Consuming External TCP Services

Describes a simple scenario based on Istio's Bookinfo example.

Feb 6, 2018 | By Vadim Eisenberg

In my previous blog post, Consuming External Web Services, I described how external services can be consumed by in-mesh Istio applications via HTTPS. In this post, I demonstrate consuming external services over TCP. You will use the Istio Bookinfo sample application, the version in which the book ratings data is persisted in a MySQL database. You deploy this database outside the cluster and configure the ratings microservice to use it. You define a Service Entry to allow the in-mesh applications to access the external database.

Bookinfo sample application with external ratings database

First, you set up a MySQL database instance to hold book ratings data outside of your Kubernetes cluster. Then you modify the Bookinfo sample application to use your database.

Setting up the database for ratings data

For this task you set up an instance of MySQL. You can use any MySQL instance; I used Compose for MySQL. I used mysqlsh (MySQL Shell) as a MySQL client to feed the ratings data.

Set the MYSQL_DB_HOST and MYSQL_DB_PORT environment variables:
```
$ export MYSQL_DB_HOST=<your MySQL database host>
$ export MYSQL_DB_PORT=<your MySQL database port>
```
In case of a local MySQL database with the default port, the values are localhost and 3306, respectively.

To initialize the database, run the following command entering the password when prompted. The command is performed with the credentials of the admin user, created by default by Compose for MySQL.

$ curl -s https://raw.githubusercontent.com/istio/istio/release-1.19/samples/bookinfo/src/mysql/mysqldb-init.sql | mysqlsh --sql --ssl-mode=REQUIRED -u admin -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT

When using the mysql client and a local MySQL database, run:

$ curl -s https://raw.githubusercontent.com/istio/istio/release-1.19/samples/bookinfo/src/mysql/mysqldb-init.sql | mysql -u root -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT

Create a user with the name bookinfo and grant it SELECT privilege on the test.ratings table:
```
$ mysqlsh --sql --ssl-mode=REQUIRED -u admin -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "CREATE USER 'bookinfo' IDENTIFIED BY '<password you choose>'; GRANT SELECT ON test.ratings to 'bookinfo';"
```
OR
For mysql and the local database, the command is:
```
$ mysql -u root -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "CREATE USER 'bookinfo' IDENTIFIED BY '<password you choose>'; GRANT SELECT ON test.ratings to 'bookinfo';"
```
Here you apply the principle of least privilege. This means that you do not use your admin user in the Bookinfo application. Instead, you create a special user for the Bookinfo application , bookinfo, with minimal privileges. In this case, the bookinfo user only has the SELECT privilege on a single table.
After running the command to create the user, you may want to clean your bash history by checking the number of the last command and running history -d <the number of the command that created the user>. You don’t want the password of the new user to be stored in the bash history. If you’re using mysql, remove the last command from ~/.mysql_history file as well. Read more about password protection of the newly created user in MySQL documentation.

Inspect the created ratings to see that everything worked as expected:

$ mysqlsh --sql --ssl-mode=REQUIRED -u bookinfo -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "select * from test.ratings;"
Enter password:
+----------+--------+
| ReviewID | Rating |
+----------+--------+
|        1 |      5 |
|        2 |      4 |
+----------+--------+

For mysql and the local database:

$ mysql -u bookinfo -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "select * from test.ratings;"
Enter password:
+----------+--------+
| ReviewID | Rating |
+----------+--------+
|        1 |      5 |
|        2 |      4 |
+----------+--------+

Set the ratings temporarily to 1 to provide a visual clue when our database is used by the Bookinfo ratings service:

$ mysqlsh --sql --ssl-mode=REQUIRED -u admin -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "update test.ratings set rating=1; select * from test.ratings;"
Enter password:

Rows matched: 2  Changed: 2  Warnings: 0
+----------+--------+
| ReviewID | Rating |
+----------+--------+
|        1 |      1 |
|        2 |      1 |
+----------+--------+

For mysql and the local database:

$ mysql -u root -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "update test.ratings set rating=1; select * from test.ratings;"
Enter password:
+----------+--------+
| ReviewID | Rating |
+----------+--------+
|        1 |      1 |
|        2 |      1 |
+----------+--------+

You used the admin user (and root for the local database) in the last command since the bookinfo user does not have the UPDATE privilege on the test.ratings table.

