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Add example request init listener
PYTHON-284
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‎cassandra/cluster.py‎

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@@ -2140,6 +2140,9 @@ def add_request_init_listener(self, fn, *args, **kwargs):
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synchronization if you have multiple threads. Any callbacks added to the response future will be executed
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on the event loop thread, so the normal advice about minimizing cycles and avoiding blocking apply (see Note in
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:meth:`.ResponseFuture.add_callbacks`.
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See `this example <https://github.com/datastax/python-driver/blob/master/examples/request_init_listener.py>`_ in the
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source tree for an example.
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"""
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self._request_init_callbacks.append((fn, args, kwargs))
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‎examples/README.rst‎

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Driver Examples
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===============
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This directory will contain a set of scripts demonstrating driver APIs or integration techniques. It will not be exhaustive, but will contain examples where they are too involved, or
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open-ended to include inline in the docstrings. In that case, they should be referenced from the docstrings
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Features
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--------
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* `request_init_listener.py <request_init_listener.py>`_ A script demonstrating how to register a session request listener and use it to track alternative metrics about requests (size, for example).

‎examples/request_init_listener.py‎

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#!/usr/bin/env python
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# Copyright 2013-2016 DataStax, Inc.
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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# This script shows an example "request init listener" which can be registered to track certain request metrics
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# for a session. In this case we're just accumulating total request and error counts, as well as some statistics
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# about the encoded request size. Note that the counts would be available using the internal 'metrics' tracking --
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# this is just demonstrating a way to track a few custom attributes.
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from __future__ import print_function
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from cassandra.cluster import Cluster
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from greplin import scales
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import pprint
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pp = pprint.PrettyPrinter(indent=2)
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class RequestAnalyzer(object):
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"""
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Class used to track request and error counts for a Session.
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Also computes statistics on encoded request size.
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"""
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requests = scales.PmfStat('request size')
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errors = scales.IntStat('errors')
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def __init__(self, session):
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scales.init(self, '/cassandra')
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# each instance will be registered with a session, and receive a callback for each request generated
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session.add_request_init_listener(self.on_request)
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def on_request(self, rf):
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# This callback is invoked each time a request is created, on the thread creating the request.
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# We can use this to count events, or add callbacks
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rf.add_callbacks(self.on_success, self.on_error, callback_args=(rf,), errback_args=(rf,))
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def on_success(self, _, response_future):
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# future callback on a successful request; just record the size
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self.requests.addValue(response_future.request_encoded_size)
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def on_error(self, _, response_future):
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# future callback for failed; record size and increment errors
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self.requests.addValue(response_future.request_encoded_size)
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self.errors += 1
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def __str__(self):
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# just extracting request count from the size stats (which are recorded on all requests)
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request_sizes = dict(self.requests)
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count = request_sizes.pop('count')
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return "%d requests (%d errors)\nRequest size statistics:\n%s" % (count, self.errors, pp.pformat(request_sizes))
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# connect a session
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session = Cluster().connect()
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# attach a listener to this session
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ra = RequestAnalyzer(session)
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session.execute("SELECT release_version FROM system.local")
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session.execute("SELECT release_version FROM system.local")
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print(ra)
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# 2 requests (0 errors)
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# Request size statistics:
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# { '75percentile': 74,
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# '95percentile': 74,
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# '98percentile': 74,
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# '999percentile': 74,
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# '99percentile': 74,
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# 'max': 74,
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# 'mean': 74.0,
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# 'median': 74.0,
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# 'min': 74,
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# 'stddev': 0.0}
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try:
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# intentional error to show that count increase
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session.execute("syntax err")
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except Exception as e:
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pass
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print()
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print(ra) # note: the counts are updated, but the stats are not because scales only updates every 20s
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# 3 requests (1 errors)
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# Request size statistics:
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# { '75percentile': 74,
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# '95percentile': 74,
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# '98percentile': 74,
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# '999percentile': 74,
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# '99percentile': 74,
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# 'max': 74,
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# 'mean': 74.0,
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# 'median': 74.0,
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# 'min': 74,
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# 'stddev': 0.0}

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