04. Ownership and Borrowing

What we will learn in this post?

• 👉 Understanding Ownership in Rust
• 👉 Move Semantics and Transfers
• 👉 References and Borrowing
• 👉 Mutable References and The Rules
• 👉 Lifetimes Basics
• 👉 String Slices and Ownership
• 👉 Common Ownership Patterns

Rust’s Ownership: Your Memory Superpower! 🦸‍♀️

Rust’s ownership system is the secret sauce that makes it special. It’s a clever set of rules that lets Rust manage memory safely without a “garbage collector.” Think of it as a super-organized library manager for your program’s data! It prevents common bugs like data races and null pointers, delivering blazing performance.

The Three Golden Rules of Ownership 📜

Let’s break down how this powerful concept works, using a real-world book analogy:

Rule 1: Each Value Has an Owner 🧑‍💻

Every piece of data (a value) in Rust has a specific part of your program (a variable within a scope) that’s responsible for it.

• Book Analogy: When you buy a new book, you become its designated owner. It belongs to you.

Rule 2: Only One Owner at a Time 🔒

At any given moment, only one owner can have control over a specific piece of data. If you pass data to another part of your program, ownership is transferred.

• Book Analogy: You can’t both own the exact same physical copy of a book simultaneously. If you lend it to a friend, they become its temporary owner. Rust ensures only one part of your code controls data, preventing conflicts!

Rule 3: Value is Dropped When Owner Goes Out of Scope 🗑️

When the part of your program that owns a piece of data finishes its job (the owner variable goes out of scope), Rust automatically cleans up that data from memory.

• Book Analogy: Once you’re done with the book, you might donate it or recycle it. Rust automatically “disposes” of the data, freeing memory. No manual cleanup!

Visualizing Ownership Flow 📉

graph TD
    A["🚀 Program Starts"]:::style1 --> B{"📦 Data Created"}:::style2
    B --> C["👤 Ownership → x"]:::style3
    C --> D["⚙️ Use Data"]:::style4
    D --> E{"❓ Scope Ends?"}:::style5
    E -- "Yes" --> F["🗑️ Data Dropped"]:::style6
    E -- "No" --> D
    F --> G["➡️ Continue"]:::style7
    
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Why It’s Revolutionary! ✨

This system guarantees memory safety and thread safety at compile time, meaning bugs are caught before runtime! You get C-like performance and robust safety without a garbage collector. Truly revolutionary!

Rust’s Ownership Magic: No More Double-Frees! ✨

Rust’s unique “ownership” system ensures memory safety without a garbage collector. It tackles common errors like “double-free” by precisely deciding who “owns” data.

Moving Ownership by Default 🚚

When you assign a complex value (like String) or pass it to a function, Rust moves its ownership. This means the original variable can no longer be used, preventing multiple pointers to the same data and thus a double-free!

fn main() {
    let s1 = String::from("Hello"); // s1 owns "Hello"
    let s2 = s1;                    // Ownership MOVED from s1 to s2
    // println!("{}", s1);         // 🚨 ERROR: s1 is now invalid!
    println!("{}", s2);             // ✅ OK: s2 now owns "Hello"
}

Why Not Automatic Copies? 🙅‍♀️

Complex types store their main data on the heap. Automatically making deep copies would be slow and wasteful. Rust prefers explicit control, like using .clone(), when you truly want a duplicate.

The Copy Trait for Simple Types 🔄

For simple, fixed-size data (like i32 integers, booleans, characters), Rust copies their values by default. These types implement the Copy trait, meaning the original variable remains perfectly valid after assignment.

fn main() {
    let x = 5; // x owns 5
    let y = x; // 5 is COPIED, x is still valid!
    println!("x: {}, y: {}", x, y); // ✅ OK: Both x and y can be used
}

Visualizing Ownership Transfer:

graph TD
    A["📦 Variable A: Data"]:::style1 --> B{"🔄 Assign to B?"}:::style2
    B --> C{"❓ Copy Trait?"}:::style3
    C -- "No (Complex)" --> D["🚚 MOVED to B"]:::style4
    C -- "Yes (Simple)" --> E["📋 COPIED to B"]:::style5
    D --> F["❌ A INVALID"]:::style6
    E --> G["✅ A VALID"]:::style7
    
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&T Borrowing: Your Data’s Friendly Librarian! 📚

Borrowing in Rust is like checking out a library book! You get to use (read) the data, but you don’t own it. This clever system uses references to access values without transferring ownership, making your code super efficient and safe.

