metrics-agent

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JVM agent based metrics with Prometheus and Dropwizard support (Java, Scala, Clojure, Kotlin, etc)

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- [Overview](#overview)
  - [Motivation](#motivation)
    - [Code Bloat](#code-bloat)
  - [Instrumentation Metadata](#instrumentation-metadata)
    - [Annotations](#annotations)
    - [Configuration](#configuration)
      - [Class Imports](#class-imports)
    - [Metric Labels](#metric-labels)
      - [Dynamic Label Values](#dynamic-label-values)
    - [What we actually Transform](#what-we-actually-transform)
    - [Supported Languages](#supported-languages)
  - [Supported Metrics Systems](#supported-metrics-systems)
    - [Prometheus](#prometheus)
    - [Dropwizard](#dropwizard)
    - [Metric System Configuration](#metric-system-configuration)
      - [Adding JVM Level Metric Information](#adding-jvm-level-metric-information)
      - [Agent Reporting](#agent-reporting)
    - [Black & Black Lists](#black-and-white-lists)
    - [Logger Configuration](#logger-configuration)
  - [Performance](#performance)
  - [Dependencies](#dependencies)
- [Binaries & Releases](#binaries-releases)
- [Building](#building)
- [Usage](#usage)
- [Debugging](#debugging)
- [Examples](#examples)

# Overview

## Motivation
Agent based bytecode instrumentation is a far more elegant, faster and safer approach to instrumenting code on the JVM. Programmatic addition of metrics into client code leads to severe code bloat and lack of clarity of the underlying business logic.

Annotation driven instrumentation using dependency injection frameworks such as Spring or Guice are an attempt to reduce the coat bloat caused by manual instrumentation. However, a clear advantage of agent based bytecode instrumentation is quite simple, you don't need to be using Spring or Guice to benefit from it. Another issue with such annotation driven DI frameworks is that they can only inject the logic on code you own and 3rd party libraries cannot be instrumented. The agent does not care if the code you want to instrument is yours, a third party library or the JDK itself.

The ability to quickly update a configuration file indicating the metrics and code locations we want to measure, and simply restart the application to begin gathering new measurements is invaluable. It saves a considerable amount of developer time and results in faster performance debugging sessions.

Finally, because this library provides a plugable provider interface, it means switching between different reporting systems does not need to be an ordeal. I've worked on projects that have had two or three different metrics libraries used and converters going between each to the backend monitoring system. Simply put, this is vile and makes code bloat even worse. With this library, if you wish to change provider, simply add an implementation of the provider of choice. As the metrics are not in the source code, there is no further change required and no technical debt added.

### Code Bloat Problem
	
Say we want to instrument a method which calls some third party library or service and tracks the number of failures (as exceptions thrown). To do this we need to track both the total number of method invocations and the number of failed invocations. Most of the time in modern Java libraries, exceptions are unchecked which allows them to propagate up to an appropriate handler without polluting the code base.

The following is an example of a basic block of code which performs a basic service call prior 
to instrumentation

```java
public Result performSomeTask() {
    return callSomeServiceMethodWhichCanThrowException(createArgs());
}
```

To instrument this programmatically we perform the following

```java
// add class fields

static final Counter total = Metrics.createCounter("requests_total");
static final Counter failed = Metrics.createCounter("requests_failed");

public Result performSomeTask() {
    total.inc();

Result result = null;
    try {
        //perform actual original call
        result = callSomeServiceMethodWhichCanThrowException(createArgs());
    } catch (Exception e) {
        failed.inc();
        throw e;
    }

return result;
}
```

Now lets add a timer to this also so we can see how long the method call takes.

```java
// add class fields

static final Counter total = Metrics.createCounter("requests_total");
static final Counter failed = Metrics.createCounter("requests_failed");
static final Timer timer = Metrics.createTimer("requests_timer");

public Result performSomeTask() {
    long startTime = System.nanoTime();
    total.inc();

Result result = null;
    try {
        result = callSomeServiceMethodWhichCanThrowException(createArgs());
    } catch (Exception e) {
        failed.inc();
        throw e;
    } finally {
        timer.record(System.nanoTime() - startTime);
    }

return result;
}
```
		
WOW! That turned ugly fast! We started with 3 LOC (lines of code) representing the business logic and ended up with 17 LOC, 14 of which were due to our metrics. This has the potential to destroy the clarity of a code base.

With agent based instrumentation, we can inject the exact same method bytecode as would be produced by writing it manually, but without touching the source.

