diff --git a/spack.yaml b/spack.yaml
index 008075be5fc3dbb82b1901902a5505ecedf0beb3..10d66c7baec7a1ac3cd299f27d05d78eab36e3d0 100644
--- a/spack.yaml
+++ b/spack.yaml
@@ -2,26 +2,37 @@ spack:
   view: false
   concretization: separately
 
-  # would need gcc@6:
-  #config:
-  #  concretizer: clingo
+  packages:
+    mpi:
+      # disable mpi lib builds altogether
+      buildable: false
+    mpich:
+      buildable: false
+      externals:
+      # centos7: mpich-3.0-3.0.4-10.el7.x86_64
+      # maybe add arch=linux-centos7-sandybridge later?
+      - spec: mpich@3.0.4
+        prefix: /usr/lib64/mpich
 
   definitions:
   - compiler-pkgs:
-#    - 'gcc@4.8.5 os=centos7'
-    - 'gcc@9.3.1 os=centos7'
-#    - 'gcc@10.3.0 os=centos7'
+    - gcc@10.3.0 os=centos7
   - pkgs:
+    # some EBRAINS-related simulators
     - py-numpy ^python@3.8.10 
     #- py-scipy ^python@3.8.10 
     #- py-pip ^python@3.8.10 
     #- nest +gsl +python ^python@3.8.10 %gcc@9.3.1
     #- arbor ^python@3.8.10
 
+    #- nest +gsl +mpi ^py-scipy@:1.6.999
+    #- arbor +mpi
+    #- neuron +mpi
+    # BrainScaleS (plus some constraining)
+    #- visionary-dls-core ^py-setuptools-scm+toml ^python@3.8.0:3.8.999 ^binutils+ld ^py-astroid@2.5.6:2.6.999 ^py-scipy@:1.6.999
   - compilers:
-#    - '%gcc@4.8.5'
-    - '%gcc@9.3.1'
-#    - '%gcc@10.3.0'
+    # for now build on one target compiler only
+    - '%gcc@10.3.0'
   - oses:
     - os=centos7
 
@@ -32,10 +43,8 @@ spack:
     - [$oses]
 
   mirrors:
-    mirror: https://spack-llnl-mirror.s3-us-west-2.amazonaws.com/
-    #cloud_gitlab: https://mirror.spack.io
-    #local_something: /builds/emuller/brainscales-spack-test/local_mirror
-    # build_cache/ gets added
+    local_something: /builds/emuller/brainscales-spack-test/jobs_scratch_dir/local_mirror
+    # some random cache :p (build_cache/ gets added)
     hacky_cache: https://brainscales-r.kip.uni-heidelberg.de:7443/nmpi/tmp
 
   gitlab-ci:
@@ -50,17 +59,31 @@ spack:
           tags:
             - docker-runner
           image: docker-registry.ebrains.eu/clb-jupyter-image/base
+
     # needed as the spack command isn't globally available
     before_script:
       - git clone ${SPACK_REPO}
       # ${SPACK_REF} ?
       - pushd spack && git checkout ${SPACK_CHECKOUT_VERSION} && git log -n 1 && popd
       - . "./spack/share/spack/setup-env.sh"
-    # for default (env activate + ci rebuild) remove it
+
+    # looks like default => remove it?
     script:
-      #- spack env activate --without-view ${SPACK_CONCRETE_ENV_DIR}
-      - spack env activate --without-view .
+      - spack env activate --without-view ${SPACK_CONCRETE_ENV_DIR}
       - spack -d ci rebuild
+
+    # all in artifacts (no shared filesystem...)
     enable-artifacts-buildcache: False
+
+    # trigger service job below
     rebuild-index: True
 
+    # post-build runner performing index (and potentially triggering deployment later)
+    service-job-attributes:
+      tags:
+        - docker-runner
+      image: docker-registry.ebrains.eu/clb-jupyter-image/base
+      before_script:
+        - git clone ${SPACK_REPO}
+        - pushd spack && git checkout ${SPACK_CHECKOUT_VERSION} && popd
+        - . "./spack/share/spack/setup-env.sh"