An asynchronous, pipelined, PostgreSQL client.

Example

use futures::{Future, Stream};
use tokio_postgres::NoTls;

# #[cfg(not(feature = "runtime"))]
# let fut = futures::future::ok(());
# #[cfg(feature = "runtime")]
let fut =
// Connect to the database
tokio_postgres::connect("host=localhost user=postgres", NoTls)

.map(|(client, connection)| {
// The connection object performs the actual communication with the database,
// so spawn it off to run on its own.
let connection = connection.map_err(|e| eprintln!("connection error: {}", e));
tokio::spawn(connection);

// The client is what you use to make requests.
client
})

.and_then(|mut client| {
// Now we can prepare a simple statement that just returns its parameter.
client.prepare("SELECT $1::TEXT")
.map(|statement| (client, statement))
})

.and_then(|(mut client, statement)| {
// And then execute it, returning a Stream of Rows which we collect into a Vec
client.query(&statement, &[&"hello world"]).collect()
})

// Now we can check that we got back the same string we sent over.
.map(|rows| {
let value: &str = rows[0].get(0);
assert_eq!(value, "hello world");
})

// And report any errors that happened.
.map_err(|e| {
eprintln!("error: {}", e);
});

// By default, tokio_postgres uses the tokio crate as its runtime.
tokio::run(fut);

Behavior

Calling a method like Client::query on its own does nothing. The associated request is not sent to the database until the future returned by the method is first polled. Requests are executed in the order that they are first polled, not in the order that their futures are created.

Pipelining

The client supports pipelined requests. Pipelining can improve performance in use cases in which multiple, independent queries need to be executed. In a traditional workflow, each query is sent to the server after the previous query completes. In contrast, pipelining allows the client to send all of the queries to the server up front, minimizing time spent by one side waiting for the other to finish sending data:

Sequential                              Pipelined
| Client         | Server          |    | Client         | Server          |
|----------------|-----------------|    |----------------|-----------------|
| send query 1   |                 |    | send query 1   |                 |
|                | process query 1 |    | send query 2   | process query 1 |
| receive rows 1 |                 |    | send query 3   | process query 2 |
| send query 2   |                 |    | receive rows 1 | process query 3 |
|                | process query 2 |    | receive rows 2 |                 |
| receive rows 2 |                 |    | receive rows 3 |                 |
| send query 3   |                 |
|                | process query 3 |
| receive rows 3 |                 |

In both cases, the PostgreSQL server is executing the queries sequentially - pipelining just allows both sides of the connection to work concurrently when possible.

Pipelining happens automatically when futures are polled concurrently (for example, by using the futures join combinator):

use futures::Future;
use tokio_postgres::{Client, Error, Statement};

fn pipelined_prepare(
client: &mut Client,
) -> impl Future<Item = (Statement, Statement), Error = Error>
{
client.prepare("SELECT * FROM foo")
.join(client.prepare("INSERT INTO bar (id, name) VALUES ($1, $2)"))
}

Runtime

The client works with arbitrary AsyncRead + AsyncWrite streams. Convenience APIs are provided to handle the connection process, but these are gated by the runtime Cargo feature, which is enabled by default. If disabled, all dependence on the tokio runtime is removed.