## Instrumentation Metadata

For those who like marking methods to measure programmatically, we provide annotations to do just that. We also provide a configuration driven system where you define the methods you want to instrument in a yaml format file. We encourage the configuration driven approach over annotations.

How all the metric types are use should be self explanatory with the exception of Gauges. We use Gauges to track the number of invocations of a particular method or constructor that are `in flight`. That effectively means we increment the gauge value as the method enters and decrements it when it exits. This is very useful for things like Http Request Handlers etc where you want to know the number of in flight requests.

### Annotations

```java
@Counted (name = "", labels = { }, doc = "")
@Gauged (name = "", mode=in_flight, labels = { }, doc = "")
@Timed (name = "", labels = { }, doc = "")
@ExceptionCounted (name = "", labels = { }, doc = "")
```

Annotations are provided for all metric types and can be added to methods including
constructors.

```java
@Counted(name = "taskx_total", doc = "total invocations of task x")
@Timed (name = "taskx_time", doc = "duration of task x")
public Result performSomeTask() {
    //...
}
```

### Configuration

metrics:
	  {class name}.{method name}{method signature}:
	    - type: Counted
		  name: {name}
		  doc: {metric documentation}
		  labels: ['{name:value}', '{name:value}']
	    - type: Gauged
		  name: {name}
		  mode: {mode}
		  doc: {metric documentation}
		  labels: ['{name:value}']
	    - type: ExceptionCounted
		  name: {name}
		  doc: {metric documentation}
		  labels: ['{name:value}']
	    - type: Timed
		  name: {name}
		  doc: {metric documentation}
		  labels: ['{name:value}']

Each metric is defined on a per method basis. A method is uniquely identified by the 
combination of `{class name}.{method name}{method signature}`. As an example, if we 
wanted to instrument the following method via configuration instead of annotations

```java
package com.fleury.test;
....

public class TestClass {
    ....
    
    @Counted(name = "taskx_total", doc = "total invocations of task x")
    public Result performSomeTask() {
        ...
    }
}
```

We write the configuration as follows

metrics:
	  com/fleury/test/TestClass.performSomeTask(Ljava/lang/String;)V:
	    - type: Counted
		  name: taskx_total
		  doc: total invocations of task x

Note the method signature is based on the method parameter types and return type. The parameter types are between the brackets `()` with the return type after. In this case we have a single String argument and the return type is void which results in `(Ljava/lang/String;)V`. [Here](http://journals.ecs.soton.ac.uk/java/tutorial/native1.1/implementing/method.html) is a good overview of Java method signature mappings.

In previous versions we allowed the package name to be specified using `.` instead of the internal `/` separator. While this is still supported for the metrics configuration section, it is not supported anywhere else and should be updated to only have the `/` package separator.

#### Class Imports

To simplify the metrics definition section of the configuration, we allow an imports section. Here we can define the fully qualified class names for any classes we use or re-use in the definitions. This includes method type descriptors.

imports:
      - com/fleury/test/TestClass
      - java/lang/Object
      - java/lang/String

metrics:
      TestClass.performSomeTask(LString;)V:
        - type: Counted
          name: taskx_total
          doc: total invocations of task x
          
       TestClass.performSomeOtherTask(LString;)LObject;:
         - type: Counted
           name: tasky_total
           doc: total invocations of task y

### Metric Labels

Labels are a concept in some reporting systems that allow for multi-dimensional metric capture and analysis. Labels are composed of name value pairs `({name}:{value})`. You can have up to a maximum of five labels per metric. See the Prometheus metric library guidelines on metric and label naming [here](https://prometheus.io/docs/practices/naming/). Dropwizard metrics doesn't support the concept of labels and so we use the label values as part of the metric name. We apply these in order and so a metric definition of

```java
@Counted(name = "taskx_total", labels = {"name1:value1", "name2:value2"})
```

would result in a metric name in the Dropwizard registry of `taskx_total.value1.value2`.

#### Dynamic Label Values

A powerful feature is the ability to set label values dynamic based on variables available on the method stack. Metric names cannot be dynamic. The way we specify dynamic label values is using the `${index}` syntax followed by the method argument index. The special value `$this` can be used to access the current instance reference in non static methods. We prevent usage of `$this` in constructors to prevent initialisation leakage.

