//! Provides the connection pool for asynchronous SQLx connections.
//!
//! Opening a database connection for each and every operation to the database can quickly
//! become expensive. Furthermore, sharing a database connection between threads and functions
//! can be difficult to express in Rust.
//!
//! A connection pool is a standard technique that can manage opening and re-using connections.
//! Normally it also enforces a maximum number of connections as these are an expensive resource
//! on the database server.
//!
//! SQLx provides a canonical connection pool implementation intended to satisfy the majority
//! of use cases.
//!
//! See [Pool][crate::pool::Pool] for details.
//!
//! Type aliases are provided for each database to make it easier to sprinkle `Pool` through
//! your codebase:
//!
//! * [MssqlPool][crate::mssql::MssqlPool] (MSSQL)
//! * [MySqlPool][crate::mysql::MySqlPool] (MySQL)
//! * [PgPool][crate::postgres::PgPool] (PostgreSQL)
//! * [SqlitePool][crate::sqlite::SqlitePool] (SQLite)
//!
//! # Opening a connection pool
//!
//! A new connection pool with a default configuration can be created by supplying `Pool`
//! with the database driver and a connection string.
//!
//! ```rust,ignore
//! use sqlx::Pool;
//! use sqlx::postgres::Postgres;
//!
//! let pool = Pool::<Postgres>::connect("postgres://")?;
//! ```
//!
//! For convenience, database-specific type aliases are provided:
//!
//! ```rust,ignore
//! use sqlx::mssql::MssqlPool;
//!
//! let pool = MssqlPool::connect("mssql://")?;
//! ```
//!
//! # Using a connection pool
//!
//! A connection pool implements [`Executor`][crate::executor::Executor] and can be used directly
//! when executing a query. Notice that only an immutable reference (`&Pool`) is needed.
//!
//! ```rust,ignore
//! sqlx::query("DELETE FROM articles").execute(&pool)?;
//! ```
//!
//! A connection or transaction may also be manually acquired with
//! [`Pool::acquire`] or
//! [`Pool::begin`].
use SharedPool;
use crate Connection;
use crate Database;
use crate Error;
use crate Transaction;
use fmt;
use Arc;
use ;
pub use PoolConnection;
pub use MaybePoolConnection;
pub use PoolOptions;
/// An asynchronous pool of SQLx database connections.
///
/// Create a pool with [Pool::connect] or [Pool::connect_with] and then call [Pool::acquire]
/// to get a connection from the pool; when the connection is dropped it will return to the pool
/// so it can be reused.
///
/// You can also pass `&Pool` directly anywhere an `Executor` is required; this will automatically
/// checkout a connection for you.
///
/// See [the module documentation](crate::pool) for examples.
///
/// The pool has a maximum connection limit that it will not exceed; if `acquire()` is called
/// when at this limit and all connections are checked out, the task will be made to wait until
/// a connection becomes available.
///
/// You can configure the connection limit, and other parameters, using [PoolOptions][crate::pool::PoolOptions].
///
/// Calls to `acquire()` are fair, i.e. fulfilled on a first-come, first-serve basis.
///
/// `Pool` is `Send`, `Sync` and `Clone`. It is intended to be created once at the start of your
/// application/daemon/web server/etc. and then shared with all tasks throughout the process'
/// lifetime. How best to accomplish this depends on your program architecture.
///
/// In Actix-Web, for example, you can share a single pool with all request handlers using [web::Data].
///
/// Cloning `Pool` is cheap as it is simply a reference-counted handle to the inner pool state.
/// When the last remaining handle to the pool is dropped, the connections owned by the pool are
/// immediately closed (also by dropping). `PoolConnection` returned by [Pool::acquire] and
/// `Transaction` returned by [Pool::begin] both implicitly hold a reference to the pool for
/// their lifetimes.
///
/// If you prefer to explicitly shutdown the pool and gracefully close its connections (which
/// depending on the database type, may include sending a message to the database server that the
/// connection is being closed), you can call [Pool::close] which causes all waiting and subsequent
/// calls to [Pool::acquire] to return [Error::PoolClosed], and waits until all connections have
/// been returned to the pool and gracefully closed.
///
/// Type aliases are provided for each database to make it easier to sprinkle `Pool` through
/// your codebase:
///
/// * [MssqlPool][crate::mssql::MssqlPool] (MSSQL)
/// * [MySqlPool][crate::mysql::MySqlPool] (MySQL)
/// * [PgPool][crate::postgres::PgPool] (PostgreSQL)
/// * [SqlitePool][crate::sqlite::SqlitePool] (SQLite)
///
/// [web::Data]: https://docs.rs/actix-web/3/actix_web/web/struct.Data.html
///
/// ### Why Use a Pool?
///
/// A single database connection (in general) cannot be used by multiple threads simultaneously
/// for various reasons, but an application or web server will typically need to execute numerous
/// queries or commands concurrently (think of concurrent requests against a web server; many or all
/// of them will probably need to hit the database).