Immutable References: The “Read-Only” Pass 👓

When you create an immutable reference using &T, you’re essentially getting a “read-only” pass to a piece of data.

• Accessing Data: You can see and use the data. For example: let value = 10; let r = &value;
• No Changes Allowed: You cannot modify the data through an &T reference. It’s like looking at a document without editing permissions!

Many Readers, No Chaos! ✨

Here’s the cool part: Rust allows you to have multiple immutable references (&T) to the same data at the same time. Think of it as many people reading the same book simultaneously. Since no one is writing or making changes, there’s absolutely no risk of conflicting edits or data corruption.

Compile-Time Safety: Preventing Data Races 🛡️

Rust’s powerful borrow checker enforces a fundamental rule at compile time (before your program even runs!):

graph TD
    A["📦 Data"]:::style1 --> B{"❓ Access Type?"}:::style2
    B --> C["👥 Many &T"]:::style3
    B --> D["✏️ One &mut T"]:::style4
    C -- "✅ Allowed" --> A
    D -- "✅ Allowed" --> A
    C -- "❌ CONFLICT" --> D
    D -- "❌ CONFLICT" --> C
    
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• You can have many immutable readers (&T).
• OR you can have one mutable writer (&mut T).
• You cannot have both a mutable reference and any immutable references simultaneously.

This “one writer OR many readers” rule guarantees that data races (when multiple parts of your code try to access and modify data at the same time, leading to unpredictable bugs) are simply impossible in safe Rust code. It’s Rust’s secret to amazing safety and concurrency!

---

🎯 Real-World Example: Zero-Copy String Parser with Borrowing

Production parsers use borrowing to avoid expensive allocations!

use std::collections::HashMap;

#[derive(Debug, PartialEq)]
enum HttpMethod {
    Get,
    Post,
    Put,
    Delete,
    Unknown,
}

struct HttpRequest<'a> {
    method: HttpMethod,
    path: &'a str,
    headers: HashMap<&'a str, &'a str>,
    body: Option<&'a str>,
}

impl<'a> HttpRequest<'a> {
    // Zero-copy parsing - borrows from original buffer
    fn parse(buffer: &'a str) -> Result<Self, String> {
        let mut lines = buffer.lines();
        
        // Parse request line
        let request_line = lines.next()
            .ok_or("Empty request")?;
        
        let parts: Vec<&str> = request_line.split_whitespace().collect();
        if parts.len() < 2 {
            return Err("Invalid request line".to_string());
        }
        
        let method = match parts[0] {
            "GET" => HttpMethod::Get,
            "POST" => HttpMethod::Post,
            "PUT" => HttpMethod::Put,
            "DELETE" => HttpMethod::Delete,
            _ => HttpMethod::Unknown,
        };
        
        let path = parts[1];
        
        // Parse headers (zero-copy)
        let mut headers = HashMap::new();
        for line in lines.by_ref() {
            if line.is_empty() {
                break; // End of headers
            }
            
            if let Some(pos) = line.find(':') {
                let key = &line[..pos].trim();
                let value = &line[pos + 1..].trim();
                headers.insert(*key, *value);
            }
        }
        
        // Parse body (zero-copy)
        let body = lines.next();
        
        Ok(HttpRequest {
            method,
            path,
            headers,
            body,
        })
    }
    
    fn get_header(&self, key: &str) -> Option<&str> {
        self.headers.get(key).copied()
    }
}

fn main() {
    let request_buffer = "GET /api/users HTTP/1.1\nHost: example.com\nUser-Agent: Rust/1.0\n\n";
    
    match HttpRequest::parse(request_buffer) {
        Ok(req) => {
            println!("✅ Method: {:?}", req.method);
            println!("✅ Path: {}", req.path);
            println!("✅ Host: {:?}", req.get_header("Host"));
            println!("✅ Headers: {} total", req.headers.len());
        }
        Err(e) => eprintln!("❌ Parse error: {}", e),
    }
}

// This pattern is used in Hyper, Actix-web, and Tokio for HTTP parsing!
// Zero allocations - all data borrowed from original buffer

---

Lifetimes ('a): Your Rust Companion for Safety! 🦀

Don’t let lifetimes scare you! While they might seem a bit daunting at first, they’re Rust’s superpower for guaranteeing memory safety without a garbage collector. Think of them as helpful labels ('a) that tell the compiler how long a reference is valid.

