Aether Consumer that performs operations based on streaming input.
At a high level the purpose of this consumer is to subscribe to input from a system (currently either Apache Kafka or Zeebe) perform some actions based on the input, possibly transforming it, and then return a signal to the original system. This can be arranged in semi-arbitrary ways using a defined Pipeline. A pipeline manages a subscription, and orchestrates the movement of data through stages. Each stage has a transformation. This sounds more complicated than it is in practice, and the flexibility allows us to do all sorts of things, combining the power of Kafka, and Zeebe (BPMN) with the ability to perform arbitrary computations.
If you want to get started right away, you can play with this system easily using the build tools in the Zeebe Tester Repo. I highly recommend this as the basic artifacts are already there, and can deploy automatically to the local instance if you modify them. It's by far the best way to get familiar with the system, even if you plan to use Kafka as your source.
Zeebe Instance is a basic resource that holds connection information to a Zeebe cluster (or single broker) and allows you to interact with the broker with Pipelines to send messages, start workflows, and subscribe to Service Tasks.
You can also communicate directly with the broker through this API to test some functionality.
zeebe/test?id={resource-id} : GET
Tests if the broker can be connected to. Returns a boolean
zeebe/send_message?id={resource-id} : POST
Send a message to the broker
Options (JSON):
- listener_name : (required) the intended message listener on the broker
- message_id=None : optional unique_id for the message (uniqueness enforced if present)
- correlationKey=None : a key used to deliver this message to a running workflow
- ttl=600_000 : how long the message should be held if it can't be delivered immediately
- variables = {} : any data that you want to include with the message
zeebe/start_workflow?id={resource-id}: POST
Start a workflow on the broker
Options (JSON):
- process_id : the ID of the process you want to start
- variables = {} : and date you want to include in the workflow context
- version = 1 : the deployed version of the process to target
The most basic in this application is the Transformation. There are a few types. Some are only useful in conjunction with Zeebe, but others are general purpose. They're all basically small functional units that take input and provide an output. Unlike other Aether Consumers, these resource don't require a lot of definition up front. Except for JSCall, you likely only need one of each with a definition of:
{
"id": "default",
"name": "The default implementation"
}
But you know that since you already checked out the Zeebe Tester Repo
Currently we have the following types of Transformations (XFs). Scroll down to see the section on testing XFs directly.
Make a http request based on the input parameters. This XF is based on the python requests library. It can take any of the following arguments:
method,
url,
headers=None,
token=None,
basic_auth=None,
query_params=None,
json_body=None,
form_body=None,
allow_redirects=False,
On success, RestCall returns:
encoding
headers
status_code
text
json (if applicable)
request_failed = False
On failure, RestCall returns:
encoding
reason
status_code
request_failed = True
Please note that all url must be resolved by the Google DNS service. This attempts to disallow access to network internals via this transform.
JS Call is powered by a sandboxed javascript environment based on quickjs. This is the only Transform that requires a specific definition. You define include an arbitrary piece of javascript which is executed at runtime on the input you define. quickjs does not have networking and some other higher level functions enabled, so you are limited to calculations you can perform in context, which is quite a lot. You can also require libraries which can be used by your function. You must define your own JSCalls before they can be run. There are a few in /app/fixtures/examples.py. Just remember if it needs to communicate (send an email, write to S3, etc), you can't do it with JSCall.
Here is a definition for a JSCall that turns a piece of JSON into a CSV. Do note that the value of the script field must be a valid JSON string. That is to say, it should be minified, then escaped. The following is presented expanded to illustrate and is not valid as it has line breaks.
{
"id": "parser",
"name": "CSV Parser",
"entrypoint": "f",
"script": "
function f(myData) {
const Parser = json2csv.Parser;
const fields = [/"a/", /"b/"];
const opts = { fields };
try {
const parser = new Parser(opts);
return parser.parse(myData);
} catch (err) {
console.error(err);
}
}
",
"arguments": ["jsonBody"],
"libraries": ["https://cdn.jsdelivr.net/npm/[email protected]/dist/json2csv.umd.js"]
}
This function would require the argument jsonBody and then return the stringified CSV as "value": {the csv}.
You can also type you arguments like:
"arguments": {"arg_a": "str", "arg_b": "int"}
This transform will broadcast a message to Zeebe. You cannot test this transform directly, as it requires a Zeebe Broker be in context. You can send adhoc messages to a Zeebe broker by posting to /zeebe/send_message?id=broker-id
listener_name # the message listener on the broker you're targeting
message_id # (optional, ids must be unique at any given time)
variables # (optional) an object with any data you want to pass
correlationKey # (optional) the Zeebe correlation key
This transform will attempt to kick off a new workflow instance in Zeebe with data your provide. You cannot test this transform directly, as it requires a Zeebe Broker be in context. You can adhoc spawn workflows on a Zeebe broker by posting to /zeebe/start_workflow?id=broker-id
process_id # the ID of the Zeebe Process
variables # (optional) an object with any data you want to pass
version # (optional) the workflow version you're trying to start
This is the most specific transform, and will complete an in process ZeebeJob. This only works in a pipeline context that subscribed to a ZeebeJob. It takes a look at the passed in data, and based on a jsonpath evaluation either tells the broker that the job was successful or failed.