Note that we restrict the stack usage to the method arguments only. That is, we don't allow use of variables created within the method as that is a very fragile thing to do. The String representation as given by `String.valueOf()` of the parameter is used as the label value. That means for primitive types we perform boxing first and null objects will result in the String `"null"`. Argument indexes start at index `0` up to the number of `args.length - 1` (i.e. array index syntax). We manage any special logic that occurs with the actual location on the stack due to non static methods (`this` is index `0` on the stack) and static methods (no `this`). Therefore you can always assume index `0` is the first method argument.

```java
@Counted (name = "service_total", labels = { "client:$0" })
public void callService(String client) 
```

Each time this method is invoked it will use the value of the `client` parameter as the metric label value. We also support accessing nested property values. For example, `($1.httpMethod)` where `$1` is the first method parameter and is e.g. of type `HttpRequest`. This means you are essentially doing `httpRequest.getHttpMethod().toString();`. This nesting can be arbitrarily deep. We use `PropertyUtils` from the `commons-beanutils` library to perform the nested property reading. Typically this means you can only use JavaBeans conforming properties, however, we have added a `GenericBeanIntrospector` which allows for accessing properties in methods like `name()`  via e.g. `$1.name` etc. This gives better cross languages support.

### What we actually Transform
As we allow the use of annotations to register metrics to track, if no black/white lists are defined we must scan all classes as they are loaded and check for the annotations. However, we do not want to have to rewrite all of these classes if we have not changed anything. There are many reasons you want to modify as little as possible with an agent but the general motto is, only touch what you have to. Hence, we only rewrite classes which have been changed due to the addition of metrics and all other classes, even though scanned, are returned untouched to the classloader.

### Supported Languages
As the agent works at the bytecode level, we support any language which runs on the JVM. Every language which compiles and runs on the JVM must obey by the bytecode rules. This simply means we need to understand the translation mechanisms of each language for the language level method name to the bytecode level. In Java this is usually 1:1 (excluding some generics fun). You can always examine the `javap` (the [Java Disassembler](http://docs.oracle.com/javase/7/docs/technotes/tools/windows/javap.html)) command to view the bytecode contents in a more `Java` centric way.

As an example. Lets take the followin Scala class 
```scala
class Person(val name:String) {
}
```
If we run `javap` on this

javap Person.class
    
we get

Compiled from "Person.scala"
    public class Person {
      private final java.lang.String name;   // field
      public java.lang.String name();        // getter method
      public Person(java.lang.String);       // constructor
    }

You can do the same for Kotlin, Clojure and any other JVM language. Be aware that there may be some quirks in the naming translations and it may not always be as simple as shown above.

## Supported Metrics Systems

### Prometheus
Prometheus is the primary supported metric system. The design of the metric meta data was driven by its design (name, labels, doc). It also has the most powerful reporting system and does not require the development / integration of a multitude of reporting sinks like Codeahale does (e.g. Graphite, Librato, etc).

Also, given the fact that we use the multidimensional labels concept from Prometheus, if we use a system which doesn't support this notion then it is not nearly as powerful.

### Dropwizard

Sometimes however, we don't want to go to all the hassle of setting up and entire metrics monitoring system if we don't have one already in place and are only interested in the metrics on a single JVM. 
This is where we would recommend Dropwizard as it provides summary statistics (percentiles, rates etc) out of the box for certain metric types. These are automatically exposed via JVM and can be viewed and graphed with the likes of the VisualVM JMX plugin.

Some differences in metric types exist between Prometheus and Dropwizard. In particular, with Prometheus you don't decrement counters, instead you use gauges for values that increase and decrease. Supporting this approach with Dropwizard gauges with an agent is tricky as we need to maintain a class variable. Hence, we simply use counters to back gauges with Dropwizard.

Profiling agents can sometimes be far to heavy to attach to a JVM for a prolonged period of time and impact performance when we only want to monitor certain methods / hotspots. Also, most of the time these tools do not provide summary metrics exception counts which can be recorded with this library.

### Metric System Configuration

The metric systems configuration is passed as simple key-value pairs `(Map<String,String>)` to the constructor of each MetricSystem via their provider. These key-values are defined in the "system" section of the agent configuration.

metrics:
        .....

system:
        key1: value1
        
The dropwizard implementation supports the property `domain` which allows to change the exposed JMX domain name. It defaults to the dropwizard default of `metrics`. See the next section for some shared properties around JVM level metrics.
       