What are Lifetimes ('a)? 🤔

Simply put, a lifetime annotation like 'a is a name we give to the scope a reference lives within. Rust uses this information to ensure that a reference never points to data that has already been deallocated – preventing a common bug called a dangling reference.

Why Rust Needs Your Help (Sometimes!) ✨

Rust’s compiler is incredibly smart. Most of the time, it can infer lifetimes using lifetime elision rules for simple cases like:

fn print_str(s: &str) { // 's' has an implicit lifetime
    println!("{}", s);
}

However, when functions take multiple references or return a reference, Rust might need a nudge to understand their relationships:

fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    // This tells Rust that 'x', 'y', and the return value all live
    // for at least the lifetime 'a'.
    if x.len() > y.len() { x } else { y }
}

This ensures that the returned reference is valid for the same duration as the shortest-lived input reference, preventing it from dangling.

Preventing Dangling References 🛡️

This is the core benefit! By enforcing lifetime rules at compile time, Rust guarantees that your references will always point to valid data. No more runtime surprises or crashes due to memory issues!

Here’s a simplified view of how it works:

graph TD
    A["📦 Data (my_string)"]:::style1
    B["🔗 Reference (&my_string)"]:::style2
    C["⏰ Lifetime 'a"]:::style3

    A -- "'a defines validity" --> B
    B -- "Rust validates 'a" --> D{"✅ Valid?"}:::style4

    D -- "Yes" --> E["✅ Compiles Safely"]:::style5
    D -- "No" --> F["❌ Compile Error"]:::style6
    
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It Gets Easier! 🚀

Lifetimes can feel like a steep learning curve, but they quickly become intuitive. They are a powerful tool that empowers you to write extremely safe and performant code. Keep practicing, and they’ll feel completely natural!

---

Unveiling Rust’s &str Slices: Your Window into Data!

Ever wonder how Rust handles parts of text without copying everything? Meet &str, your efficient string slice! It’s a powerful way to work with sequences of characters without taking ownership.

---

🧐 &str vs. String: Ownership Explained!

Think of String as a car you own (String::from("Hello")). It lives on the heap and you’re responsible for it. An &str is like borrowing a specific seat in that car (&car[0..5] for “Hello”). It’s a reference to a part of String (or static text), showing where it starts and how long it is, but it doesn’t own the data. This makes &str always immutable.

---

✨ Crafting Slices: [start..end]

Creating slices is super intuitive! You use the [start..end] syntax to specify a range.

• Example:

  let sentence = String::from("Rust is fun!");
  let word_slice: &str = &sentence[0..4]; // "Rust"
  println!("{}", word_slice); // Output: Rust
  

  This word_slice is just a pointer and a length. It doesn’t copy “Rust”; it simply points to the first four characters of sentence. This is highly efficient for tasks like parsing text!

---

🛡️ Why Slices are Super Safe!

Rust’s compiler ensures &str slices are always valid. It guarantees that the data &str points to (sentence in our example) outlives the slice itself. This compile-time check prevents “dangling references” (where a pointer points to freed memory), making &str incredibly safe. You can confidently use slices for operations like parsing email domains or extracting filenames without worrying about runtime memory errors.

Here’s a visual of ownership vs. borrowing:

graph LR
    A["📦 String (Owner)"]:::style1 -- "owns" --> B["💾 Heap"]:::style2
    B -- "contains" --> C["🔤 'H','e','l','l','o'"]:::style3
    D["🔗 &str (Borrower)"]:::style4 -- "points to" --> C
    
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---

Rust Ownership demystified! 🤝

Rust’s unique ownership system might seem a bit challenging initially, but it’s a superpower for memory safety without a garbage collector! Understanding these core patterns will help you write robust, efficient code. Let’s explore practical ways to work with owned values and borrowing.

