Slack vs Mattermost: An Open-Source Alternative for Enterprises (GDPR-Compliant)
Slack has been the standard for modern team communication for years. However, many companies are wondering how secure their data is in US clouds and whether alternatives exist that better cover data protection, integration, and customizability.
One of these is Mattermost – a fully self-hostable open-source platform for secure team chats, workflows, and automation.
This article compares Slack and Mattermost from the perspective of IT decision-makers and developers. It is not just about technical differences, but also about the user experience in day-to-day business, data protection (→ GDPR Article 44 ff.), and integration capability in existing IT landscapes.
Table of Contents
- Brief Overview of the Solutions
- Functional and Technical Comparison
- User Experience and Practice in Corporate Everyday Life
- Data Protection and GDPR
- Operations and Costs
- Conclusion
- Contact Us
- Sources and Further Links
Brief Overview of the Solutions
Slack
Slack is a cloud-based communication service from Salesforce. The tool combines chat channels, file sharing, audio/video functions (Slack Huddles), and integrations in a central platform.
- License model: Proprietary, subscription-based (Free, Pro, Business+, Enterprise Grid).
- Architecture: Multi-tenant cloud, operated on AWS infrastructure.
- Target group: Small to large companies, remote teams, SaaS environments.
- Strengths: Large app platform, intuitive user interface, ready to use immediately.
- Weaknesses: No complete data sovereignty, limited control over updates, potential GDPR issues with US hosting.
More information: Slack Trust Center →
Mattermost
Mattermost is an open-source communication platform that can be operated on-premises or in your own cloud. The source code is open, and operations can be carried out entirely within your own infrastructure.
- License model: Open source (MIT, Team Edition) + commercial enterprise extensions.
- Architecture: Modular, containerizable, REST API-based, scalable.
- Target group: Companies with high data protection requirements, DevOps teams, government agencies, organizations with their own IT infrastructure.
- Strengths: Full data control, LDAP / OIDC integration, plugin system, API access, self-hosting.
- Weaknesses: Self-operated, somewhat steeper learning curve for administration.
Learn more: Mattermost Homepage →
Functional and Technical Comparison
| Category | Slack | Mattermost |
|---|---|---|
| Deployment | SaaS cloud, centrally managed | Self-hosting, private cloud, managed hosting |
| Authentication | SAML SSO, Google Workspace, SCIM (Enterprise) | SAML, OpenID Connect, LDAP native |
| Integrations & APIs | Web API, Workflow Builder, App Marketplace | REST API, Webhooks, Plugins, Mattermost Marketplace |
| Data Protection & GDPR | Data Residency optional, CLOUD Act risk | Full control, EU hosting, no third-country transfer |
| Cost Model | License / subscription per user | Open source (free), enterprise plan optional |
| Operations & Updates | Maintenance by Slack | Self-managed or managed service |
| Extensibility | Proprietary app store | Open plugin system, custom extensions possible |
| Deployment Flexibility | No control over environment | Docker, Kubernetes, bare metal |
Integrations and Automation
Slack: Slack offers extensive options for managing messages, channels, or users with the Web API and the Events API. The Workflow Builder allows you to create simple automations, such as automating notifications or status messages.
Mattermost: Mattermost uses REST APIs and webhooks (incoming / outgoing). This allows you to integrate CI/CD pipelines, monitoring alerts, or ChatOps workflows into systems such as Jenkins, GitLab, or Prometheus.
Example:
# Example: Send Mattermost message via webhook
curl -X POST -H 'Content-Type: application/json' \
-d '{"text":"✅ Deployment completed"}' \
https://mattermost.example.com/hooks/WEBHOOK_TOKEN
User Experience and Practice in Corporate Everyday Life
Interface and Operation
Slack impresses with a modern, consistent interface with clear navigation and high user-friendliness. Mattermost is visually oriented towards Slack, but also offers additional customization options (branding, themes, sidebar structure).
Conclusion: Slack is somewhat "smoother," Mattermost is more flexible and customizable – ideal for companies with their own corporate design.
