Pushing apps with sidecar processes

About sidecars
Use cases
Limitations
Requirements for Java apps
- Reserving memory
- Packaging binaries
Push an app with a sidecar using an app manifest
- Prerequisites
- Procedure
Sidecar tutorial

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You can run additional processes in the same container as your app. These additional processes are called sidecar processes, or sidecars. An example of a sidecar is an Application Performance Monitoring (APM) tool.

Important The cf CLI v6 commands described in this topic are unsupported, but are supported in cf CLI v7. The latest supported cf CLI release is cf CLI v8. To upgrade to cf CLI v7, see Upgrading to cf CLI v7. To upgrade to cf CLI v8, see Upgrading to cf CLI v8.

Note This feature requires that your Cloud Foundry deployment uses capi-release 1.790 or later.

About sidecars

When you provide a sidecar for your app, Cloud Foundry Application Runtime packages the required code and configuration needed to run the sidecar and app in the same droplet. It deploys this droplet in a single container on Diego. Both processes within the container undergo health checks independently.

You can push sidecar processes with your app by using one of two methods:

Using an app manifest. For instructions, see Push an app with a sidecar Using an app manifest.
With a custom buildpack. For instructions, see Sidecar buildpacks.

For additional information about sidecars, see Sidecars in the Cloud Foundry API (CAPI) documentation.

For sample apps that use sidecars, see the capi-sidecar-samples repository on GitHub. These sample apps use an app manifest.

Use cases

You can use sidecars for processes that depend on each other or must run in the same container.

For example, you can use sidecars for processes that must:

Communicate over a UNIX socket or through localhost
Share the same file system
Be scaled and placed together
Have fast interprocess communication

Limitations

Sidecars have these limitations:

The start and stop order of app processes and their sidecars is undefined.
App processes and sidecars are codependent. If either fails or exits, the other does also.
Sidecars are not independently scalable. Sidecars share resources with the main app process and other sidecars within the container.
Sidecars only support PID-based health checks. HTTP health checks for sidecars are not supported.

Requirements for Java apps

These sections describe several requirements that are specific to pushing sidecars with Java apps.

Reserving memory

You must allocate memory to the sidecar. If you do not, the Java buildpack allocates all of the available memory to the app. As a result, the sidecar does not have enough memory and the app fails to start.

To allocate memory to the sidecar, use the memory property in the app manifest. For example:

sidecars:
- name: SIDECAR-NAME
    process_types: [ 'PROCESS-TYPES' ]
    command: START-COMMAND
    memory: 256MB

Where:

SIDECAR-NAME is a name you give your sidecar.
PROCESS-TYPES is a list of app processes for the sidecar to attach to, such as web or worker. You can attach multiple sidecars to each process type your app uses.
START-COMMAND is the command used to start the sidecar. For example, ./binary or java -jar java-file.jar.

You must also allocate memory to sidecars that you push with a custom buildpack. For more information, see Sidecar buildpacks.

Packaging binaries

If your sidecar is a binary file rather than a set of buildable source files, then you must package the binary file with your Java app.

In some cases, the Java buildpack requires you to push a .jar file. If this is the case with your app, you must include the sidecar binary in the .jar file.

To package the sidecar binary with the .jar file, run:

zip JAR -u SIDECAR-BINARY

Where:

JAR is your .jar file.
SIDECAR-BINARY is your sidecar binary.

For more information about packaging assets with your Java app, see Cloud Foundry documentation.

Push an app with a sidecar using an app manifest

These sections explain how to push an app with a sidecar using an app manifest. For an example that you can try yourself, see Sidecar tutorial.

When pushing a Java app, follow the requirements listed in Requirements for Java Apps.

Prerequisites

Before you can push an app with a sidecar with an app manifest, you must have:

An app that is running or ready to be pushed.
A file that Cloud Foundry can run inside the application container as a sidecar process. For example, an executable binary, a Java .jar file, or Ruby scripts.

Procedure

To push an app with a sidecar:

Create an app or use an existing app. To create an app:
```
cf create-app APP-NAME
```
Where APP-NAME is the name you give your app.
Create a manifest file in the root directory of your app, such as manifest.yml. Otherwise, use an existing manifest file for your app. For more information, see Deploying with app manifests.
Add the following values to your app manifest file under the applications key:
```
  sidecars:
    - name: SIDECAR-NAME
      process_types: [ 'PROCESS-TYPES' ]
      command: START-COMMAND
```
Where:
- SIDECAR-NAME is a name you give your sidecar.
- PROCESS-TYPES is a list of app processes for the sidecar to attach to, such as web or worker. You can attach multiple sidecars to each process type your app uses.
- START-COMMAND is the command used to start the sidecar. For example, ./binary or java -jar java-file.jar.
This example manifest file includes multiple sidecars:
```
---
applications:
- name: my-app
  sidecars:
     - name: authenticator
       process_types: [ 'web', 'worker' ]
       command: bundle exec run-authenticator
     - name: performance monitor
       process_types: [ 'web' ]
       command: bundle exec run-performance-monitor
```
To apply the manifest file to your app:
```
cf apply-manifest -f PATH-TO-MANIFEST
```
Where PATH-TO-MANIFEST is the path to your manifest file.
To push your app:
```
cf push APP-NAME
```
Where APP-NAME is the name of your app.

