formatting-stores.md

Formatting Stores

The store was designed to work well with JSON data. This makes it easy to deploy models into microservices, run them as serverless functions, and many other things. A store can be directly encoded into JSON as a representation of its variable assignments.

Let's create a new store and encode it into JSON.

import (
    "encoding/json"
    "os"

    "github.com/nextmv-io/sdk/store"
)

func main() {
    enc := json.NewEncoder(os.Stdout)

    s := store.New()
    enc.Encode(s)
}

This should write an empty list to standard out because the store is empty.

[]

Let's add some variables to the store before the end of our main function.

x := store.NewVar(s, 42)
y := store.NewVar(s, "foo")
pi := store.NewVar(s, 3.14)
enc.Encode(s)

By default, the JSON representation of a store contains its variable assignments in order of declaration.

[
  42,
  "foo",
  3.14
]

This may be fine, or we may rather reshape the format into something more convenient. Let's turn that JSON list into a map with the variable names. To do this, we use the Format method. Format is similar to Apply, in that it doesn't change the existing store, but applies a change to create a new one. The difference is that now we are adding logic to the store instead of a change to a variable assignment. Stores give us very specific ways to introduce logic.

s = s.Format(func(s store.Store) any {
    return map[string]any{
        "x":  x.Get(s),
        "y":  y.Get(s),
        "pi": pi.Get(s),
    }
})
enc.Encode(s)

Now we should see our store encoded as a map. This isn't that far from where we started, but is more useful! Since Format can reshape a store into anything that can be encoded in JSON, its easy to make the output match anything we might expect in a production environment.

{
  "pi": 3.14,
  "x": 42,
  "y": "foo"
}

Finally, let's apply a change to a variable assignment and encode the resulting store.

enc.Encode(s.Apply(y.Set("bar")))

Note how the new store inherits our formatting logic.

{
  "pi": 3.14,
  "x": 42,
  "y": "bar"
}

Exercises

• Start with the source above and reshape the output into something more complex than a map. Can you format it as a map of maps or as a user-defined structure? How do the rules of the encoding/json library apply to the output?
• What happens if you override the formatting logic of a child store? Does that impact the parent or any sibling stores?

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