network-wait
Lightweight library for waiting on networked services to become available.
https://github.com/mbg/network-wait#readme
LTS Haskell 22.33: | 0.2.0.0 |
Stackage Nightly 2024-09-07: | 0.2.0.0 |
Latest on Hackage: | 0.2.0.0 |
network-wait-0.2.0.0@sha256:c9fd762f125b46a9fef5ff477b766c7ee38def659cdc05a2add8f4c6830db54a,3031
Module documentation for 0.2.0.0
- Network
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.
Example: Redis
The network-wait
package can be compiled with the network-wait:redis
flag (e.g. stack build --flag network-wait:redis
) which enables support for checking the readiness of Redis servers specifically. Unlike the functions in the Network.Wait
module, which only check that connections can be established, the functions in Network.Wait.Redis
also check that a Redis server is ready to accept commands. To wait for a Redis server to become available and accept commands:
import Control.Retry (retryPolicyDefault)
import Database.Redis (defaultConnectInfo)
import Network.Wait.Redis (waitRedis)
main :: IO ()
main = do
waitRedis retryPolicyDefault defaultConnectInfo
putStrLn "Yay, the Redis server is ready to accept commands!"
Internally, this uses hedis
to connect to the specified server (defaultConnectInfo
in the example above) and send a ping. If the ping is answered, we consider the server to be in a state ready to accept commands.
The Network.Wait.Redis
module is gated behind the network-wait:redis
flag so that the Redis-specific dependencies are only required when Redis 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.
Changes
Changelog for network-wait
0.2.0
- Add
Network.Wait.Redis
module with functions to wait for Redis servers to become ready to accept connections. This module and its dependency onhedis
are not enabled by default. Thenetwork-wait:redis
flag must be enabled for this package’s Redis support. - Generalise the
recoveringWith
function to allow it to return the result of a computation. - The functions for TCP/Sockets now return the
Socket
if they are successful. - The functions for PostgreSQL now return the
Connection
if they are successful.
0.1.2
- Compatibility with GHC 8.2 and Stack LTS 11
0.1.1
- Add
Network.Wait.PostgreSQL
module with functions to wait for PostgreSQL servers to become ready to accept connections. This module and its dependency onpostgresql-simple
are not enabled by default. Thenetwork-wait:postgres
flag must be enabled for this package’s PostgreSQL support.
0.1.0
- First release