Add response timing middleware

[brettlangdon]

The first thing I did in my app with this client was to add before_request and after_request handlers to get response time metrics into datadog. Seems like it might be a decent/common functionality people would want?

The gist of it looks like this:

import time

from flask import Flask, g, request
from flask.ext.datadog import StatsD

app = Flask(__name__)
statsd = StatsD(app=app)

def before_request():
    g.request_start_time = time.time()

def after_request(response):
    # Return early if we don't have the start time
    if hasattr(g, 'request_start_time'):
        return response

    # Get elapsed time in milliseconds
    elapsed = time.time() - g.request_start_time
    elapsed = int(round(1000 * elapsed))

    # Collect request/response tags
    tags = [
        'endpoint:{endpoint}'.format(endpoint=request.endpoint),
        'request_method:{method}'.format(method=request.method.lower()),
        'status_code:{status_code}'.format(status_code=response.status_code),
    ]

    # Record our response time metric
    statsd.timing('flask.response.time', elapsed, tags=g.request_tags + tags)

    # Return the original unmodified response
    return response

app.before_request(before_request)
app.after_request(after_request)

This might be something we can do by default when running init_app, or maybe just a method setup_middleware(app) for handling it.

Does this seem like a reasonable thing to try and tackle, or should we leave this middleware stuff as an exercise for the implementer?

[volker48]

Its definitely common functionality as its pretty much the first thing we did as well. If its done by default it should be able to be disabled. I kind of like the setup_middleware approach. I can see different projects wanting different tags or wanting/needing to use different names for their tags so as long as its customizable it would be a good thing to have.

Feel free to tackle it.

[brettlangdon]

Yeah, maybe we can do something like:

def before_request():
    g.request_tags = []

def add_request_tags(tag):
    g.request_tags.append(tag)

def after_request():
    tags = [
        # Common request/response tags
    ] + g.request_tags

This way people can add custom tags in an endpoint or in other middleware.