Understanding the Core Concepts 📚

1️⃣ Returning Owned Values 🎁

When a function creates new data or transforms data it took ownership of, it often gives back ownership of the new result. The caller then fully owns this value and is responsible for it.

fn create_message() -> String {
    "Hello, Rustacean!".to_string() // Creates and returns ownership
}
let my_message = create_message(); // `my_message` now owns the String

graph LR
    A["📞 Function Call"]:::style1 --> B["🏗️ Create Value"]:::style2
    B --> C["🎁 Return Ownership"]:::style3
    C --> D["👤 Caller Owns"]:::style4
    
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2️⃣ Borrowing for Read Access (Immutable) 📖

If you just need to look at data without changing it, you can borrow it using an immutable reference (&T). Multiple immutable borrows can exist simultaneously, allowing many parts of your code to read the same data safely.

fn print_length(text: &String) { // Borrows 'text' for reading
    println!("Length: {}", text.len());
}
let greeting = String::from("Hi!");
print_length(&greeting); // Pass a reference

3️⃣ Mutable Borrowing for Modifications (Exclusive) ✏️

To change data, you need a mutable reference (&mut T). This is exclusive: only one mutable reference can exist to a piece of data at any given time. This rule prevents tricky data races!

fn add_exclamation(text: &mut String) { // Borrows 'text' mutably
    text.push('!');
}
let mut welcome = String::from("Welcome");
add_exclamation(&mut welcome); // Pass a mutable reference

4️⃣ When to use .clone() 👯‍♀️

The clone() method creates a deep, independent copy of your data. Use it only when you genuinely need two separate, modifiable versions that don’t share ownership. Remember, cloning can impact performance, so prefer borrowing when possible!

let original_data = vec![1, 2, 3];
let copied_data = original_data.clone(); // Creates a new, separate Vec

Tips for Taming the Borrow Checker ✨

Working with the borrow checker gets easier with practice!

• Keep scopes small: Shorter lifetimes for borrows reduce the chances of conflicts.
• Pass references: Default to passing &T or &mut T unless your function needs to own the data.
• Refactor: If the borrow checker persistently complains, it’s often a helpful hint to improve your code’s design or break down a function.

You’ve got this! For more in-depth learning, check out The Rust Book’s chapter on Ownership.

---

🎯 Real-World Example: Arena Allocator with Ownership

Game engines and compilers use arena allocators to manage owned memory efficiently!

use std::cell::RefCell;
use std::rc::Rc;

// Arena allocator - owns all allocated memory
struct Arena {
    storage: RefCell<Vec<String>>,
}

impl Arena {
    fn new() -> Self {
        Arena {
            storage: RefCell::new(Vec::new()),
        }
    }
    
    // Allocates owned String in the arena
    fn alloc(&self, s: String) -> usize {
        let mut storage = self.storage.borrow_mut();
        let index = storage.len();
        storage.push(s);
        index
    }
    
    // Borrows data from the arena
    fn get(&self, index: usize) -> Option<String> {
        let storage = self.storage.borrow();
        storage.get(index).cloned()
    }
    
    // Gets reference without cloning
    fn get_ref(&self, index: usize) -> Option<std::cell::Ref<str>> {
        let storage = self.storage.borrow();
        if index < storage.len() {
            Some(std::cell::Ref::map(storage, |s| s[index].as_str()))
        } else {
            None
        }
    }
    
    // Clear all owned memory at once
    fn clear(&self) {
        self.storage.borrow_mut().clear();
    }
    
    fn len(&self) -> usize {
        self.storage.borrow().len()
    }
}

// Game Entity using arena allocation
struct GameEntity {
    name_index: usize,
    description_index: usize,
}

impl GameEntity {
    fn new(arena: &Arena, name: String, desc: String) -> Self {
        GameEntity {
            name_index: arena.alloc(name),
            description_index: arena.alloc(desc),
        }
    }
    
    fn display(&self, arena: &Arena) {
        if let Some(name) = arena.get(self.name_index) {
            if let Some(desc) = arena.get(self.description_index) {
                println!("🎮 {}: {}", name, desc);
            }
        }
    }
}

fn main() {
    let arena = Arena::new();
    
