Policies
A Clustered Example
We'll now look at a more complex example that better shows the capabilities of Brooklyn, including management of a running clustered application.
Below is the annotated blueprint. Download the blueprint into a
text file, mycluster.yaml, in your workspace. Before you create an application with it,
review and/or change the the location where the application will be deployed.
You will need four machines for this example: one for the load-balancer (nginx), and three for the
Tomcat cluster (but you can reduce this by changing the maxPoolSize below).
Start by giving it a name, optionally adding a version and other metadata.
Choose your cluster type.
Customize with config keys, such as the initial size. Define the members of the cluster.
On the cluster, handle a member's failure by replacing it with a brand new member.
Auto-scale the cluster, based on runtime metrics of the cluster.
For a simplistic demonstration, this uses requests per second.
Add an enricher to aggregate the member's requests per second.
For a simplistic demonstration, this uses requests per second.
Add a load balancer entity.
Configure it to monitor and balance across the cluster of Tomcat servers, which was given:
id: cluster
The Tomcat Cluster
The DynamicCluster can dynamically increase or decrease the number of members. Resizing the
cluster can either be carried out manually via effectors or automatically by attaching an
AutoScalerPolicy.
It is configured with a dynamiccluster.memberspec, which defines the type and configurtion of members
in the cluster. In our example, each is a Tomcat server with a WAR deployed at the root URL.
Deploy the app:
$ br deploy mycluster.yaml
Id: nGY58ZZN
Name: Tomcat Cluster
Status: In progress
And wait for the app to be running, viewing its state with:
$ br application
Id Name Status Location
nGY58ZZN Tomcat Cluster RUNNING Mf0CJac6
You can view the list of entities within the cluster with the command below (which drills into the application named "Tomcat Cluster", then into its child entity named "Cluster", and then lists its entities):
$ br application "Tomcat Cluster" entity "Cluster" entity
Id Name Type
dYfUvLIw quarantine org.apache.brooklyn.entity.group.QuarantineGroup
tOpMeYYr Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
The "quarantine" entity is used when Tomcat servers fail to start correctly - this entity is by
default added to the quarantine group, where it can later be investigated. This can be disabled using
the configuration dynamiccluster.quarantineFailedEntities: false.
Tomcat auto-repair
Each Tomcat entity has a ServiceFailureDetector enricher and a ServiceRestarter policy.
An enricher generates new events or sensor values (metrics) for the entity by modifying or aggregating data from one or more other sensors. A policy coordinates the runtime management of entities by carrying out actions, initiated by specific triggers. Policies are often used to keep the system in a healthy state, such as handling failures and auto-scaling.
The built-in functionality of the Tomcat entity is to use the "service.state" sensor to report its status. It will be "on-fire" when a failure is detected, or "running" when healthy, or one of the other lifecycle states such as "starting", "stopping" or "stopped".
The ServiceFailureDetector enricher emits an "Entity Failed" event whenever a failure is detected,
and similarly an "Entity Recovered" event when recovered. The configuration option
serviceOnFireStabilizationDelay will suppress reporting of failure until the entity is detected
as failed for the given duration. This is very useful so as not to over-react to temporary failures.
The ServiceRestarter policy attaches to an entity in order to restart the service on failure. If
there are subsequent failures within a configurable time interval, or if the restart fails, the
service entity is marked as failed and no futher restarts are attempted.
Try killing the Tomcat process for one of the members in the cluster. The command below will kill Tomcat on the vagrant VMs named "byon1" to "byon4":
for i in byon{1..4}; do
vagrant ssh ${i} --command 'ps aux | grep -i tomcat | grep -v grep | awk '\''{print $2}'\'' | xargs kill -9'
done
You can view the state of the Tomcat server with the command below (which drills into the
application named "Tomcat Cluster", then into its child entity named "Cluster", and then into the
first member of the cluster named "Tomcat Server"):
$ br application "Tomcat Cluster" entity "Cluster" entity "Tomcat Server"
Id: tOpMeYYr
Name: Tomcat Server
Status: ON_FIRE
ServiceUp: false
Type: org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
CatalogItemId: org.apache.brooklyn.entity.webapp.tomcat.TomcatServer:0.0.0.SNAPSHOT
After the given stabilizationDelay, the Tomcat server will be automatically restarted.
