# \[mechanics\] When is Central Certification Vector cleared?

**URL:** <https://forums.percona.com/t/mechanics-when-is-central-certification-vector-cleared/7249>\
**Category:** Percona XtraDB Cluster 5.x\
**Created:** [October 18, 2019, 10:10am UTC](https://forums.percona.com/t/mechanics-when-is-central-certification-vector-cleared/7249 "2019-10-18T10:10:13Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![zmusiato](https://avatars.discourse-cdn.com/v4/letter/z/e79b87/32.png) [@zmusiato](https://forums.percona.com/u/zmusiato)\
**Post date:** [October 18, 2019, 10:10am UTC](https://forums.percona.com/t/mechanics-when-is-central-certification-vector-cleared/7249/1 "2019-10-18T10:10:13Z")

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I understand that the crucial part of the proces of certification is looking up the reference transactions accordingly to certification keys in CCV. And it is done when a locally accepted (in terms of integrity) transaction is broadcasted to the whole cluster so that the other nodes may accept it or not. But after a reference transaction is certified by the whole cluster, does every node delete it from its local CCV? Do the nodes that’ve certified the transaction, wait for positive/negative reply from other nodes? If not, how do other transactions get certified positively if they modify the same row, e. g., a day later?