I need some clarification on how to monitor BDR nodes.  In particular 
determining replication lag.  As an example,  I have a two node cluster with 
nodes ‘A’ and ‘B’.    I need to be able to look at node ‘B’ and determine if it 
is lagging behind node ‘A’,  by interrogating node ‘B’ only.  

From the BDR documentation on monitoring:

SELECT pg_xlog_location_diff(pg_current_xlog_insert_location(), flush_location) 
AS lag_bytes,
       pid, 
       application_name
  FROM pg_stat_replication;

Because it is querying the pg_stat_replication table,  I will need to run this 
query on node ‘A’ to check the lag on node ‘B’, is that true?  I need to be 
able run a query on node ‘B’ to determine if it node ‘B’ is behind.  I am not 
sure the above query will work for that use case.




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