# Cockroach Database

CockroachDB (CRDB) is well supported by peewee.

    from playhouse.cockroachdb import CockroachDatabase

    db = CockroachDatabase('my_app', user='root', host='10.1.0.8')

If you are using Cockroach Cloud, you may find it easier to specify the connection parameters using a connection-string:

    db = CockroachDatabase('postgresql://root:secret@host:26257/defaultdb...')

Note

CockroachDB requires the `psycopg2` (postgres) Python driver.

The `playhouse.cockroachdb` extension module provides the following classes and helpers:

- `CockroachDatabase` - a subclass of `PostgresqlDatabase`, designed specifically for working with CRDB.

- `PooledCockroachDatabase` - like the above, but implements connection-pooling.

- `run_transaction()` - runs a function inside a transaction and provides automatic client-side retry logic.

Special field-types that may be useful when using CRDB:

- `UUIDKeyField` - a primary-key field implementation that uses CRDB’s `UUID` type with a default randomly-generated UUID.

- `RowIDField` - a primary-key field implementation that uses CRDB’s `INT` type with a default `unique_rowid()`.

- `JSONField` - same as the Postgres `BinaryJSONField`, as CRDB treats JSON as JSONB.

- `ArrayField` - same as the Postgres extension (but does not support multi-dimensional arrays).

CRDB is compatible with Postgres’ wire protocol and exposes a very similar SQL interface, so it is possible (though not recommended) to use `PostgresqlDatabase` with CRDB:

1. CRDB does not support nested transactions (savepoints), so the `atomic()` method has been implemented to enforce this when using `CockroachDatabase`. For more info CRDB Transactions.

2. CRDB may have subtle differences in field-types, date functions and introspection from Postgres.

3. CRDB-specific features are exposed by the `CockroachDatabase`, such as specifying a transaction priority or the `AS OF SYSTEM TIME` clause.

## CRDB Transactions

CRDB does not support nested transactions (savepoints), so the `atomic()` method on the `CockroachDatabase` has been modified to raise an exception if an invalid nesting is encountered. If you would like to be able to nest transactional code, you can use the `transaction()` method, which will ensure that the outer-most block will manage the transaction (e.g., exiting a nested-block will not cause an early commit).

Example:

    @db.transaction()
    def create_user(username):
        return User.create(username=username)

    def some_other_function():
        with db.transaction() as txn:
            # do some stuff...

            # This function is wrapped in a transaction, but the nested
            # transaction will be ignored and folded into the outer
            # transaction, as we are already in a wrapped-block (via the
            # context manager).
            create_user('some_user@example.com')

            # do other stuff.

        # At this point we have exited the outer-most block and the transaction
        # will be committed.
        return

CRDB provides client-side transaction retries, which are available using a special `run_transaction()` helper. This helper method accepts a callable, which is responsible for executing any transactional statements that may need to be retried.

Simplest possible example of `run_transaction()`:

    def create_user(email):
        # Callable that accepts a single argument (the database instance) and
        # which is responsible for executing the transactional SQL.
        def callback(db_ref):
            return User.create(email=email)

        return db.run_transaction(callback, max_attempts=10)

    huey = create_user('huey@example.com')

Note

The `cockroachdb.ExceededMaxAttempts` exception will be raised if the transaction cannot be committed after the given number of attempts. If the SQL is mal-formed, violates a constraint, etc., then the function will raise the exception to the caller.

Example of using `run_transaction()` to implement client-side retries for a transaction that transfers an amount from one account to another:

    from playhouse.cockroachdb import CockroachDatabase

    db = CockroachDatabase('my_app')

    def transfer_funds(from_id, to_id, amt):
        """
        Returns a 3-tuple of (success?, from balance, to balance). If there are
        not sufficient funds, then the original balances are returned.
        """
        def thunk(db_ref):
            src, dest = (Account
                         .select()
                         .where(Account.id.in_([from_id, to_id])))
            if src.id != from_id:
                src, dest = dest, src  # Swap order.

            # Cannot perform transfer, insufficient funds!
            if src.balance < amt:
                return False, src.balance, dest.balance

            # Update each account, returning the new balance.
            src, = (Account
                    .update(balance=Account.balance - amt)
                    .where(Account.id == from_id)
                    .returning(Account.balance)
                    .execute())
            dest, = (Account
                     .update(balance=Account.balance + amt)
                     .where(Account.id == to_id)
                     .returning(Account.balance)
                     .execute())
            return True, src.balance, dest.balance

        # Perform the queries that comprise a logical transaction. In the
        # event the transaction fails due to contention, it will be auto-
        # matically retried (up to 10 times).
        return db.run_transaction(thunk, max_attempts=10)

## CRDB APIs

class CockroachDatabase(database[, **kwargs])

  CockroachDB implementation, based on the `PostgresqlDatabase` and using the `psycopg2` driver.

  Additional keyword arguments are passed to the psycopg2 connection constructor, and may be used to specify the database `user`, `port`, etc.

  Alternatively, the connection details can be specified in URL-form.

  run_transaction(callback[, max_attempts=None[, system_time=None[, priority=None]]])
    Parameters

      - callback – callable that accepts a single `db` parameter (which will be the database instance this method is called from).

      - max_attempts (int) – max number of times to try before giving up.

      - system_time (datetime) – execute the transaction `AS OF SYSTEM TIME` with respect to the given value.

      - priority (str) – either “low”, “normal” or “high”.

    Returns

      returns the value returned by the callback.

    Raises

      `ExceededMaxAttempts` if `max_attempts` is exceeded.

    Run SQL in a transaction with automatic client-side retries.

    User-provided `callback`:

    - Must accept one parameter, the `db` instance representing the connection the transaction is running under.

    - Must not attempt to commit, rollback or otherwise manage the transaction.

    - May be called more than one time.

    - Should ideally only contain SQL operations.

    Additionally, the database must not have any open transactions at the time this function is called, as CRDB does not support nested transactions. Attempting to do so will raise a `NotImplementedError`.

    Simplest possible example:

        def create_user(email):
            def callback(db_ref):
                return User.create(email=email)

            return db.run_transaction(callback, max_attempts=10)

        user = create_user('huey@example.com')

class PooledCockroachDatabase(database[, **kwargs])

  CockroachDB connection-pooling implementation, based on `PooledPostgresqlDatabase`. Implements the same APIs as `CockroachDatabase`, but will do client-side connection pooling.

run_transaction(db, callback[, max_attempts=None[, system_time=None[, priority=None]]])

  Run SQL in a transaction with automatic client-side retries. See `CockroachDatabase.run_transaction()` for details.

  Parameters

    - db (CockroachDatabase) – database instance.

    - callback – callable that accepts a single `db` parameter (which will be the same as the value passed above).

  Note

  This function is equivalent to the identically-named method on the `CockroachDatabase` class.

class UUIDKeyField

  UUID primary-key field that uses the CRDB `gen_random_uuid()` function to automatically populate the initial value.

class RowIDField

  Auto-incrementing integer primary-key field that uses the CRDB `unique_rowid()` function to automatically populate the initial value.

See also:

- `BinaryJSONField` from the Postgresql extension (available in the `cockroachdb` extension module, and aliased to `JSONField`).

- `ArrayField` from the Postgresql extension.