Init and Sidecar Containers

Overview

This page details how to set up init and sidecar containers on Appian custom resources for self-managed customers.

Init containers run before applications containers run in a pod, and sidecar containers run alongside application containers in a pod. One use for init containers is to bootstrap Appian with RDBMS/JDBC drivers not included in the Webapp container image (for example, MySQL or IBM Db2).

Init Containers

Init containers are specialized containers that run before application containers run in a pod. Init containers support many of the features of application containers. Like application containers, init containers can access their Pods' shared storage volumes.

The Appian operator and Appian custom resource definition (CRD) support specifying init containers via the .spec.[COMPONENT].initContainers fields on Appian custom resources.

apiVersion: crd.k8s.appian.com/v1beta1
kind: Appian
metadata:
  name: appian
spec:
  serviceManager:
    initContainers:
    - name: test
      image: alpine
      command:
      - "/bin/sh"
      - "-c"
      - "echo 'I am an init container'"

Using init containers to add JARs to Tomcat's classpath

Init containers for Webapp pods can be used to add JARs to Tomcat's classpath prior to site startup. This is useful, for example, for bootstrapping Appian with RDBMS/JDBC drivers not included in the Webapp container image (for example, MySQL or IBM Db2).

Appian configures Tomcat to search for JARs in /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext. All Webapp pods define a volume mount for the webapp container in which Tomcat runs. If you specify a RWX volume using .spec.webapp.haExistingClaim, the volume mount will mount from the distributed volume to this directory with a sub-path of tomcat/apache-tomcat/lib/ext:

- name: distributed
  subPath: tomcat/apache-tomcat/lib/ext
  mountPath: /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext

Otherwise, it will mount from the storage volume with a sub-path of ae/tomcat/apache-tomcat/lib/ext:

- name: storage
  subPath: ae/tomcat/apache-tomcat/lib/ext
  mountPath: /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext

By defining an init container with the same volume mount, it's possible to bootstrap Tomcat with JARs prior to site startup by placing them in /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext in the init container. The following custom resource, for example, uses init containers to download the MySQL RDBMS/JDBC driver. Note how it also leverages the tmp volume so that /tmp is writable in case the container's file system is read-only.

apiVersion: crd.k8s.appian.com/v1beta1
kind: Appian
metadata:
  name: appian
spec:
  webapp:
    initContainers:
    - name: mysql-jdbc-download
      image: alpine
      command:
      - "/bin/sh"
      - "-c"
      - "set -exo pipefail;
         if ! [ -f /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext/mysql-connector-java-5.1.49.jar ]; then
           wget https://dev.mysql.com/get/Downloads/Connector-J/mysql-connector-java-5.1.49.tar.gz -O /tmp/mysql-connector-java-5.1.49.tar.gz;
           [ \"$(md5sum /tmp/mysql-connector-java-5.1.49.tar.gz | awk '{print $1}')\" == \"e7bc11a55398bad0ea8548163deabaa8\" ];
           tar -xzf /tmp/mysql-connector-java-5.1.49.tar.gz -C /tmp;
           cp /tmp/mysql-connector-java-5.1.49/mysql-connector-java-5.1.49.jar /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext;
         fi"
      volumeMounts:
      - name: storage
        subPath: ae/tomcat/apache-tomcat/lib/ext
        mountPath: /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext
      - name: tmp
        mountPath: /tmp

While the init container above uses the alpine Docker image to download the MySQL RDBMS/JDBC driver, it's also possible to build and publish your own image containing the driver instead. Imagine, for example, that you've built the following Dockerfile and published the resulting container image as registry.example.com/mysql-jdbc:

FROM alpine

RUN \
  wget https://dev.mysql.com/get/Downloads/Connector-J/mysql-connector-java-5.1.49.tar.gz -O /tmp/mysql-connector-java-5.1.49.tar.gz && \
  [ \"$(md5sum /tmp/mysql-connector-java-5.1.49.tar.gz | awk '{print $1}')\" == \"e7bc11a55398bad0ea8548163deabaa8\" ] && \
  tar -xzf /tmp/mysql-connector-java-5.1.49.tar.gz -C /tmp && \
  mkdir /opt/jdbc && \
  cp /tmp/mysql-connector-java-5.1.49/mysql-connector-java-5.1.49.jar /opt/jdbc

Then, you could simplify your init container like so:

apiVersion: crd.k8s.appian.com/v1beta1
kind: Appian
metadata:
  name: appian
spec:
  webapp:
    initContainers:
    - name: mysql-jdbc-copy
      image: registry.example.com/mysql-jdbc
      command:
      - "/bin/sh"
      - "-c"
      - "set -exo pipefail;
         if ! [ -f /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext/mysql-connector-java-5.1.49.jar ]; then
           cp /opt/jdbc/mysql-connector-java-5.1.49.jar /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext;
         fi"
      volumeMounts:
      - name: storage
        subPath: ae/tomcat/apache-tomcat/lib/ext
        mountPath: /usr/local/appian/ae/tomcat/apache-tomcat/lib/ext

Sidecar Containers

Sidecar containers are containers that run along with the main container in a pod. You can define any number of sidecar containers to run along side the main container. Like application containers, sidecar containers can access their pods' shared storage volumes.

The Appian operator and Appian custom resource definition (CRD) support specifying sidecar containers via the .spec.[COMPONENT].sidecarContainers fields on Appian custom resources.

apiVersion: crd.k8s.appian.com/v1beta1
kind: Appian
metadata:
  name: appian
spec:
  serviceManager:
    sidecarContainers:
    - name: test
      image: alpine
      command:
      - "/bin/sh"
      - "-c"
      - "while true; do echo 'I am a sidecar container'; sleep 5; done"

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