network-wait

A lightweight Haskell library for waiting on networked services to become available. This is useful if you are e.g. building a web application which relies on a database server to be available, but which may not be immediately available on application startup.

Example

All functions provided by this library work with retry policies from Control.Retry which provide good control over the retry behaviour. To wait for a PostgreSQL server to become available on the same machine:

import Control.Retry
import Network.Wait

main :: IO ()
main = do
    waitTcp retryPolicyDefault "localhost" "5432"
    putStrLn "Yay, the server is available!"

The haddock documentation for this library contains more examples.

Example: PostgreSQL

The network-wait package can be compiled with the network-wait:postgres flag (e.g. stack build --flag network-wait:postgres) which enables support for checking the readiness of PostgreSQL servers specifically. Unlike the functions in the Network.Wait module, which only check that connections can be established, the functions in Network.Wait.PostgreSQL also check that a PostgreSQL server is ready to accept commands. To wait for a PostgreSQL server to become available and accept commands:

import Control.Retry (retryPolicyDefault)
import Database.PostgreSQL.Simple (defaultConnectInfo)
import Network.Wait.PostgreSQL (waitPostgreSQL)

main :: IO ()
main = do
    waitPostgreSQL retryPolicyDefault defaultConnectInfo
    putStrLn "Yay, the PostgreSQL server is ready to accept commands!"

Internally, this uses postgresql-simple to connect to the specified server (defaultConnectInfo in the example above) and send a SELECT 1; query. If the query is answered correctly, we consider the server to be in a state ready to accept commands.

The Network.Wait.PostgreSQL module is gated behind the network-wait:postgres flag so that the PostgreSQL-specific dependencies are only required when PostgresSQL support is required by users of this library.

See also

• wait-for is a popular shell script with the same objectives as this library.
• The port-utils package has some functions for waiting on TCP servers to become available, with a fixed timeout.