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Most Used Docker Commands:Interview Question

Pre-Requisite:

==> HOW TO INSTALL DOCKER IN WINDOWS AND MAC

==> How to create Docker Image From Scratch

What is Prometheus?

Prometheus is a free software application used for event monitoring and alerting. It records real-time metrics in a time series database (allowing for high dimensionality) built using a HTTP pull model, with flexible queries and real-time alerting.

Monitor Java Applications with Prometheus

This section will explain how an existing Java application can be updated to publish metrics and monitored by Prometheus.

Prometheus collects metrics from monitored targets by scraping metrics HTTP endpoints on these targets.

As discussed earlier, Prometheus collects metrics from monitored targets by scraping from an HTTP endpoint on these targets. By default, these metrics are expected to be published at /metrics. Any existing Java application can be updated to publish Prometheus-style metrics at this endpoint.

An earlier chapter explained a simple Java EE application that talks to a MySQL database. This application also publishes Prometheus-style metrics for the underlying JVM at /metrics. It also publishes application-specific metrics such as total number of times GET / and GET /{id} is called.

The complete set of JVM metrics are explained at https://github.com/prometheus/client_java. Refer to https://github.com/arun-gupta/docker-javaee/tree/master/employees/src/main/java/org/javaee/samples/employees/metrics for more details on how these metrics are enabled.

Start Java application

Use the Compose file to deploy a simple the Java EE application. This will start WildFly Swarm application and MySQL database.
```
docker stack deploy --compose-file=docker-compose.yml webapp
```
This will create webapp_default overlay network, and start the webapp_web and webapp_db services.

Verify the network:

$ docker network ls
NETWORK ID          NAME                DRIVER              SCOPE
u6ybdaqx5h5y        webapp_default      overlay             swarm

Other networks may be shown here as well.

Verify the services:

$ docker service ls
ID                  NAME                MODE                REPLICAS            IMAGE                            PORTS
ucztcpf1vw0a        webapp_db           replicated          1/1                 mysql:8                          *:3306->3306/tcp
jttfgvr09kre        webapp_web          replicated          1/1                 arungupta/docker-javaee:latest   *:8080->8080/tcp,*:9990->9990/tcp

Verify that the endpoint is accessible:

$ curl http://localhost:8080/resources/employees
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><collection><employee><id>1</id><name>Penny</name></employee><employee><id>2</id><name>Sheldon</name></employee><employee><id>3</id><name>Amy</name></employee><employee><id>4</id><name>Leonard</name></employee><employee><id>5</id><name>Bernadette</name></employee><employee><id>6</id><name>Raj</name></employee><employee><id>7</id><name>Howard</name></employee><employee><id>8</id><name>Priya</name></employee></collection>

Access the metrics published by the endpoint using curl http://localhost:8080/metrics to see the output:

# HELP jvm_info JVM version info
# TYPE jvm_info gauge
jvm_info{version="1.8.0_141-8u141-b15-1~deb9u1-b15",vendor="Oracle Corporation",} 1.0
# HELP jvm_gc_collection_seconds Time spent in a given JVM garbage collector in seconds.
# TYPE jvm_gc_collection_seconds summary
jvm_gc_collection_seconds_count{gc="PS Scavenge",} 25.0
jvm_gc_collection_seconds_sum{gc="PS Scavenge",} 0.386
jvm_gc_collection_seconds_count{gc="PS MarkSweep",} 6.0
jvm_gc_collection_seconds_sum{gc="PS MarkSweep",} 0.546
# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 25.5
# HELP process_start_time_seconds Start time of the process since unix epoch in seconds.
# TYPE process_start_time_seconds gauge
process_start_time_seconds 1.508056592419E9
# HELP process_open_fds Number of open file descriptors.
# TYPE process_open_fds gauge
process_open_fds 499.0
# HELP process_max_fds Maximum number of open file descriptors.
# TYPE process_max_fds gauge
process_max_fds 1048576.0
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 4.244393984E9
# HELP process_resident_memory_bytes Resident memory size in bytes.
# TYPE process_resident_memory_bytes gauge
process_resident_memory_bytes 5.06601472E8
# HELP jvm_classes_loaded The number of classes that are currently loaded in the JVM
# TYPE jvm_classes_loaded gauge
jvm_classes_loaded 13096.0
# HELP jvm_classes_loaded_total The total number of classes that have been loaded since the JVM has started execution
# TYPE jvm_classes_loaded_total counter
jvm_classes_loaded_total 13096.0
# HELP jvm_classes_unloaded_total The total number of classes that have been unloaded since the JVM has started execution
# TYPE jvm_classes_unloaded_total counter
jvm_classes_unloaded_total 0.0
# HELP jvm_threads_current Current thread count of a JVM
# TYPE jvm_threads_current gauge
jvm_threads_current 60.0
# HELP jvm_threads_daemon Daemon thread count of a JVM
# TYPE jvm_threads_daemon gauge
jvm_threads_daemon 12.0
# HELP jvm_threads_peak Peak thread count of a JVM
# TYPE jvm_threads_peak gauge
jvm_threads_peak 67.0
# HELP jvm_threads_started_total Started thread count of a JVM
# TYPE jvm_threads_started_total counter
jvm_threads_started_total 93.0
# HELP jvm_threads_deadlocked Cycles of JVM-threads that are in deadlock waiting to acquire object monitors or ownable synchronizers
# TYPE jvm_threads_deadlocked gauge
jvm_threads_deadlocked 0.0
# HELP jvm_threads_deadlocked_monitor Cycles of JVM-threads that are in deadlock waiting to acquire object monitors
# TYPE jvm_threads_deadlocked_monitor gauge
jvm_threads_deadlocked_monitor 0.0
# HELP jvm_memory_bytes_used Used bytes of a given JVM memory area.
# TYPE jvm_memory_bytes_used gauge
jvm_memory_bytes_used{area="heap",} 1.2072508E8
jvm_memory_bytes_used{area="nonheap",} 9.3550048E7
# HELP jvm_memory_bytes_committed Committed (bytes) of a given JVM memory area.
# TYPE jvm_memory_bytes_committed gauge
jvm_memory_bytes_committed{area="heap",} 2.69484032E8
jvm_memory_bytes_committed{area="nonheap",} 1.0133504E8
# HELP jvm_memory_bytes_max Max (bytes) of a given JVM memory area.
# TYPE jvm_memory_bytes_max gauge
jvm_memory_bytes_max{area="heap",} 4.66092032E8
jvm_memory_bytes_max{area="nonheap",} -1.0
# HELP jvm_memory_pool_bytes_used Used bytes of a given JVM memory pool.
# TYPE jvm_memory_pool_bytes_used gauge
jvm_memory_pool_bytes_used{pool="Code Cache",} 1.4589888E7
jvm_memory_pool_bytes_used{pool="Metaspace",} 6.9998048E7
jvm_memory_pool_bytes_used{pool="Compressed Class Space",} 8962112.0
jvm_memory_pool_bytes_used{pool="PS Eden Space",} 2.3732032E7
jvm_memory_pool_bytes_used{pool="PS Survivor Space",} 6073592.0
jvm_memory_pool_bytes_used{pool="PS Old Gen",} 9.0919456E7
# HELP jvm_memory_pool_bytes_committed Committed bytes of a given JVM memory pool.
# TYPE jvm_memory_pool_bytes_committed gauge
jvm_memory_pool_bytes_committed{pool="Code Cache",} 1.47456E7
jvm_memory_pool_bytes_committed{pool="Metaspace",} 7.5800576E7
jvm_memory_pool_bytes_committed{pool="Compressed Class Space",} 1.0788864E7
jvm_memory_pool_bytes_committed{pool="PS Eden Space",} 9.2274688E7
jvm_memory_pool_bytes_committed{pool="PS Survivor Space",} 3.8797312E7
jvm_memory_pool_bytes_committed{pool="PS Old Gen",} 1.38412032E8
# HELP jvm_memory_pool_bytes_max Max bytes of a given JVM memory pool.
# TYPE jvm_memory_pool_bytes_max gauge
jvm_memory_pool_bytes_max{pool="Code Cache",} 2.5165824E8
jvm_memory_pool_bytes_max{pool="Metaspace",} -1.0
jvm_memory_pool_bytes_max{pool="Compressed Class Space",} 1.073741824E9
jvm_memory_pool_bytes_max{pool="PS Eden Space",} 9.699328E7
jvm_memory_pool_bytes_max{pool="PS Survivor Space",} 3.8797312E7
jvm_memory_pool_bytes_max{pool="PS Old Gen",} 3.49700096E8

It shows all the JVM metrics that are published by the Prometheus JVM Client. The metrics generated by the application are not shown yet. It requires for the application to be accessed first.

Let’s access the JVM metrics in Prometheus dashboard first, and then we’ll access the app to show app-specific metrics.

Start Prometheus service

Make sure to terminate any previously running Prometheus endpoints:
```
docker service rm metrics
```
Create a directory prometheus and change into that directory.
Create a text file prometheus.yml and add the following content:
```
global:
  scrape_interval: 10s
scrape_configs:
  - job_name: 'webapp'
    dns_sd_configs:
      - names: ['tasks.webapp_web']
        type: 'A'
        port: 8080
```
This defines the configuration for the HTTP endpoint that publishes Prometheus-style metrics from the Java application.

Start Prometheus service:

docker service create \
  --replicas 1 \
  --network webapp_default \
  --name metrics \
  --mount type=bind,source=`pwd`/prometheus.yml,destination=/etc/prometheus/prometheus.yml \
  --publish 9090:9090 \
  prom/prometheus

Note, this service is using the webapp_default overlay network that is created when the application stack was deployed.

Access Prometheus dashboard at http://localhost:9090
Check the configured targets at http://localhost:9090/targets:
It shows that the application metrics HTTP endpoint is configured as a Prometheus target.

View application metrics

On Prometheus dashboard, click on -insert metric at cursor- to see the list of metrics available:
JVM metrics shown earlier are displayed here as well.
Select jvm_memory_pool_bytes_used metric and click on Execute to view the metric.
Select Graph to view the graphical representation
Now access the application using curl http://localhost:8080/resources/employees a few times.
Refresh Prometheus dashboard and see the updated list of metrics:
Note, app* and requests* that are generated by the application.
Select requests_get_all metric and view the graph:
Access the application a few times using curl http://localhost:8080/resources/employees/5 and then watch the requests_get_one metric.