The driver exposes metadata about the Cassandra cluster via the Session#getMetadata method. It returns a Metadata object, which contains three types of information:
Metadata is mostly immutable (except for the fields of the Node class, see the “node metadata” link above for details). Each call to getMetadata() will return a new copy if something has changed since the last call. Do not cache the result across usages:
Metadata metadata = session.getMetadata(); session.execute("CREATE TABLE test.foo (k int PRIMARY KEY)"); // WRONG: the metadata was retrieved before the CREATE TABLE call, it does not reflect the new table TableMetadata fooMetadata = metadata .getKeyspace(CqlIdentifier.fromCql("test")) .getTable(CqlIdentifier.fromCql("foo")); assert fooMetadata == null;
On the other hand, the advantage of immutability is that a Metadata instance provides a consistent view of the cluster at a given point in time. In other words, the token map is guaranteed to be in sync with the node and schema metadata:
Metadata metadata = session.getMetadata(); // Pick up any node and keyspace: Node node = metadata.getNodes().values().iterator().next(); KeyspaceMetadata keyspace = metadata.getKeyspaces().values().iterator().next(); TokenMap tokenMap = metadata.getTokenMap().get(); // The token map is guaranteed to have the corresponding data: Set<TokenRange> tokenRanges = tokenMap.getTokenRanges(keyspace.getName(), node);
This is a big improvement over previous versions of the driver, where it was possible to observe a new keyspace in the schema metadata before the token metadata was updated.