NIOPostgres: A NIO-based PostgreSQL Driver
- Proposal: SSWG-0003
- Author(s): Tanner Nelson
- Review Manager: Tomer Doron
- Status: Implemented
- Implementation: vapor/nio-postgres
- Forum Threads: Pitch, Discussion, Feedback
- Review ends: May 16th, 2019
What goes into a review of a proposal?
The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the evolution of the server-side Swift ecosystem.
When reviewing a proposal, here are some questions to consider:
- What is your evaluation of the proposal?
- Is the problem being addressed significant enough?
- Does this proposal fit well with the feel and direction of Swift on Server?
- If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
- How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
Thank you for contributing to the Swift Server Work Group!
What happens if the proposal gets accepted?
If this proposal gets accepted, the official repository will be created and the code (minus examples, the proposal text, etc) will be submitted. The repository will then become usable as a SwiftPM package and a version (likely 0.1.0 ) will be tagged. The development (in form of pull requests) will continue as a regular open-source project.
Introduction
NIOPostgres is a client package for connecting to, authorizing, and querying a PostgreSQL server. At the heart of this module are channel handlers for parsing and serializing messages in PostgreSQL's proprietary wire protocol. These channel handlers are combined in a request / response style connection type that provides a convenient, client-like interface for performing queries. Support for both simple (text) and parameterized (binary) querying is provided out of the box alongside a PostgresData type that handles conversion between PostgreSQL's wire format and native Swift types.
Motiviation
Most Swift implementations of Postgres clients are based on the libpq C library which handles transport internally. Building a library directly on top of Postgres' wire protocol using SwiftNIO should yield a more reliable, maintainable, and performant interface for PostgreSQL databases.
Dependencies
This package has four dependencies:
-
swift-niofrom2.0.0 -
swift-nio-sslfrom2.0.0 -
swift-logfrom1.0.0 -
swift-metricsfrom1.0.0
This package has no additional system dependencies.
Proposed Solution
This section goes into detail on a few distinct types from this module to give an idea of how they work together and what using the package looks like.
PostgresConnection
The base connection type, PostgresConnection, is a wrapper around NIO's ClientBootstrap that initializes the pipeline to communicate via Postgres messages using a request / response pattern.
import NIOPostgres
// create a new event loop group
let elg = MultiThreadedEventLoopGroup(numberOfThreads: 1)
defer { try! elg.syncShutdownGracefully() }
// create a new connection
let address = try SocketAddress(ipAddress: "127.0.0.1", port: 5432)
let conn = try PostgresConnection.connect(
to: address,
// optionally configure TLS
tlsConfiguration: .forClient(certificateVerification: .none),
serverHostname: "127.0.0.1"
on: elg.eventLoop
).wait()
defer { try! conn.close().wait() }
// authenticate the connection using credentials
try conn.authenticate(username: "username", database: "database", password: "password").wait()
// ready to query
print(conn) // PostgresConnection
Closing
A connection must be closed before it deinitializes. PostgresConnection ensures this by asserting that it has been closed in its deinit handler. This is meant to help developers implement proper graceful shutdown early and avoid leaking memory or sockets.
Simple Query
Assuming we have an active, authenticated PostgresConnection, we can query the connected server using PostgreSQL's simple, text format.
import NIOPostgres
let conn: PostgresConnection ...
// select the current version
let rows = try conn.simpleQuery("SELECT version()").wait()
print(rows) // [PostgresRow]
// fetch the version column from the first row casting it to a Swift string
let version = rows[0].column("version")?.string
print(version) // String?
This format does not support parameterizing input and returns all data in string format. To bind values, insert them into the string:
try conn.simpleQuery("SELECT * FROM planets WHERE name = 'Earth'")
Query
We can also perform parameterized queries with an active PostgresConnection. These queries support binding input parameters and return data in a more compact binary format.
Input parameters are passed as an array of PostgresData following the SQL string. In the query string, input parameters are referenced by incrementing placeholders, starting with $1.
import NIOPostgres
let conn: PostgresConnection ...
// selects all planets where name is equal to the first bound parameter
let rows = try conn.query("SELECT * FROM planets WHERE name = $1", ["Earth"]).wait()
// fetch the "name" column from the first row, casting it to a string
let foo = rows[0].column("name")?.string
print(foo) // "Earth"
PostgresData
PostgresData represents data both going to and coming from Postgres.
