Hit Calculation Tutorial for Xignite Web Services
First Published Monday, 28th March 2011 06:59 am from Xand : Joel York
The opinions expressed by this blogger and those providing comments are theirs alone, this does not reflect the opinion of Automated Trader or any employee thereof. Automated Trader is not responsible for the accuracy of any of the information supplied by this article.
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One of the huge benefits of cloud computing is low risk
pay-as-you-go, usage-based pricing. Unfortunately, if you are
counting usage by anything other than users, then choosing the
best subscription plan for your needs can get a little
tricky.
At Xignite, we use a uniform usage
metric known as "hits" across all of our Web services to simplify
things. Estimating the number of hits you need for your
application is straightforward, once you get the basic idea.
However, for the first time customer it can be a little cryptic.
So, we're providing this short tutorial and some href="http://xignite.web-services-blog.com/media/xignite-hit-estimation-worksheet.xlsx"
target="_blank">handy tools to help you through the
hit calculation process. The short story is that if you can
convert inches to miles, ounces to liters, and Fahrenheit to
Celsius, then you'll have no trouble understanding how to
estimate your hits, because the math required is essentially the
same.
Before slugging through the general
approach for calculating hits, let's do a simple example to get
familiar with the concepts involved. Suppose you have a website
stock index widget that shows five common stock indices. And, the
pages that have the widget receive 10,000 pageviews per day. If
you use the href="http://www.xignite.com/xindices.asmx?op=GetDelayedValues"
target="_blank">GetDelayedValues operation of the
target="_blank">XigniteIndices Web service, then
each request to create the widget will cost 5 hits, one for each
index. Then your total monthly hits are calculated as
follows:
1.5 million hits/month =
5 hits/widget x 1 widget/pageview x 10,000 pageviews/day x 30
days/month
Not so hard. In particular, notice
how the units attached to each factor cancel out to the proper
units of the final answer (hits/month).
1.5
million hits/month =
5
hits/widget
x 1 widget/pageview
x 10,000
pageviews/day x
30
days/month
Checking the units is always a good QA method to test
the accuracy of your calculation. Just like converting inches to
miles!
The post that follows generalizes this
simple formula to provide a general method for calculating hits
for any application using any Xignite Web service.
src="http://xignite.web-services-blog.com/media/xignite-web-services-usage-hits.png"
alt="Xignite Web service hit calculation model" width="423px"
height="309px" />
Each data block
processed by a Web service operation entails a specific number of
hits. Total hits are determined by the total number of data
blocks processed in a given time window. Each time a trigger
event occurs (request frequency), the Web service operation
processes a certain number of data blocks (request population).
In most cases, there is a fixed ratio between input data blocks
and output data blocks and either can be used for hit
calculation.
Relationship between
Requests and Hits
Each time you call an
Xignite Web service operation, you rack up some hits. The number
of hits depends on the data returned by the service, so you only
pay for what you get. If you request a
stock quote for a single symbol using the href="http://www.xignite.com/xquotes.asmx?op=GetQuotes"
target="_blank">GetQuotes operation of the href="http://www.xignite.com/xquotes.asmx"
target="_blank">XigniteQuotes Web service, then a
successful return of the quote requested will cost exactly one
hit. If you request ten stock quotes using the GetQuotes
operation, then a successful return of the ten quotes will cost
ten hits. If only nine quotes are available, you are only charged
nine hits for the successfully returned quotes.
For every implementation, there is a "request data
block" that determines the number of hits for a single request to
an operation. In the examples above, the request data blocks are
single symbols and ten symbol sets with a hit cost per request of
one hit and ten hits, respectively. As in these two examples, the
request data block is the input data set for the operation in the
vast majority of cases.
However, there are
situations where the output data set must be used as the request
data block. In the simplest case, the operation has no inputs and
the request data block is the fixed output data set from the
operation, as with the href="http://www.xignite.com/xquotes.asmx?op=GetTopMovers"
target="_blank">GetTopMovers operation of
XigniteQuotes. For some of the more complex operations, such as
the href="http://www.xignite.com/xoptions.asmx?op=GetEquityOptionChain"
target="_blank">GetEquityOptionChain operation of
target="_blank">XigniteOptions Web service, the hit
count is determined by the number of option contracts returned by
the operation, which is not a fixed number and cannot be
determined by the input data set. In this case, you must use the
output data set. In addition, since the number of contracts in
the output data set changes with each request, you must use the
average number of contracts in the output, e.g., the request data
block is an average of five contracts returned with an average
hit cost of five hits based on the particular set of symbols that
will be requested over time.
Request
Frequency and Request Population
All Xignite
Subscription plans are quoted in maximum hits per month. So, once
you've identified the request data block and its associated hits,
the next step is to estimate the number of times an operation
will handle a request of this data block in a given month.
Multiplying the number of requests per month by the hits for each
data block gives the total number of monthly hits.
Monthly Hits = Hits per Request Data Block x Monthly
Requests
The total number of monthly requests
can be broken down into two components: request frequency and
request population. Request frequency is the speed at which
requests are cycled and is usually determined by some external
trigger event that precipitates the request on a regular basis,
like a pageview or cache refresh. Request population is the
number of data blocks processed in each request cycle.
Monthly Requests = Request Population x Request
Frequency x 1 month
For example, a real-time
quote caching process that runs every 5 seconds has a request
frequency of 12 cache refreshes / minute. If this cache process
handles 2,000 of the most popular stocks in blocks of 50 symbols,
it will have a request population of 40 50 symbol blocks per
cache refresh. Thereby, it will process 480 cache requests per
minute or about 21M requests per month. Note that the unit for
the request frequency is trigger events per unit time (cache
refreshes per minute), whereas the unit for the request
population is data blocks / trigger event (50 symbol blocks per
cache refresh).
Substituting the Monthly
Requests formula into our formula for Monthly Hits gives the
following general formula:
Monthly Hits
=
Hits per Data Block x Request Population x Request
Frequency x 1 month
Which, upon closer
inspection mirrors the formula for the the simple Web widget
example given at the beginning of this post. The request data
block is a 5 indices set or a "widget" with a
cost of 5 hits per widget. The request population is 1 widget per
pageview. The request frequency is 10,000 pageviews per day. And,
there are 30 days in 1 month.
That's
all there is to it! To make life a little easier, we've also
created a handy href="http://xignite.web-services-blog.com/media/xignite-hit-estimation-worksheet.xlsx"
target="_blank">Xignite hit estimation Excel
worksheet based on this simple model. In the next few
posts in this series, we'll use the spreadsheet to provide
complete end-to-end hit calculation for common applications, such
as stock portfolios, currency converters, EOD updates to
historical databases, real-time price caching, etc. In the
meantime, you can always get help from your account manager at
sales@xigntie.com.
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