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# Publish-Subscribe pattern and PubSub in ejabberd
- URL: https://www.process-one.net/blog/publish-subscribe-pattern-and-pubsub-in-ejabberd/
- Published: 2021-03-19T15:21:00.000Z
- Updated: 2024-09-16T14:21:55.000Z
- Author: ProcessOne
Publish–Subscribe is a messaging pattern where senders of messages, called *publishers*, do not send the messages directly to specific receivers, called *subscribers*. Instead, publishers categorize messages into channels without knowledge of which subscribers, if any, there may be. Similarly, subscribers express interest in one or more channels and only receive messages that are of interest, without knowledge of which publishers, if any, there are.
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# ejabberd and PubSub
In case of ejabberd, the Publish-Subscribe pattern (PubSub) is implemented by the native module `mod_pubsub`. From the outside, it works very similar to ejabberd MQTT module `mod_mqtt`. However, `mod_pubsub` uses XML and `iq` stanzas for communication. Therefore, [ejabberd PubSub](https://www.process-one.net/blog/ejabberd%5Fpubsub%5Ffeatures/) brings all the advantages as well as complexities that come with XML.
# Create a PubSub node
Before we start, make sure you are using an XMPP client that has an option of sending raw XML input to your server. One of such clients is [Psi](https://github.com/psi-im/psi?ref=process-one.net), available for many platforms.
Next, we need to keep in mind that in ejabberd `mod_pubsub` is enabled by default with plugins `flat` and `pep`. The permission to create nodes is limited to local accounts.
To create your first node, send the following command using the “XML Input” within the Psi XML Console. Substitute `marekfoss.org` in `to` and `node` params with your server domain, and `mf` in the `/home/marekfoss.org/mf/open` with your username. The node in this example is called `open`.
```xml
```
To verify that the node was created, you can send the above command again, at which point you should get a 409 error saying “Node already exists”. To create more nodes, remember to increment or change the `id` value of the `iq` element, like `create1`, `create2` etc. and change the node’s name in the `/home/marekfoss.org/mf/...` path.
Please note that the correct node path: `/home/[domain]/[username]/[nodename]` is important. With the default `mod_pubsub` permissions, the plugin will allow you to create this node. If you would like to create your node in another path, [refer to the docs](https://docs.ejabberd.im/archive/20%5F04/modules/?ref=process-one.net#mod-pubsub).
# Subscribe the PubSub client
Now we can subscribe our client to receive messages from the node we just created. To do this, we use the following command, where we `subscribe` the specified `jid` to the specified `node`:
```xml
```
If successful, you should see in your XML terminal a response containing a section like this:
```xml
```
# Publish with PubSub client
Now we can test our new PubSub communication channel by sending the first message to the node we created. To do this, we use the following command:
```xml
-
Hello Brave New World
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles,
And by opposing end them?
```
In response, you should see the message relied back to you, as well as an acknowledgement looking like the following:
```xml