15.2.3 Viewing application activity

It can be useful to keep an eye on activity in a running application, including the kinds of HTTP requests that the application is handling and the activity of all of the threads in the application. For this, Actuator provides the /httptrace, /threaddump, and /heapdump endpoints.

The /heapdump endpoint is perhaps the most difficult Actuator endpoint to describe in any detail. Put succinctly, it downloads a gzip-compressed HPROF heap dump file that can be used to track down memory or thread issues. For the sake of space and because use of the heap dump is a rather advanced feature, I’m going to limit coverage of the /heapdump endpoint to this paragraph.

TRACING HTTP ACTIVITY

The /httptrace endpoint reports details on the most recent 100 requests handled by an application. Details included are the request method and path, a timestamp indicating when the request was handled, headers from both the request and the response, and the time taken handling the request.

The following snippet of JSON code shows a single entry from the response of the /httptrace endpoint:

{
  "traces": [
    {
      "timestamp": "2020-06-03T23:41:24.494Z",
      "principal": null,
      "session": null,
      "request": {
        "method": "GET",
        "uri": "http://localhost:8081/ingredients",
        "headers": {
          "Host": ["localhost:8081"],
          "User-Agent": ["curl/7.54.0"],
          "Accept": ["*/*"]
        },
        "remoteAddress": null
      },
      "response": {
        "status": 200,
        "headers": {
          "Content-Type": ["application/json;charset=UTF-8"]
        }
      },
      "timeTaken": 4
    },
  ...
  ]
}

Although this information may be useful for debugging purposes, it’s even more interesting when the trace data is tracked over time, providing insight into how busy the application was at any given time as well as how many requests were successful compared to how many failed, based on the value of the response status. In chapter 16, you’ll see how Spring Boot Admin captures this information into a running graph that visualizes the HTTP trace information over a period of time.

MONITORING THREADS

In addition to HTTP request tracing, thread activity can also be useful in determining what’s going on in a running application. The /threaddump endpoint produces a snapshot of current thread activity. The following snippet from a /threaddump response gives a taste of what this endpoint provides:

{
  "threadName": "reactor-http-nio-8",
  "threadId": 338,
  "blockedTime": -1,
  "blockedCount": 0,
  "waitedTime": -1,
  "waitedCount": 0,
  "lockName": null,
  "lockOwnerId": -1,
  "lockOwnerName": null,
  "inNative": true,
  "suspended": false,
  "threadState": "RUNNABLE",
  "stackTrace": [
    {
      "methodName": "kevent0",
      "fileName": "KQueueArrayWrapper.java",
      "lineNumber": -2,
      "className": "sun.nio.ch.KQueueArrayWrapper",
      "nativeMethod": true
    },
    {
      "methodName": "poll",
      "fileName": "KQueueArrayWrapper.java",
      "lineNumber": 198,
      "className": "sun.nio.ch.KQueueArrayWrapper",
      "nativeMethod": false
    },
    ...
    ],
  "lockedMonitors": [
    {
      "className": "io.netty.channel.nio.SelectedSelectionKeySet",
      "identityHashCode": 1039768944,
      "lockedStackDepth": 3,
      "lockedStackFrame": {
        "methodName": "lockAndDoSelect",
        "fileName": "SelectorImpl.java",
        "lineNumber": 86,
        "className": "sun.nio.ch.SelectorImpl",
        "nativeMethod": false
      }
    },
  ...
  ],
  "lockedSynchronizers": [],
  "lockInfo": null
}

The complete thread dump report includes every thread in the running application. To save space, the thread dump here shows an abridged entry for a single thread. As you can see, it includes details regarding the blocking and locking status of the thread, among other thread specifics. There’s also a stack trace that gives some insight into which area of the code the thread is spending time on.

Because the /threaddump endpoint provides a snapshot of thread activity only at the time it was requested, it can be difficult to get a full picture of how threads are behaving over time. In chapter 16, you’ll see how Spring Boot Admin can monitor the /threaddump endpoint in a live view.