Input
An array of PostgresData is supplied alongside parameterized queries, one for each parameter. There are many initializers for creating PostgresData from Swift's standard types. For example:
import NIOPostgres
let string = PostgresData(string: "Hello")
let double = PostgresData(double: 3.14)
let date = PostgresData(date: Date(timeIntervalSince1970: 42))
PostgresData also conforms to Swift's Expressible... protocols, allowing for conversion between Swift literals.
import NIOPostgres
let inputs: [PostgresData] = ["hello", 3.14]
Output
Likewise, PostgresData can be converted back to Swift types. This is useful for converting data returned by Postgres queries into meaningful types. There are many methods for Swift's standard types, for example:
import NIOPostgres
let data: PostgresData
print(data.string) // String?
Here is a full list of types supported currently:
Swift.StringSwift.IntSwift.Int64Swift.Int32Swift.Int16Swift.Int8Swift.UIntSwift.UInt64Swift.UInt32Swift.UInt16Swift.UInt8Swift.FloatSwift.DoubleFoundation.DateFoundation.DataFoundation.UUID
PostgresRow
Both simpleQuery and query return an array of PostgresRow. Each row can be thought of as a dictionary with column names as the key and data as the value. While the actual storage implementation is private, PostgresRow gives the following method for accessing column data:
struct PostgresRow {
func column(_ column: String) -> PostgresData?
}
If no column with that name is contained by the row, nil is returned. Matching columns from any table will be returned on a first match basis.
PostgresError
The PostgresError type represents errors thrown from both the Postgres package itself (during parsing, for example) and errors returned by the server:
public enum PostgresError: Error {
case proto(String)
case server(PostgresMessage.Error)
case connectionClosed
var code: Code { ... }
}
The PostgresError.Code type is a large enum-like struct containing all recognized Postgres error codes. This is useful for error handling:
let conn: PostgresConnection ...
do {
_ = try conn.simpleQuery("SELECT &").wait()
} catch let error as PostgresError {
switch error.code {
case .syntaxError: ...
default: ...
}
}
PostgresClient
While PostgresConnection is the main type to use for connecting, authorizing, and requesting TLS, the PostgresClient protocol is sufficient for performing both text and parameterized queries.
protocol PostgresClient {
var eventLoop: EventLoop { get }
func send(_ request: PostgresRequest) -> EventLoopFuture<Void>
}
PostgresConnection is the only conformer that NIOPostgres provides, but it is expected that dependencies will add additional conformers. For example, a consumer of this package might add conformance to a pool of connections, allowing for automatic recycling as needed, a crucial feature for long-running applications.
Note on usage
Since most of NIOPostgres's convenience methods are added to PostgresClient instead of PostgresConnection directly, any additional conformers should feel exactly the same to use. Because of this, it is expected that PostgresClient should be used any place where you need to make queries. For example, in a theoretical controller:
final class UserController: Controller {
let db: PostgresClient
init(db: PostgresClient) {
self.db = db
}
func names(_ req: HTTPRequest) -> EventLoopFuture<[String]> {
return self.db.query("SELECT name FROM users").map { rows in
return rows.map { $0.column("name")?.string! }
}
}
}
Because this controller relies on PostgresClient, any of the following could be supplied to it:
- Connected
PostgresConnection - Pool of
PostgresConnections - Dummy conformer for testing
PostgresRequest
Postgres' wire protocol uses a request / response pattern, but unlike HTTP or Redis, one request can yield one or more responses. PostgresRequest conformers handle this with the following protocol.
protocol PostgresRequest {
func respond(to message: PostgresMessage) throws -> [PostgresMessage]?
func start() throws -> [PostgresMessage]
}
PostgresRequest is responsible for sending zero or more initial messages and handling the server's responses. When the request is complete, nil is returned by respond, causing the client's send future to complete.
CMD5
MD5 hashing is required for PostgreSQL's authentication flow. This module follows NIO's approach and embeds a private C-based implementation rather than relying on external Swift crypto libraries.
Todo / Discussion
Here are some things that are still a work in progress:
- Prepared Statement API: Postgres allows for parameterized queries to be re-used multiple times with different inputs. An API for doing this in NIO Postgres would be useful.
- PostgresRequest edge cases: Finer grain input / output from this protocol would be useful in assisting with protocol edge cases. For example, sometimes a Postgres error message can signal request completion depending on state.
How to use
To try this package out at home, add the following dependency to your Package.swift file:
.package(url: "https://github.com/vapor/nio-postgres.git", .branch("master")),
Then add "NIOPostgres" to your module target's dependencies array.
Seeking Feedback
- If anything, what does this proposal not cover that you will definitely need?
- If anything, what could we remove from this and still be happy?
- API-wise: what do you like, what don't you like?
Feel free to post feedback as response to this post and/or GitHub issues on vapor/nio-postgres.
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