24. Systemd Unit Without Service Dependencies

Date: 2026-02-06

Status

Accepted

Context

The Trento Agent runs as a systemd service on monitored hosts. Depending on the host platform, a metrics provider service must also be running: node_exporter on SLES 15 or Grafana Alloy on SLES 16. The current systemd unit file declares node_exporter as a dependency using Wants= and After= directives.

With the introduction of Alloy support (see RFC 0003 - Grafana Alloy Integration for SLES 16 System Monitoring), maintaining conditional systemd dependencies based on the target platform was considered. This would require two separate unit files: one referencing prometheus-node_exporter.service and another referencing alloy.service, with the appropriate one installed based on the SLES version during RPM packaging.

Decision

We decided to use a single systemd unit file for the Trento Agent with no dependencies on external services (node_exporter or alloy).

[Unit]
Description=Trento agent service

[Service]
ExecStart=/usr/bin/trento-agent start
Type=simple
User=root
Restart=on-failure
RestartSec=5

[Install]
WantedBy=multi-user.target

The effort of maintaining two separate systemd unit configurations and the conditional packaging logic required to install the correct one outweighs the benefit of automatic dependency management. A single unit file simplifies packaging, testing, and maintenance.

Additionally, this approach provides greater flexibility for user environments. It naturally supports SLES 15 SP7, where Alloy is available as an optional package alongside node_exporter, without requiring special-cased packaging logic. Users are free to choose and manage their preferred metrics provider independently from the Trento Agent lifecycle.

Design details

See RFC 0003 - Grafana Alloy Integration for SLES 16 System Monitoring for the complete design, including the alternative approach with conditional unit files and automatic dependency management.

Consequences

  • Users are responsible for starting and managing the metrics provider service (node_exporter or alloy) independently from the Trento Agent. The agent will start successfully even if the metrics provider is not running, but system metrics will not be available until the provider is properly configured and started.

  • On Alloy-based systems, users must review and deploy the Alloy configuration manually. This has the benefit of requiring an explicit review step, reducing the risk of unwanted misconfigurations being applied automatically.

Alternatives Considered

Conditional Unit Files With Automatic Dependency Management

Two separate systemd unit files would be maintained: one for SLES 15 declaring a dependency on prometheus-node_exporter.service, and one for SLES 16 declaring a dependency on alloy.service with an ExecStartPre directive to automatically generate and deploy the Alloy configuration. The RPM spec would conditionally install the appropriate unit file based on the sle_version OBS macro.

This approach was discarded because it doubles the packaging and testing effort for the systemd unit, introduces platform-specific branching in the build system, and reduces user control over the metrics provider lifecycle.