Sidecar tutorial

You can explore sidecars using the app in the capi-sidecar-samples repository on GitHub. The following sections describe the app, how to build and push the app, and some ways to observe the app and its processes after pushing.

Important In this tutorial, you are pushing the Ruby sample app. You can also follow this tutorial for a Java app using the sidecar-dependent-java-app and push_java_app_with_binary_sidecar.sh in the samples repository. When pushing a Java app, follow the requirements listed in Requirements for Java apps.

About the sample app

The capi-sidecar-samples repository contains:

A simple Ruby app: This app is named sidecar-dependent-app. It includes a /config endpoint that calls to the sidecar and prints the response, as shown in this code snippet:

get '/config' do
puts "Sending a request to the config-server sidecar at localhost:#{ENV['CONFIG_SERVER_PORT']}/config/"
response = Typhoeus.get("localhost:#{ENV['CONFIG_SERVER_PORT']}/config/")
puts "Received #{response.body} from the config-server sidecar"
response.body
end

A Golang sidecar: The config-server-sidecar produces a config-server binary. It provides apps with their required configuration over its /config endpoint. It also accepts connections only over localhost on the CONFIG_SERVER_PORT port. This means the sidecar must be co-located in the same container as the app, so that it shares the same network namespace as the main app.

The following diagram illustrates the app architecture:

App Process and Sidecar Process are inside an App Container, which is inside a Diego cell.

Push the app and sidecar

If you do not have Go installed, download the config-serverlinuxx86-64 binary from CAPI Sidecar sample releases in the capi-sidecar-samples repository in GitHub.

To push the app and sidecar:

In a terminal window, clone the Git repository to your workspace by running:

git clone https://github.com/cloudfoundry-samples/capi-sidecar-samples.git

Go to the config-server-sidecar directory.

Build the binary for the sidecar by running:

GOOS=linux GOARCH=amd64 go build -o config-server .

To create the app:
```
cf create-app sidecar-dependent-app
```
Go to the sidecar-dependent-app directory.
Open and review the manifest.yml file. Under sidecars, the sidecar is specified with a name, process type, and start command. Under env, an environment variable defines the port on which the app and sidecar communicate.
To apply the manifest to the app:
```
cf apply-manifest
```
To push the app:
```
cf push sidecar-dependent-app
```

After you push the app, you can further explore it as described in the sections below:

View processes running in the app container via SSH
View the web URL and app logs via SSH
View and debug processes running in the app container via HTTP

View processes running in the app container via SSH

To view the app and sidecar process running in the container:

SSH into the application container by running:
```
cf ssh sidecar-dependent-app
```

To see both the rackup process for the main app and config-server process for the sidecar, run:

ps aux

The output you might see resembles this example output:

vcap@f00949bd-6601-4731-6f7e-e859:~$ ps aux
USER         PID %CPU %MEM    VSZ      RSS   TTY   STAT   START TIME  COMMAND
root           1 0.0  0.0     1120     0     ?     S      22:17 0:00  /tmp/garden-init
vcap           7 0.0  0.0     106716   4508  ?     S      22:17 0:00  ./config-server
vcap          13 0.0  0.1     519688   35412 ?     S      22:17 0:00  /home/vcap/deps/0/vendor_bundle/ruby/2.4.0/bin/rackup config.ru -p 8080
vcap          24 0.0  0.0     116344   10792 ?     S      22:17 0:00  /tmp/lifecycle/diego-sshd --allowedKeyExchanges= --address=0.0.0.0:2222 --allowUnauthenticatedClients=false --inhe
root          82 0.0  0.0     108012   4548  ?     S      22:17 0:00  /etc/cf-assets/healthcheck/healthcheck -port=8080 -timeout=1000ms -liveness-interval=30s
vcap         215 0.3  0.0     70376    3756  pts/0 S      23:12 0:00  /bin/bash
vcap         227 0.0  0.0     86268    3116  pts/0 R      23:12 0:00  ps aux

