> ## Documentation Index
> Fetch the complete documentation index at: https://site.aspect.build/llms.txt
> Use this file to discover all available pages before exploring further.

# Hermeticity: trade-offs

> Understand the trade-offs of Bazel hermeticity, including external test tags and spawn strategies that relax sandboxing for legacy or non-hermetic workloads.

Bazel often gets conflated with the dogma of how Blaze is used in google3, Google’s internal monorepo. Hermeticity is an example: Googlers usually expect a build to be completely hermetic. But Bazel doesn’t actually require this.

* Intentionally non-hermetic tests can be tagged with `external`. This tells Bazel that the test depends on undeclared systems (such as a service you’ve deployed before running `bazel`). It won’t be cached, since any cache key would obviously be inadequate to determine when the test needs to be re-executed.
* The spawn strategies we mentioned in the previous section try to enforce hermeticity. You might have a legitimate reason to run with `--spawn_strategy=local` for example, if you want to run a build without having to declare dependencies, such as when you bridge Bazel to a legacy build tool that doesn’t have a correct dependency graph.