Deep dive into the Amazon Managed Service for Apache Flink software lifecycle – Half 2

In Half 1 of this sequence, we mentioned elementary operations to regulate the lifecycle of your Amazon Managed Service for Apache Flink software. In case you are utilizing higher-level instruments comparable to AWS CloudFormation or Terraform, the device will execute these operations for you. Nonetheless, understanding the basic operations and what the service routinely does can present some stage of Mechanical Sympathy to confidently implement a extra strong automation.

Within the first a part of this sequence, we centered on the completely happy paths. In a super world, failures don’t occur, and each change you deploy works completely. Nonetheless, the true world is much less predictable. Quoting Werner Vogels, Amazon’s CTO, “All the pieces fails, on a regular basis.”

On this put up, we discover failure situations that may occur throughout regular operations or if you deploy a change or scale the applying, and monitor operations to detect and recuperate when one thing goes unsuitable.

The much less completely happy path

A strong automation should be designed to deal with failure situations, specifically throughout operations. To do this, we have to perceive how Apache Flink can deviate from the completely happy path. As a result of nature of Flink as a stateful stream processing engine, detecting and resolving failure situations requires totally different strategies in comparison with different long-running purposes, comparable to microservices or short-lived serverless capabilities (comparable to AWS Lambda).

Flink’s conduct on runtime errors: The fail-and-restart loop

When a Flink job encounters an sudden error at runtime (an unhandled exception), the traditional conduct is to fail, cease the processing, and restart from the newest checkpoint. Checkpoints permit Flink to help knowledge consistency and no knowledge loss in case of failure. Additionally, as a result of Flink is designed for stream processing purposes, which run constantly, if the error occurs once more, the default conduct is to maintain restarting, hoping the issue is transient and the applying will ultimately recuperate the traditional processing.In some circumstances, the issue isn’t transient, nevertheless. For instance, if you deploy a code change that incorporates a bug, inflicting the job to fail as quickly because it begins processing knowledge, or if the anticipated schema doesn’t match the data within the supply, inflicting deserialization or processing errors. The identical situation may also occur for those who mistakenly modified a configuration that forestalls a connector to achieve the exterior system. In these circumstances, the job is caught in a fail-and-restart loop, indefinitely, or till you actively force-stop it.

When this occurs, the Managed Service for Apache Flink software standing is perhaps RUNNING, however the underlying Flink job is definitely failing and restarting. The AWS Administration Console offers you a touch, pointing that the applying would possibly want consideration (see the next screenshot).

Within the following sections, we discover ways to monitor the applying and job standing, to routinely react to this example.

When beginning or updating the applying goes unsuitable

To grasp the failure mode, let’s evaluation what occurs routinely if you begin the applying, or when the applying restarts after you issued UpdateApplication command, as we explored in Half 1 of this sequence. The next diagram illustrates what occurs when an software begins.

The workflow consists of the next steps:

1. Managed Service for Apache Flink provisions a cluster devoted to your software.
2. The code and configuration are submitted to the Job Supervisor node.
3. The code within the foremost() methodology of your software runs, defining the dataflow of your software.
4. Flink deploys to the Job Supervisor nodes the substasks that make up your job.
5. The job and software standing change to RUNNING. Nonetheless, subtasks begin initializing now.
6. Subtasks restore their state, if relevant, and initialize any sources. For instance, a Kafka connector’s subtask initializes the Kafka consumer and subscribes the subject.
7. When all subtasks are efficiently initialized, they modify to RUNNING standing and the job begins processing knowledge.

