YT is Yandex’s core platform for storing and processing big data, analogous to Hadoop MapReduce and HBase in terms of its features and functionality.
The bottom layer of YT consists of a distributed file system (DFS), similar to HDFS or GFS. Crucial differences include support of transactionality, data storage in tables, and a blocking system on nodes, which allows using YT as a coordination service (analogous to Apache Zookeeper).
The layer above DFS is a scheduler, which can manage a group of hosts with thousands of GPUs and more than a million CPU cores. It splits large computations (operations) into separate blocks (jobs), distributes resources between them, monitors their execution, and restarts those jobs that fail.
The top layer of YT is a distributed key-value storage (comparable to BigTable and HBase). The storage and the file system share a common namespace, which looks for the end user like a special type of tables in DFS. These tables support efficient read and write operations on strings by primary key. The key features of the KV storage include transactionality, strict consistency (the snapshot isolation level), and support for distributed transactions.