Now you are ready to deploy a version of the Bookinfo application that will use your database.

Initial setting of Bookinfo application

To demonstrate the scenario of using an external database, you start with a Kubernetes cluster with Istio installed. Then you deploy the Istio Bookinfo sample application, apply the default destination rules, and change Istio to the blocking-egress-by-default policy.

This application uses the ratings microservice to fetch book ratings, a number between 1 and 5. The ratings are displayed as stars for each review. There are several versions of the ratings microservice. Some use MongoDB, others use MySQL as their database.

The example commands in this blog post work with Istio 0.8+, with or without mutual TLS enabled.

As a reminder, here is the end-to-end architecture of the application from the Bookinfo sample application.

Use the database for ratings data in Bookinfo application

Modify the deployment spec of a version of the ratings microservice that uses a MySQL database, to use your database instance. The spec is in samples/bookinfo/platform/kube/bookinfo-ratings-v2-mysql.yaml of an Istio release archive. Edit the following lines:
```
- name: MYSQL_DB_HOST
  value: mysqldb
- name: MYSQL_DB_PORT
  value: "3306"
- name: MYSQL_DB_USER
  value: root
- name: MYSQL_DB_PASSWORD
  value: password
```
Replace the values in the snippet above, specifying the database host, port, user, and password. Note that the correct way to work with passwords in container’s environment variables in Kubernetes is to use secrets. For this example task only, you may want to write the password directly in the deployment spec. Do not do it in a real environment! I also assume everyone realizes that "password" should not be used as a password…

Apply the modified spec to deploy the version of the ratings microservice, v2-mysql, that will use your database.

$ kubectl apply -f @samples/bookinfo/platform/kube/bookinfo-ratings-v2-mysql.yaml@
deployment "ratings-v2-mysql" created

Route all the traffic destined to the reviews service to its v3 version. You do this to ensure that the reviews service always calls the ratings service. In addition, route all the traffic destined to the ratings service to ratings v2-mysql that uses your database.
Specify the routing for both services above by adding two virtual services. These virtual services are specified in samples/bookinfo/networking/virtual-service-ratings-mysql.yaml of an Istio release archive. Important: make sure you applied the default destination rules before running the following command.
Zip
```
$ kubectl apply -f @samples/bookinfo/networking/virtual-service-ratings-mysql.yaml@
```

The updated architecture appears below. Note that the blue arrows inside the mesh mark the traffic configured according to the virtual services we added. According to the virtual services, the traffic is sent to reviews v3 and ratings v2-mysql.

The Bookinfo application with ratings v2-mysql and an external MySQL database

Note that the MySQL database is outside the Istio service mesh, or more precisely outside the Kubernetes cluster. The boundary of the service mesh is marked by a dashed line.

Access the webpage

Access the webpage of the application, after determining the ingress IP and port.

You have a problem… Instead of the rating stars, the message “Ratings service is currently unavailable” is currently displayed below each review:

As in Consuming External Web Services, you experience graceful service degradation, which is good. The application did not crash due to the error in the ratings microservice. The webpage of the application correctly displayed the book information, the details, and the reviews, just without the rating stars.

You have the same problem as in Consuming External Web Services, namely all the traffic outside the Kubernetes cluster, both TCP and HTTP, is blocked by default by the sidecar proxies. To enable such traffic for TCP, a mesh-external service entry for TCP must be defined.

Mesh-external service entry for an external MySQL instance

TCP mesh-external service entries come to our rescue.

Get the IP address of your MySQL database instance. As an option, you can use the host command:
```
$ export MYSQL_DB_IP=$(host $MYSQL_DB_HOST | grep " has address " | cut -d" " -f4)
```
For a local database, set MYSQL_DB_IP to contain the IP of your machine, accessible from your cluster.

Define a TCP mesh-external service entry:

$ kubectl apply -f - <<EOF
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: mysql-external
spec:
  hosts:
  - $MYSQL_DB_HOST
  addresses:
  - $MYSQL_DB_IP/32
  ports:
  - name: tcp
    number: $MYSQL_DB_PORT
    protocol: tcp
  location: MESH_EXTERNAL
EOF

Review the service entry you just created and check that it contains the correct values:

$ kubectl get serviceentry mysql-external -o yaml
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
...

