Algorithms for doing leader election and name resolving with the help of another HA system, serves as anticorruption layer.

• I: intervalfor heartbeat (e.g. 1 sec)
• T: heartbeat timeout (e.g. 5 secs)
• T > 2 * I

• Heartbeat Table: A table in the external HA system contains heartbeat entry.
• Heartbeat Entry: in the format {uuid, utype, timestamp}
• ha_time: current date time of the external HA system
• All time is in UTC time

• UpdateHeartBeat(uuid, utype):

  For each type, update entry {old_uuid, utype, old_timestamp} in heartbeat table with {uuid, utype, ha_time}.

  For each type, if uuid is not equal to old_uuid, then (ha_time – old_timestamp > T) must be satisfied.

  The update process uses optimistic concurrency control. e.g. if the heartbeat entry has been updated before another heartbeat reaches, the later heartbeat is discarded.

• GetPrimary(utype):

  Return (uuid, utype) in heartbeat entry with the corresponding query utype if (ha_time - timestamp <= T). Else return empty value.

1. After a client S started, it generates a unique instance ID uuid to identify itself and marks itself with the exact utype, which it will work as in the future.

2. S calls GetPrimary(utype) every I secs.

3. If GetPrimary(utype) returned empty value, S calls UpdateHeartbeat(uuid, utype).

4. Continue to call GetPrimary(utype) every I secs.

   a. If subsequent call to GetPrimary(utype) returns (uuid, utype) generated in 1, S will then work as primary.

   b. If subsequent call to GetPrimary(utype) returns a unique ID which is different from uuid and the same type with utype generated in 1, go back to 2.

   c. If subsequent call to GetPrimary(utype) returns an empty value / a corrupted message, error occurred in 3. Retry 3.

5. S call UpdateHeartBeat(uuid, utype) and GetPrimary(utype) every I sec.

   a. If GetPrimary(utype) returns anything except (uuid, utype), or didn't return for (T - I) secs, exit itself and restart.

Source Code

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