Lightweight Transactions (Compare-and-set)¶

Specialized Results¶

The result returned from a LWT request is always a single row result. It will always have prepended a special column named [applied]. How this value appears in your results depends on the row factory in use. See below for examples.

The value of this [applied] column is boolean value indicating whether or not the transaction was applied. If True, it is the only column in the result. If False, the additional columns depend on the LWT operation being executed:

• When using a UPDATE ... IF "col" = ... clause, the result will contain the [applied] column, plus the existing columns and values for any columns in the IF clause (and thus the value that caused the transaction to fail).

• When using INSERT ... IF NOT EXISTS, the result will contain the [applied] column, plus all columns and values of the existing row that rejected the transaction.

• UPDATE .. IF EXISTS never has additional columns, regardless of [applied] status.

How the [applied] column manifests depends on the row factory in use. Considering the following (initially empty) table:

CREATE TABLE test.t (
    k int PRIMARY KEY,
    v int,
    x int
)

… the following sections show the expected result for a number of example statements, using the three base row factories.

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
Row(applied=True)

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
Row(applied=False, k=0, v=0, x=None)

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0")
Row(applied=True)

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0 AND x = 1")
Row(applied=False, v=1, x=2)

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
(True,)

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
(False, 0, 0, None)

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0")
(True,)

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0 AND x = 1")
(False, 1, 2)

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
{u'[applied]': True}

>>> session.execute("INSERT INTO t (k,v) VALUES (0,0) IF NOT EXISTS")
{u'x': 2, u'[applied]': False, u'v': 1}

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0")
{u'x': None, u'[applied]': False, u'k': 0, u'v': 0}

>>> session.execute("UPDATE t SET v = 1, x = 2 WHERE k = 0 IF v =0 AND x = 1")
{u'[applied]': True}