cache_diff

Generate clean, human readable diffs between two cache structs

What

The CacheDiff trait provides a way to compare two structs and generate a list of differences between them. When it returns an empty list, the two structs are identical.

You can manually implement the trait, or you can use the #[derive(CacheDiff)] macro to automatically generate the implementation.

Top level struct configuration (Container attributes):

• #[cache_diff(custom = <function>)] Specify a function that receives references to both current and old values and returns a Vec of strings if there are any differences. This function is only called once. It can be in combination with #[cache_diff(custom)] on fields to combine multiple related fields into one diff (for example OS distribution and version) or to split apart a monolithic field into multiple differences (for example an "inventory" struct that contains a version and CPU architecture information).

Attributes for fields are:

• #[cache_diff(rename = "<new name>")] Specify custom name for the field
• #[cache_diff(ignore)] or #[cache_diff(ignore = "<reason>")] Ignores the given field with an optional comment string. If the field is ignored because you're using a custom diff function (see container attributes) you can use cache_diff(ignore = "custom") which will check that the container implements a custom function.

Why

Cloud Native Buildpacks (CNBs) written in Rust using libcnb.rs use a serializable struct to represent the state of a cache. When that data changes, we need to invalidate the cache, but also report back to the user what changed.

Due to the CNB layer implementation, this struct is often called "metadata".

Install

$ cargo add cache_diff

For ANSI colored output, add the bullet_stream feature:

$ cargo add cache_diff --features bullet_stream

Derive usage

By default a #[derive(CacheDiff)] will generate a diff function that compares each field in the struct. You can disable this dependency by specifying features = [].

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
}
let diff = Metadata { version: "3.4.0".to_string() }
    .diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "version (`3.3.0` to `3.4.0`)");

Struct fields must implement PartialEq and Display. Also note that PartialEq on the top level cache struct is not used or required. If you want to customize equality logic, you can implement the CacheDiff trait manually:

use cache_diff::CacheDiff;

#[derive(Debug)]
struct Metadata {
    version: String,
}

// Implement the trait manually
impl CacheDiff for Metadata {
   fn diff(&self, old: &Self) -> Vec<String> {
        let mut diff = vec![];
        // This evaluation logic differs from the derive macro
        if !self.custom_compare_eq(old) {
            diff.push(format!("Cache is different ({old:?} to {self:?})"));
        }

        diff
   }
}

impl Metadata {
  fn custom_compare_eq(&self, old: &Self) -> bool {
      todo!()
  }
}

Ordering

The order of output will match the struct field definition from top to bottom:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
    distro: String,
}
let now = Metadata { version: "3.4.0".to_string(), distro: "Ubuntu".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string(), distro: "Alpine".to_string() });

assert_eq!(diff.join(", "), "version (`3.3.0` to `3.4.0`), distro (`Alpine` to `Ubuntu`)");

Rename attributes

If your field name is not descriptive enough, you can rename it:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(rename="Ruby version")]
    version: String,
}
let now = Metadata { version: "3.4.0".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "Ruby version (`3.3.0` to `3.4.0`)");

Ignore attributes

If the struct contains fields that should not be included in the diff comparison, you can ignore them:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,

    #[cache_diff(ignore)]
    changed_by: String
}
let now = Metadata { version: "3.4.0".to_string(), changed_by: "Alice".to_string() };
let diff = now.diff(&Metadata { version: now.version.clone(), changed_by: "Bob".to_string() });

assert!(diff.is_empty());

Handle structs missing display

Not all structs implement the Display trait, for example std::path::PathBuf requires that you call display() on it.

The #[derive(CacheDiff)] macro will automatically handle the following conversions for you:

• std::path::PathBuf (via std::path::Path::display)

However, if you have a custom struct that does not implement Display, you can specify a function to call instead:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(display = my_function)]
    version: NoDisplay,
}

#[derive(PartialEq)]
struct NoDisplay(String);
fn my_function(s: &NoDisplay) -> String {
    format!("custom {}", s.0)
}

let now = Metadata { version: NoDisplay("3.4.0".to_string())};
let diff = now.diff(&Metadata { version: NoDisplay("3.3.0".to_string())});

assert_eq!(diff.join(" "), "version (`custom 3.3.0` to `custom 3.4.0`)");

Customize one or more field differences

You can provide a custom implementation for a diffing a subset of fields without having to roll your own implementation.

Custom logic for one field example

Here's an example where someone wants to bust the cache after N cache calls. Everything else other than cache_usage_count can be derived. If you want to keep the existing derived difference checks, but add on a custom one you can do it like this:

use cache_diff::CacheDiff;
const MAX: f32 = 200.0;

#[derive(Debug, CacheDiff)]
#[cache_diff(custom = diff_cache_usage_count)]
pub(crate) struct Metadata {
    #[cache_diff(ignore = "custom")]
    cache_usage_count: f32,

    binary_version: String,
    target_arch: String,
    os_distribution: String,
    os_version: String,
}

fn diff_cache_usage_count(_old: &Metadata, now: &Metadata) -> Vec<String> {
    let Metadata {
        cache_usage_count,
        binary_version: _,
        target_arch: _,
        os_distribution: _,
        os_version: _,
    } = now;

    if cache_usage_count > &MAX {
        vec![format!("Cache count ({}) exceeded limit {MAX}", cache_usage_count)]
    } else {
        Vec::new()
    }
}

In this example, four fields are derived automatically, saving us time, while one field is custom using the #[cache_diff(custom = diff_cache_usage_count)] attribute on the struct. This tells [CacheDiff] to call this function and pass in the old and current values. It expects a vector with some strings if there is a difference and an empty vector if there are none.

Don't forget to #[cache_diff(ignore = "custom")] any fields you're implementing yourself. You can also use this feature to combine several fields into a single diff output, for example using the previous struct, if you only wanted to have one output for a combined os_distribution and os_version in one output like "OS (ubuntu-22 to ubuntu-24)". Alternatively, you can use https://github.com/schneems/magic_migrate to re-arrange your struct to only have one field with a custom display.

Releasing

Releases can be performed via cargo release:

$ cargo install cargo-release

Release readiness for all crates can be checked by running:

$ cargo release --workspace --exclude usage --dry-run

When satisfied, contributors with permissions can release by running:

$ cargo release --workspace --exclude usage --execute