celery/celery/backends/cassandra.py

# -* coding: utf-8 -*-
"""Apache Cassandra result store backend using the DataStax driver."""
from celery import states
from celery.exceptions import ImproperlyConfigured
from celery.utils.log import get_logger

from .base import BaseBackend

try:  # pragma: no cover
    import cassandra
    import cassandra.auth
    import cassandra.cluster
except ImportError:  # pragma: no cover
    cassandra = None   # noqa

__all__ = ['CassandraBackend']

logger = get_logger(__name__)

E_NO_CASSANDRA = """
You need to install the cassandra-driver library to
use the Cassandra backend. See https://github.com/datastax/python-driver
"""

E_NO_SUCH_CASSANDRA_AUTH_PROVIDER = """
CASSANDRA_AUTH_PROVIDER you provided is not a valid auth_provider class.
See https://datastax.github.io/python-driver/api/cassandra/auth.html.
"""

Q_INSERT_RESULT = """
INSERT INTO {table} (
    task_id, status, result, date_done, traceback, children) VALUES (
        %s, %s, %s, %s, %s, %s) {expires};
"""

Q_SELECT_RESULT = """
SELECT status, result, date_done, traceback, children
FROM {table}
WHERE task_id=%s
LIMIT 1
"""

Q_CREATE_RESULT_TABLE = """
CREATE TABLE {table} (
    task_id text,
    status text,
    result blob,
    date_done timestamp,
    traceback blob,
    children blob,
    PRIMARY KEY ((task_id), date_done)
) WITH CLUSTERING ORDER BY (date_done DESC);
"""

Q_EXPIRES = """
    USING TTL {0}
"""


def buf_t(x):
    return bytes(x, 'utf8')


class CassandraBackend(BaseBackend):
    """Cassandra backend utilizing DataStax driver

    Raises:
        celery.exceptions.ImproperlyConfigured:
            if module :pypi:`cassandra-driver` is not available,
            or if the :setting:`cassandra_servers` setting is not set.
    """

    #: List of Cassandra servers with format: ``hostname``.
    servers = None

    supports_autoexpire = True      # autoexpire supported via entry_ttl

    def __init__(self, servers=None, keyspace=None, table=None, entry_ttl=None,
                 port=9042, **kwargs):
        super().__init__(**kwargs)

        if not cassandra:
            raise ImproperlyConfigured(E_NO_CASSANDRA)

        conf = self.app.conf
        self.servers = servers or conf.get('cassandra_servers', None)
        self.port = port or conf.get('cassandra_port', None)
        self.keyspace = keyspace or conf.get('cassandra_keyspace', None)
        self.table = table or conf.get('cassandra_table', None)

        if not self.servers or not self.keyspace or not self.table:
            raise ImproperlyConfigured('Cassandra backend not configured.')

        expires = entry_ttl or conf.get('cassandra_entry_ttl', None)

        self.cqlexpires = (
            Q_EXPIRES.format(expires) if expires is not None else '')

        read_cons = conf.get('cassandra_read_consistency') or 'LOCAL_QUORUM'
        write_cons = conf.get('cassandra_write_consistency') or 'LOCAL_QUORUM'

        self.read_consistency = getattr(
            cassandra.ConsistencyLevel, read_cons,
            cassandra.ConsistencyLevel.LOCAL_QUORUM)
        self.write_consistency = getattr(
            cassandra.ConsistencyLevel, write_cons,
            cassandra.ConsistencyLevel.LOCAL_QUORUM)

        self.auth_provider = None
        auth_provider = conf.get('cassandra_auth_provider', None)
        auth_kwargs = conf.get('cassandra_auth_kwargs', None)
        if auth_provider and auth_kwargs:
            auth_provider_class = getattr(cassandra.auth, auth_provider, None)
            if not auth_provider_class:
                raise ImproperlyConfigured(E_NO_SUCH_CASSANDRA_AUTH_PROVIDER)
            self.auth_provider = auth_provider_class(**auth_kwargs)

        self._connection = None
        self._session = None
        self._write_stmt = None
        self._read_stmt = None
        self._make_stmt = None

    def process_cleanup(self):
        if self._connection is not None:
            self._connection.shutdown()  # also shuts down _session
        self._connection = None
        self._session = None

    def _get_connection(self, write=False):
        """Prepare the connection for action

        Arguments:
            write (bool): are we a writer?
        """
        if self._connection is not None:
            return
        try:
            self._connection = cassandra.cluster.Cluster(
                self.servers, port=self.port,
                auth_provider=self.auth_provider)
            self._session = self._connection.connect(self.keyspace)

            # We are forced to do concatenation below, as formatting would
            # blow up on superficial %s that will be processed by Cassandra
            self._write_stmt = cassandra.query.SimpleStatement(
                Q_INSERT_RESULT.format(
                    table=self.table, expires=self.cqlexpires),
            )
            self._write_stmt.consistency_level = self.write_consistency

            self._read_stmt = cassandra.query.SimpleStatement(
                Q_SELECT_RESULT.format(table=self.table),
            )
            self._read_stmt.consistency_level = self.read_consistency

            if write:
                # Only possible writers "workers" are allowed to issue
                # CREATE TABLE. This is to prevent conflicting situations
                # where both task-creator and task-executor would issue it
                # at the same time.

                # Anyway; if you're doing anything critical, you should
                # have created this table in advance, in which case
                # this query will be a no-op (AlreadyExists)
                self._make_stmt = cassandra.query.SimpleStatement(
                    Q_CREATE_RESULT_TABLE.format(table=self.table),
                )
                self._make_stmt.consistency_level = self.write_consistency

                try:
                    self._session.execute(self._make_stmt)
                except cassandra.AlreadyExists:
                    pass

        except cassandra.OperationTimedOut:
            # a heavily loaded or gone Cassandra cluster failed to respond.
            # leave this class in a consistent state
            if self._connection is not None:
                self._connection.shutdown()     # also shuts down _session

            self._connection = None
            self._session = None
            raise   # we did fail after all - reraise

    def _store_result(self, task_id, result, state,
                      traceback=None, request=None, **kwargs):
        """Store return value and state of an executed task."""
        self._get_connection(write=True)

        self._session.execute(self._write_stmt, (
            task_id,
            state,
            buf_t(self.encode(result)),
            self.app.now(),
            buf_t(self.encode(traceback)),
            buf_t(self.encode(self.current_task_children(request)))
        ))

    def as_uri(self, include_password=True):
        return 'cassandra://'

    def _get_task_meta_for(self, task_id):
        """Get task meta-data for a task by id."""
        self._get_connection()

        res = self._session.execute(self._read_stmt, (task_id, ))
        if not res:
            return {'status': states.PENDING, 'result': None}

        status, result, date_done, traceback, children = res[0]

        return self.meta_from_decoded({
            'task_id': task_id,
            'status': status,
            'result': self.decode(result),
            'date_done': date_done.strftime('%Y-%m-%dT%H:%M:%SZ'),
            'traceback': self.decode(traceback),
            'children': self.decode(children),
        })

    def __reduce__(self, args=(), kwargs={}):
        kwargs.update(
            dict(servers=self.servers,
                 keyspace=self.keyspace,
                 table=self.table))
        return super().__reduce__(args, kwargs)