Does AWS lambda store global variables?

Question

I have created simple increment counter as below.

global.counter = 0;
exports.handler = (event, context, callback) => {
    // TODO implement
    callback(null, ++global.counter);
};

Whenever I test this function, I get incremented value as expected.

Is this right approach or do I need to store counter in cloud database?

Answer 1

When you call your Lambda function for the first time, a container is bootstrapped in the background by AWS.

When you call your Lambda function several times, you MIGHT get the same container to optimise run duration and setup delay.

You can't rely on this behavior but you must be aware it exists.

For you global vars, if you need to run you logic in Lambda, it should be working for several function to call the same code at the same time and should therefore be locked somewhere outside lambda code(DB or cache).

Regards,

Answer 2

I had the same issue, and I ended up using AWS Elasticache service (Memcached) to store (key,value) pairs. It's quick, pretty easy to use (from python at least) and reliable.

Otherwise I completely agree with Thomas, you should be aware as it can cause issues, but do not trust this "feature".