Note that for a TCP service entry, you specify tcp as the protocol of a port of the entry. Also note that you have to specify the IP of the external service in the list of addresses, as a CIDR block with suffix 32.

I will talk more about TCP service entries below. For now, verify that the service entry we added fixed the problem. Access the webpage and see if the stars are back.

It worked! Accessing the web page of the application displays the ratings without error:

Note that you see a one-star rating for both displayed reviews, as expected. You changed the ratings to be one star to provide us with a visual clue that our external database is indeed being used.

As with service entries for HTTP/HTTPS, you can delete and create service entries for TCP using kubectl, dynamically.

Motivation for egress TCP traffic control

Some in-mesh Istio applications must access external services, for example legacy systems. In many cases, the access is not performed over HTTP or HTTPS protocols. Other TCP protocols are used, such as database-specific protocols like MongoDB Wire Protocol and MySQL Client/Server Protocol to communicate with external databases.

Next let me provide more details about the service entries for TCP traffic.

Service entries for TCP traffic

The service entries for enabling TCP traffic to a specific port must specify TCP as the protocol of the port. Additionally, for the MongoDB Wire Protocol, the protocol can be specified as MONGO, instead of TCP.

For the addresses field of the entry, a block of IPs in CIDR notation must be used. Note that the hosts field is ignored for TCP service entries.

To enable TCP traffic to an external service by its hostname, all the IPs of the hostname must be specified. Each IP must be specified by a CIDR block.

Note that all the IPs of an external service are not always known. To enable egress TCP traffic, only the IPs that are used by the applications must be specified.

Also note that the IPs of an external service are not always static, for example in the case of CDNs. Sometimes the IPs are static most of the time, but can be changed from time to time, for example due to infrastructure changes. In these cases, if the range of the possible IPs is known, you should specify the range by CIDR blocks. If the range of the possible IPs is not known, service entries for TCP cannot be used and the external services must be called directly, bypassing the sidecar proxies.

Relation to virtual machines support

Note that the scenario described in this post is different from the Bookinfo with Virtual Machines example. In that scenario, a MySQL instance runs on an external (outside the cluster) machine (a bare metal or a VM), integrated with the Istio service mesh. The MySQL service becomes a first-class citizen of the mesh with all the beneficial features of Istio applicable. Among other things, the service becomes addressable by a local cluster domain name, for example by mysqldb.vm.svc.cluster.local, and the communication to it can be secured by mutual TLS authentication. There is no need to create a service entry to access this service; however, the service must be registered with Istio. To enable such integration, Istio components (Envoy proxy, node-agent, _istio-agent_) must be installed on the machine and the Istio control plane (Pilot, Mixer, Citadel) must be accessible from it. See the Istio VM-related tasks for more details.

In our case, the MySQL instance can run on any machine or can be provisioned as a service by a cloud provider. There is no requirement to integrate the machine with Istio. The Istio control plane does not have to be accessible from the machine. In the case of MySQL as a service, the machine which MySQL runs on may be not accessible and installing on it the required components may be impossible. In our case, the MySQL instance is addressable by its global domain name, which could be beneficial if the consuming applications expect to use that domain name. This is especially relevant when that expected domain name cannot be changed in the deployment configuration of the consuming applications.

Cleanup

Drop the test database and the bookinfo user:

$ mysqlsh --sql --ssl-mode=REQUIRED -u admin -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "drop database test; drop user bookinfo;"

For mysql and the local database:

$ mysql -u root -p --host $MYSQL_DB_HOST --port $MYSQL_DB_PORT -e "drop database test; drop user bookinfo;"

Remove the virtual services:

$ kubectl delete -f @samples/bookinfo/networking/virtual-service-ratings-mysql.yaml@
Deleted config: virtual-service/default/reviews
Deleted config: virtual-service/default/ratings

Undeploy ratings v2-mysql:

$ kubectl delete -f @samples/bookinfo/platform/kube/bookinfo-ratings-v2-mysql.yaml@
deployment "ratings-v2-mysql" deleted

Delete the service entry:

$ kubectl delete serviceentry mysql-external -n default
Deleted config: serviceentry mysql-external

Conclusion

In this blog post, I demonstrated how the microservices in an Istio service mesh can consume external services via TCP. By default, Istio blocks all the traffic, TCP and HTTP, to the hosts outside the cluster. To enable such traffic for TCP, TCP mesh-external service entries must be created for the service mesh.