Metadata

The driver exposes metadata about the Cassandra cluster via the Session#getMetadata method. It returns a Metadata object, which contains three types of information:

• node metadata
• schema metadata
• token metadata

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.