Read-only Transactions via Snapshots

A Snapshot represents a read-only transaction: when multiple read operations are peformed 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:

snapshot = database.snapshot() 

You can also specify a weaker bound, which can either be to perform all reads as of a given timestamp:

import datetime from pytz import UTC TIMESTAMP = datetime.datetime.utcnow().replace(tzinfo=UTC) snapshot = database.snapshot(read_timestamp=TIMESTAMP) 

or as of a given duration in the past:

import datetime DURATION = datetime.timedelta(seconds=5) snapshot = database.snapshot(exact_staleness=DURATION) 

Single Use and Multiple Use Snapshots

In the context of read only transactions, read and execute_sql methods can be used multiple times if you specify multi_use=True in the constructor of the snapshot. However, multi_use=True is incompatible with either max_staleness and/or min_read_timestamp.

Otherwise multi_use defaults to False and the snapshot cannot be reused.

snapshot = database.snapshot(multi_use=True) 

begin() can only be used on a snapshot with multi_use=True. In which case it is also necessary to call if you need to have multiple pending operations.

Read Table Data

Read data for selected rows from a table in the database. Calls the Read API, which returns all rows specified in key_set, or else fails if the result set is too large,

with database.snapshot() as snapshot: result = snapshot.read( table='table-name', columns=['first_name', 'last_name', 'age'], key_set=['phred@example.com', 'bharney@example.com']) for row in result.rows: print(row) 

NOTE: Perform all iteration within the context of the with database.snapshot() block.

Execute a SQL Select Statement

Read data from a query against tables in the database. Calls the ExecuteSql API, which returns all rows matching the query, or else fails 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 list(result): print(row) 

NOTE: Perform all iteration within the context of the with database.snapshot() block.

Next Step

Next, learn about Read-write Transactions.