///
/// You could place the connection in a `Mutex` but this will make it a huge bottleneck.
///
/// Naively, you might also think to just open a new connection per request, but this
/// has a number of other caveats, generally due to the high overhead involved in working with
/// a fresh connection. Examples to follow.
///
/// Connection pools facilitate reuse of connections to _amortize_ these costs, helping to ensure
/// that you're not paying for them each time you need a connection.
///
/// ##### 1. Overhead of Opening a Connection
/// Opening a database connection is not exactly a cheap operation.
///
/// For SQLite, it means numerous requests to the filesystem and memory allocations, while for
/// server-based databases it involves performing DNS resolution, opening a new TCP connection and
/// allocating buffers.
///
/// Each connection involves a nontrivial allocation of resources for the database server, usually
/// including spawning a new thread or process specifically to handle the connection, both for
/// concurrency and isolation of faults.
///
/// Additionally, database connections typically involve a complex handshake including
/// authentication, negotiation regarding connection parameters (default character sets, timezones,
/// locales, supported features) and upgrades to encrypted tunnels.
///
/// If `acquire()` is called on a pool with all connections checked out but it is not yet at its
/// connection limit (see next section), then a new connection is immediately opened, so this pool
/// does not _automatically_ save you from the overhead of creating a new connection.
///
/// However, because this pool by design enforces _reuse_ of connections, this overhead cost
/// is not paid each and every time you need a connection. In fact you set the `min_connections`
/// option in [PoolOptions], the pool will create that many connections up-front so that they are
/// ready to go when a request comes in.
///
/// ##### 2. Connection Limits (MySQL, MSSQL, Postgres)
/// Database servers usually place hard limits on the number of connections that it allows open at
/// any given time, to maintain performance targets and prevent excessive allocation of resources,
/// namely RAM.
///
/// These limits have different defaults per database flavor, and may vary between different
/// distributions of the same database, but are typically configurable on server start;
/// if you're paying for managed database hosting then the connection limit will typically vary with
/// your pricing tier.
///
/// In MySQL, the default limit is typically 150, plus 1 which is reserved for a user with the
/// `CONNECTION_ADMIN` privilege so you can still access the server to diagnose problems even
/// with all connections being used.
///
/// In MSSQL the only documentation for the default maximum limit is that it depends on the version
/// and server configuration.
///
/// In Postgres, the default limit is typically 100, minus 3 which are reserved for superusers
/// (putting the default limit for unprivileged users at 97 connections).
///
/// In any case, exceeding these limits results in an error when opening a new connection, which
/// in a web server context will turn into a `500 Internal Server Error` if not handled, but should
/// be turned into either `403 Forbidden` or `429 Too Many Requests` depending on your rate-limiting
/// scheme. However, in a web context, telling a client "go away, maybe try again later" results in
/// a sub-optimal user experience.
///
/// Instead with a connection pool, clients are made to wait in a fair queue for a connection to
/// become available; by using a single connection pool for your whole application, you can ensure
/// that you don't exceed the connection limit of your database server while allowing response
/// time to degrade gracefully at high load.
///
/// Of course, if multiple applications are connecting to the same database server, then you
/// should ensure that the connection limits for all applications add up to your server's maximum
/// connections or less.
///
/// ##### 3. Resource Reuse
/// The first time you execute a query against your database, the database engine must first turn
/// the SQL into an actionable _query plan_ which it may then execute against the database. This
/// involves parsing the SQL query, validating and analyzing it, and in the case of Postgres 12+ and
/// SQLite, generating code to execute the query plan (native or bytecode, respectively).
///
/// These database servers provide a way to amortize this overhead by _preparing_ the query,
/// associating it with an object ID and placing its query plan in a cache to be referenced when
/// it is later executed.
///
/// Prepared statements have other features, like bind parameters, which make them safer and more
/// ergonomic to use as well. By design, SQLx pushes you towards using prepared queries/statements
/// via the [Query][crate::query::Query] API _et al._ and the `query!()` macro _et al._, for
/// reasons of safety, ergonomics, and efficiency.
///
/// However, because database connections are typically isolated from each other in the database
/// server (either by threads or separate processes entirely), they don't typically share prepared
/// statements between connections so this work must be redone _for each connection_.
///
/// As with section 1, by facilitating reuse of connections, `Pool` helps to ensure their prepared
/// statements (and thus cached query plans) can be reused as much as possible, thus amortizing
/// the overhead involved.
///
/// Depending on the database server, a connection will have caches for all kinds of other data as
/// well and queries will generally benefit from these caches being "warm" (populated with data).
;
/// Returns a new [Pool] tied to the same shared connection pool.
/// get the time between the deadline and now and use that as our timeout
///
/// returns `Error::PoolTimedOut` if the deadline is in the past