Concepts Used in FULL ACID v2 Tables

Before beginning to use FULL ACID v2 tables you must be aware of these new concepts like transactions, WriteIds, rowIDs, delta delete directories, locks, etc. that are added to FULL ACID tables to achieve ACID semantics.

Write IDs

For every transaction, both read and write, Hive will assign a globally unique ID. For transactional writes like INSERT and DELETE, it will also assign a table-wise unique ID, a write ID. The write ID range will be encoded in the delta and delete directory names. Results of a DML transactional query are allocated to a location under partition/table. This location is derived by Write ID allocated to the transaction. This provides Isolation of DML queries and such queries can run in parallel without interfering with each other.

New Sub-directories

New data files resulting from a DML query are written to a unique location derived from WriteId of the transaction. You can find the results of an INSERT query in delta directories under partition/table location. Depending on the operation type there can be two types of delta directories:

  • Delta Directory: This type is created for the results of INSERT statements and is named delta_<writeId>_<writeId> under partition/table location.
  • Delete Delta Directory: This delta directory is created for results of DELETE statements and is named delete_delta_<writeId>_<writeId> under partition/table location.

UPDATE operations create both delete and delta directories.

Row IDs

rowId is the auto-generated unique ID within the transaction and bucket. This is added to each row to identify each row in a table. RowID is used during a DELETE operation. When a record is deleted from a table, the rowId of the deleted row will be written to the delete_delta directory. So for all subsequent READ operations all rows will be read except these rows.

Schematic differences between INSERT-ONLY and FULL ACID tables

INSERT-ONLY tables do not have a special schema. They store the data just like plain original files from the non-ACID world. However, their files are organized differently. For every INSERT statement the created files are put into a transactional directory which has transactional information in its name.

Full ACID tables do have a special schema. They have row identifiers to support row-level DELETEs. So a row in Full ACID format looks like this:

   "operation": 0,
   "originalTransaction": 1,
   "bucket": 536870912,
   "rowId": 0,
   "currentTransaction": 1,
   "row": {"i": 1}
  • The green columns are the hidden/system ACID columns.
  • Field “row” holds the user data.
  • operation 0 means INSERT, 1 UPDATE, and 2 DELETE. UPDATE will not appear because of the split-update technique (INSERT + DELETE).
  • originalTransaction is the write ID of the INSERT operation that created this row.
  • bucket is a 32-bit integer defined by BucketCodec class.
  • rowId is the auto-generated unique ID within the transaction and bucket.
  • currentTransaction is the current write ID. For INSERT, it is the same as currentTransaction. For DELETE, it is the write ID when this record is first created.
  • row contains the actual data. For DELETE, row will be null.