Building a Reliable Redis Connection Pool in Rust: A Guide

Imagine you’re managing a team of bees buzzing around, collecting data from fields of flowers. Each bee (or connection) has to be quick, efficient, and ready to switch to the next task without delay.

But here’s the catch: you only have so many bees, and if they’re all out, the others waiting around won’t be too thrilled! This is precisely the situation with database connections in an application — particularly with Redis, a lightning-fast key-value store widely used for caching, session management, and more.

A connection pool helps us manage these busy bees, making sure our hive (or application) hums smoothly, even under load.

The Problem: Lack of a Maintained Redis Connection Pool Library

In Rust, while there’s a growing ecosystem for database pooling, we face a gap when it comes to Redis. Existing libraries aren’t actively maintained, leaving developers in a bind.

Writing a Redis connection pool from scratch might sound like a tough nut to crack, but with the right approach, it can be manageable, efficient, and customized to your specific needs. You can find the full code here.

Why We Need a Connection Pool

A connection pool is more than just a convenience. It’s critical for:

1. Efficient Resource Management: Connections are reused, avoiding the overhead of creating and tearing down connections with each request.

2. Improved Performance: With connections at the ready, response times are reduced.

3. Scalability: A pool of connections makes it easier to handle spikes in demand without overloading your Redis instance.

Enter deadpool: Our Pooling Backbone

The deadpool crate provides a simple and flexible framework for managing pools of objects, including asynchronous database connections.

It abstracts the pooling logic, so we only need to focus on defining how connections are created, checked, and recycled. Deadpool works by associating a manager with a pool. This manager defines how to create, validate, and recycle connections.

Now, let’s dive into our code and build our Redis connection pool using deadpool.

Code Walkthrough: Building the Redis Connection Pool

Here’s how the connection pool comes together, piece by piece.

1. Setting Up the RedisConnectionManager

The RedisConnectionManager struct is the main manager of our connection pool. It’s responsible for creating and managing connections to Redis. Let’s look at the fields:

• client: This holds the Redis client, which is used to spawn connections.

• check_on_recycle: This boolean flag indicates if we should check the connection’s health when recycling it.

• connection_ttl: Defines the connection’s time-to-live (TTL), which determines how long a connection should remain in the pool.

Here’s the constructor:

impl RedisConnectionManager {
    pub fn new(client: redis::Client, check_on_recycle: bool, connection_ttl: Option<Ttl>) -> Self {
        Self {
            client,
            check_on_recycle,
            connection_ttl,
        }
    }
}

The new function initializes a manager with a Redis client and optional TTL settings.

2. Implementing the Manager Trait

The Manager trait is the heart of any poolable object in deadpool. Here, we define:

• create: Creates a new connection. If the TTL is specified, it calculates an expiration timestamp based on Simple, Fuzzy, or Once types.

• recycle: Revalidates a connection, ensuring it’s not expired.

• detach: (Optional) Cleans up the connection when it’s removed from the pool.

Let’s break down create and recycle:

async fn create(&self) -> Result<RedisConnection, RedisError> {
    Ok(RedisConnection {
        actual: self.client.get_multiplexed_async_connection().await?,
        expires_at: self.connection_ttl.as_ref().map(|ttl| {
            match ttl {
                Ttl::Simple(ttl) => Instant::now() + *ttl,
                Ttl::Fuzzy { min, fuzz } => Instant::now() + *min + Duration::from_secs_f64(rand::thread_rng().gen_range(0.0, fuzz.as_secs_f64())),
                Ttl::Once => Instant::now(),
            }
        }),
    })
}

This function initiates a new Redis connection with optional TTL expiration.

The recycle function, shown below, checks a connection’s health when recycling it and verifies if it’s expired.

async fn recycle(&self, mut conn: &mut Self::Type, _metrics: &Metrics) -> RecycleResult<Self::Error> {
    if self.check_on_recycle {
        let _r: bool = conn.exists(b"key").await?;
    }

    match &conn.expires_at {
        Some(expires_at) if &Instant::now() >= expires_at => {
            Err(RecycleError::Message(Cow::from("Connection expired")))
        }
        _ => Ok(()),
    }
}

Convenience Features: Treating the Connection Manager as a Redis Connection

To make our Redis connection pool even easier to use, we can implement several traits on RedisConnection. This allows RedisConnection to be treated like a direct Redis connection, making our code cleaner and more flexible.

• Deref and DerefMut: These traits let RedisConnection act as a reference to MultiplexedConnection, allowing it to be used directly for Redis commands.

• AsRef and AsMut: These traits let us pass a reference to the underlying connection easily.

• ConnectionLike: By implementing this trait, we integrate fully with the Redis command interface.

Here’s the implementation of these traits:

impl Deref for RedisConnection {
    type Target = redis::aio::MultiplexedConnection;
    fn deref(&self) -> &Self::Target {
        &self.actual
    }
}

impl DerefMut for RedisConnection {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.actual
    }
}
impl AsMut<redis::aio::MultiplexedConnection> for RedisConnection {
    fn as_mut(&mut self) -> &mut redis::aio::MultiplexedConnection {
        &mut self.actual
    }
}
impl AsRef<redis::aio::MultiplexedConnection> for RedisConnection {
    fn as_ref(&self) -> &redis::aio::MultiplexedConnection {
        &self.actual
    }
}
impl ConnectionLike for &mut RedisConnection {
    fn req_packed_command<'a>(&'a mut self, cmd: &'a Cmd) -> RedisFuture<'a, Value> {
        (**self).req_packed_command(cmd)
    }
    fn req_packed_commands<'a>(
        &'a mut self,
        cmd: &'a Pipeline,
        offset: usize,
        count: usize,
    ) -> RedisFuture<'a, Vec<Value>> {
        (**self).req_packed_commands(cmd, offset, count)
    }
    fn get_db(&self) -> i64 {
        (**self).get_db()
    }
}

With these trait implementations, RedisConnection can be seamlessly used wherever a MultiplexedConnection or ConnectionLike is required, making it incredibly versatile. This setup abstracts away the pool details, letting you work directly with Redis commands, just like you would with a raw Redis connection.

Wrapping Up

In this tutorial, we’ve created a Redis connection pool in Rust using the deadpool crate. By implementing custom connection expiration and recycling, this pool is efficient and resource-conscious. Not only is our Redis connection pool capable of handling a variety of TTL settings, but it’s also simple to extend or tweak as needed.

The next time you’re dealing with Redis connections in Rust, try building a pool — you’ll notice improved performance, a more responsive application, and happy “bees” ready to work. 🐝