Our greatest glory is not in never failing, but in rising every time we fail.
Confucius.
What are Supervision Trees?
A Supervision Tree is a design pattern where developers build applications as a composable group of nodes that form a tree.
flowchart TD Root --> workera["Worker A"] Root --> subsystemb["System B"] subsystemb --> workerb1["Worker B.1"] subsystemb --> workerb2["Worker B.2"] Root --> subsystemc["System C"] subsystemc --> workerc1["Worker C.1"] subsystemc --> workerc2["Worker C.2"]
By designing applications this way, we accomplish a few desirable properties:
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Every sub-tree node is responsible of supervising it's children failures, restarting them when needed.
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Every sub-tree node encapsulates a specific scope of the application.
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Every leaf node encapsulates some business logic that may use resources shared across sibling nodes.
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When we nest one sub-tree into another, we compose multiple sub-applications together in a way that facilitates splitting them apart when necessary (when doing microservices, for example.)
Why Restarts fix (most of) my problems?
You may find that one of the top 3 culprits of failing systems is invalid states at program runtime. It is pretty much inevitable, a system may be able to have infinite combinations of values in their runtime state, and some of these combinations are not valid or expected, making our systems crash with broken assumptions.
A way to deal with this issue is to make invalid states impossible, but that is a costly endeavour, more so in dynamic typed languages or languages that fill up empty values with "sane defaults" for the sake of simplicity.
When we develop software, we start from a state that is known to work, and our system would later evolve into an invalid one, crashing again. By restarting our components, we can guarantee that when your application encounters an error state, it will reset itself to a healthy state as soon as possible.
Later, we can assess through error reports what went wrong and fix our code so that it doesn't crash unnecessarily. The main idea is to let our applications crash fast to be back in business again.
The folks at Ericson had great success with this approach by designing Erlang to have this feature front-and-center via Supervision Trees.
Long Term Goal
Capataz wants to offer a supervision tree specification that can be implemented across multiple programming languages and provide cross-language test-suites.