To new Flink customers, it may be complicated {that a} RUNNING standing doesn’t essentially suggest the job is wholesome and processing knowledge.When one thing goes unsuitable in the course of the means of beginning (or restarting) the applying, relying on the section when the issue arises, you would possibly observe two several types of failure modes:

• (a) An issue prevents the applying code from being deployed – Your software would possibly encounter this failure situation if the deployment fails as quickly because the code and configuration are handed to the Job Supervisor (step 2 of the method), for instance if the applying code package deal is malformed. A typical error is when the JAR is lacking a mainClass or if mainClass factors to a category that doesn’t exist. This failure mode may also occur if the code of your foremost() methodology throws an unhandled exception (step 3). In these circumstances, the applying fails to vary to RUNNING, and reverts to READY after the try.
• (b) The applying is began, the job is caught in a fail-and-restart loop – An issue would possibly happen later within the course of, after the applying standing has modified RUNNING. For instance, after the Flink job has been deployed to the cluster (step 4 of the method), a part would possibly fail to initialize (step 6). This would possibly occur when a connector is misconfigured, or an issue prevents it from connecting to the exterior system. For instance, a Kafka connector would possibly fail to hook up with the Kafka cluster due to the connector’s misconfiguration or networking points. One other attainable situation is when the Flink job efficiently initializes, nevertheless it throws an exception as quickly because it begins processing knowledge (step 7). When this occurs, Flink reacts to a runtime error and would possibly get caught in a fail-and-restart loop.

The next diagram illustrates the sequence of software standing, together with the 2 failure situations simply described.

Troubleshooting

We’ve got examined what can go unsuitable throughout operations, specifically if you replace a RUNNING software or restart an software after altering its configuration. On this part, we discover how we will act on these failure situations.

Roll again a change

Whenever you deploy a change and understand one thing isn’t fairly proper, you usually wish to roll again the change and put the applying again in working order, till you examine and repair the issue. Managed Service for Apache Flink supplies a sleek option to revert (roll again) a change, additionally restarting the processing from the purpose it was stopped earlier than making use of the fault change, offering consistency and no knowledge loss.In Managed Service for Apache Flink, there are two forms of rollbacks:

• Computerized – Throughout an computerized rollback (additionally known as system rollback), if enabled, the service routinely detects when the applying fails to restart after a change, or when the job begins however instantly falls right into a fail-and-restart loop. In these conditions, the rollback course of routinely restores the applying configuration model earlier than the final change was utilized and restarts the applying from the snapshot taken when the change was deployed. See Enhance the resilience of Amazon Managed Service for Apache Flink software with system-rollback function for extra particulars. This function is disabled by default. You may allow it as a part of the applying configuration.
• Handbook – A guide rollback API operation is sort of a system rollback, nevertheless it’s initiated by the consumer. If the applying is operating however you observe one thing not behaving as anticipated after making use of a change, you may set off the rollback operation utilizing the RollbackApplication API motion or the console. Handbook rollback is feasible when the applying is RUNNING or UPDATING.

Each rollbacks work equally, restoring the configuration model earlier than the change and restarting with the snapshot taken earlier than the change. This prevents knowledge loss and brings you again to a model of the applying that was working. Additionally, this makes use of the code package deal that was saved on the time you created the earlier configuration model (the one you’re rolling again to), so there isn’t any inconsistency between code, configuration, and snapshot, even when within the meantime you have got changed or deleted the code package deal from the Amazon Easy Storage Service (Amazon S3) bucket.

Implicit rollback: Replace with an older configuration

A 3rd option to roll again a change is to easily replace the configuration, bringing it again to what it was earlier than the final change. This creates a brand new configuration model, and requires the right model of the code package deal to be out there within the S3 bucket if you subject the UpdateApplication command.

Why is there a 3rd possibility when the service supplies system rollback and the managed RollbackApplication motion? As a result of most high-level infrastructure-as-code (IaC) frameworks comparable to Terraform use this technique, explicitly overwriting the configuration. It is very important perceive this chance despite the fact that you’ll in all probability use the managed rollback for those who implement your automation based mostly on the low-level actions.

The next are two essential caveats to think about for this implicit rollback:

• You’ll usually wish to restart the applying from the snapshot that was taken earlier than the defective change was deployed. If the applying is at present RUNNING and wholesome, this isn’t the newest snapshot (RESTORE_FROM_LATEST_SNAPSHOT), however moderately the earlier one. You should set the restart from RESTORE_FROM_CUSTOM_SNAPSHOT and choose the right snapshot.
• UpdateApplication solely works if the applying is RUNNING and wholesome, and the job will be gracefully stopped with a snapshot. Conversely, if the applying is caught in a fail-and-restart loop, it’s essential to force-stop it first, change the configuration whereas the applying is READY, and later begin the applying from the snapshot that was taken earlier than the defective change was deployed.