This XF allows the production of a message to Kafka. The reasons for doing this are many, you might want to consume some conditionally filtered information as part of a different pipeline, or handle errors in that occur in another XF in another pipeline or consumer, or maybe you want to add some context to the error_handling you've setup in your Pipeline. For example, on a failure from a RESTCall XF, you could send the context to a new topic which would be retried by a separate pipeline.
id
name
topic # The name of the topic to be written to. Will be created if it doesn't exist.
schema # The avro schema of messages to be produced (NOT stringified, but as a child object of this key)
Each type of transform can be accessed, created and tested individually through the API.
If you had created the above jscall, you would test is by posting a JSON payload to:
{consumer_url}/jscall/test?id=parser
In this case, it would fail if you don't have "jsonBody" as one of the keys in your JSON payload.
If you post a properly formatted payload (for the instance) like:
{
"jsonBody": {
"a": 1, "b": 2
}
}
You the the correct result, a CSV (as an escaped string):
{
"result": "\"a\",\"b\"\n1,2"
}
One level up from a transform, we have a Stage. A stage describes the input and output from a Transformation, and optionally provides validation criteria to be evaluated against an output.
Stages are not themselves resources, as they operate on context specific data. I.E. the proper input map of a stage will be dependent on the outputs of previous stages, or the expected input from a subscription. Stages are combined in the next level of the hierarchy, Pipeline, but since they are highly configurable, they get their own section here. They include the following:
- name : name of the stage, used as the key for it's output
- type : type of the Transformation to be used
- id : instance of the Transform to be used
- transition : the transition definition (input / output map && validation)
Here is an example of a single stage:
{
'name': 'one',
'type': 'jscall',
'id': 'strictadder',
'transition': {
'input_map': {
'a': '$.source.value',
'b': '$.const.one'
},
'output_map': {
'result': '$.result'
},
'fail_condition': '$.result.`match(0, null)`'
}
}
The input map and output map are both expressed as a set of JSONPath expressions, which can incorporate extensions as described at the eHA JSONPath Extensions page.
The input map describes the parameters to be passed to a Transformation.
Take the referenced JSCall Transformation for example:
{
'id': 'strictadder',
'name': 'Adder with Type Checking',
'entrypoint': 'f',
'script': '''
function adder(a, b) {
return a + b;
}
function f(a, b) {
return adder(a, b);
}
''',
'arguments': {'a': 'int', 'b': 'int'}
}
It expects integer arguments named a and b, so those must be included in out input map from some source. A reasonable input map then might be:
{
"a": "$.some.path.to.an.integer",
"b": "$.a.path.to.another.integer"
}
Additionally you can add validation to the transition. This is expressed as a JSONPath expression that should evaluate to a boolean.
'fail_condition': '$.result.`match(0, null)`'
This isn't a very useful condition, but we've indicated here that we should fail if the result of strictadder is 0. This is more useful for status_codes from the RESTCall transform, etc.
A Pipeline is where concepts come together into an actionable set of behaviors.
It consists of two primary things.
- A
subscription. A data source that which at the moment can either be a set of Kafka Topics or a cursor to incoming instances of a Zeebe Service Task of a particular type. - An ordered set of
stageswhich transform the input.
As a pipeline runs, it carries a context, which allows each state to reference the source message from the subscription, a set of user defined constants, and the output of each previous stage. The constants for the pipeline are set when the resource is defined.
Here is a very simple and pretty dumb pipeline, which uses the strictadder JSCall Transformation we used in the stages example.
{
'id': 'adder_example',
'zeebe_instance': 'default',
'zeebe_subscription': 'adder-worker',
'const': {
'one': 1,
},
'error_handling': {
'error_topic': 'my_log_stream',
'log_failure': True,
'log_success': True
}
'stages': [
{
'name': 'one',
'type': 'jscall',
'id': 'strictadder',
'transition': {
'input_map': {
'a': '$.source.value',
'b': '$.const.one'
},
'output_map': {
'result': '$.result'
}
}
},
{
'name': 'two',
'type': 'jscall',
'id': 'strictadder',
'transition': {
'input_map': {
'a': '$.one.result',
'b': '$.const.one'
},
'output_map': {
'result': '$.result'
}
}
},
{
'name': 'three',
'type': 'jscall',
'id': 'strictadder',
'transition': {
'input_map': {
'a': '$.two.result',
'b': '$.const.one'
},
'output_map': {
'result': '$.result'
}
}
},
{
'name': 'four',
'type': 'zeebecomplete',
'id': 'default',
'transition': {
'input_map': {
'added_value': '$.three.result'
}
}
}
]
}
You can see that the source of data here is a Zeebe ServiceTask of type adder-worker from which we expect an integer as input ($.source.value). From there, we have three identical stages that each add 1 to the value of the previous stage. At stage 4, we return the value from stage three via a zeebecomplete Transformation.
The schema also allows for constants to be included in the pipeline definition in the const section. These are then available to all stages at the path $.const.
