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# development
w

witty-crayon-22786

09/25/2020, 6:25 PM
@hundreds-father-404: re: why
Specs
should probably not be a `SessionValue`: https://github.com/pantsbuild/pants/issues/6845 and https://github.com/pantsbuild/pants/blob/master/src/rust/engine/graph/README.md#inputs-and-identity attempt to explain the connection between identity and cacheability, but they don’t do a great job of it
if a node has a stable identity because it’s based on a small Param, re-requesting that exact node is quicker, because we can use cleaning and dirtying
but “large” Params (such as the OptionsBootstrapper) tend to be less stable… there are more ways that they can change
and the effect of using the OptionsBootstrapper (effectively “the whole env and args”) as a Param is that any change to that large object represents a brand new node, which cannot be cleaned
it’s a spectrum: what is “large” exactly? but the gut check on
Specs
is: if i ran
./pants test x
and then
./pants test y
, would i want those to be considered “the same run”?
no, probably not.
would i want
./pants test x -- -k some_test
to be the same run?
yes, probably. just with slightly different options.
and “same run” isn’t quite accurate: it’s really, “same node”. this is a decision made on a node-by-node basis all the way down the Graph:
as a Param, the
Specs
don’t actually go that deep in the graph: take a look at the visualization of the graph, and you can see which portion the
Specs
create the identity/memoization key for
ie, only nodes in the graph that indicate
for Specs
are actually dependent on the specs.