nathanbrock / exploring-systemd-start-order

Explore the various start order options in systemd and systemctl replacement.

Exploring SystemD Replacement Start Order

Repo aiming to explore the various start order options in SystemD and how they operate with SystemD replacement.

Table of contents

• Exploring SystemD Replacement Start Order
  • Table of contents
  • Prerequisite
  • Running tests
  • Results
    • Summary table
  • Reading

Prerequisite

• Docker
• CentOS7 with SystemD (Run ./build-centos7-systemd.sh)

Running tests

You can run a scenario by running the following command.

./test-scenario.sh {SCENARIO_PATH} eg ./test-scenario.sh scenarios/1-oneshot-a-before-b

The test scenario process is very manual as it stands. It expects you to tell it when you think the start-up process is complete and to move onto the next test step.

This command will;

• Run the scenario four times;
  1. Run with pure SystemD with service A exit 0
  2. Run with replacement SystemD with service A exit 0
  3. Run with pure SystemD with service A exit 1
  4. Run with replacement SystemD with service A exit 1
• Expect you to hit CMD+C when you deemed each run "finished".
• After hitting CMD+C it'll take a screenshot, request the systemctl status of each service and screenshot that.
• It'll then move onto the next scenario step.

Results

The screenshot results of each scenario can be found in the ./scenarios/**/results directory.

Summary table

Scenario #	Description	SystemD	Service A exit code	Expected B status	Actual B status	Pass
1	Loose dependency. A sets Before=b	Pure	0	Running after start-up	Running after start-up	✅
		Replacement	0	Running after start-up	Running after start-up	✅
		Pure	1	Running after start-up	Running after start-up	✅
		Replacement	1	Running after start-up	Running after start-up	✅
2	A sets Before=b. B sets BindsTo=a	Pure	0	Running after start-up	Running after start-up	✅
		Replacement	0	Running after start-up	Running after start-up	✅
		Pure	1	Not running	Not running	✅
		Replacement	1	Not running	Running after start-up	❌
3	Same as above but with a wait
4	A sets Before=b. B sets Requires=a	Pure	0	Running after start-up	Running after start-up	✅
		Replacement	0	Running after start-up	Running after start-up	✅
		Pure	1	Not running	Not running	✅
		Replacement	1	Not running	Running after start-up	❌
5	B sets After&Requires=a	Pure	0	Running after start-up	Running after start-up	✅
		Replacement	0	Running after start-up	Running after start-up	✅
		Pure	1	Not running	Not running	✅
		Replacement	1	Not running	Running after start-up	❌
6	B sets After&Requires&PartOf=a	Pure	0	Running after start-up	Running after start-up	✅
		Replacement	0	Running after start-up	Running after start-up	✅
		Pure	1	Not running	Not running	✅
		Replacement	1	Not running	Running after start-up	❌

Reading

• SystemCTL replacement start order notes
• SystemD section options