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Subject: availability and accessibility


Dear all
I was asked to express my opinion "about ‘availability’ and 
‘accessibility’ in the review document of WSQM & FWSI joint meeting" as 
action item #0012
Please find below my thoughts on this issues and on availability and 
accessibility in general.

1. Availability

1.1 First, availability is about the presence and ready for immediate 
use of a service. When we talk about the "presence" of a service two 
aspects might be considered: location and time. Therefor we have two 
kinds of availability: in time (e.g. the service is available 24 hours 
per day) and in space (e.g. the service is available at this location). 
What we capture in our definition is just the temporal aspect.

1.2 In section 5.2.2. in WSQM document, the Availability is defined once 
as "the average Up Time". However the formula dose not contain any
average function. I think this part from the definition "average Up 
Time" should be removed.

1.3 Our formula is right but I think the terminology we use is maybe not 
the best one. I propose to use "measurement_time" instead of "unit time" .

Coming back to the differences between WSQM and FWSI availability 
formulas, I dont understand why in FWSI formula they relate to the 
number of invocations. A closer look at the availability and 
accessibility (here I consider the slides available at: 
http://www.oasis-open.org/apps/org/workgroup/wsqm/download.php/17560/QoS-WSQM-Analysis-03.ppt) 
in FWSI shows that these two are basically the same: (1) Availability = 
Number of successful invocations/Total invocations and (2) Accessibility 
= Successful invocations/Total invocations.

I think that WSQM describes better temporal availability in a given 
period of time. However if we think about a set of periods of time then 
we need some average functions on availabilities to measure the overall 
availability.

2. Accessibility as it is defined in WSQM document is fine.
The formulas for accessibility in WSQM and FWSI in my opinion are the 
same. FWSI just impose the restriction that invocations are fired 
simultaneously and therefor the answer to "•Can we deduce that the FWSI 
formula is a sub-set of WSQM formula?" question is "yes". WSQM formula 
dose not impose such a restriction and thus applies also when 
invocations are fired simultaneous or not.

I hope it helps a bit.
Best regards,
Ioan

-- 
Ioan Toma
Digital Enterprise Research Institute (DERI)
http://www.deri.org

Tel:   +43 512 5076461
Email: ioan.toma@deri.org 



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