iWF project - main & server repo

iWF will make you a 10x developer!

iWF is a platform for developing resilient, fault-tolerant, scalable long-running applications. It offers a convenient abstraction for durable timers, background execution with backoff retry, customized persisted data(with optional caching, indexing), message queues, RPC, and more. You will build long-running reliable processes faster than ever.

iWF is built on top of Cadence/Temporal.

Related projects:

What is iWF

Basic Concepts

The top level concept is ObjectWorkflow -- nearly any "object" can be an ObjectWorkflow, as long as it's long-lasting, at least a few seconds.

User application creates ObjectWorkflow by implementing the Workflow interface, e.g. in Java or Golang. An implementation of the interface is referred to as a WorkflowDefinition, consisting below components:

Name	Description
Data Attribute	Persistence field to storing data
Search Attribute	"Searchable data attribute" -- attribute data is persisted and also indexed in search engine backed by ElasticSearch or OpenSearch
Workflow State	A background execution unit. State is super powerful like a small workflow of two steps: waitUntil(optional) and execute with default infinite retry
RPC	Remote procedure call. Invoked by client, executed in worker, and interact with data/search attributes, internal channel and state execution
Signal Channel	Asynchronous message queue for the workflow object to receive message from external
Internal Channel	"Internal Signal Channel" -- An internal message queue for workflow states/RPC

A workflow definition can be outlined like this:

These are all the concepts that you need to build a super complicated workflow. See this engagement workflow example in Java or Golang for how it looks like!

Below are the detailed explanation of the concepts. They are powerful, also extremely simple to learn and use (as the philosophy of iWF).

Persistence

iWF let you store customized data as a database during the workflow execution. This eliminates the needs of depending on a database to implement your workflow.

The data are stored as data attributes and search attributes. Both are defined as "persistence schema". The schema just defined and maintained in the code along with other business logic.

Search attribute works like infinite indexes in traditional database. You only need to specify which attributes should be indexed, without worrying about things in a traditional database like the number of indexes, and the order of the fields in an index.

Logically, the above workflow definition example will have a persistence schema like below:

Workflow Execution	Search Attr A	Search Attr B	Data Attr C	Data Attr D
Workflow Execution 1	val 1	val 2	val 3	val 4
Workflow Execution 2	val 5	val 6	val 7	val 8
...	...	...	...	...

With Search attributes, you can write customized SQL-like query to find out any workflow executions, just like using a database query.

Note that after workflows are closed(completed, timeout, terminated, canceled, failed), all the data will be deleted after the retention period.

Caching

By default, RPC will load data/search attributes with Cadence/Temporal query API, which is not optimized for very high volume requests on a single workflow execution(like 100 rps), because it could cause too many replay with history, especially when workflows are closed.

You can enable the caching to support those high volume requests.

NOTES:

The read after write will become eventual consistent, unless set bypassCachingForStrongConsistency=true in RPC options
Caching will be more useful for read-only RPC(no persistence.SetXXX API or communication API calls in RPC implementation) or GetDataAttributes API.
- A read-only RPC can still invoke any other RPCs(like calling other microservices, or DB operation) in the RPC implementation
It will cost extra event in history (upsertMemo operation for WorkflowPropertiesModified event) for updating the persisted data attributes
This feature is currently only supported if the backend is Temporal, because Cadence doesn't support mutable memo

Workflow State

WorkflowState is to implement the asynchronous process as "workflow".
It will be running in the background, with default infinite backoff retry.

A WorkflowState is like “a small workflow” of 1~2 steps:

[ waitUntil ] → execute

The waitUntil API can returns some commands to wait for. When the commands are completed, the execute API will be invoked. The two APIs have access to read/write the persistence defined in the workflow.

The full detailed execution flow is like this:

The waitUntil API is optional. If not defined, then execute API will be invoked instead when the state started.

Note: the two APIs are invoked by iWF service with infinite backoff retry by default. See WorkflowStateOptions section for customization.

The execute API will return a StateDecision:

Single next state
- Go to to different state
- Go to the same state as a loop
- Go the the previous state as a loop
Multiple next states, executing as multi threads in parallel
Dead end -- Just stop the thread
Graceful complete -- Stop the thread, and also will stop the workflow when all other threads are stopped
Force complete -- Stop the workflow immediately
Force fail -- Stop the workflow immediately with failure

With decisions, the workflow definitions can have flows like these:

or even more complicated as needed.

Commands for WorkflowState's WaitUntil API

iWF provides three types of commands:

SignalCommand: will wait for a signal to be published to the workflow signal channel. External application can use SignalWorkflow API to signal a workflow.
TimerCommand: will wait for a durable timer to fire.
InternalChannelCommand: will wait for a message from InternalChannel.

