Strategy (aka Policy)

Description

The Strategy design pattern is a technique that enables separation of concerns. It also allows to decouple software modules through Dependency Inversion.

The basic idea behind the Strategy pattern is that, given an algorithm solving a particular problem, we define only the skeleton of the algorithm at an abstract level, and we separate the specific algorithm’s implementation into different parts.

In this way, a client using the algorithm may choose a specific implementation, while the general algorithm workflow remains the same. In other words, the abstract specification of the class does not depend on the specific implementation of the derived class, but specific implementation must adhere to the abstract specification. This is why we call it "Dependency Inversion".

Motivation

Imagine we are working on a project that generates reports every month. We need the reports to be generated in different formats (strategies), e.g., in JSON or Plain Text formats. But things vary over time, and we don't know what kind of requirement we may get in the future. For example, we may need to generate our report in a completely new format, or just modify one of the existing formats.

Example

In this example our invariants (or abstractions) are Context, Formatter, and Report, while Text and Json are our strategy structs. These strategies have to implement the Formatter trait.

use std::collections::HashMap;

type Data = HashMap<String, u32>;

trait Formatter {
    fn format(&self, data: &Data, buf: &mut String);
}

struct Report;

impl Report {
    // Write should be used but we kept it as String to ignore error handling
    fn generate<T: Formatter>(g: T, s: &mut String) {
        // backend operations...
        let mut data = HashMap::new();
        data.insert("one".to_string(), 1);
        data.insert("two".to_string(), 2);
        // generate report
        g.format(&data, s);
    }
}

struct Text;
impl Formatter for Text {
    fn format(&self, data: &Data, buf: &mut String) {
        for (k, v) in data {
            let entry = format!("{} {}\n", k, v);
            buf.push_str(&entry);
        }
    }
}

struct Json;
impl Formatter for Json {
    fn format(&self, data: &Data, buf: &mut String) {
        buf.push('[');
        for (k, v) in data.into_iter() {
            let entry = format!(r#"{{"{}":"{}"}}"#, k, v);
            buf.push_str(&entry);
            buf.push(',');
        }
        buf.pop(); // remove extra , at the end
        buf.push(']');
    }
}

fn main() {
    let mut s = String::from("");
    Report::generate(Text, &mut s);
    assert!(s.contains("one 1"));
    assert!(s.contains("two 2"));

    s.clear(); // reuse the same buffer
    Report::generate(Json, &mut s);
    assert!(s.contains(r#"{"one":"1"}"#));
    assert!(s.contains(r#"{"two":"2"}"#));
}

Advantages

The main advantage is separation of concerns. For example, in this case Report does not know anything about specific implementations of Json and Text, whereas the output implementations does not care about how data is preprocessed, stored, and fetched. The only thing they have to know is context and a specific trait and method to implement, i.e,Formatter and run.

Disadvantages

For each strategy there must be implemented at least one module, so number of modules increases with number of strategies. If there are many strategies to choose from then users have to know how strategies differ from one another.

Discussion

In the previous example all strategies are implemented in a single file. Ways of providing different strategies includes:

All in one file (as shown in this example, similar to being separated as modules)
Separated as modules, E.g. formatter::json module, formatter::text module
Use compiler feature flags, E.g. json feature, text feature
Separated as crates, E.g. json crate, text crate

Serde crate is a good example of the Strategy pattern in action. Serde allows full customization of the serialization behavior by manually implementing Serialize and Deserialize traits for our type. For example, we could easily swap serde_json with serde_cbor since they expose similar methods. Having this makes the helper crate serde_transcode much more useful and ergonomic.

However, we don't need to use traits in order to design this pattern in Rust.

The following toy example demonstrates the idea of the Strategy pattern using Rust closures:

struct Adder;
impl Adder {
    pub fn add<F>(x: u8, y: u8, f: F) -> u8
    where
        F: Fn(u8, u8) -> u8,
    {
        f(x, y)
    }
}

fn main() {
    let arith_adder = |x, y| x + y;
    let bool_adder = |x, y| {
        if x == 1 || y == 1 {
            1
        } else {
            0
        }
    };
    let custom_adder = |x, y| 2 * x + y;

    assert_eq!(9, Adder::add(4, 5, arith_adder));
    assert_eq!(0, Adder::add(0, 0, bool_adder));
    assert_eq!(5, Adder::add(1, 3, custom_adder));
}

In fact, Rust already uses this idea for Options's map method:

fn main() {
    let val = Some("Rust");

    let len_strategy = |s: &str| s.len();
    assert_eq!(4, val.map(len_strategy).unwrap());

    let first_byte_strategy = |s: &str| s.bytes().next().unwrap();
    assert_eq!(82, val.map(first_byte_strategy).unwrap());
}

Rust Design Patterns