To see that the sidecar is listening on the port specified by CONFIG_SERVER_PORT and that the main ruby process is connected to it, run:

lsof -i | grep $CONFIG_SERVER_PORT

The output you see might resemble this example output:

vcap@f00949bd-6601-4731-6f7e-e859:~$ lsof -i | grep $CONFIG_SERVER_PORT
config-se   7 vcap 3u  IPv4 17265901     0t0 TCP *:8082 (LISTEN)
config-se   7 vcap 5u  IPv4 17265992     0t0 TCP localhost:8082->localhost:42266 (ESTABLISHED)
ruby       13 vcap 11u  IPv4 17274965    0t0 TCP localhost:42266->localhost:8082 (ESTABLISHED)

View the web URL and app logs via SSH

To view the web URL and logs for the app:

In a browser, go to the config endpoint of the sidecar-dependent-app. For example: https://sidecar-dependent-app.example.com/config.
See that the browser displays Scope and Password information. This is the configuration that the app fetches from the config-server sidecar.
Begin streaming logs for the app by running:
```
cf logs sidecar-dependent-app
```
In your browser, refresh the /config endpoint page and observe that the log stream in your terminal displays logs for both the sidecar and the main app process.
In a separate terminal from your log stream, SSH into the application container by running:
```
cf ssh sidecar-dependent-app
```
Stop the sidecar process by running:
```
kill -9 $(pgrep config-server)
```

View the output in the terminal where you are streaming the app logs. The app logs indicate that the sidecar process failed and that Diego restarted the application container. For example:

2019-04-17T16:48:55.41-0700 [API/0] OUT App instance exited with guid
21df1eb8-f25d-43b2-990b-c1a417310553 payload:
{"instance"=>"a8db0eed-7371-4805-5ad3-4596", "index"=>0,
"cellid"=>"86808ce7-afc2-47da-9e79-522a62a48cff", "reason"=>"CRASHED",
"exitdescription"=>"APP/PROC/WEB/SIDECAR/CONFIG-SERVER: Exited with status 137",
"crashcount"=>1, "crashtimestamp"=>1555544935367052708,
"version"=>"50892dcb-274d-4cf6-b944-3eda1e000283"}

View and debug processes running in the app container via HTTP

Sidecars can be used to collect debug information (e.g. thread dumps) from another process running in the same container. This is especially useful if you want to debug a running process using an HTTP request, for instance if you do not have ssh access to the application container.

This is an example of a simple routable debug sidecar for java applications, but the same concept can be adapted to more advanced debug tooling and other languages or frameworks.

Prerequisites

Before you begin, you must have:

A java app with a manifest that is ready to be pushed. For example, the java sample app from CF Acceptance Tests.
A route to map to the debug sidecar

Procedure

To add a debug sidecar to the java app:

Create a sidecar that responds to HTTP requests and calls kill -3 on the java process. When a java process receives the resulting SIGQUIT signal, it will print a thread dump to STDOUT. For an example, see the javadebugsidecar on GitHub.
Place the sidecar binary in the root directory of your java app
Add the following to your app manifest:
```
sidecars:
- name: debug-sidecar
  process_types: [ 'web' ]
  command: START-COMMAND
  memory: 256MB
```
Where START-COMMAND is the command used to start the sidecar. For the above example sidecar, ./java_debug_sidecar.
Push the java application. For example:
```
cf push APP-NAME -f manifest.yml -m 1G
```
Where APP-NAME is the name of your app.
Map a route to the sidecar process’s port. For detailed steps, see Configuring Cloud Foundry to route traffic to apps on custom ports.
```
cf curl -X PATCH /v3/routes/ROUTE-GUID/destinations -d '{
"destinations": [
    {
      "app": {
        "guid": "APP-GUID",
        "process": {
          "type": "web"
        }
      },
      "port": PORT,
      "protocol": "http1"
    }
  ]
}'
```
Where:
- APP-GUID is the GUID of your app.
- ROUTE-GUID is the GUID of the route to map to the sidecar.
- PORT is a custom port on which your app is configured to receive requests. This is 8081 for the above example.
Restart the app to complete the port mapping
```
cf restart APP_NAME
```
Where APP-NAME is the name of your app.
Tail the app logs:
```
cf logs APP_NAME
```
Where APP-NAME is the name of your app.
In another terminal, make a HTTP request to the sidecar, to trigger the thread dump. For example:
```
curl https://debug-sidecar.example.com/threaddump
```
Observe the thread dump in the app logs

Next Steps

Now that you have a simple debugging sidecar working, here are some ideas for next steps:

Use more sophisticated debugging tools
Return the debug information in the HTTP response, rather than logging to STDOUT
Inject the debug sidecar via a Sidecar buildpacks. For example: java-debug-buildpack on GitHub.
Add authentication for the debug sidecar’s HTTP route, for instance using an authentication Route Service.

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