I built a tool that shows you exactly which method slowed down after your last deploy published: false

You deployed. p99 latency spiked. Now what?

Open Grafana. Check Jaeger. Dig through logs. Read the commit diff. Connect the dots yourself — every single time.

I got tired of that loop, so I built lofi: a zero-config library that links deploy events to method-level latency and lets you diff them from the terminal.

$ lofi diff a3f9c1..d82e04



Deploy Diff  a3f9c1 → d82e04

───────────────────────────────────────────────────────────────────

Method                          Before     After      Delta



───────────────────────────────────────────────────────────────────

OrderService.createOrder()      14.23ms  → 91.00ms   +76.77ms  ▲

PaymentClient.validate()        22.10ms  → 58.40ms   +36.30ms  ▲

UserService.findById()          3.05ms   → 3.12ms    +0.07ms   —

───────────────────────────────────────────────────────────────────

2 regression(s) detected

Regressed methods show in red. No dashboards required.

How it works

lofi uses Spring AOP to automatically instrument every @Service, @Component, @Repository, @Controller, and @RestController bean in your application. No annotations on your business code. No agent. No code changes beyond adding the dependency.

When your app starts, it reads a GIT_COMMIT_HASH environment variable to know which deploy is running. Every method call gets timed (in nanoseconds) and flushed asynchronously to a local SQLite database

at ~/.lofi/metrics.db. When you're ready to compare two deploys, you run lofi diff and it calls the actuator endpoint to compute the regression diff.

The library is careful about what it instruments:

• Spring internals (org.springframework.*) → skipped


• Jakarta Servlet filters and MVC interceptors → skipped (avoids Security filter chain conflicts)


• AspectJ @aspect classes → skipped (avoids proxy-on-proxy chaos)
  


• JDK dynamic proxies like Spring Data JPA repositories → skipped (their time is already captured through the enclosing service call)
  

Getting started in 5 steps

1. Add the dependency

Gradle:



implementation 'io.github.closeup1202:lofi-spring-boot-starter:0.1.7'

Maven:



<dependency>

  <groupId>io.github.closeup1202</groupId>

  <artifactId>lofi-spring-boot-starter</artifactId>

  <version>0.1.7</version>

</dependency>

2. Expose actuator endpoints

  management:

    endpoints:

      web:

        exposure:

          include: lofi, lofiDiff

3. Set the commit hash and run

export GIT_COMMIT_HASH=$(git rev-parse --short HEAD)

./gradlew bootRun

# [LO-FI] Monitoring active — commit: a3f9c1 | store: sqlite | regression-threshold: 0.2

4. Verify metrics via actuator

Send some traffic to your app, then check the raw JSON directly:

curl http://localhost:8080/actuator/lofi/a3f9c1



  {

    "commitHash": "a3f9c1",

    "deployedAt": "2024-11-01T09:00:00Z",

    "metrics": [

      {

        "className": "com.example.OrderService",

        "methodName": "createOrder",

        "elapsedMs": 14.23,

        "recordedAt": "2024-11-01T09:01:23Z"

      }

    ]

  }

Once you have two deploys' worth of data, you can also diff them directly:

curl "http://localhost:8080/actuator/lofiDiff?base=a3f9c1&head=d82e04"

5. Install the CLI for a better view

The CLI renders the same data as a formatted table with regression highlighting — easier to read at a glance than raw JSON.

Install the CLI

npm install -g @closeup1202/lofi-cli

Two commands:

• lofi diff .. — compare latency between two deploys
  


• lofi snapshot — inspect metrics for a single deploy
  

lofi diff a3f9c1..d82e04 --url http://localhost:9090

lofi snapshot a3f9c2 --url http://localhost:9090

What it stores (and where)

All data stays local. lofi writes a SQLite file to ~/.lofi/metrics.db. No telemetry, no cloud, nothing leaves your machine unless you opt in to a dashboard (coming later).

If you're running in Docker or Kubernetes, mount a volume at /root/.lofi so the database survives container restarts:

  docker run \ 

    -e GIT_COMMIT_HASH=$(git rev-parse --short HEAD) \

    -v $HOME/.lofi:/root/.lofi \

    my-app

Spring Security note

If your app uses Spring Security, the actuator endpoints return 403 by default. The cleanest fix is management port isolation — run actuator on a separate internal port that's never exposed publicly:

management:

  server:

    port: 9090



lofi diff a3f9c1..d82e04 --url http://localhost:9090

No security config changes needed.

Configuration

  lofi:

    store-type: sqlite              # or in-memory (for tests/dev)

    regression-threshold: 0.2       # 20% increase = regression

    buffer:

      flush-threshold: 100

      flush-delay-ms: 5000

      queue-capacity: 1000

JSR-303 validation runs at startup — if you misconfigure a value, all violations are reported at once rather than stopping at the first one.

Current state and roadmap

lofi is in early development. It currently works best in single-pod environments (each pod has its own SQLite file). Multi-pod metric aggregation and a team dashboard are on the roadmap.

Requires Spring Boot 3.x and Java 17+.

• GitHub: https://github.com/closeup1202/lofi
  


• npm: https://www.npmjs.com/package/@closeup1202/lofi-cli
  


• Maven Central:
  https://central.sonatype.com/artifact/io.github.closeup1202/lofi-spring-boot-starter
  

Feedback welcome — open an issue or drop a comment below.