Transactions and Transaction Operators

Owned by Jason Kinchen
Jul 06, 2022
3 min read

SciDB transactions overcome the limitations of mquery() by empowering users to:

  1. Create a transaction apart from any query

  2. Arbitrarily roll back (abort) a transaction

  3. Commit the transaction explicitly

Users do not need to know beforehand all queries they'd like to execute in the transaction, allowing users to interactively explore their data in new and flexible ways.

Transactions cannot be nested and mquery() may not be used within a transaction. Many SciDB operators are disallowed inside a transaction because they have effects (especially on metadata) which cannot easily be rolled back. A disallowed operator will result in an error and an aborted transaction. See for a list of disallowed operators.

Changes to arrays modified by a user’s transaction are protected by array locks held by that transaction. Array locking is the biggest change from mquery() to transactions because the array locks are acquired as needed rather than up front. The array locks won’t be released until the transaction either commits or rolls back. This will prevent a transaction from removing an array or removing array versions out from underneath another transaction.

If a transaction has modified an array and another session (whether in a transaction or not) attempts to use that array in a query, the query in that other session will block until the transaction modifying the array is committed or rolled back, or a configurable timeout [TODO: ADD config variable name] is reached. If two transactions attempt to lock arrays in an order that would cause a deadlock, SciDB detects the deadlock and causes an exception in the query causing the deadlock.

Transactions are automatically rolled back when the user disconnects (iquery) or when the user session expires (shim replacement).

If a query results in an error while in a transaction, the transaction is not rolled back automatically. Instead, the transaction becomes invalid and is said to be in “pending rollback” state: all queries in this state are forbidden and result in an error with error code SCIDB_LE_PENDING_ROLLBACK. The only way to proceed out of this state is to issue a rollback(), which ends the transaction and undoes any work that was performed in the transaction. Requiring a rollback() in this manner prevents the session from getting into a state where the user thinks the transaction is still active but the transaction has actually ended.

SciDB transactions provide Read Committed isolation [SQL 1992]. Arrays created, removed, or altered by a new version will be visible to other queries and transactions only after the transaction commits. Once committed, the change immediately becomes visible within other transactions since the isolation is not Read Repeatable or Serializable [SQL 1992].

Transaction error messages

Long error message code

Meaning

Long error message code

Meaning

SCIDB_LE_NO_TRANSACTION

The query contains an operator that can only be used within a transaction, but your session is not in a transaction. For example: You tried to commit() but you are not in a transaction.

SCIDB_LE_NOT_ALLOWED_IN_TRANSACTION

The query contains an operator that isn’t allowed in a transaction. Most operators that affect metadata aren’t allowed in transactions. See for a list of such operators. In general, the only metadata operation you can perform in a transaction is to create a new array.

This error invalidates the transaction; to continue, you must rollback(), then begin() a new transaction and repeat any work you attempted in the original transaction.

SCIDB_LE_PENDING_ROLLBACK

The transaction was invalidated due to a previous error. No operations can be performed until you issue a rollback().

SCIDB_LE_TRANSACTION_DEADLOCK

The transaction tried to lock an array that was already locked by another transaction, while that other transaction was waiting for an array locked by this transaction. SciDB has invalidated your transaction and released its locks so that the other transaction can proceed.

This error invalidates the transaction; to continue, you must rollback(), wait for the locked arrays to be released, then begin() a new transaction and repeat any work you attempted in the original transaction.

SCIDB_LE_TRANSACTION_ROLLBACK_SUCCESS

This message indicates that a rollback() was successful; it is not an error condition.

Most SciDB clients, such as the iquery command-line tool and all tools provided by Paradigm4 customer service, will handle this message internally and will not expose it to users. However, if you are writing a client that communicates directly with a SciDB server, you might need to detect this message and present an appropriate “success” message to the user.