Cluster auto-replace
The cluster has a ServiceReplacer policy. This attaches to a DynamicCluster and replaces a
failed member; if this fails, it sets the Cluster state to on-fire.
To simulate a terminal failure of a member, repeatedly kill the process (using the command above).
The Tomcat server should be replaced by a new member of the cluster, and then the old failed member removed.
You can view the list of Tomcat servers in the cluster with the command below (which drills into the
application named "Tomcat Cluster", then into its child entity named "Cluster", and then lists the
child entities):
$ br application "Tomcat Cluster" entity "Cluster" entity
Id Name Type
dYfUvLIw quarantine org.apache.brooklyn.entity.group.QuarantineGroup
tOpMeYYr Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
mgoRpkKH Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
Auto-scaling
The AutoScalerPolicy attaches to a Resizable entity (in this case the DynamicCluster) and
dynamically adjusts its size in response to keeping a metric within a given range. It adds/removes
members (e.g. Tomcat instances) automatically.
In this example policy, the metricUpperBound for requests per second per server is configured
very low at just 3. This makes it simple to trigger auto-scaling. The resizeUpStabilizationDelay
of 2 seconds means the load must be sustained for at least that length of time. TheresizeDownStabilizationDelay of 1 minute means there must be low load for a full one minute
before it will scale back down.
Find the URL of the load-balancer - see the NginxController sensor named main.uri.
To generate load, you can use your web-browser by repeatedly refreshing that page. Alternatively, you could use a load generator like jmeter, or use a script such as the one shown below (changing URL for the URL of your load-balancer):
URL=http://10.10.10.101:8000/
for i in {1..600}; do
for j in {1..50}; do
curl --fail --silent ${URL} > /dev/null || echo "Curl failed with exit code $?"
done
echo "Finished batch $i"
sleep 1
done
While those curl commands run in a separate terminal, you can look at the metrics for the first Tomcat server using the command:
$ br application "Tomcat Cluster" entity "Cluster" entity "Tomcat Server" sensor
Name Description Value
...
webapp.reqs.perSec.last Reqs/sec (last datapoint) 0.9980039920159681
webapp.reqs.perSec.windowed Reqs/sec (over time window) 0.9326571333555038
webapp.reqs.processingTime.fraction.last Fraction of time spent processing, reported by webserver (percentage, last datapoint) "0.067%"
webapp.reqs.processingTime.fraction.windowed Fraction of time spent processing, reported by webserver (percentage, over time window) "0.073%"
webapp.reqs.processingTime.max Max processing time for any single request, reported by webserver (millis) ""
webapp.reqs.processingTime.total Total processing time, reported by webserver (millis) "3.12s"
webapp.reqs.total Request count 5575
webapp.tomcat.connectorStatus Catalina connector state name "STARTED"
webapp.url URL "http://10.10.10.103:18082/"
...
You can look at the average requests per second on the cluster with the command:
$ br application "Tomcat Cluster" entity "Cluster" sensor "webapp.reqs.perSec.perNode"
25.765557404326124
When this value exceeds 3 for two seconds, the cluster with scale up. You can see the new instance using the command:
$ br application "Tomcat Cluster" entity "Cluster" entity
Id Name Type
dYfUvLIw quarantine org.apache.brooklyn.entity.group.QuarantineGroup
mgoRpkKH Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
xpLeJufy Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
CpabLxZE Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer
Cancel the curl commands (or wait for them to finish), and then wait for the one minute
resizeDownStabilizationDelay. The cluster will scale back to the minimum one instance.
$ br application "Tomcat Cluster" entity "Cluster" entity
Id Name Type
dYfUvLIw quarantine org.apache.brooklyn.entity.group.QuarantineGroup
mgoRpkKH Tomcat Server org.apache.brooklyn.entity.webapp.tomcat.TomcatServer