#### Adding JVM Level Metric Information

Both Dropwizard and Prometheus support adding JVM level metrics information obtained from the JVM via MBeans for

- gc
- memory
- classloading
- threads

To enable each, simply add the metrics you want to a `jvm` property in the `system` section of the configuration yaml. For example, to add `gc` and `memory` information to the registry used:

system:
        jvm:
           - gc
           - memory

#### Agent Reporting

We start the default reporting (endpoint) methods on both metrics systems. For Dropwizard that is JMX (which can be scraped via other services or agents), and for Prometheus that is the HttpServer. The default port for the Prometheus endpoint is `9899` and it can be changed by specifying the property `httpPort` in the system configuration section as follows

system:
        httpPort: 9899

Additional reporting systems can be added for each agent programmatically if required. Alternatively an additional Java agent could be attached to send the JMX metrics to e.g. Graphite for Dropwizard. The benefit of this approach is that it doesn't care how many metric registries are started within the application or by the agent(s).

### <a name="black-and-white-lists"></a>Black & Black Lists

Sometimes we only want to scan certain packages or classes which we wish to instrument. This could be to reduce the agent startup time or to work around problematic instrumentation situations. Note that the black and white lists do not take any annotations or metric configuration into account and essentially override them.

To white list a class or package include the fully qualified class or package name under the `whiteList` property. If no white list is specified, then all classes are scanned and eligible for transforming.

whiteList:
      - com/fleury/test/ClassName
      - com/fleury/package2
        
To black list a class or package add the fully qualified class or package name under the `blackList` property. If a class or package is in both white and black list, the black list wins and the class will not be touched.

blackList:
       - com/
        
### Logger Configuration

j.u.l is used for logging and can be configured by passing the agent argument `log-config:<properties path>` to the agent with the path to the logger properties file.

## Performance
We use the Java ASM bytecode manipulation library. This is the lowest level and fastest of all the bytecode libraries which is used by the likes of cglib. It allows us to inject bytecode in a precise way which means we can craft the exact same bytecode as if it was hand written. To make the metric system plugable, we chose to abstract the metric work behind a generic SPI interface. The bytecode which we inject uses the SPI which can in turn be swapped out without any change to our bytecode. This makes it very flexible but it comes at the cost of not being able to keep field level static variable references for our metrics. Instead we perform a lookup from a ConcurrentHashMap to get the metric by name in the SPIs. Note that JIT takes care of the additional method dispatches up to the Map by performing inlining. If we were using a single implementation we would inject the metrics fields as static variables at the top of each class. If someone wishes to fork this for a single metric system that would be the best way to go. See the Prometheus fork implementation for an example [https://github.com/willfleury/prometheus-metrics-agent](https://github.com/willfleury/prometheus-metrics-agent).

It should be noted that as with hand crafted metrics, the additional bytecode and hence method size required to handle capturing all metrics could potentially lead to methods which might otherwise have been inlined or compiled by the JIT being skipped instead. This should be considered regardless off the instrumentation choice and if unsure, the appropriate JVM output should be checked (-XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -XX:+PrintCompilation).

## Dependencies 
Very lightweight.
	
	asm
	jackson

The client libraries for whatever metric provider you choose are also included. Note that the final agent binaries are shaded and all dependencies relocated to prevent possible conflicts.

# <a name="binaries-releases"></a>Binaries & Releases

See the releases section of the github repository for releases along with the prebuilt agent binaries for dropwizard and prometheus.

# Building

The module metrics-agent-dist has build profiles for both Prometheus and Dropwizard.

mvn clean package -Pprometheus

mvn clean package -Pdropwizard
	
The uber jar can be found under `/target/metrics-agent.jar`

# Usage

The agent must be attached to the JVM at startup. It cannot be attached to a running JVM.

-javaagent:metrics-agent.jar

Example
	
	java -javaagent:metrics-agent.jar -jar myapp.jar

Using the configuration file config.yaml is performed as follows

java -javaagent:metrics-agent.jar=agent-config:agent.yaml -jar myapp.jar

Using the configuration file config.yaml and logging configuration logger.properties is performed as follows

java -javaagent:metrics-agent.jar=agent-config:agent.yaml,log-config:logger.properties -jar myapp.jar

# Debugging

Note if you want to debug the metrics agent you should put the debugger agent first.

-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=<port> -javaagent:metrics-agent.jar myapp.jar

# Examples

We provide some example configurations for popular frameworks. They serve as examples for how to instrument others and can be combined as desired (e.g. you can have both dropwizard request metrics and hibernate in the same configuration). As you will see, it is very simple and light weight to add new frameworks. In the case where different major versions of frameworks required different classes and methods to be instrumented, this simply becomes the addition of a new configuration file for that version.

- [Jersey](example-configurations/jersey.yaml)
 - [Dropwizard (via Jersey)](example-configurations/dropwizard.yaml)
 - [Tomcat Servlet, JSP, Jersey](example-configurations/tomcat.yaml)
 - [Hibernate](example-configurations/hibernate.yaml)

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