khepri_machine: Add API to wait for a behaviour to be effective#374
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khepri_machine: Add API to wait for a behaviour to be effective#374
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[Why] It is responsible for the mapping from an API behaviour to the state machine version the API behaviour was introduced in. We will be able to use it in a future API. The `does_api_comply_with/2` function is simplified by using `api_behaviour_to_machine_version/1`.
[Why] It allows to wait that a Khepri store runs the given machine version or more.
[Why] It allows to wait that a Khepri store supports a specific behaviour.
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Why
In the client code of the Khepri state machine, we need to know what version of the state machine is currently effective in the cluster before we can send some new commands. This is to avoid sending a new command to a member of the cluster that runs an older version of Khepri, causing it to possibly crash.
So far, we always had a fallback code path: if the new behaviour is not supported by the cluster, we could use an old way of doing somethin, or not doing it at all (keeping the old behaviour).
In a future patch, we will need to return an error to the caller to indicate the cluster is not ready for the behaviour it wants. Therefore, we need an API to allow the client code of the state machine (or the caller) to wait for a specific behaviour to be supported by the cluster.
How
The patch consists of three commits. The first one refactors the mapping between a given behaviour and the state machine version it wa introduced in.
The remaining two commits add functions to wait for the cluster to run a minimum version or a support a specific behaviour:
khepri_machine:wait_for_effective_machine_version/2khepri_machine:wait_for_effective_behaviour/2; this relies on the refactored behaviour/version mapping and thewait_for_effective_machine_version/2