    // Create multiple entities - all strings owned by arena
    let player = GameEntity::new(
        &arena,
        "Knight".to_string(),
        "A brave warrior with shining armor".to_string(),
    );
    
    let enemy = GameEntity::new(
        &arena,
        "Dragon".to_string(),
        "A fearsome beast breathing fire".to_string(),
    );
    
    player.display(&arena);
    enemy.display(&arena);
    
    println!("\n📊 Arena Statistics:");
    println!("   Total allocations: {}", arena.len());
    
    // Clear all memory at once - efficient bulk deallocation
    arena.clear();
    println!("   After clear: {}", arena.len());
}

// This pattern is used in:
// - Unity ECS (Entity Component System)
// - Unreal Engine memory pools
// - Rust compiler's arena allocator
// - Database query engines

---

🎯 Real-World Example: Thread-Safe Reference Counting

Shared ownership across threads using Arc (Atomic Reference Counting)!

use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;

#[derive(Debug, Clone)]
struct SharedCache {
    data: Arc<Mutex<Vec<String>>>,
}

impl SharedCache {
    fn new() -> Self {
        SharedCache {
            data: Arc::new(Mutex::new(Vec::new())),
        }
    }
    
    fn add(&self, item: String) {
        let mut data = self.data.lock().unwrap();
        data.push(item);
        println!("✅ Added item. Cache size: {}", data.len());
    }
    
    fn get_all(&self) -> Vec<String> {
        let data = self.data.lock().unwrap();
        data.clone()
    }
    
    fn len(&self) -> usize {
        let data = self.data.lock().unwrap();
        data.len()
    }
}

fn main() {
    let cache = SharedCache::new();
    let mut handles = vec![];
    
    // Spawn 5 threads, each with shared ownership
    for i in 0..5 {
        let cache_clone = cache.clone(); // Arc clone - cheap reference count increment
        
        let handle = thread::spawn(move || {
            for j in 0..3 {
                cache_clone.add(format!("Thread-{} Item-{}", i, j));
                thread::sleep(Duration::from_millis(10));
            }
            println!("🧵 Thread {} finished", i);
        });
        
        handles.push(handle);
    }
    
    // Wait for all threads to complete
    for handle in handles {
        handle.join().unwrap();
    }
    
    println!("\n📊 Final Cache Contents:");
    let items = cache.get_all();
    for item in items {
        println!("   - {}", item);
    }
    println!("   Total items: {}", cache.len());
}

// This pattern is used in:
// - Tokio runtime for shared state
// - Web servers for connection pools
// - Redis clients for shared connections
// - Database connection pools

---

🎯 Hands-On Assignment: Build a Memory-Safe File Cache System 🚀

📝 Your Mission

Build a production-ready file cache system demonstrating ownership, borrowing, lifetimes, and memory safety!

🎯 Requirements

1. Create a FileCache struct that:
   • Owns a HashMap<String, String> to store file contents
   • Tracks cache hits/misses
   • Has a maximum capacity
2. Implement these methods:
   • new(capacity: usize) -> Self
   • load(&mut self, path: &str) -> Result<&str, String> - Returns borrowed content
   • get(&self, path: &str) -> Option<&str> - Borrows cached content
   • clear(&mut self) - Clears all cached files
   • stats(&self) -> CacheStats - Returns statistics
3. Implement proper ownership:
   • Cache owns the file contents
   • Methods return borrowed slices (no unnecessary clones)
   • Use lifetimes to tie borrowed data to cache lifetime
4. Create a CacheStats struct with:
   • hits: usize
   • misses: usize
   • total_files: usize
   • hit_rate(&self) -> f64 method
5. Implement eviction policy when cache is full (LRU - Least Recently Used)
6. Add logging for cache operations (hits, misses, evictions)
7. Write comprehensive unit tests
8. Handle file read errors gracefully with Result

💡 Implementation Hints

1. Use HashMap::get() to return borrowed &str
2. Use std::fs::read_to_string() for file I/O
3. Consider using std::collections::VecDeque for LRU tracking
4. Return Option<&str> for cache lookups
5. Use Result<&str, String> for operations that can fail

