Chapter 23. Issues with Report Merging
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## Chapter 23. Issues with Report Merging

In some cases, a merged report doesn't display the right information. We outline some worst case scenarios, and justify our implementation.

Suppose log file 1 (“requests” with “sizes”) looks like:

request | size
--- | ---
A | 12
B | 11
C | 10

while log file 2 looks like:

request | size
--- | ---
D | 3
E | 2
F | 1

We report on the top 2 biggest requests, so the report from log 1 looks like:

request | size
--- | ---
A | 12
B | 11

while the report from log 2 would look like:

request | size
--- | ---
D | 3
E | 2

Now we change the superservice.cfg file to list the top-4 biggest items. A naive merge would lead to:

request | size
--- | ---
A | 12
B | 11
D | 3
E | 2

Of course, this should've been:

request | size
--- | ---
A | 12
B | 11
C | 10
D | 3

This effect does not occur when keeping the top-limit to the same value. However, when we're not reporting on distinct values in the log, but are summing, more horrible things might happen. Consider this: We want to report on the total size by client. Logs look like:

client | size
--- | ---
a | 12
b | 11
c | 10

and

client | size
--- | ---
d | 4
e | 4
c | 3

Reports from these logs would look like:

client | size
--- | ---
a | 12
b | 11

client | size
--- | ---
d | 4
e | 4

After naively merging, one would get:

client | size
--- | ---
a | 12
b | 11

In fact, the complete report should look like:

client | size
--- | ---
c | 13
a | 12

Luckily, the Lire merging algorithm is not this naive: in fact, the XML reports store a little more records than actually needed. This heuristic trick leads to sane merged reports in most cases. However, since this is merely a heuristic trick, it is no waterproof guarantee.

See the description of the guess_extra_entries routine in the Lire::Group manpage for more implementation details.

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