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TC Subgroup: Distributed vs. Centralized Configuration

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Subgroup Members:

Florian Gleixner 

Craig McNally 

Julian Ladisch 

Mike Taylor 

Meetings: TBD

Meeting room: https://meet.lrz.de/tcdistributedvscentralized or Slack Huddle.


Links to former proposals and documents

Scope of the group

Shall tenant level configuration stored in a central module or distributed in modules.

Give guidance when to use central or distributed configuration.

Types of configurations

System settings (the same for all Tenants, all modules)

Examples:

  • Common Kafka URL and Access
  • Common Elasticsearch URL and Access
  • Common Database URL and Access

Module System/deployment settings

Examples:

  • specific Kafka URL and Access
  • specific Elasticsearch URL and Access
  • specific Database URL and Access
  • S3 Access


Tenant specific settings

Examples:

  • Tenant Default Language
  • Circulation Rules
  • Locations
  • Usergroups

Tenant and Module specific settings

Examples:

  • KB Credentials in mod-kb-ebsco-java

Module specific settings (the same for all Tenants)

Examples:

  • S3 Bucket Access

User/Usergroup specific settings

  • User Default Language
  • Circulation Desk

Configuration storage locations

Deployment Settings (central)

For deployment options like Ansible, Kubernetes/Helm.

Examples:

  • Common Kafka Url
  • Okapi Url

Module environment variables (distributed)

Examples:

  • Kafka URL
  • Database Access

Module configuration files (distributed)

Mounted at deploy time.

Examples:

  • edge SIP2 configuration files

Module managed settings (distributed)

Module stores configuration in its database schema. Module may offer API endpoints to modify configuration

Examples:

  • Circulation rules


Configuration Module managed settings (central)

Examples:

Central vs. Distributed

Pros of central configuration:

  • No need to implement API endpoints and storage for module specific configuration.
  • All configuration variables of a tenant can be accessed for backup/clone/configuration of new tenant ... But: differentiating between configuration and data depends on point of view.
  • Possibility for a central UI for configuration
  • Can store configuration for values that have no "natural" owner module. Example: default tenant locale
  • Can handle shared configuration, but do we need a owner module?

Drawback of mod-configuration:

  • A big institution need config write permissions with module granularity. One member of staff may be allowed to edit circulation config but not acquisition config. Solved in mod-settings

Drawbacks of central configuration like mod-settings and mod-configuration

  • No validation. The module cannot validate a POST or PUT request because it doesn't know a schema. Only the module it belongs to knows this. Relevant use case: Using curl/wget/postman/...
  • No documentation. mod-configuration has no documentation, one needs to search, maybe the module's README has some? A dedicated module API always publishes the API documentation at https://dev.folio.org/reference/api/
  • No explicit dependency. If more than one module uses a configuration this dependency should be make explicit with an interface dependency in the ModuleDescriptor.
  • Performance. Requests to mod-configuration result in latency. If the config API belongs to the module the module can cache it and can invalidate the cache if the config is changed. Caching requests to mod-configuration will always result in a time period with outdated values. In mod-inventory-storage we've combined fetching the HRID config and HRID generation into a single SQL query.
  • Coupling. Modules should be loosely coupled and therefore each module should store its own configs.

Pros of distributed configuration

  • Offers possibility for write-only configuration values like passwords

Drawbacks of distributed configuration

  • Adding configuration APIs adds interface dependencies and APIs are not consistent - need a guide for module configuration APIs


Conclusions

  • mod-configuration shall not be used any more due to security issues
    • module is already deprecated, no more configuration settings shall be added.




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