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What are 'Concurrent Users'?
Why do you care?
How do you correlate
the business perspective of ‘Numbers of Visitors’ to the IT focused
requirements of ‘concurrent users’, ‘transactions per second’ or
‘volumes of data’?
Marketers and
company executives gauge the popularity of their e-businesses in terms
of ‘initial interest’, ‘unique visitors’, ‘annual footfall’
and ‘registered users’. Engineers work with technical data:
‘numbers of sessions’, ‘concurrent users’, ‘concurrent
connections’, ‘requests per second’ and ‘numbers of
sockets’.
Misinterpretation of
the requirements by the engineers, or of the expected capacity by the
business, can lead to incorrectly sizing the system. The results of
wrong sizing can be both very public and very expensive. To prevent such
misunderstandings the relationships between the business requirements
and the technical measurements must be analysed and understood.
Often the first areas
of confusion is an inconsistent use of terms:
| Hits |
The download
of a single file is a single hit. The download of a Web page
including two images will be 3 hits; 1 hit for the web page, and
2 additional hits for the image files. Total hits is an
indication of visitors, page views and data, it is not a
precise measurement of these.
|
| Page |
Web pages.
All documents, static & dynamic pages and forms.
|
| Visits |
The number
of user sessions. Each user session represents an individual
viewing the site. Visitors that are idle for longer than preset
limits are assumed to have ended their session. Visit counts may
include automated sessions such as search spiders and monitors.
|
| Unique
Visitor |
An
individual who visits the site more than once are counted only
the first time they visit (within a particular time period)
|
| Annual
footfall |
Total number
of visits per year.
|
| Registered
user |
The number
of visitors that have completed a registration procedure. May
include pre-registered visitors.
|
| Logged
on users |
Number of
visitors who have completed a logging in procedure. May be an
instantaneous measurement or cumulative.
|
| Concurrent
users |
Number of
visitors active on the site at a particular instant in time. A
dynamic measurement, often expressed as a peak or average.
|
| Transactions
per second |
Number of
events or actions being processed by the web site system within
1 second. A dynamic measurement, often expressed as a peak or
average.
|
| Bandwidth |
A dynamic
measurement of the amount of information that flows to and from
a web site system. The requests made on the system are called
inbound, the information sent by the system as outbound. Usually
measured as a peak or average. Also used as an estimation of the
capability of a network and it’s components. Expressed in bits
per second (bps) or Bytes per second (Bps).
|
| Data
volume |
A static and
cumulative measurement of bandwidth usage over time. Expressed
in Megabytes or Gigabytes. The size of a database may also be
expressed in data volume.
|
| Sockets |
Each
visitor creates socket connections to the web site server.
Web browser IE creates 2 sockets per session window and Netscape
4. The number of open sockets is a dynamic measurement of web
site server usage.
|
| Concurrent
connections |
Usually
refers to connections made to a database. Due to connection
pooling may not correlate directly to concurrent sessions. A
dynamic measurement.
|
| Requests
per second |
A dynamic
measurement of number of requests made to a web server within 1
second. Each ‘hit’ will make a request.
|
Further articles in this series will focus on the
relationship between the business and technical perspectives of web
application performance requirements & measurement.
Oct 2002
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