Introduction
When working with web services that involve sending data and receiving responses, it’s often crucial to understand how long different phases of a request take. This information can help diagnose performance issues or optimize service interactions. One powerful tool for this purpose is cURL, which allows detailed measurement of the various stages in an HTTP transaction.
What You Need to Know About cURL
cURL is a command-line tool used to transfer data using various protocols, most commonly HTTP and HTTPS. It provides options to customize requests extensively, including setting headers, choosing methods like GET or POST, and sending data payloads. Moreover, cURL can report detailed timing information for different phases of an HTTP transaction.
Timing Metrics in cURL
The following are common timing metrics you can measure with cURL:
- time_namelookup: Time taken to resolve the domain name.
- time_connect: Time taken to establish a connection to the server.
- time_appconnect: Time taken for establishing SSL/TLS handshake (if applicable).
- time_pretransfer: Time elapsed before the first byte is received, after the request has been made.
- time_redirect: Total time spent following redirects.
- time_starttransfer: Time at which the first byte is transferred.
- time_total: The total time taken for the entire transaction.
Measuring Timing with cURL
To measure these timings, cURL uses the -w or --write-out option to specify an output format. This option allows you to capture and display various timing metrics during a request.
Basic Usage Example
Here’s how to use cURL to send a POST request with data while measuring response times:
curl -X POST -d @file server:port -w 'time_namelookup: %{time_namelookup}s\n\
time_connect: %{time_connect}s\n\
time_appconnect: %{time_appconnect}s\n\
time_pretransfer: %{time_pretransfer}s\n\
time_redirect: %{time_redirect}s\n\
time_starttransfer: %{time_starttransfer}s\n\
time_total: %{time_total}s'
Explanation
@file: Specifies the file containing data to be sent with the request.server:port: The target server and port to which the request is directed.-X POST: Sets the HTTP method to POST.-w '...': Defines a format string to output timing metrics.
Advanced Usage
For more convenience, you can create a shell function or script to encapsulate this functionality. Here’s an example of adding such a function to your .bashrc file:
curl_time() {
curl -so /dev/null -w "\
namelookup: %{time_namelookup}s\n\
connect: %{time_connect}s\n\
appconnect: %{time_appconnect}s\n\
pretransfer: %{time_pretransfer}s\n\
redirect: %{time_redirect}s\n\
starttransfer: %{time_starttransfer}s\n\
-------------------------\n\
total: %{time_total}s\n" "$@"
}
How to Use the Function
Once you’ve added curl_time to your shell configuration, you can use it like so:
curl_time -X POST -H "Content-Type: application/json" -d '{"key": "val"}' https://postman-echo.com/post
This will output detailed timing metrics similar to the following:
namelookup: 0.125000s
connect: 0.250000s
appconnect: 0.609000s
pretransfer: 0.609000s
redirect: 0.000000s
starttransfer: 0.719000s
-------------------------
total: 0.719000s
Conclusion
By leveraging cURL‘s timing capabilities, you can gain valuable insights into the performance characteristics of your web requests. Whether you are debugging latency issues or optimizing application performance, these techniques provide a straightforward approach to measuring request and response times effectively.