Power-stop the applying

In regular situations, you cease the applying gracefully, with the automated snapshot creation. Nonetheless, this won’t be attainable in some situations, comparable to if the Flink job is caught in a fail-and-restart loop. This would possibly occur, for instance, if an exterior system the job makes use of stops working, or as a result of the AWS Identification and Entry Administration (IAM) configuration was erroneously modified, eradicating permissions required by the job.

When the Flink job will get caught in a fail-and-restart loop after a defective change, your first possibility must be utilizing RollbackApplication, which routinely restores the earlier configuration and begins from the right snapshot. Within the uncommon circumstances you may’t cease the applying gracefully or use RollbackApplication, the final resort is force-stopping the applying. Power-stop makes use of the StopApplication command with Power=true. You may also force-stop the applying from the console.

Whenever you force-stop an software, no snapshot is taken (if that had been attainable, you’ll have been capable of gracefully cease). Whenever you restart the applying, you may both skip restoring from a snapshot (SKIP_RESTORE_FROM_SNAPSHOT) or use a snapshot that was beforehand taken, scheduled utilizing Snapshot Supervisor, or manually, utilizing the console or CreateApplicationSnapshot API motion.

We strongly suggest organising scheduled snapshots for all manufacturing purposes you can’t afford restarting with no state.

Monitoring Apache Flink software operations

Efficient monitoring of your Apache Flink purposes throughout and after operations is essential to confirm the result of the operation and permit lifecycle automation to lift alarms or react, in case one thing goes unsuitable.

The principle indicators you need to use throughout operations embody the FullRestarts metric (out there in Amazon CloudWatch) and the applying, job, and process standing.

Monitoring the result of an operation

The only option to detect the result of an operation, comparable to StartApplication or UpdateApplication, is to make use of the ListApplicationOperations API command. This command returns an inventory of the latest operations of a selected software, together with upkeep occasions that pressure an software restart.

For instance, to retrieve the standing of the latest operation, you need to use the next command:

aws kinesisanalyticsv2 list-application-operations 
    --application-name MyApplication 
   | jq '.ApplicationOperationInfoList 
   | sort_by(.StartTime) | final'

The output will likely be just like the next code:

{
  "Operation": "UpdateApplication",
  "OperationId": "12abCDeGghIlM",
  "StartTime": "2025-08-06T09:24:22+01:00",
  "EndTime": "2025-08-06T09:26:56+01:00",
  "OperationStatus": "IN_PROGRESS"
}

OperationStatus will comply with the identical logic as the applying standing reported by the console and by DescribeApplication. This implies it won’t detect a failure in the course of the operator initialization or whereas the job begins processing knowledge. As we now have realized, these failures would possibly put the applying in a fail-and-restart loop. To detect these situations utilizing your automation, it’s essential to use different strategies, which we cowl in the remainder of this part.

Detecting the fail-and-restart loop utilizing the FullRestarts metric

The only option to detect whether or not the applying is caught in a fail-and-restart loop is utilizing the fullRestarts metric, out there in CloudWatch Metrics. This metric counts the variety of restarts of the Flink job after you began the applying with a StartApplication command or restarted with UpdateApplication.

In a wholesome software, the variety of full restarts ought to ideally be zero. A single full restart is perhaps acceptable throughout deployment or deliberate upkeep; a number of restarts usually point out some subject. We suggest to not set off an alarm on a single restart, and even a few consecutive restarts.

The alarm ought to solely be triggered when the applying is caught in a fail-and-restart loop. This means checking whether or not a number of restarts have occurred over a comparatively brief time frame. Deciding the interval isn’t trivial, as a result of the time the Flink job takes to restart from a checkpoint is dependent upon the dimensions of the applying state. Nonetheless, if the state of your software is decrease than a number of GB per KPU, you may safely assume the applying ought to begin in lower than a minute.

