Functions as Queries @ SwayDB

Conditional updates & deletes are performed by submitting any Java, Scala, Kotlin or any native JVM code/function. There is no query language.

Being just JVM functions/code we can

• Be more expressive in writing our query logic as we are not restricted by rules of a query language.
• Use external Java libraries within our queries like Joda-Time for time management, Jackson or Circe for JSON parsing etc.
• Write test-cases using familiar testing libraries like JUnit, kotlin.test, ScalaTest etc.
• Type-safe – catch errors during compile time.
• Fast – No runtime parsing & compiling performance cost. See these benchmarks that show 482,000+ writes per second. Function updates follow the same write path as any basic write and do not incur any extra cost.

Example

A simple example which creates a Map<String, Double> that stores productName as key and price as value. You can use custom classes (Product.java for values) here but for simplicity String and Double is used.

Update logic: If a product has less than 2 days left to expiration, give a generous 50% discount!

//Our function that implements the update logic. 
//You can also use PureFunction.OnValue or PureFunction.OnKey.
PureFunction.OnKeyValue<String, Double, Return.Map<Double>> discount =  
  (String product, Double price, Optional<Deadline> deadline) -> {
    //if there are less than 2 days to expiry then apply discount. 
    if (deadline.isPresent() && deadline.get().timeLeft().minusDays(2).isNegative()) {  
      double discountedPrice = price * 0.50; //50% discount.
      if (discountedPrice <= 10) { //If the price is below $10
        //return Return.expire(Duration.ZERO); //expire it
        return Return.remove(); //or remove the product
      } else {  
        return Return.update(discountedPrice); //else update with the discounted price
      }  
    } else {
      return Return.nothing(); //else do nothing.
    }
  };  

//create our map with functions enabled. 
Map<String, Double, PureFunction<String, Double, Return.Map<Double>>> products =  
  MapConfig.functionsOn(stringSerializer(), doubleSerializer())  
    .registerFunction(discount) //register the discount function
    .get();  

//insert two products that expire after a day. 
products.put("MacBook Pro", 2799.00, Duration.ofDays(1));  
products.put("Tesla", 69275.0, Duration.ofDays(1));  

//apply the discount function.
products.applyFunction("MacBook Pro", "Tesla", discount);  

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We can print the content of the Map with the following

products.stream().forEach(System.out::println);    

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Before the discount is applied will print

KeyVal(MacBook Pro, 2799.0)
KeyVal(Tesla, 69275.0)

After the discount is applied prints

KeyVal(MacBook Pro, 1399.5)
KeyVal(Tesla, 34637.5)

See the above example on GitHub – DiscountApp.java.

What is function ID?

Each PureFunction type also implement a String id value which gets stored in the database. Currently the id is defaulted to this.getClass().getName(); which can be overridden by your function.

Note & TODOs

• The function itself is not serialised. Only the function’s id gets serialised and stored in the database. TODO – Serialisable functions.
• .mightContainFunction can be used to check if a function is in use. TODO – Automatic function removal.
• TODO – stronger types when registering and applying function.

Useful links

• SwayDB on GitHub.
• Java examples repo.
• Kotlin examples repo.
• Scala examples repo.
• Documentation.

原文链接：Functions as Queries @ SwayDB