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Hi,

at OVH, we kept the mongodb backend (understand: we are currently running an old version of ceilometer-collector). But we modified it to implement real-time aggregation so that we can then get the interresting values immediatly instead of running long calculations when we need them (we use Ceilometer for billing).

To do that, mongodb provides some operators such as $inc and $max:
https://docs.mongodb.com/manual/reference/operator/update-field/

This implementation scales well, we currently handle more than 20 000 mongodb updates per seconds without problems. (The issue is actually ceilometer-collector consuming too many CPU, forcing us to scale the number of servers to handle the load)
  
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Romain LE DISEZ