# nodeattribexpressions(5) - Confluent attribute expression syntax

## DESCRIPTION

In confluent, any attribute may either be a straightforward value, or an expression to generate the value.

An expression will contain some directives wrapped in `{}` characters. Within `{}` are a number of potential substitute values and operations.

Note that syntax of expressions can have overlap with the shell syntax. For example:

`$ echo (n2)`  
`-bash: syntax error near unexpected token`n2'\`

In such a case, it helps to quote the expression to allow it to be passed:

`$ echo '(n2)'`  
`(n2)`

The most common operation is to extract a number from the nodename. These values are available as n1, n2, etc. So for example attributes for a node named b1o2r3u4 would have {n1} as 1, {n2} as 2, {n3} as 3, and {n4} as 4. Additionally, {n0} is special as representing the last number in a name, so in the b1o2r3u4 example, {n0} would be 4.

Frequently a value derives from a number in the node name, but must undergo a transform to be useful. As an example, if we have a scheme where nodes are numbered n1-n512, and they are arranged 1-42 in rack1, 43-84 in rack2, and so forth, it is convenient to perform arithmetic on the extracted number. Here is an example of codifying the above scheme, and setting the u to the remainder:

`location.rack=rack{(n1-1)/42+1}`  
`location.u={(n1-1)%42+1}`

Note how text may be mixed into expressions, only data within {} will receive special treatment. Here we also had to adjust by subtracting 1 and adding it back to make the math work as expected.

It is sometimes the case that the number must be formatted a different way, either specifying 0 padding or converting to hexadecimal. This can be done by a number of operators at the end to indicate formatting changes.

`{n1:02x} - Zero pad to two decimal places, and convert to hexadecimal, as might be used for generating MAC addresses` `{n1:x} - Hexadecimal without padding, as may be used in a generated IPv6 address`  
`{n1:X} - Uppercase hexadecimal`  
`{n1:02d} - Zero pad a normal numeric representation of the number.`

Another common element to pull into an expression is the node name in whole:

`hardwaremanagement.manager={node}-imm`

Additionally other attributes may be pulled in:

`hardwaremanagement.switchport={location.u}`

Multiple expressions are permissible within a single attribute:

`hardwaremanagement.manager={node}-{hardwaremanagement.method}`

A note to developers: in general the API layer will automatically recognize a generic set attribute to string with expression syntax and import it as an expression. For example, submitting the following JSON:

`{ 'location.rack': '{n1}' }`

Will auto-detect {n1} as an expression and assign it normally. If wanting to set that value verbatim, it can either be escaped by doubling the {} or by explicitly declaring it as a value:

`{ 'location.rack': '{{n1}}' }`

`{ 'location.rack': { 'value': '{n1}' } }`