Add support for multiple directors / HA federations #2023
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When `Range` was called on an empty ttlcache object, the older version of ttlcache would trigger a process panic.
We will need version information initialized in the `server_structs` module; having `server_structs` include the `config` module for the version information causes a circular dependency. This factors out the version-related calls (keeping backward API compatibility) into a new module, breaking the dependency.
With this, an empty configuration string indicates to use the default.
The common base includes the name -- but also the version number, a process/instance identifier, and a generation number. The instance ID and generation number provide a total ordering for all ads generated by the name/type of ad. This will allow a process to compare any two ads and determine which one is the "newer" of the two. This will allow the director to keep the right ad in memory even if it receives them out-of-order.
The director ad needs to be able to determine its own name, just like the cache and origin. Pull these into a single location. There's a need for future improvements -- the service should persist its currently registered name -- but this simple refactor should suffice for this line of work.
This provides a helper function to ensure we consistently calculate the audience value for our token configuration.
Provide a framework for any service to detect the presence of multiple director services in a federation, whether from static configuration, from metadata discovery, or from querying known directors. All the sources are merged together to create one set of known directors. Once the directors are known, and advertisements from the service are sent to all directors directly from the service. Additionally, the directors will forward the ads they receive from a service to all other known directors. This prevents a director from missing updates just because the service hadn't discovered it yet. The directors will compare the generation and instance IDs of the incoming ads to remove duplicates and ensure they only keep the latest copy of a given service description. A simple unit test is included.
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Add preliminary support for having multiple directors in a single federation.
This PR allows the services in a federation to calculate all the available directors in a federation and advertise to all available ones. If the director knows of more directors than the service, it will also forward received ads to the director.
This way, all available directors - whether configured statically or discovered through advertisement - will eventually receive all ads. It's left as an exercise for the operations to figure out how to allow fallback of the default URLs to a different director if one goes down.