amend service-target-cache.md

Remove wrong (un-needed) design specifications. When we compute the
TargetCacheKey of an export target, there is no need to check if any
of (recursively) referenced trees is absent. As soon as we deal with
an "absent target" we query just serve. If it is not able to provide
the target cache value, we error out.

1 files changed, 4 insertions, 11 deletions

diff --git a/doc/future-designs/service-target-cache.md b/doc/future-designs/service-target-cache.md
index 139fc00a..8dafcea2 100644
--- a/doc/future-designs/service-target-cache.md
+++ b/doc/future-designs/service-target-cache.md
@@ -223,17 +223,10 @@ If the _target root_ is marked as absent:
    serve` cannot answer, we break the analysis and inform the user with a proper
    error message.
 
-Once we know the flexible configuration variables, during the computation of the
-`TargetCacheKey`, we need to verify if any of the (recursively) referenced trees
-is absent (`ComputeTargetCacheKey`) -- this is checked while building the
-repository graph. Once the target cache key is computed, we first verify if we
-have it locally. If not and if at least one tree is absent:
- - query `just serve` to get the target cache value for the given key. Note that
-   even if `just serve` cannot provide the target cache value, we can continue
-   the analysis in the hope that the absent tree is guarded by a closer (inner)
-   export target. The analysis will fail if, in the continuation, no suitable
-   target cache values can be retrieved from `just serve`.
-
+ - once we know the flexible configuration variables, we compute the
+   `TargetCacheKey`. If it is not in the local target cache, we query `just
+   serve` to get the target cache value for the given key. If it is not able to
+   provide the target cache value, we error out.
 
 ### Auxiliary changes