by a dot ('.') as shown in the example below.

```sql
Token1.Token2.Token3
```

| Wildcard | Wildcard Definition |
|----------|---------------------|
| * | Can accept all values for one token, e. g. : A.*.B |
| > | Accepts all values and tokens beyond the wildcard, e. g.: A.> |

Table: Message Bus wildcards

!!! info
	Subjects that start with CMF are reserved and must not be used by any application.

Following this structure, each customer should come up with a naming structure convention that makes sense for their particular case.

## Sending Messages

:lock: The Send Message functionality is associated with the security feature MessageBus.Publish.

Messages are complex self-described structures. Messages can consist of simple scalar types or other messages, thus constructing nested messages.

To send a message, it is necessary to follow the sequence:

1. Go to *Views* and select the *Message Sender* option.
2. Specify the *Type* of message - **Notification** or **Request**.

   * If the Message is a request for which a reply is expected, a *Timeout* must be specified. If the response is received within the specified timeframe, the reply is displayed on the right pane.

3. Optionally, enter the *Subject* of the Message.
4. Enter the *Message* and click **Send**.

![messagebus02][messagebus02]

## Listening To Messages

:lock: MessageBus.**Subscribe**

Following the subject name structures and specifications described above, it is possible to subscribe to many subjects at the same time.

To subscribe a subject, it is necessary to:

1. Go to the *Message Listener* view.
2. Add the desired *Subject* to be listened to.
3. Press the **Add** button to add it to the list of registered subjects.

To unsubscribe a subject, it is necessary to:

1. Select the subject to be unsubscribed from the list of registered subjects.
2. Press the **Remove** button to remove it.

When there are active subjects being listened to, whenever a message is received, it will be placed in a list that can be accessed on the bottom-left pane of the *Message Listener* tab. Clicking on the message will display its contents on the message details pane on the right side.

![messagebus01][messagebus01]

To clear the list of received messages, it is necessary to press the **Reset** button on the top right of the grid.

[messagebus00]:./images/MessageBus00.gif
[messagebus01]:./images/MessageBus01.png
[messagebus02]:./images/MessageBus02.png