- Backends in ejabberd are pluggable modules that store data to provide to feature modules. This allows ejabberd to be more stateless. Common backends include Mnesia, MySQL, PostgreSQL, Riak, and custom backends. - Message carbons allow messages to be copied to all connected clients so the full conversation history is visible, but they do not work in MUC rooms due to the anonymous nature of MUC chats. - To learn XMPP, it is best to start at the protocol level by using a client like PSI that allows inspecting and crafting XMPP packets directly rather than starting from libraries that abstract the protocol.

1. Academy #2
4th November 2015
Mickaël Rémond, @mickael

2. Questions
ejabberd questions
• What are backends ? What backends are available in ejabberd and how do they work ?
XMPP questions
• How can I learn and experiment with XMPP ?
• What are message carbons and how do they work ?
• Why do avatars / carbons not work in MUC rooms ?

3. ejabberd: Routing messages in a statefull world
ejabberd leverage both stateless and statefull features:
• Stateless:
• ejabberd is an XMPP server: Its goal is to route XMPP packets between JID.
• Statefull:
• In most case ejabberd depends on data (user base, contact list, …) or produce data (message archive, ...)
To deploy an ejabberd that is as stateless as possible, ejabberd leverages backends.

4. What are ejabberd backends ?
• Backends are pluggable modules you can configure to define where you would like to store part or all of your
data.
• Backends provide the data to the feature modules of ejabberd.
• They can be read-write or read-only if you do not need some of the ejabberd associated features:
• For example, if you handle user management elsewhere in your infrastructure, you can use a user back-
end that can only authenticate users but not create them.

5. Available backends
• Modules that need data backends generally support:
• Internal database: Mnesia
• RDBMS: MySQL / MariaDB, Postgres, SQLServer / Azure SQL, SQLite
• NOSQL: Riak
• Some of them support additional backends (i.e authentication, shared roster):
• LDAP / Active Directory / Azure Directory
• ejabberd SaaS provides ReST API description to implement HTTPS backend for
• User authentication,
• Contact list (rosters),
• Message archiving
• Custom backends in ejabberd-contribs: i.e. HTTP authentication.

6. Example configuration
You generally need a global parameter for RDBMS:
• Example of MySQL connection:
odbc_type: mysql
odbc_server: "server.company.com"
odbc_port: 3306 # the default
odbc_database: "mydb"
odbc_username: "user1"
odbc_password: "**********"
odbc_pool_size: 5
You can then reference that back-end for authentication:
auth_method: odbc
Or in some modules configuration on a granular basis:
modules:
mod_roster: [{db_type: odbc}]

7. ejabberd
architecture
model
Mobile - Desktop - Web Browser
XMPP - 5222
Websocket / Bosh:
HTTP - 80
HTTPS - 443
ejabberd Instant Messaging
ejabberd cluster
...
Load balancers
XMPP - 5222
User base
(i.e. MySQL)
User calls
ejabberd backend API
select one or several backends
ejabberd service
ejabberd architecture
model
ejabberd internal database
Offline messages
Last seen
Privacy lists
Pubsub nodes
Push tokens (APNS / GCM)
…
Contact service
(i.e. webservice)
Roster calls
Message archive
(i.e. Riak)
Message
Archiving calls
…
Data backends
to integrate
with infrastructure
and scale
Data can be moved to backends
to make ejabberd more stateless

8. ejabberd internals: Feature modules rely on data modules -
backends

9. Properties of ejabberd backends
• Backend can be combined:
• Different features can use different backends to leverage different backend properties:
• Massive amount of data
• Speed
• …
• Backend are API based:
• They are developed using ejabberd API.
• It means you can write your own backend to meet special needs.
• => Selecting the right backend or deciding to write your own for the each module is part of ejabberd solution
design.

10. What are sessions backends ?
• We have a special other type of backend call session backend.
• Sessions are transient data that serve to locate a user connection in the the cluster when he is online.
• As a default in ejabberd Community Edition, sessions are stored in Mnesia database.
• This is not permanent data.
• However, for scalability or convenience, you can select other backends:
• Redis
• RDBMS: MySQL, MariaDB, Postgres
• ejabberd Business Edition clustering

11. How can I learn and experiment with XMPP ?
• Developers often get confused when dealing with XMPP because it is quite a large protocol with a lot of
extensions.
• A common approach is to learn from the client libraries. This is often a bad idea to start from here are libraries
often try to abstract XMPP protocol.
• => Starting from the library hide what is XMPP about and can limit understanding of the protocol.
• I recommend to learn XMPP at the XMPP packet / text protocol level.
• To experiment, I am often using PSI client XMPP console to check traffic, analyse packets received and send,
and craft and send my own packets.

12. What are Message Carbons and how do they work ?
• Message Carbons are defined in XEP-0280.
• This is a feature that is intended so that all connected clients can receive all parts of the message
conversation to avoid online clients having only a part of the messages.
• It does so by having the server
• Send copies of your own messages to your other connected clients that want to receive the copies.
• Send copies of incoming messages to your other connected resources.
• Enabled by sending XMPP IQ to explicitely enable carbons for your session.

13. PSI XMPP debug
session demo

14. Why do avatars / carbons not work in MUC rooms ?
• One-to-one chat in XMPP rely on different features set than Multi-User chat.
• Multi-User Chat is a kind of proxy that allows you to discuss indirectly with other users.
• It means that some features that works on one-to-one chat may or may not be usable, depending on the
context:
• Avatars: Avatars are available for contact that are:
• in your roster
• or that you can at least know the JID.
MUC being semi anonymous, if you are not admin, you may not be able to retrieve VCard avatars as you
do not know the user JID.
Even if you have the JID, if you use PEP based avatars, user must have accepted you so that you can
see the avatar.
• Message carbons: XEP-0280 explicitely exclude MUC messages.

15. See you at next