Some of the subdirectories in the E3SM code contain code that was brought in from another repository.

Terms

external any subdirectory in E3SM whose entire content is from another repository
upstream master the repository everyone is sending their changes to. Only one so that everyone pulling from the same source is working with all the same changes. The source of an external.

Methods for including externals

submodule a link that points to another git repository. See Sharing External Code using Git Submodules .
recursive submodule: a submodule that is inside another submodule.
subtree a set of files and commits from another git repo, brought in to your repo so that it looks like original content. See Sharing External Code using Git Subtree.
monorepo the entire repo is added once (usally with subtree) and developed within E3SM. Might maintain a separate build/test system.
buildtime the repo is checked out to the $CIME_OUTPUT_ROOT/$CASE/bld when compiling a case for the first time. Code does not exist in E3SM repo.

Rules for adding externals

Group leads must approve adding a new external. The group needing the external can decide on the method of inclusion. A POC for the upstream master must be identified and the IG group leads notified.
The upstream master for the external must be build-able and testable on its own.
For submodules, the linked repo must be readable by all E3SM developers. If the upstream master can not be made readable, a fork must be maintained in E3SM-Project github and E3SM then links to that fork.

Development in externals

In general, there are several ways to develop with external code

strict 1-way: all development is merged to the upstream master first and flows down to E3SM. No exceptions.
mostly 1-way: prefer strict 1-way but some exceptions can be made for emergency fixes
2-way: developers can make changes in E3SM or the upstream master. POC keeps them in sync.

More specific instructions for externals are below.

Table of Externals currently in ACME

Name	Subdirectory in ACME	external method	upstream master	development method	POC	Notes

Name	Subdirectory in ACME	external method	upstream master	development method	POC	Notes
CIME	cime	subtree	https://github.com/ESMCI/cime	2-way	@James Foucar
MCT	cime/externals/MCT	subtree	https://github.com/MCSclimate/MCT	mostly 1-way	@Robert Jacob	usually brought in with CIME
PIO	cime/externals/pio*	subtree	https://github.com/NCAR/ParallelIO	mostly 1-way	@Jayesh Krishna	usually brought in with CIME
SCORPIO	externals/scorpio externals/scorpio_classic	submodule	https://github.com/E3SM-Project/scorpio	strict 1-way	@Jayesh Krishna	E3SM uses this and ignores cime/externals/pio
Kokkos	externals/kokkos	submodule	git@github.com:E3SM-Project/kokkos.git branch e3sm/kokkos	strict 1-way	@James Foucar
MPAS-Source	components/mpas-source	submodule	https://github.com/MPAS-Dev/MPAS-Model	strict 1-way	@Jon Wolfe	MPAS-Ocean, Seaice and land ice all use code from here.
RRTMGP	components/cam/src/physics/rrtmgp/external	submodule	git@github.com:RobertPincus/rte-rrtmgp.git branch develop	strict 1-way	@Ben Hillman
COSP2	components/cam/src/physics/cosp2/external	submodule	git@github.com:CFMIP/COSPv2.0.git branch CESM_v2.1.4	strict 1-way
MPP	components/clm/src/external_models/mpp	submodule	https://github.com/MPP-LSM/MPP branch alm/develop	strict 1-way	@Gautam Bisht
FATES	components/clm/src/external_models/fates	submodule	https://github.com/NGEET/fates	strict 1-way	@Gautam Bisht
SBETr	components/clm/src/external_models/sbetr	submodule	git@github.com:BeTR-biogeochemistry-modeling/sbetr.git	strict 1-way	@Gautam Bisht
CVMix	components/mpas-source/src/core_ocean/cvmix	rescursive submodule	https://github.com/CVMix/CVMix-src	strict 1-way	@Jon Wolfe	used to be not in-source and fetched at build time.
ocean BGC	components/mpas-source/src/core_ocean/BGC	recursive submodule	https://github.com/E3SM-Project/Ocean-BGC	strict 1-way	@Jon Wolfe	used to be not in-source and fetched at build time.

Externals planned for E3SM

Name	Subdirectory in ACME	external method	upstream master	development method	POC	Notes

Name	Subdirectory in ACME	external method	upstream master	development method	POC	Notes
CICE column physics	TBD	TBD	?	?	?
GCAM	TBD	submodule?	https://github.com/JGCRI/gcam-core	strict 1-way	@Katherine Calvin

Specific external development instructions

CIME:

Code location: E3SM/cime

Any code development in E3SM/cime/config/acme can be committed directly to E3SM through the normal PR process.

Code development in other parts of CIME should be committed first with a PR in https://github.com/ESMCI/cime. It will be then brought to E3SM by a subtree merge as in Maintaining the CIME subtree in ACME.

Report issues about the Case Control System, driver or coupler directly to https://github.com/ESMCI/cime. config file issues can be reported to E3SM gitub.