Training and UX
Slack offers guided tutorials, quickstart guides, and intuitive user guidance. Mattermost initially requires a little more technical understanding, but scores with clear documentation (End-User Guide).
Performance and Stability
Slack benefits from global cloud infrastructure (AWS, CDN) but is dependent on internet connections. Mattermost performance depends on your own infrastructure – with well-dimensioned hardware, it can be on par with or faster than Slack.
Communication and Notifications
Both platforms offer threads, @mentions, direct messages, and channel-based communication. Slack allows more detailed notification settings, Mattermost offers granularly controllable server policies and role models.
File Management and Search
Slack uses cloud storage with powerful full-text search and archiving. Mattermost allows file uploads and full-text search, which can be extended with OpenSearch or Elasticsearch integration.
Meeting Integration
Slack offers Huddles for spontaneous audio/video sessions and screen sharing. Mattermost supports audio and screen sharing natively via plugins and integrates video tools such as Jitsi Meet or BigBlueButton.
Extensions in Daily Work
Slack scores with thousands of apps (e.g., Jira, Trello, Google Drive). Mattermost offers integrations via the Mattermost Marketplace – including GitLab, Jenkins, Prometheus, and Grafana.
Mobile Use and Offline Mode
Both platforms offer mobile apps (iOS / Android). Slack is more stable offline, Mattermost offers controllable synchronization and no dependency on US servers.
Administration and Role Rights
Slack: Central role control via workspaces (owner, admin, channel manager). Mattermost: Fine-grained rights management via LDAP or AD, customizable depending on edition.
Data Protection and GDPR
Slack stores customer data on AWS servers, mostly outside the EU. Even with activated Data Residency, metadata and backups partially remain in the USA – thus the CLOUD Act applies.
Mattermost can be operated entirely in EU data centers (e.g., Hetzner Cloud). This eliminates third-country transfers according to Art. 44 ff. GDPR, and access control lies with the company or managed provider (e.g., WZ-IT Mattermost Hosting).
Operations and Costs
| Aspect | Slack | Mattermost |
|---|---|---|
| Cost Model | Subscription per user (monthly / yearly) | Free (Team Edition) or license per user (Enterprise) |
| Operational Effort | No own effort (SaaS) | Self-operation or managed service required |
| Scaling | Automatic via Slack cloud | Freely scalable (cluster, Docker, Kubernetes) |
| Updates & Support | Automatic, central | Manual or via support contract |
| Data Backup | By Slack | Full control over backup & retention |
Slack offers lower operational effort, Mattermost more flexibility and long-term plannable cost structures.
Conclusion
Slack convinces with user-friendliness, performance, and integrations – optimal for teams that want to start quickly and rely on a global SaaS infrastructure.
Mattermost offers the same functionality, supplemented by data sovereignty, integration depth, and customizability – ideal for companies that value data protection, control, and internal integration.
In short:
- Slack: fast, polished, but controlled by the provider.
- Mattermost: open, flexible, completely company-owned.
Which solution fits better depends on the focus: Quick start in the cloud – Slack. Long-term control & GDPR compliance – Mattermost.
WZ-IT offers Mattermost as a managed service with hosting in German data centers, automated updates, monitoring, SSO integration, and individual branding.
👉 Learn more about Mattermost at WZ-IT
Contact Us
Would you like to switch from Slack to Mattermost or build a secure communication platform? We'll be happy to advise you on suitable solutions.
Book your free, no-obligation initial consultation with us here: Schedule appointment.
You can also reach us by phone at +49 2922 875068 or by email at [email protected]. We look forward to hearing from you!
Sources and Further Links
- Slack API Documentation
- Slack Data Residency (Help Center)
- Slack Trust Center
- Mattermost Documentation
- Mattermost REST API Reference
- Mattermost Marketplace
- Mattermost Elasticsearch Config
- Mattermost Security & Compliance
- Wikipedia: Slack (Software)
- Wikipedia: Mattermost
- CLOUD Act – EU Commission
- GDPR – Art. 44 ff. (EUR-Lex)
Let's Talk About Your Project
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