mollydb hooks in action

Along this post we will be guided on how to proceed to take advantage of mollydb hooks.

A couple of scenarios that we usually face when we are working with micro-services are:

• Try to increase the log level to the services with no down time(and this could be a pain in the neck).
• Being able to change database connection without restarting the application.

MollyDB in action

We are going to create a storage, named ms, and this storage will host a document for our application, the document for the application (ms-users) will look like this:

ms-users.yml

logLevel: debug
db:
  connection: mongodb://mongodb.wesovilabs.com:27017/users

Launch a real scenario

To run the sample we will take advantage of docker-compose and the descriptor will look like this:

version: '3'

services:

  mollydb:
    image: wesovilabs/mollydb:0.0.1-alpha
    volumes:
      - "./mollydb/data:/var/mollydb/storage/ms"
    ports:
      - 7000:9090

  users:
    image: golang:1.9
    container_name: ms-users
    volumes:
      - "./ms-users:/go/src/ms-users"
    ports:
      - 7001:3333
    working_dir: /go/src/ms-users
    depends_on:
      - mollydb
    links:
      - mollydb
    entrypoint: go run main.go

  initializer:
    image: centos:7
    volumes:
      - "./initializer/setup.sh:/setup.sh"
    depends_on:
      - mollydb
    entrypoint: sh /setup.sh

As we can observe in the above docker-compose.yml, we launch a service, named ms-users, mollydb and also an ephemeral container that will be only used to register a storage in mollydb

We can get the environment up an running by:

git clone https://github.com/wesovilabs/mollydb.git
cd mollydb/samples/001-microservices_configuration
docker-compose up -d

The application ms-users make an initial connection to mollydb to take their configuration. (have a look at function init in files ./ms-users/main.go. It makes the below GraphQL queries:

query FindProperties {
  properties(storage: "ms", document: "ms-users) {
    path
    key
    value
  }
}

Once the docker-compose is up and running we can monitor the docker-compose logs by running:

docker-compose logs -f users

then, from a another terminal we can create a user

curl http://localhost:7001/users  \
-d '{"name":"john.doe"}'  \
-H  "Contenty-type:application/json"  -i

and the logs displayed will look like these (important the line in blond style)

ms-users | 2018/04/02 05:55:21 “PUT http://users:3333/configuration HTTP/1.1” from 172.29.0.2:56836–405 0B in 32.284µs
ms-users | time=”2018–04–02T05:55:30Z” level=debug msg=”retrieving request from localhost:7001"
ms-users | time=”2018–04–02T05:55:30Z” level=debug msg=”read request body”
ms-users | time=”2018–04–02T05:55:30Z” level=debug msg=”unmarshal request body”
ms-users | time=”2018–04–02T05:55:30Z” level=info msg=”establishing connection wtih mongodb://mongodb.wesovilabs.com:27017/users”
ms-users | 2018/04/02 05:55:30 “POST http://localhost:7001/users HTTP/1.1" from 172.29.0.1:46412–200 13B in 593.603µs
ms-users | time=”2018–04–02T05:55:30Z” level=info msg=”user john.doe was created successfully”

Deep into mollydb

In the previous step, we said that an ephemeral container was registering a storage in mollydb. The registered storage is hosted in path ./mollydb/data. ( We mount a volume in docker-compose) We can interact with mollydb by running GraphQL queries, for example the list of properties grouped by document and storage:

curl -XPOST http://localhost:7000/graphql \
-H 'Content-Type: application/graphql' \
-d "query StorageQuery {
  storageList {
    name
    len
    documents {
      name
      len
      properties {
        key
        value
        path
      }
    }
  }
}
" \
-i

and we’ll get the response

{
  "data": {
    "storageList": [
      {
        "documents": [
          {
            "len": 2,
            "name": "ms-users",
            "properties": [
              {
                "key": "logLevel",
                "path": "mollydb://ms/ms-users?key=logLevel",
                "value": "DEBUG"
              },
              {
                "key": "db.connection",
                "path": "mollydb://ms/ms-users?key=db.connection",
                "value": "mongodb://mongodb.wesovilabs.com:27017/users"
              }
            ]
          }
        ],
        "len": 1,
        "name": "ms"
      }
    ]
  }
}

Full GraphQL API can be found on https://github.com/wesovilabs/mollydb#graphql

Creating a hook

So now, we are going to register a hook that will notify service ms-users when db.connection is changed.

curl -XPOST http://localhost:7000/graphql \
-H 'Content-Type: application/graphql' \
-d "mutation AddHook { 
  propertyRestHook(
    uri: \"http://users:3333/configuration\", 
    verb: \"POST\", 
    path: \"mollydb://ms/ms-users?key=db.connection\"
  )
}" \
-i

What we are saying with the previous script can be translated as “When property logLevel in document ms-user from storage ms changes, then mollydb will make a POST request to URI http://users:3333/configuration

so if we modify file mollydb/data/ms-users.yml and update property db.connection to mongodb://mongodb.wesovilabs.com:27030/users

then when creating a user again

curl http://localhost:7001/users  \
-d '{"name":"john.doe"}'  \
-H  "Contenty-type:application/json"  -i

we will observe that the database connection url in the log has changed.

ms-users       | 2018/04/02 06:06:46 "POST http://localhost:7001/users HTTP/1.1" from 172.29.0.1:46454 - 200 13B in 174.213µs
ms-users       | time="2018-04-02T06:06:46Z" level=debug msg="retrieving request from localhost:7001"
ms-users       | time="2018-04-02T06:06:46Z" level=debug msg="read request body"
ms-users       | time="2018-04-02T06:06:46Z" level=debug msg="unmarshal request body"
ms-users       | time="2018-04-02T06:06:46Z" level=info msg="establishing connection wtih mongodb://mongodb.wesovilabs.com:27030/users"
ms-users       | time="2018-04-02T06:06:46Z" level=info msg="user john.doe was created successfully"

Challenge

Feel free to fork this sample and make the changes you prefer. Yo could add a new service or maybe modify the logLevel by hooks registration!

External links

• MollyDB code can be found on https://github.com/wesovilabs/mollydb

Any feedback will be more than welcome!!