khepri_machine: Use process dict instead of persistent_term to track the need for a fence preliminary query#317
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... as the arbitrary fence preliminary query. [Why] It allows to distinguish it from the other arbitrary query used in `fence/2` during debugging session.
... to track the need for a fence preliminary query. [Why] There was a small bug with our use of the persistent_term: it should have been a key per store ID and process PID combination, not just per store ID. But anyway, a persistent_term is designed for a value that is mostly read, rarely written to. This was not the case here. [How] I thought about using an ETS table instead but it had a problem: if the calling process is on a different node that the one running Khepri, Khepri can't create an ETS table on that remote node. Each call could initialize that table if it didn’t exist, but this would be too much overhead in that code path to my taste. Moreover, we would have to put in place a cleaning mechanism for processes that exited if they still had an entry in that ETS table. The solution retained is the use of the calling process dictionary. It is fast, can handle frequent updates and will be automatically cleaned up when the process exits. It also does not need an initial setup.
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Additional details and impacted files@@ Coverage Diff @@
## main #317 +/- ##
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+ Coverage 89.14% 89.20% +0.06%
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Files 22 22
Lines 3299 3299
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+ Hits 2941 2943 +2
+ Misses 358 356 -2
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Why
There was a small bug with our use of the persistent_term: it should have been a key per store ID and process PID combination, not just per store ID.
But anyway, a persistent_term is designed for a value that is mostly read, rarely written to. This was not the case here.
How
I thought about using an ETS table instead but it had a problem: if the calling process is on a different node that the one running Khepri, Khepri can't create an ETS table on that remote node. Each call could initialize that table if it didn’t exist, but this would be too much overhead in that code path to my taste. Moreover, we would have to put in place a cleaning mechanism for processes that exited if they still had an entry in that ETS table.
The solution retained is the use of the calling process dictionary. It is fast, can handle frequent updates and will be automatically cleaned up when the process exits. It also does not need an initial setup.