Spring Boot - Warm Me Up

Belonging StackOverflow question: https://stackoverflow.com/q/69760176/324531

Thing to Demonstrate

Small example to demonstrate that the first request hitting a Spring Boot app is slow.

To experience the problem, following these steps:

• build & run the app (see below)
• execute REST request twice (see below)
• notice that 1st request was much slower than sub-sequent requests
• stop app
• enable pre-loading in source code (see below)
• build & run the app
• execute REST request twice
• notice that 1st request is now roughly as fast as sub-sequent requests

Possible Cause

It seems that on 1st request, Jackson is initializing "something" needed for parsing request and response messages. On every sub-sequent request, this instance is re-used. This seems to happen in Spring's AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters calling into AbstractJackson2HttpMessageConverter

Question

How to initialize Jackson in PreloadComponent without doing an actual REST request???

Answer

By adding the JVM option

-verbose:class

I can see what classes get loaded on each request. I noticed that a lot of Jackson validation classes are in the output when doing the first request.

To have a fast 1st request, one must do:

• create a distinct warm-up endpoint
• this endpoint must do input validation (@Valid) on request body and input parameters
• it can use an own independent DTO
• this DTO should contain all Java validation annotations also used in the real DTOs
• query this warm-up endpoint during start up

When enabling this preloading, 1st request is only 2-3 times slower than sub-sequent requests. Without preloading, 1st request is about 20-40 times slower than sub-sequent requests.

Enable Preloading in Source Code

Edit PreloadComponent.java and uncomment the following line:

//        sendWarmUpRestRequest();

PreloadComponent is executed when Spring was fully initialized. It sends a REST request to /warmup endpoint. This endpoint is in a different controller then the "production" endpoint and has its own independent DTO, etc.

Don't forget to build & run the app after changing the source.

Build & Run

Use Maven to build the application:

mvn clean package

Afterwards, you can run the app via command line:

java -jar target/warm-me-up*.jar

To get output when classes get loaded by the JVM, use:

java -verbose:class -jar target/warm-me-up*.jar

REST Request

Use curl to run REST requests. Use time to measure execution time.

time curl --location --request POST 'http://localhost:8080/api' \
--header 'Content-Type: application/json' \
--data-raw '{
"inputMessage": "abc",
"someNumber": 123.4,
"patternString": "this is a fixed string",
"selectOne": "TWO"
}'

The response is printed on standard out and will look similar to:

{"message":"abc","uuid":"892c5f35-10ac-4439-8ba4-088d5ea1ae05"}curl --location --request POST 
'http://localhost:8080/' --header  --data-raw   0,01s user 0,00s system 3% cpu 0,221 total

The number at the end is the runtime in milli-seconds, 221ms in the given example.