🚀 Example Starter Code

use std::collections::HashMap;
use std::fs;

struct FileCache {
    cache: HashMap<String, String>,
    capacity: usize,
    hits: usize,
    misses: usize,
}

#[derive(Debug)]
struct CacheStats {
    hits: usize,
    misses: usize,
    total_files: usize,
}

impl CacheStats {
    fn hit_rate(&self) -> f64 {
        let total = self.hits + self.misses;
        if total == 0 {
            0.0
        } else {
            (self.hits as f64 / total as f64) * 100.0
        }
    }
}

impl FileCache {
    fn new(capacity: usize) -> Self {
        FileCache {
            cache: HashMap::new(),
            capacity,
            hits: 0,
            misses: 0,
        }
    }
    
    fn load(&mut self, path: &str) -> Result<&str, String> {
        // Check if file is in cache
        if let Some(content) = self.cache.get(path) {
            self.hits += 1;
            println!("✅ Cache HIT: {}", path);
            return Ok(content.as_str());
        }
        
        // Cache miss - load from disk
        self.misses += 1;
        println!("❌ Cache MISS: {} - Loading from disk", path);
        
        let content = fs::read_to_string(path)
            .map_err(|e| format!("Failed to read file: {}", e))?;
        
        // Check capacity
        if self.cache.len() >= self.capacity {
            // TODO: Implement LRU eviction
            println!("⚠️  Cache full - evicting oldest entry");
            if let Some(key) = self.cache.keys().next().cloned() {
                self.cache.remove(&key);
            }
        }
        
        self.cache.insert(path.to_string(), content);
        Ok(self.cache.get(path).unwrap().as_str())
    }
    
    fn get(&self, path: &str) -> Option<&str> {
        self.cache.get(path).map(|s| s.as_str())
    }
    
    fn stats(&self) -> CacheStats {
        CacheStats {
            hits: self.hits,
            misses: self.misses,
            total_files: self.cache.len(),
        }
    }
    
    fn clear(&mut self) {
        self.cache.clear();
        println!("🗑️  Cache cleared");
    }
}

fn main() {
    let mut cache = FileCache::new(3);
    
    // Test cache operations
    match cache.load("Cargo.toml") {
        Ok(content) => println!("📄 File length: {} bytes", content.len()),
        Err(e) => eprintln!("Error: {}", e),
    }
    
    // Second load should hit cache
    cache.load("Cargo.toml").ok();
    
    let stats = cache.stats();
    println!("\n📊 Cache Statistics:");
    println!("   Hits: {}", stats.hits);
    println!("   Misses: {}", stats.misses);
    println!("   Hit Rate: {:.2}%", stats.hit_rate());
    println!("   Total Files: {}", stats.total_files);
}

🏆 Bonus Challenges

• Level 2: Add TTL (time-to-live) for cache entries with automatic expiration
• Level 3: Implement thread-safe caching using Arc<Mutex<>>
• Level 4: Add async file loading with tokio
• Level 5: Implement multiple eviction policies (LRU, LFU, FIFO)
• Level 6: Add cache persistence (save/load cache state to disk)

📚 Learning Goals

• Master ownership rules with HashMap 🎯
• Return borrowed slices without cloning ✨
• Understand lifetime annotations in practice 🔄
• Implement memory-safe cache eviction 🛡️
• Handle errors with Result types 💡
• Build production-ready systems 🚀

💡 Pro Tip: This cache pattern is used in web browsers, databases, compilers, and CDN edge servers!

Share Your Solution! 💬

Completed the project? Post your code in the comments below! Show us your Rust ownership mastery! 🦀✨

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Conclusion: Master Memory Safety with Rust’s Ownership 🎓

Rust’s ownership system, borrowing rules, and lifetime annotations form a revolutionary approach to memory safety that eliminates entire classes of bugs at compile time. By understanding these core concepts – ownership transfer, borrowing constraints, and zero-cost abstractions – you can build blazing-fast, memory-safe applications without garbage collection, from embedded systems to high-performance web servers powering production infrastructure.