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Strangling the Monolith Design Pattern

• Strangling - incrementally replacing functionality in app with something better (cheaper, faster, easier to maintain).
• As functionality is replaced, “dead” parts of monolith can be removed/retired.
• You can also wait for all functionality to be replaced before retiring anything!
• You can optionally include new functionality during strangulation to make it more attractive to business stakeholders.

https://paulhammant.com/2013/07/14/legacy-application-strangulation-case-studies/

Steps for Strangling the Monolith

1. Strangle Monolith

2. Add a Microservice

3. And Another

4. Squeeze Monolith

5. And Remove Dead

Step 1

• In this lab, you will begin to ‘strangle’ the coolstore monolith by implementing two of its services as external microservices, split along business boundaries
• Once implemented, traffic destined to the original monolith’s services will be redirected (via OpenShift software-defined routing) to the new services
• We will be using the Fabric8 Maven Plugin to automatically deploy to openshift

• First, build and deploy the catalog microservice (Based on Spring Boot)

$ cd ~/coolstore

$ git clone -b app-partner https://github.com/epe105/catalog

$ cd catalog

Please update <USERNAME> below with your assigned username

$ oc project coolstore-<USERNAME>

$ mvn clean fabric8:deploy -Popenshift -DskipTests

Step 2

Once the new catalog service is deployed, test it with curl:

$ oc get route/catalog
NAME      HOST/PORT                                              PATH      SERVICES   PORT      TERMINATION
catalog   catalog-coolstore-<USERNAME>.apps.ocp.naps-redhat.com            catalog    8080

Curl using the HOST/PORT from the previous step

$ curl http://catalog-coolstore--USERNAME>.apps.ocp.naps-redhat.com/api/catalog

[{"itemId":"329299","name":"Red Fedora","desc":"Official Red Hat Fedora","price":34.99},{"itemId":"329199","name":"Forge Laptop Sticker","desc":"JBoss Community Forge Project Sticker","price":8.5}
...

You can also test it by clicking on the route to the catalog service and adding /api/catalog to the URL:

Step 3

Now we will do the same for the Inventory service, based on WildFly Swarm:

$ cd ~/coolstore

$ git clone -b app-partner https://github.com/epe105/inventory

$ cd inventory

Please update <USERNAME> below with your assigned username

$ oc project coolstore-<USERNAME>

$ mvn clean fabric8:deploy -Popenshift  -DskipTests

• The catalog microservice is responsible for retrieving and returning a list of products and their descriptions (price, images, etc).
• The inventory service is responsible for retrieving and returning inventory for a given product.
• The catalog service does not specifically know about or care about inventory. In the next lab we will combine the two!

Step 4

• The inventory microservice has a separate database from the current monolith.
• This allows teams split along business concerns to independently develop, test, deploy and scale the service and its database

Once the new inventory service is deployed, dump its database:

Get the POSTGRESQL_PASSWORD

$ oc env dc/inventory-database --list # get the POSTGRESQL_PASSWORD

Get the postgresql pod

$ oc get pods

oc rsh into the postgresql pod from the previous step

$ oc rsh inventory-database-1-kkxs2
sh-4.2$ psql -h $HOSTNAME -d $POSTGRESQL_DATABASE -U $POSTGRESQL_USER
Password for user userV31:XXXXXXXX
psql (9.5.4)
Type "help" for help.

monolith=> select * from inventory;
 itemid |               link                | location | quantity
--------+-----------------------------------+----------+----------
 329299 | http://maps.google.com/?q=Raleigh | Raleigh  |      736
...

Exit out of psql and then the postgresql pod

monolith=> \q
sh-4.2$ exit
exit

Step 5

Once the new service is deployed, test it with curl
You should get a JSON object representing the inventory availability of product 329299

$ oc get route/inventory
NAME        HOST/PORT                                                  PATH      SERVICES    PORT      TERMINATION
inventory   inventory-coolstore-<USERNAME>.apps.ocp.naps-redhat.com              inventory   8080

$ curl http://inventory-coolstore-<USERNAME>.apps.ocp.naps-redhat.com/api/inventory/329299
{"itemId":"329299","location":"Raleigh","quantity":736,"link":"http://maps.google.com/?q=Raleigh"}

You can also test it by clicking on the route to the inventory service and adding /api/inventory/329299 to the URL

Strangling Your Monolith Lab Architecture

Once the new Microservices are deployed, this is the Architecture of this Lab

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