Read-only Transactions via Snapshots
Snapshot represents a read-only
transaction: when multiple read operations are performed via a Snapshot,
the results are consistent as of a particular point in time.
Beginning a Snapshot
To begin using a snapshot using the default “bound” (which is “strong”), meaning all reads are performed at a timestamp where all previously-committed transactions are visible:
with database.snapshot() as snapshot: ...
You can also specify a weaker bound, which can either be to perform all reads as of a given timestamp:
import datetime TIMESTAMP = datetime.datetime.utcnow().replace(tzinfo=datetime.timezone.utc) with database.snapshot(read_timestamp=TIMESTAMP) as snapshot: ...
or as of a given duration in the past:
import datetime DURATION = datetime.timedelta(seconds=5) with database.snapshot(exact_staleness=DURATION) as snapshot: ...
Single Use and Multiple Use Snapshots
In the context of read only transactions,
methods can be used multiple times if you specify
in the constructor of the snapshot. However,
incompatible with either
multi_use defaults to
False and the snapshot cannot be
with database.snapshot(multi_use=True) as snapshot: ...
begin() can only be used on a
multi_use=True. In which case it is also necessary
to call if you need to have multiple pending operations.
Read Table Data
To read data for selected rows from a table in the database, call
read() which will return
all rows specified in
keyset, or fail if the result set is too large,
with database.snapshot() as snapshot: result = snapshot.read( table='table-name', columns=['first_name', 'last_name', 'age'], keyset=spanner.KeySet([['firstname.lastname@example.org'], ['email@example.com']])) for row in result: print(row)
NOTE: Perform all iterations within the context of the
Execute a SQL Select Statement
To read data from tables in the database using a query, call
which will return all rows matching the query, or fail if the
result set is too large,
with database.snapshot() as snapshot: QUERY = ( 'SELECT e.first_name, e.last_name, p.telephone ' 'FROM employees as e, phones as p ' 'WHERE p.employee_id == e.employee_id') result = snapshot.execute_sql(QUERY) for row in result: print(row)
NOTE: Perform all iteration within the context of the
Next, learn about Read-write Transactions.