# Between workflow caching

Within certain data analysis fields, there are certain intermediate results that reoccur in exactly the same way in many analysis. For example, in bioinformatics, reference genomes or annotations are downloaded, and read mapping indexes are built. Since such steps are independent of the actual data or measurements that are analyzed, but still computationally or timely expensive to conduct, it has been common practice to externalize their computation and assume the presence of the resulting files before execution of a workflow.

From version 5.8.0 on, Snakemake offers a way to keep those steps inside the actual analysis without requiring from redundant computations. By hashing all steps, parameters, software stacks (in terms of conda environments or containers), and raw input required up to a certain step in a blockchain, Snakemake is able to recognize before the computation whether a certain result is already available in a central cache on the same system. Note that this is explicitly intended for caching results between workflows! There is no need to use this feature to avoid redundant computations within a workflow. Snakemake does this already out of the box.

Such caching has to be explicitly activated per rule, which can be done via the command line interface. For example,

    $ export SNAKEMAKE_OUTPUT_CACHE=/mnt/snakemake-cache/
    $ snakemake --cache download_data create_index

would instruct Snakemake to cache and reuse the results of the rules `download_data` and `create_index`. The environment variable definition that happens in the first line (defining the location of the cache) should of course be done only once and system wide in reality. When Snakemake is executed without a shared filesystem (e.g., in the cloud, see Cloud execution), the environment variable has to point to a location compatible with the given remote provider (e.g. an S3 or Google Storage bucket). In any case, the provided location should be shared between all workflows of your group, institute or computing environment, in order to benefit from the reuse of previously obtained intermediate results.

Alternatively, rules can be marked as eligible for caching via the `cache` directive:

    rule download_data:
        output:
            "results/data/worldcitiespop.csv"
        cache: True # allowed values: "all", "omit-software", True
        shell:
            "curl -L https://burntsushi.net/stuff/worldcitiespop.csv > {output}"

Here, the given value defines what information shall be considered for calculating the hash value. With `"all"` or `True`, all relevant rule information is used as outlined above (this is the recommended default). With `"omit-software"`, the software stack is not considered, which is useful if the software stack is not relevant for the result (e.g., if the rule is only a data download).

For workflows defining cache rules like this, it is enough to invoke Snakemake with

    $ snakemake --cache

without explicit rulenames listed.

Note that only rules with just a single output file (or directory) or with multiext output files are eligible for caching. The reason is that for other rules it would be impossible to unambiguously assign the output files to cache entries while being agnostic of the actual file names. Also note that the rules need to retrieve all their parameters via the `params` directive (except input files). It is not allowed to directly use `wildcards`, `config` or any global variable in the shell command or script, because these are not captured in the hash (otherwise, reuse would be unnecessarily limited).

Also note that Snakemake will store everything in the cache as readable and writeable for all users on the system (except in the remote case, where permissions are not enforced and depend on your storage configuration). Hence, caching is not intended for private data, just for steps that deal with publicly available resources.

Finally, be aware that the implementation should be considered experimental until this note is removed.