detecting high server load using toobusy.js18 Mar 2017 | node.js
toobusy.js is a nice little Node.js library that helps you detect when your server is experiencing high load. The idea is that when your server is overwhelmed with requests, it is better to drop some requests than to stop working altogether.
toobusy.js works by measuring the lag, the duration an event is delayed in the event loop until it gets served. A server is considered busy if the lag gets above some predefined threshold.
There are two ways you can interact with toobusy.js: you can either register to a
LAG_EVENT event which fires once the server is found to be busy,
or you can actively query the busy state by calling the
Two examples for places you can use toobusy are in a middleware which returns Service Unavailable (HTTP status code 503) to client if server is busy, or in a health check endpoint which your load balancer pings.
Measuring the event loop lag
toobusy.js works by setting up a repeated event to fire every
interval ms, and measuring the time difference between successive
invocations. As we know, once an event is triggered it gets pushed to the back of the events queue,
but it only gets handled when all prior event handlers have finished and it reaches the head of the queue. Thus the event loop lag is
equal to the difference between the time passed since the previous event was handled and the interval itself.
Smoothing the measurement
It is normal for the measured lags time series to be quite erratic. Just like your CPU usage, it experiences many small spikes and they should not immediately indicate a high load event.
To reduce false positives, toobusy.js uses two techniques:
It smoothes the lag series fluctuations by applying exponential smoothing. Assuming
currentLagis the ongoing lag series and
lagis the lag just measured, we use .
Use some randomness. Even if
currentLaggoes over the threshold, the process is not always ‘too busy’. The farther it goes over the threshold, the more likely the process will be considered too busy. The percentage is equal to the percent over the max lag threshold. So 1.25x over the maxLag will indicate too busy 25% of the time. 2x over the maxLag threshold will indicate too busy 100% of the time.