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-h2(#main). development main (as of 2020-10-28)
+h2(#main). development main (as of 2020-12-10)
"Upgrading from 2.1.0":#v2_1_0
+h3. Changes on the collection's @preserve_version@ attribute semantics
+
+The @preserve_version@ attribute on collections was originally designed to allow clients to persist a preexisting collection version. This forced clients to make 2 requests if the intention is to "make this set of changes in a new version that will be kept", so we have changed the semantics to do just that: When passing @preserve_version=true@ along with other collection updates, the current version is persisted and also the newly created one will be persisted on the next update.
+
+h3. System token requirements
+
+System services now log a warning at startup if any of the system tokens (@ManagementToken@, @SystemRootToken@, and @Collections.BlobSigningKey@) are less than 32 characters, or contain characters other than a-z, A-Z, and 0-9. After upgrading, run @arvados-server config-check@ and update your configuration file if needed to resolve any warnings.
+
+The @API.RailsSessionSecretToken@ configuration key has been removed. Delete this entry from your configuration file after upgrading.
+
h3. Centos7 Python 3 dependency upgraded to python3
Now that Python 3 is part of the base repository in CentOS 7, the Python 3 dependency for Centos7 Arvados packages was changed from SCL rh-python36 to python3.