The objective is making a CloudWatch alarm that triggers when fullRestarts retains growing over a time interval adequate for a number of restarts. For instance, assuming your software restarts in lower than 1 minute, you may create a CloudWatch alarm that depends on the DIFF math expression of the fullRestarts metric. The next screenshot exhibits an instance of the alarm particulars.

This instance is a conservative alarm, solely triggering if the applying retains restarting for over 5 minutes. This implies you detect the issue after no less than 5 minutes. You would possibly think about lowering the time to detect the failure earlier. Nonetheless, watch out to not set off an alarm after only one or two restarts. Occasional restarts would possibly occur, for instance throughout regular upkeep (patching) that’s managed by the service, or for a transient error of an exterior system. Flink is designed to recuperate from these circumstances with minimal downtime and no knowledge loss.

Detecting whether or not the job is up and operating: Monitoring software, job, and process standing

We’ve got mentioned how you have got totally different statuses: the standing of the applying, job, and subtask. In Managed Service for Apache Flink, the applying and job standing change to RUNNING when the subtasks are efficiently deployed on the cluster. Nonetheless, the job isn’t actually operating and processing knowledge till all of the subtasks are RUNNING.

Observing the applying standing throughout operations

The applying standing is seen on the console, as proven within the following screenshot.

In your automation, you may ballot the DescribeApplication API motion to watch the applying standing. The next command exhibits use the AWS Command Line Interface (AWS CLI) and jq command to extract the standing string of an software:

aws kinesisanalyticsv2 describe-application  
    --application-name  
    | jq -r '.ApplicationDetail.ApplicationStatus'

Observing job and subtask standing

Managed Service for Apache Flink offers you entry to the Flink Dashboard, which supplies helpful info for troubleshooting, together with the standing of all subtasks. The next screenshot, for instance, exhibits a wholesome job the place all subtasks are RUNNING.

Within the following screenshot, we will see a job the place subtasks are failing and restarting.

In your automation, if you begin the applying or deploy a change, you wish to ensure the job is ultimately up and operating and processing knowledge. This occurs when all of the subtasks are RUNNING. Observe that ready for the job standing to grow to be RUNNING after an operation isn’t fully secure. A subtask would possibly nonetheless fail and trigger the job to restart after it was reported as RUNNING.

After you execute a lifecycle operation, your automation can ballot the substasks standing ready for one among two occasions:

• All subtasks report RUNNING – This means the operation was profitable and your Flink job is up and operating.
• Any subtask experiences FAILING or CANCELED – This means one thing went unsuitable, and the applying is probably going caught in a fail-and-restart loop. You should intervene, for instance, force-stopping the applying after which rolling again the change.

In case you are restarting from a snapshot and the state of your software is kind of massive, you would possibly observe subtasks will report INITIALIZING standing for longer. In the course of the initialization, Flink restores the state of the operator earlier than altering to RUNNING.

The Flink REST API exposes the state of the subtasks, and can be utilized in your automation. In Managed Service for Apache Flink, this requires three steps:

1. Generate a pre-signed URL to entry the Flink REST API utilizing the CreateApplicationPresignedUrl API motion.
2. Make a GET request to the /jobs endpoint of the Flink REST API to retrieve the job ID.
3. Make a GET request to the /jobs/ endpoint to retrieve the standing of the subtasks.

The next GitHub repository supplies a shell script to retrieve the standing of the duties of a given Managed Service for Apache Flink software.

Monitoring subtasks failure whereas the job is operating

The method of polling the Flink REST API can be utilized in your automation, instantly after an operation, to watch whether or not the operation was ultimately profitable.

We strongly suggest to not constantly ballot the Flink REST API whereas the job is operating to detect failures. This operation is useful resource consuming, and would possibly degrade efficiency or trigger errors.

