https://github.com/mbg/network-wait
A lightweight Haskell library for waiting on networked services to become available
https://github.com/mbg/network-wait
automation docker haskell haskell-library networking
Last synced: about 1 month ago
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A lightweight Haskell library for waiting on networked services to become available
- Host: GitHub
- URL: https://github.com/mbg/network-wait
- Owner: mbg
- License: mit
- Created: 2022-02-21T20:40:43.000Z (over 3 years ago)
- Default Branch: main
- Last Pushed: 2024-08-20T01:20:05.000Z (10 months ago)
- Last Synced: 2025-05-05T20:20:03.307Z (about 1 month ago)
- Topics: automation, docker, haskell, haskell-library, networking
- Language: Haskell
- Homepage: https://mbg.github.io/network-wait/
- Size: 149 KB
- Stars: 1
- Watchers: 2
- Forks: 2
- Open Issues: 3
-
Metadata Files:
- Readme: README.md
- Changelog: ChangeLog.md
- License: LICENSE
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README
# network-wait
[](https://hackage.haskell.org/package/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.
[**Full Haddock documentation for all modules**](https://mbg.github.io/network-wait/)
## Example
All functions provided by this library work with retry policies from [`Control.Retry`](https://hackage.haskell.org/package/retry) which provide good control over the retry behaviour. To wait for a PostgreSQL server to become available on the same machine:
```haskell
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:
```haskell
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:
```haskell
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](https://github.com/eficode/wait-for) is a popular shell script with the same objectives as this library.
- The [port-utils](https://hackage.haskell.org/package/port-utils) package has some functions for waiting on TCP servers to become available, with a fixed timeout.