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//! This module provides the functionality to cache the aggregated results fetched and aggregated
//! from the upstream search engines in a json format.
use error_stack::Report;
use futures::future::try_join_all;
use md5::compute;
use redis::{aio::ConnectionManager, AsyncCommands, Client, RedisError};
use super::error::CacheError;
/// A named struct which stores the redis Connection url address to which the client will
/// connect to.
#[derive(Clone)]
pub struct RedisCache {
/// It stores a pool of connections ready to be used.
connection_pool: Vec<ConnectionManager>,
/// It stores the size of the connection pool (in other words the number of
/// connections that should be stored in the pool).
pool_size: u8,
/// It stores the index of which connection is being used at the moment.
current_connection: u8,
}
impl RedisCache {
/// A function which fetches the cached json results as json string.
///
/// # Arguments
///
/// * `redis_connection_url` - It takes the redis Connection url address.
/// * `pool_size` - It takes the size of the connection pool (in other words the number of
/// connections that should be stored in the pool).
///
/// # Error
///
/// Returns a newly constructed `RedisCache` struct on success otherwise returns a standard
/// error type.
pub async fn new(
redis_connection_url: &str,
pool_size: u8,
) -> Result<Self, Box<dyn std::error::Error>> {
let client = Client::open(redis_connection_url)?;
let mut tasks: Vec<_> = Vec::new();
for _ in 0..pool_size {
tasks.push(client.get_tokio_connection_manager());
}
let redis_cache = RedisCache {
connection_pool: try_join_all(tasks).await?,
pool_size,
current_connection: Default::default(),
};
Ok(redis_cache)
}
/// A helper function which computes the hash of the url and formats and returns it as string.
///
/// # Arguments
///
/// * `url` - It takes an url as string.
fn hash_url(&self, url: &str) -> String {
format!("{:?}", compute(url))
}
/// A function which fetches the cached json results as json string from the redis server.
///
/// # Arguments
///
/// * `url` - It takes an url as a string.
///
/// # Error
///
/// Returns the results as a String from the cache on success otherwise returns a `CacheError`
/// on a failure.
pub async fn cached_json(&mut self, url: &str) -> Result<String, Report<CacheError>> {
self.current_connection = Default::default();
let hashed_url_string: &str = &self.hash_url(url);
let mut result: Result<String, RedisError> = self.connection_pool
[self.current_connection as usize]
.get(hashed_url_string)
.await;
// Code to check whether the current connection being used is dropped with connection error
// or not. if it drops with the connection error then the current connection is replaced
// with a new connection from the pool which is then used to run the redis command then
// that connection is also checked whether it is dropped or not if it is not then the
// result is passed as a `Result` or else the same process repeats again and if all of the
// connections in the pool result in connection drop error then a custom pool error is
// returned.
loop {
match result {
Err(error) => match error.is_connection_dropped() {
true => {
self.current_connection += 1;
if self.current_connection == self.pool_size {
return Err(Report::new(
CacheError::PoolExhaustionWithConnectionDropError,
));
}
result = self.connection_pool[self.current_connection as usize]
.get(hashed_url_string)
.await;
continue;
}
false => return Err(Report::new(CacheError::RedisError(error))),
},
Ok(res) => return Ok(res),
}
}
}
/// A function which caches the results by using the hashed `url` as the key and
/// `json results` as the value and stores it in redis server with ttl(time to live)
/// set to 60 seconds.
///
/// # Arguments
///
/// * `json_results` - It takes the json results string as an argument.
/// * `url` - It takes the url as a String.
///
/// # Error
///
/// Returns an unit type if the results are cached succesfully otherwise returns a `CacheError`
/// on a failure.
pub async fn cache_results(
&mut self,
json_results: &str,
url: &str,
) -> Result<(), Report<CacheError>> {
self.current_connection = Default::default();
let hashed_url_string: &str = &self.hash_url(url);
let mut result: Result<(), RedisError> = self.connection_pool
[self.current_connection as usize]
.set_ex(hashed_url_string, json_results, 60)
.await;
// Code to check whether the current connection being used is dropped with connection error
// or not. if it drops with the connection error then the current connection is replaced
// with a new connection from the pool which is then used to run the redis command then
// that connection is also checked whether it is dropped or not if it is not then the
// result is passed as a `Result` or else the same process repeats again and if all of the
// connections in the pool result in connection drop error then a custom pool error is
// returned.
loop {
match result {
Err(error) => match error.is_connection_dropped() {
true => {
self.current_connection += 1;
if self.current_connection == self.pool_size {
return Err(Report::new(
CacheError::PoolExhaustionWithConnectionDropError,
));
}
result = self.connection_pool[self.current_connection as usize]
.set_ex(hashed_url_string, json_results, 60)
.await;
continue;
}
false => return Err(Report::new(CacheError::RedisError(error))),
},
Ok(_) => return Ok(()),
}
}
}
}
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