GRAYLOG - Chapter 3, Exercise 2 Solution
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Warning-or-Worse for ws-lite, Using Explicit OR-Grouping

PROBLEM
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Write a query, using this chapter's own real syntax, that matches every
message at warning level or more severe (levels 2, 3, and 4) for the
ws-lite service - using the explicit OR-grouping form, not a range
comparison.

SOLUTION
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This chapter's own OR-grouping syntax for level is level:(X OR Y OR ...).
The three levels requested - 2 (Critical), 3 (Error), and 4 (Warning) -
combine as:

    level:(2 OR 3 OR 4)

Scoping to the ws-lite service as well, using AND (per Chapter 1's own
syntax):

    service:ws-lite AND level:(2 OR 3 OR 4)

ANSWER: service:ws-lite AND level:(2 OR 3 OR 4)

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WHY THIS WORKS AS AN ANSWER

This is a direct extension of this chapter's own real level:(2 OR 3)
example to a third value, combined with Chapter 2's own service field.
The explicit OR-grouping form was specifically requested instead of a
range comparison (level:<=4) because, as this chapter's own warn-box
demonstrated, range comparisons on level are genuinely easy to get
subtly wrong at the edges - listing every wanted value explicitly
removes any ambiguity about which levels are actually included.