Why Hard out of memory?

I have a cluster of 6 server node and suddenly in my cluster i see this log:
Hard Out Of Memory Error. Bucket “service” on node couchbasexxxx.it is full. All memory allocated to this bucket is used for metadata.
How to resolve this error ?
and why in web console the ram usage and Node ram quota for bucket service was always available?

As indicated, that means you’re out of memory for metadata for the items. You have a few options:

  1. Allocate more memory to the existing buckets, assuming you have some in the cluster
  2. Add more nodes to the cluster
  3. Add more memory to existing nodes and allocate it to the bucket
  4. Remove some items from the cluster
  5. Move to a “full eviction” bucket

See the Database Engine Architecture docs for a description of how this works.

Thanks, I already allocated more memory to the cluster and to the existing buckets.
Moreover i went from 18 millions to 8 millions entries, why the problem has appeared now?