To watch for suspicious subtask standing modifications throughout regular operations, we suggest utilizing CloudWatch Logs as an alternative. The next CloudWatch Logs Insights question extracts all subtask state transitions:

fields , message
| parse message /^(?.+) switched from (?[A-Z]+) to (?[A-Z]+)./
| filter ispresent(process) and ispresent(fromStatus) and ispresent(toStatus)
| show , process, fromStatus, toStatus
| restrict 10000

How Managed Service for Apache Flink minimizes processing downtime

We’ve got seen how Flink is designed for sturdy consistency. To ensure exactly-once state consistency, Flink briefly stops the processing to deploy any modifications, together with scaling. This downtime is required for Flink to take a constant copy of the applying state and put it aside in a savepoint. After the change is deployed, the job is restarted from the savepoint, and there’s no knowledge loss. In Managed Service for Apache Flink, updates are absolutely managed. When snapshots are enabled, UpdateApplication routinely stops the job and makes use of snapshots (based mostly on Flink’s savepoints) to retain the state.

Flink ensures no knowledge loss. Nonetheless, your small business necessities or Service Stage Targets (SLOs) may also impose a most delay for the info acquired by downstream programs, or end-to-end latency. This delay is affected by the processing downtime, or the time the job doesn’t course of knowledge to permit Flink deploying the change.With Flink, some processing downtime is unavoidable. Nonetheless, Managed Service for Apache Flink is designed to attenuate the processing downtime if you deploy a change.

We’ve got seen how the service runs your software in a devoted cluster, for full isolation. Whenever you subject UpdateApplication on a RUNNING software, the service prepares a brand new cluster with the required quantity of sources. This operation would possibly take a while. Nonetheless, this doesn’t have an effect on the processing downtime, as a result of the service retains the job operating and processing knowledge on the unique cluster till the final attainable second, when the brand new cluster is prepared. At this level, the service stops your job with a savepoint and restarts it on the brand new cluster.

Throughout this operation, you’re solely charged for the variety of KPU of a single cluster.

The next diagram illustrates the distinction between the period of the replace operation, or the time the applying standing is UPDATING, and the processing downtime, observable from the job standing, seen within the Flink Dashboard.

You may observe this course of, preserving each the applying console and Flink Dashboard open, if you replace the configuration of a operating software, even with no modifications. The Flink Dashboard will grow to be briefly unavailable when the service switches to the brand new cluster. Moreover, you may’t use the script we supplied to test the job standing for this scope. Although the cluster retains serving the Flink Dashboard till it’s tore down, the CreateApplicationPresignedUrl motion doesn’t work whereas the applying is UPDATING.

The processing time (the time the job isn’t operating on both clusters) is dependent upon the time the job takes to cease with a savepoint (snapshot) and restore the state within the new cluster. This time largely is dependent upon the dimensions of the applying state. Knowledge skew may also have an effect on the savepoint time because of the barrier alignment mechanism. For a deep dive into the Flink’s barrier alignment mechanism, confer with Optimize checkpointing in your Amazon Managed Service for Apache Flink purposes with buffer debloating and unaligned checkpoints, preserving in thoughts that savepoints are all the time aligned.

For the scope of your automation, you usually wish to wait till the job is again up and operating and processing knowledge. You usually wish to set a timeout. If each the applying and job don’t return to RUNNING inside this timeout, one thing in all probability went unsuitable and also you would possibly wish to increase an alarm or pressure a rollback. This timeout ought to think about your complete replace operation period.

Conclusion

On this put up, we mentioned attainable failure situations if you deploy a change or scale your software. We confirmed how Managed Service for Apache Flink rollback functionalities can seamlessly convey you again to a secure place after a change went unsuitable. We additionally explored how one can automate monitoring operations to watch software, job, and subtask standing, and use the fullRestarts metric to detect when the job is in a fail-and-restart loop.

For extra info, see Run a Managed Service for Apache Flink software, Implement fault tolerance in Managed Service for Apache Flink, and Handle software backups utilizing Snapshots.

Concerning the authors