Most pipelines should be quite simple, since if you're using Zeebe, you want to perform all possible logic there. Pipelines should source, transmit, transform or validate data, not sort it or perform a lot of logic.
You can also source messages from kafka to power a pipeline. Simply replace the zeebe information with a single kafka_subscription key, the value being a description of the subscription like:
kafka_subscription: {"topic_pattern": "*"},
By including an error_handling block in your pipeline, you can log the result of each pipeline run to Kafka to topic error_topic. The messages will conform to the following schema. If there is no error_handling block, errors will only appear ephemerally (last 100)in the log for the job.
log_failure will send a message when a pipeline does not complete successfully.
log_success will send a message when a pipeline is successful.
{
"name": "Event",
"type": "record",
"fields": [
{
"name": "id",
"type": "string"
},
{
"name": "event",
"type": "string"
},
{
"name": "timestamp",
"type": "string",
"@aether_extended_type": "dateTime"
},
{
"name": "pipeline",
"type": "string"
},
{
"name": "success",
"type": "boolean"
},
{
"name": "error",
"type": [
"null",
"string"
]
}
]
}
Just like individual Transformations, once you have a Pipeline Resource registered, you can test it directly without side effect by posting a message to the test endpoint at {consumer_url}/pipeline/test?id={pipeline-id}. This is highly recommended to test for behavior and edge cases before adding it to a job where it runs automatically.
In the case that a Zeebe operation is set to occur as part of the pipeline you're testing, that step will be skipped without Zeebe interaction. Zeebe interactions can be tested directly against a running broker.
To test, POST your test payload to {consumer_url}/pipeline/test?id={pipeline-id}.
If successful, the result will be the completed Pipeline Context. Otherwise, errors will help you to debug your Pipeline. For example, this 400 error that arose from an RESTCall Transform not receiving a url.
On Stage "one": TransformationError: Expected required fields, missing ['url']
A Job is a worker that allows for multiple pipelines to be executed regularly.
A job is quite simple and really just a container for managing the running of one or more pipelines. This is a valid definition of a job running one pipeline:
{
"id": "my_job",
"name": "A Stream Consumer Job",
"pipelines": ["my_pipeline"]
}
If you have more than one pipeline included, the job will process them in series, checking if there's work to be done each before going to the next. If you need a pipeline to be checked as often as possible for work, consider putting it into a job where it is the only pipeline. If it's not time sensitive, then you can group many pipelines into a single job.
This is quite a powerful and flexible use pattern. The goal should be to build your pipeline in such a way that the ServiceTask can take a variety of parameters to drive it.
For example, here is a simple pipeline that allows for arbitrary REST calls to be parameterized in the BPMN, passed to the stream-consumer for execution, and the result be registered as the Task Result.
{
"id": "rest_handler",
"zeebe_instance": "default",
"zeebe_subscription": "rest-worker",
"stages": [
{
"name": "one",
"type": "restcall",
"id": "default",
"transition": {
"input_map": {
"url": "$.source.url",
"method": "$.source.method",
"headers": "$.source.headers",
"token": "$.source.token",
"basic_auth": "$.source.basic_auth",
"query_params": "$.source.query_params",
"json_body": "$.source.json_body"
},
"output_map": {
"text": "$.text",
"failed": "$.request_failed",
"status_code": "$.status_code",
"json": "$.json"
}
}
},
{
"name": "two",
"type": "zeebecomplete",
"id": "default",
"transition": {
"input_map": {
"status_code": "$.one.status_code",
"json": "$.one.json",
"text": "$.one.text"
}
}
}
]
}
Looking at the input map to the stage "one" restcall, you can see that the behavior of the pipeline is driven directly by what is passed into the ServiceTask node's context. If you want this pipeline to service multiple ServiceTasks in the same workflow, each pointing to different URLs, you need to take advantage of Zeebe's local context by way of "Input Variable Mappings".
This would likely be part of a more complex system, but using kafka_subscription as your pipeline source and a mix of the available zeebe interaction XFs,
zeebemessageto send a message to the broker orzeebespawnto create a new workflow instance
You can create very complex logic that can signal across the two systems. For example, using the zeebe MessageReceive task to block a workflow from proceeding until the correct CorrelationKey value is emitted as a zeebemessage via Kafka.
A pipeline need not interact with Zeebe at all. Here is a simplification of the previous generic rest-worker, reconfigured for a specific Kafka Source.
{
"id": "send-reports",
kafka_subscription: {"topic_pattern": "the_report_topic"},
"const": {
"url": "https://fixed-url-for-post",
"method": "POST",
"auth": {"user": "a-user", "password": "password"}
},
"stages": [
{
"name": "one",
"type": "restcall",
"id": "default",
"transition": {
"input_map": {
"url": "$.const.url",
"method": "$.const.method",
"basic_auth": "$.const.basic_auth",
"json_body": "$.source"
},
"output_map": {
"text": "$.text",
"failed": "$.request_failed",
"status_code": "$.status_code",
"json": "$.json"
}
}
}
]
}
The downside of this approach currently is that there's not a good way to re-enqueue failed messages. We're working on a feature that will allow production to Kafka as a Transform, which should allow better failure handling.
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