Defining workflows for external codes

In httk v1 a calculation was a task-template directory you pointed at with a t: reference. In httk₂ it is a workflow package: a self-contained directory whose httk_workflow.toml manifest is the httk-owned glue around a runner, and whose runner is written against a small SDK. The whole directory is published content-addressed and pinned by digest per job, so upgrading httk₂ underneath a queued campaign cannot change what its jobs execute.

The quickstart shape

For VASP you do not have to author anything: the packaged vasp-relax workflow (id httk.vasp.relax) runs VASP through three steps — prepare, run, publish — and needs only a POSCAR and a vasp.command setting.

$ httk project init --name quickstart .
$ httk workspace init --name default .
$ httk job new --workflow vasp-relax --input structure=POSCAR --tag silicon
$ httk workspace settings set --key vasp.command --value "$PWD/examples/mock_vasp.py" default
$ httk workflow run

vasp.command is an application setting resolved most-specific-first: a job’s own vasp.command parameter, then HTTK_VASP_COMMAND in the environment, then the workspace setting. On a real machine you would set it to something like "srun -n 32 vasp_std".

In httk v1

The equivalent was a per-code shell layer. ht_steps scripts dispatched on $STEP (start → prerelax → relax1 → relax2 → cleanup) and sourced helpers such as ht_tasks_api.sh and vasptools.sh; the VASP invocation lived in shell functions like VASP_PREPARE_CALC and VASP_RUN_CONTROLLED. In httk₂ that is the packaged runner plus the one vasp.command workspace setting.

Authoring your own package

A package is a directory with a manifest and one executable entry:

my-workflow/
├── httk_workflow.toml
└── run                    # the executable entry (any language)

[workflow]
id = "example.relax"

[workflow.runner]
entry = "run"
steps = ["prepare", "relax", "publish"]
initial_step = "prepare"

[workflow.inputs.structure]
destination = "POSCAR"
entry_type = "structures"

[workflow.parameters.encut]
default = 520

The manifest declares [workflow] identity, the [workflow.runner] (an executable, or a language such as CWL, PWD, jobflow, or httk-v1), [workflow.inputs.*] (staged objects), [workflow.parameters.*] (knobs), [workflow.outputs.*], the [workflow.instantiate] and [workflow.collect] hooks, and [workflow.postprocess.<NAME>] scripts. Runners are written against the Runner/Attempt SDK (@run.step, @run.instantiate) in Python or Bash, or in C, C++, Fortran, Rust, Perl, Ada, or Java.

In httk v1

The template directory was the interface. A batch template under Execution/tasks-templates/vasp/batch/ carried its INCAR templates inline and an ht.instantiate.py executed once per structure, and was referenced as t:vasp/batch/vasp-relax-two. A one-shot run went through httk.iface.vasp_if.prepare_single_run(). httk₂ replaces the directory convention with a declared manifest and SDK steps.

In httk v1

You do not have to rewrite an existing v1 template to use it. Wrap it unchanged as a package with language = "httk-v1" and it runs under the httk₂ CLI — see the migration guide, §15.