After three (or was it almost four?) years of using Slack, we took the plunge and set up our own instance of Mattermost. The reasons for doing this include wanting more control over our data and wanting an unlimited history which Slack, as a hosted service, offers only to paying customers. This is more than fair enough – it’s not their fault that we’re too stingy. Apropos stingy: Mattermost exists in three editions – we chose the Team Edition; guess why.

Web UI

If you know how to wield Slack, you know how to use Mattermost. (Oh, please don’t mind the awful colors above – this is my test installation and I need severe optical distinction in order to not mistake the installations.) There are few differences, if any. Mattermost’s use of Markdown appears to be more comprehensive, in particular because its Webhooks support Markdown.

Installing Mattermost is easy thanks to the good documentation they provide which explains, step by step, what I have to do to install Mattermost on a machine. I chose to use PostgreSQL because I recall having read that’s Mattermost’s primary database candidate, and because it allows me to use PostgreSQL – a reason sufficient on its own. I selected to loosely follow advice given regarding location of config.json as that seemed a sensible thing to do. (config.json is Mattermost’s central configuration file which is reloaded on change.) If Ansible’s your drug, Pieter Lexis created an Ansible role for installing Mattermost on Debian/Unbuntu, and there’s playbook which does that and more also.

I can create as many teams as I want on a server, and each team can have as many channels as I want. In the Team Edition users authenticate with password and I enforced e-mail verification. (Other editions offer 2FA and LDAP.)

Mattermost users can upload files (images, code snippets, etc.) which have to be stored somewhere. By default a configurable directory on the local file system is used, but Mattermost’s system console allows me to configure Amazon S3 storage such as Minio.

Webhooks, API, Websocket, CLI, etc.

One of the things I like most about Slack are its integrations, and low and behold, Mattermost has these as well. Incoming and outgoing Webhooks as well as slash commands. Lovely.

Also very powerful is the Websocket API; there’s a Python3 driver which works very well, and next to that is Mattermost’s API with which I can create users, get their details, enumerate posts, create posts, etc. The following example using curl and jo shows how I can add a post from the command line:

json=$(jo channel=cartoons1 \
          username="my-script" \
          icon_url="" \
          text='Ha, this is _just_  an example using `curl`, :tada:')

curl -H 'Content-Type: application/json' \
     --data "$json" \

posted with curl

(For a much more flexible solution see 42wim’s mattertee.)

Programs which use Mattermost’s API must authenticate to the service and they can do so with either session tokens that expire and Personal Access Tokens which I create in my account preferences which don’t expire until I revoke them. Additionally Mattermost can act as an OAuth 2.0 provider.

Masses of messages

One of the channels we have is reserved for Nagios/Icinga-type notifications. One thing I wanted to be able to do is to delete and purge those messages; I don’t see why I need to know weeks later that something was offline for a moment. However, if I delete a message, either interactively or via the API, Mattermost soft deletes it; the message is marked as deleted with a time stamp, but it remains in the database.

So I went in search of an API to physically remove these messages, but it doesn’t exist. The solution? Use, say, the API to find the posts I want to remove, “delete” them using said API, and then use an SQL DELETE to purge:

DELETE FROM posts WHERE deleteat <> 0;

Mattermost at a console

Mattermost has a Web UI and some mobile and fat clients, but what does a person do with just a terminal at his/her disposal? Use either matterhorn or Irssi or your favorite IRC client with matterircd.

seed in Matterhorn

What you see in the screenshot above is matterhorn showing what the first Web UI screenshot shows. The program has some really cool features including scripts – just shell scripts which are given the text I enter on stdin and the stdout they produce is posted. Matterircd on the other hand is an IRC to Mattermost gateway written also by 42wim: you connect it to your remote Mattermost installation and talk to it via your IRC client.


Do I regret leaving Slack? Not really, even though their mobile apps are quite a bit more polished than Mattermost’s are – a result of development effort obviously. I now get a warm and quite fuzzy feeling knowing that we have control over our data, how we back it up, and what we do with it. And I’m confident that (other than the NSA) no third party has it.

Apropos 3rd party: while it’s possible to access dozens (or hundreds?) of integrations using an external service called Zapier we will not as that defeats the purpose of wanting to be the sole owners of said data. Similarly we’ve been discussing mobile notifications for which we could either set up mobile push or do it ourselves; we haven’t finally decided yet.

Do it yourself push

Do it ourselves, you ask? Yes, that’s possible by creating a notification endpoint which Mattermost uses whenever it’s about to notify a mobile device. The post I created with a shell script earlier, pushes to this example MQTT:

$ mosquitto_sub -v -t 'mm/#'
mm/_notif my-script in cartoons: Ha, this is _just_  an example using `curl`, :tada:

The way this happens is that Mattermost notifies a Web service I create which obtains the message and disposes of it in any way I want:

#!/usr/bin/env python

from bottle import run, request, post
import json
import paho.mqtt.publish as paho

__author__    = 'Jan-Piet Mens <jp()>'

def post1():

    data = json.loads(

    paho.single("mm/_notif", data['message'],

run(host='', port=8864)

If you’re not interested in mobile push, there’s always e-mail: when users are away or offline they can choose to be notified of new content by e-mail.

If I’d known about Mattermost before, I’d have migrated earlier.