The waitUntil API can return multiple commands along with a CommandWaitingType:

AllCommandCompleted: This option waits for all commands to be completed.
AnyCommandCompleted: This option waits for any of the commands to be completed.
AnyCommandCombinationCompleted: This option waits for any combination of the commands in a specified list to be completed.

InternalChannel: synchronization for multi threading

When there are multiple threads of workflow states running in parallel, you may want to let them wait on each other to ensure some ordering.

For example, WorkflowState 1,2,3 needs to be complete before workflow state 4.

In this case, you need to utilize the "InternalChannel". WorkflowState 4 should be waiting on an "InternalChannel" for 3 messages in "waitUntil" API. And then WorkflowState 1,2,3 will be publishing one message for each when completing.

RPC

RPC stands for "Remote Procedure Call". It's for external system to interact with the workflow execution.

It's invoked by client, executed in workflow worker, and then respond back the results to client.

RPC can have access to not only persistence read/write API, but also interact with WorkflowStates using InternalChannel, or trigger a new WorkflowState execution in a new thread.

As an example, you can even uses iWF to implement a job post system, which is much more powerful than a typical CRUD application on database:

class JobPost implements ObjectWorkflow{
    DataAttribute String title;
    DataAttribute String authorName;
    DataAttribute String body;

    @RPC
    public void update(Context context, JobInfo input, Persistence persistence, Communication communication) {
        persistence.setSearchAttributeText(SA_KEY_TITLE, input.getTitle());
        persistence.setSearchAttributeText(SA_KEY_JOB_DESCRIPTION, input.getDescription());

        persistence.setSearchAttributeInt64(SA_KEY_LAST_UPDATE_TIMESTAMP, System.currentTimeMillis());

        if (input.getNotes().isPresent()) {
            persistence.setDataAttribute(DA_KEY_NOTES, input.getNotes().get());
        }
        communication.triggerStateMovements(
                StateMovement.create(ExternalUpdateState.class)
        );
    }
    
    @RPC
    public JobInfo get(Context context, Persistence persistence, Communication communication) {
        String title = persistence.getSearchAttributeText(SA_KEY_TITLE);
        String description = persistence.getSearchAttributeText(SA_KEY_JOB_DESCRIPTION);
        String notes = persistence.getDataAttribute(DA_KEY_NOTES, String.class);

        return ImmutableJobInfo.builder()
                .title(title)
                .description(description)
                .notes(Optional.ofNullable(notes))
                .build();
    }
}

Atomicity of RPC APIs

It's important to note that in addition to read/write persistence fields, a RPC can trigger new state executions, and publish message to InternalChannel, all atomically.

Atomically sending internal channel, or triggering state executions is an important pattern to ensure consistency across dependencies for critical business – this solves a very common problem in many existing distributed system applications. Because most RPCs (like REST/gRPC/GraphQL) don't provide a way to invoke background execution when updating persistence. People sometimes have to use complicated design to acheive this.

But in iWF, it's all builtin, and user application just needs a few lines of code!

Signal Channel vs RPC

There are two major ways for external clients to interact with workflows: Signal and RPC. So what are the difference?

They are completely different:

Signal is sent to iWF service without waiting for response of the processing
RPC will wait for worker to process the RPC request synchronously
Signal will be held in a signal channel until a workflow state consumes it
RPC will be processed by worker immediately

So choose based on the situations/requirements

	Availability	Latency	Workflow Requirement
Signal Channel	High	Low	Requires a WorkflowState to process
RPC	Depends on workflow worker	Higher than signal, depends on workflow worker	No WorkflowState required

Advanced Customization

WorkflowOptions

iWF let you deeply customize the workflow behaviors with the below options.

IdReusePolicy for WorkflowId

At any given time, there can be only one WorkflowExecution running for a specific workflowId. A new WorkflowExecution can be initiated using the same workflowId by setting the appropriate IdReusePolicy in WorkflowOptions.

ALLOW_IF_NO_RUNNING
- Allow starting workflow if there is no execution running with the workflowId
- This is the default policy if not specified in WorkflowOptions
ALLOW_IF_PREVIOUS_EXISTS_ABNORMALLY
- Allow starting workflow if a previous Workflow Execution with the same Workflow Id does not have a Completed status. Use this policy when there is a need to re-execute a Failed, Timed Out, Terminated or Cancelled workflow execution.
DISALLOW_REUSE
- Not allow to start a new workflow execution with the same workflowId.
ALLOW_TERMINATE_IF_RUNNING
- Always allow starting workflow no matter what -- iWF server will terminate the current running one if it exists.

