Minimalist would be a simple Redis, like other said. You can get also persistent storage (from time to time), performance and scaling (if needed).
Slack devs are also reticent to new dependencies and avoids adding new tech in their stack, you can read about their Job Queue system here, which is exactly what you need too (more or less). The old system is on redis (previous version, probably is enough for your needs)
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Minimalist would be a simple Redis, like other said. You can get also persistent storage (from time to time), performance and scaling (if needed).
Slack devs are also reticent to new dependencies and avoids adding new tech in their stack, you can read about their Job Queue system here, which is exactly what you need too (more or less). The old system is on redis (previous version, probably is enough for your needs)