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77 lines
3.0 KiB
Markdown
77 lines
3.0 KiB
Markdown
nodeattribexpressions(5) -- Confluent attribute expression syntax
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=================================================================
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## DESCRIPTION
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In confluent, any attribute may either be a straightforward value, or an
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expression to generate the value.
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An expression will contain some directives wrapped in `{}` characters. Within
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`{}` are a number of potential substitute values and operations.
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Note that syntax of expressions can have overlap with the shell syntax.
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For example:
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`$ echo (n2)`
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`-bash: syntax error near unexpected token `n2'`
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In such a case, it helps to quote the expression to allow it to be passed:
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`$ echo '(n2)'`
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`(n2)`
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The most common operation is to extract a number from the nodename. These
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values are available as n1, n2, etc. So for example attributes for a node named
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b1o2r3u4 would have {n1} as 1, {n2} as 2, {n3} as 3, and {n4} as 4.
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Additionally, {n0} is special as representing the last number in a name, so in
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the b1o2r3u4 example, {n0} would be 4.
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Frequently a value derives from a number in the node name, but must undergo a
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transform to be useful. As an example, if we have a scheme where nodes are
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numbered n1-n512, and they are arranged 1-42 in rack1, 43-84 in rack2, and so
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forth, it is convenient to perform arithmetic on the extracted number. Here is
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an example of codifying the above scheme, and setting the u to the remainder:
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`location.rack=rack{(n1-1)/42+1}`
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`location.u={(n1-1)%42+1}`
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Note how text may be mixed into expressions, only data within {} will receive
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special treatment. Here we also had to adjust by subtracting 1 and adding it
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back to make the math work as expected.
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It is sometimes the case that the number must be formatted a different way,
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either specifying 0 padding or converting to hexadecimal. This can be done by a
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number of operators at the end to indicate formatting changes.
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`{n1:02x} - Zero pad to two decimal places, and convert to hexadecimal, as mightbe used for generating MAC addresses`
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`{n1:x} - Hexadecimal without padding, as may be used in a generated IPv6 address`
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`{n1:X} - Uppercase hexadecimal`
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`{n1:02d} - Zero pad a normal numeric representation of the number.`
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Another common element to pull into an expression is the node name in whole:
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`hardwaremanagement.manager={node}-imm`
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Additionally other attributes may be pulled in:
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`hardwaremanagement.switchport={location.u}`
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Multiple expressions are permissible within a single attribute:
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`hardwaremanagement.manager={node}-{hardwaremanagement.method}`
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A note to developers: in general the API layer will automatically recognize a
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generic set attribute to string with expression syntax and import it as an
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expression. For example, submitting the following JSON:
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`{ 'location.rack': '{n1}' }`
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Will auto-detect {n1} as an expression and assign it normally. If wanting to
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set that value verbatim, it can either be escaped by doubling the {} or by
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explicitly declaring it as a value:
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`{ 'location.rack': '{{n1}}' }`
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`{ 'location.rack': { 'value': '{n1}' } }`
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