CRON Schedule

iWF allows you to start a workflow with a fixed cron schedule like below

// CronSchedule - Optional cron schedule for workflow. If a cron schedule is specified, the workflow will run
// as a cron based on the schedule. The scheduling will be based on UTC time. The schedule for the next run only happens
// after the current run is completed/failed/timeout. If a RetryPolicy is also supplied, and the workflow failed
// or timed out, the workflow will be retried based on the retry policy. While the workflow is retrying, it won't
// schedule its next run. If the next schedule is due while the workflow is running (or retrying), then it will skip
that
// schedule. Cron workflow will not stop until it is terminated or cancelled (by returning cadence.CanceledError).
// The cron spec is as follows:
// ┌───────────── minute (0 - 59)
// │ ┌───────────── hour (0 - 23)
// │ │ ┌───────────── day of the month (1 - 31)
// │ │ │ ┌───────────── month (1 - 12)
// │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday)
// │ │ │ │ │
// │ │ │ │ │
// * * * * *

NOTE:

iWF also supports more advanced cron expressions
The crontab guru site is useful for testing your cron expressions.
To cancel a cron schedule, use terminate of cancel type to stop the workflow execution.
By default, there is no cron schedule.

RetryPolicy for workflow

Workflow execution can have a backoff retry policy which will retry on failed or timeout.

By default, there is no retry policy.

Initial Search Attributes

Client can specify some initial search attributes when starting the workflow.

By default, there is no initial search attributes.

WorkflowStateOptions

Similarly, users can customize the WorkflowState

WorkflowState WaitUntil/Execute API timeout and retry policy

By default, the API timeout is 30s with infinite backoff retry. Users can customize the API timeout and retry policy:

InitialIntervalSeconds: 1
MaxInternalSeconds:100
MaximumAttempts: 0
MaximumAttemptsDurationSeconds: 0
BackoffCoefficient: 2

Where zero means infinite attempts.

Both MaximumAttempts and MaximumAttemptsDurationSeconds are used for controlling the maximum attempts for the retry policy. MaximumAttempts is directly by number of attempts, where MaximumAttemptsDurationSeconds is by the total time duration of all attempts including retries. It will be capped to the minimum if both are provided.

Persistence loading policy

When a workflowState/RPC API loads DataAttributes/SearchAttributes, by default it will use LOAD_ALL_WITOUT_LOCKING to load everything.

For WorkflowState, there is a 2MB limit by default to load data. User can use another loading policy LOAD_PARTIAL_WITHOUT_LOCKING to specify certain DataAttributes/SearchAttributes only to load.

WITHOUT_LOCKING here means if multiple StateExecutions/RPC try to upsert the same DataAttribute/SearchAttribute, they can be done in parallel without locking.

If racing conditions could be a problem, usingPARTIAL_WITH_EXCLUSIVE_LOCK allows specifying some keys to be locked during the execution.

However, PARTIAL_WITH_EXCLUSIVE_LOCK is not supported in RPC yet. The feature is WIP with waiting for the Temporal "update" being production ready. As a workaround, RPC can kick off a WorkflowState to update the persistence data using the locking policy.

WaitUntil API failure policy

By default, the workflow execution will fail when API max out the retry attempts. In some cases that workflow want to ignore the errors.

Using PROCEED_ON_API_FAILURE for WaitUntilApiFailurePolicy will let workflow continue to execute decide API when the API fails with maxing out all the retry attempts.

Alternatively, WorkflowState can utilize attempts or firstAttemptTime from the context to decide ignore the exception/error.

Limitation

Though iWF can be used for a very wide range of use case even just CRUD, iWF is NOT for everything. It is not suitable for use cases like:

High performance transaction( e.g. within 10ms)
High frequent writes on a single workflow execution(like a single record in database) for hot partition issue
- High frequent reads on a single workflow execution is okay if using memo for data attributes
Join operation across different workflows
Transaction for operation across multiple workflows

Architecture

An iWF application is composed of several iWF workflow workers. These workers host REST APIs as "worker APIs" for server to call. This callback pattern similar to AWS Step Functions invoking Lambdas, if you are familiar with.

An application also perform actions on workflow executions, such as starting, stopping, signaling, and retrieving results by calling iWF service APIs as "service APIs".

The service APIs are provided by the "API service" in iWF server. Internally, this API service communicates with the Cadence/Temporal service as its backend.

In addition, the iWF server also runs the Cadence/Temporal workers as "worker service". The worker service hosts an interpreter workflow. This workflow implements all the core features as described above, and also things like "Auto ContinueAsNew" to let you use iWF without any scaling limitation.

How to use

Using docker image & docker-compose

Checkout this repo, go to the docker-compose folder and run it:

cd docker-compose && docker-compose up

This by default will run Temporal server with it. And it will also register a default Temporal namespace and required search attributes by iWF. Link to the Temporal WebUI: http://localhost:8233/namespaces/default/workflows

By default, iWF server is serving port 8801, server URL is http://localhost:8801/ )