Apache OpenWhisk Composer

Composer is a new programming model for composing cloud functions built on Apache OpenWhisk. With Composer, developers can build even more serverless applications including using it for IoT, with workflow orchestration, conversation services, and devops automation, to name a few examples.

Composer synthesizes OpenWhisk conductor actions to implement compositions. Compositions have all the attributes and capabilities of an action, e.g., default parameters, limits, blocking invocation, web export.

This repository includes:

• the composer Node.js module for authoring compositions using JavaScript,
• the compose and deploy commands for compiling and deploying compositions,
• documentation, examples, and tests.

Installation

Composer is distributed as Node.js package. To install this package, use the Node Package Manager:

npm install -g openwhisk-composer

We recommend to install the package globally (with -g option) if you intend to use the compose and deploy commands to compile and deploy compositions.

Defining a composition

A composition is typically defined by means of a Javascript expression as illustrated in samples/demo.js:

const composer = require('openwhisk-composer')

module.exports = composer.if(
    composer.action('authenticate', { action: function ({ password }) { return { value: password === 'abc123' } } }),
    composer.action('success', { action: function () { return { message: 'success' } } }),
    composer.action('failure', { action: function () { return { message: 'failure' } } }))

Compositions compose actions using combinator methods. These methods implement the typical control-flow constructs of an imperative programming language. This example composition composes three actions named authenticate, success, and failure using the composer.if combinator, which implements the usual conditional construct. It takes three actions (or compositions) as parameters. It invokes the first one and, depending on the result of this invocation, invokes either the second or third action.

This composition includes the definitions of the three composed actions. If the actions are defined and deployed elsewhere, the composition code can be shorten to:

composer.if('authenticate', 'success', 'failure')

Deploying a composition

One way to deploy a composition is to use the compose and deploy commands:

compose demo.js > demo.json
deploy demo demo.json -w

ok: created /_/authenticate,/_/success,/_/failure,/_/demo

The compose command compiles the composition code to a portable JSON format. The deploy command deploys the JSON-encoded composition creating an action with the given name. It also deploys the composed actions if definitions are provided for them. The -w option authorizes the deploy command to overwrite existing definitions.

Running a composition

The demo composition may be invoked like any action, for instance using the OpenWhisk CLI:

wsk action invoke demo -p password passw0rd

ok: invoked /_/demo with id 09ca3c7f8b68489c8a3c7f8b68b89cdc

The result of this invocation is the result of the last action in the composition, in this case the failure action since the password in incorrect:

wsk activation result 09ca3c7f8b68489c8a3c7f8b68b89cdc

{
    "message": "failure"
}

Execution traces

This invocation creates a trace, i.e., a series of activation records:

wsk activation list

The entry with the earliest start time (09ca3c7f8b68489c8a3c7f8b68b89cdc) summarizes the invocation of the composition while other entries record later activations caused by the composition invocation. There is one entry for each invocation of a composed action (5dceeccbdc7a4caf8eeccbdc7a9caf18 and 7efb6b7354c3472cbb6b7354c3272c98). The remaining entries record the beginning and end of the composition as well as the transitions between the composed actions.

Compositions are implemented by means of OpenWhisk conductor actions. The documentation of conductor actions explains execution traces in greater details.

While composer does not limit in principle the length of a composition, OpenWhisk deployments typically enforce a limit on the number of action invocations in a composition as well as an upper bound on the rate of invocation. These limits may result in compositions failing to execute to completion.

Parallel compositions with Redis

Composer offers parallel combinators that make it possible to run actions or compositions in parallel, for example:

composer.parallel('checkInventory', 'detectFraud')

The width of parallel compositions is not in principle limited by composer, but issuing many concurrent invocations may hit OpenWhisk limits leading to failures: failure to execute a branch of a parallel composition or failure to complete the parallel composition.

These combinators require access to a Redis instance to hold intermediate results of parallel compositions. The Redis credentials may be specified at invocation time or earlier by means of default parameters or package bindings. The required parameter is named $composer. It is a dictionary with a redis field of type dictionary. The redis dictionary specifies the uri for the Redis instance and optionally a certificate as a base64-encoded string to enable TLS connections. Hence, the input parameter object for our order-processing example should be:

{
    "$composer": {
        "redis": {
            "uri": "redis://...",
            "ca": "optional base64 encoded tls certificate"
        }
    },
    "order": { ... }
}

The intent is to store intermediate results in Redis as the parallel composition is progressing. Redis entries are deleted after completion and, as an added safety, expire after twenty-four hours.

OpenWhisk SSL configuration

Additional configuration is required when using an OpenWhisk instance with self-signed certificates to disable SSL certificate validation. The input parameter object must contain a parameter of type dictionary named $composer. This dictionary must contain a dictionary named openwhisk. The openwhisk dictionary must contain a field named ignore_certs with value true:

{
    "$composer": {
        "openwhisk": {
            "ignore_certs": true
        }
    },
    ...
}

This explicit SSL configuration is currently only necessary when using parallel combinators or the async combinator.

Installation from Source

To install composer from a source release, download the composer source code from the Apache OpenWhisk Downloads page, rename the release tarball to openwhisk-composer.tgz and install it with command:

npm install -g openwhisk-composer.tgz