Testing Simultaneous HTTP Requests in Parallel using cURL

If you're developing a web application and are worried that a similar HTTP request could come in multiple times from different users (or clients) exactly at the same time, you can see how your application will behave by using cURL (on modern versions of Linux) to create this rare (if almost impossible) circumstance:

curl --parallel --parallel-immediate --parallel-max 3 --header "Content-Type: application/json" --request POST --data '{STRINGIFIED_JSON_PAYLOAD}' --config urls.txt

The urls.txt file will contain the URL to your API endpoint you're testing the same number of times as the –parallel-max parameter.  So in our case, it would contain:

url = "https://pathtoapiendpoint"
url = "https://pathtoapiendpoint"
url = "https://pathtoapiendpoint"

Check how your application behaves and make appropriate changes to maintain concurrency if you're worried about this happening.  There are various approaches you can use to make sure concurrency is maintained such as appending the old value of the database record into your where clause when updating to see if the data has already been changed within the timeframe of the request being processed. 

Or, you can use persistent virtual columns like this:

https://stackoverflow.com/questions/54338201/mysql-prevent-insertion-of-record-if-id-and-timestamp-unique-combo-constraint 

cURL and wget Issues on Ubuntu 16.04

When using wget or curl to make HTTP requests from a no longer supported installation of Ubuntu 16.04 Xenial, if you get any of the following errors:

curl gnutls_handshake() failed: Error in protocol version
curl: (35) error:1407742E:SSL routines:SSL23_GET_SERVER_HELLO:tlsv1 alert protocol version  /home/mohan/mesg
[SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:727) 

The solution is to add SavOS Rob Savoury PPAs to get updated curl and wget packages:

sudo add-apt-repository ppa:savoury1/build-tools
sudo add-apt-repository ppa:savoury1/backports
sudo add-apt-repository ppa:savoury1/python
sudo add-apt-repository ppa:savoury1/encryption
sudo add-apt-repository ppa:savoury1/curl34
sudo apt-get update && sudo apt-get upgrade
sudo apt-get install wget curl python2.7