We are developing a pipeline in apache flink (datastream API) that needs to sends its messages to an external system using API calls. Sometimes such an API call will fail, in this case our message needs some extra treatment (and/or a retry).
We had a few options for doing this:
We map() our stream through a function that does the API call and get the result of the API call returned, so we can act upon failures subsequently (this was my original idea, and why i did this: flink scala map with dead letter queue)
We write a custom sink function that does the same.
However, both options have problems i think:
With the map() approach i won't be able to get exactly once (or at most once which would also be fine) semantics since flink is free to re-execute pieces of pipelines after recovering from a crash in order to get the state up to date.
With the custom sink approach i can't get a stream of failed API calls for further processing: a sink is a dead end from the flink APPs point of view.
Is there a better solution for this problem ?
The async i/o operator is designed for this scenario. It's a better starting point than a map.
There's also been recent work done to develop a generic async sink, see FLIP-171. This has been merged into master and will be released as part of Flink 1.15.
One of those should be your best way forward. Whatever you do, don't do blocking i/o in your user functions. That causes backpressure and often leads to performance problems and checkpoint failures.
I am trying to implement event sourcing/CQRS/DDD for the first time, mostly for learning purposes, where there is the idea of an event store and a message queue such as Apache Kafka, and you have events flowing from event store => Kafka Connect JDBC/Debezium CDC => Kafka.
I am wondering why there needs to be a separate event store when it sounds like its purpose can be fulfilled by Kafka itself with its main features and log compaction or configuring log retention for permanent storage. Should I store my events in a dedicated store like RDBMS to feed into Kafka or should I feed them straight into Kafka?
Much of the literature on event-sourcing and cqrs comes from the [domain driven design] community; in its earliest form, CQRS was called DDDD... Distributed domain driven design.
One of the common patterns in domain driven design is to have a domain model ensuring the integrity of the data in your durable storage, which is to say, ensuring that there are no internal contradictions...
I am wondering why there needs to be a separate event store when it sounds like its purpose can be fulfilled by Kafka itself with its main features and log compaction or configuring log retention for permanent storage.
So if we want an event stream with no internal contradictions, how do we achieve that? One way is to ensure that only a single process has permission to modify the stream. Unfortunately, that leaves you with a single point of failure -- the process dies, and everything comes to an end.
On the other hand, if you have multiple processes updating the same stream, then you have risk of concurrent writes, and data races, and contradictions being introduced because one writer couldn't yet see what the other one did.
With an RDBMS or an Event Store, we can solve this problem by using transactions, or compare and swap semantics; and attempt to extend the stream with new events is rejected if there has been a concurrent modification.
Furthermore, because of its DDD heritage, it is common for the durable store to be divided into many very fine grained partitions (aka "aggregates"). One single shopping cart might reasonably have four streams dedicated to it.
If Kafka lacks those capabilities, then it is going to be a lousy replacement for an event store. KAFKA-2260 has been open for more than four years now, so we seem to be lacking the first. From what I've been able to discern from the Kakfa literature, it isn't happy about fine grained streams either (although its been a while since I checked, perhaps things have changed).
See also: Jesper Hammarbäck writing about this 18 months ago, and reaching similar conclusions to those expressed here.
Kafka can be used as a DDD event store, but there are some complications if you do so due to the features it is missing.
Two key features that people use with event sourcing of aggregates are:
Load an aggregate, by reading the events for just that aggregate
When concurrently writing new events for an aggregate, ensure only one writer succeeds, to avoid corrupting the aggregate and breaking its invariants.
Kafka can't do either of these currently, since 1 fails since you generally need to have one stream per aggregate type (it doesn't scale to one stream per aggregate, and this wouldn't necessarily be desirable anyway), so there's no way to load just the events for one aggregate, and 2 fails since https://issues.apache.org/jira/browse/KAFKA-2260 has not been implemented.
So you have to write the system in such as way that capabilities 1 and 2 aren't needed. This can be done as follows:
Rather than invoking command handlers directly, write them to
streams. Have a command stream per aggregate type, sharded by
aggregate id (these don't need permanent retention). This ensures that you only ever process a single
command for a particular aggregate at a time.
Write snapshotting code for all your aggregate types
When processing a command message, do the following:
Load the aggregate snapshot
Validate the command against it
Write the new events (or return failure)
Apply the events to the aggregate
Save a new aggregate snapshot, including the current stream offset for the event stream
Return success to the client (via a reply message perhaps)
The only other problem is handling failures (such as the snapshotting failing). This can be handled during startup of a particular command processing partition - it simply needs to replay any events since the last snapshot succeeded, and update the corresponding snapshots before resuming command processing.
Kafka Streams appears to have the features to make this very simple - you have a KStream of commands that you transform into a KTable (containing snapshots, keyed by aggregate id) and a KStream of events (and possibly another stream containing responses). Kafka allows all this to work transactionally, so there is no risk of failing to update the snapshot. It will also handle migrating partitions to new servers, etc. (automatically loading the snapshot KTable into a local RocksDB when this happens).
there is the idea of an event store and a message queue such as Apache Kafka, and you have events flowing from event store => Kafka Connect JDBC/Debezium CDC => Kafka
In the essence of DDD-flavoured event sourcing, there's no place for message queues as such. One of the DDD tactical patterns is the aggregate pattern, which serves as a transactional boundary. DDD doesn't care how the aggregate state is persisted, and usually, people use state-based persistence with relational or document databases. When applying events-based persistence, we need to store new events as one transaction to the event store in a way that we can retrieve those events later in order to reconstruct the aggregate state. Thus, to support DDD-style event sourcing, the store needs to be able to index events by the aggregate id and we usually refer to the concept of the event stream, where such a stream is uniquely identified by the aggregate identifier, and where all events are stored in order, so the stream represents a single aggregate.
Because we rarely can live with a database that only allows us to retrieve a single entity by its id, we need to have some place where we can project those events into, so we can have a queryable store. That is what your diagram shows on the right side, as materialised views. More often, it is called the read side and models there are called read-models. That kind of store doesn't have to keep snapshots of aggregates. Quite the opposite, read-models serve the purpose to represent the system state in a way that can be directly consumed by the UI/API and often it doesn't match with the domain model as such.
As mentioned in one of the answers here, the typical command handler flow is:
Load one aggregate state by id, by reading all events for that aggregate. It already requires for the event store to support that kind of load, which Kafka cannot do.
Call the domain model (aggregate root method) to perform some action.
Store new events to the aggregate stream, all or none.
If you now start to write events to the store and publish them somewhere else, you get a two-phase commit issue, which is hard to solve. So, we usually prefer using products like EventStore, which has the ability to create a catch-up subscription for all written events. Kafka supports that too. It is also beneficial to have the ability to create new event indexes in the store, linking to existing events, especially if you have several systems using one store. In EventStore it can be done using internal projections, you can also do it with Kafka streams.
I would argue that indeed you don't need any messaging system between write and read sides. The write side should allow you to subscribe to the event feed, starting from any position in the event log, so you can build your read-models.
However, Kafka only works in systems that don't use the aggregate pattern, because it is essential to be able to use events, not a snapshot, as the source of truth, although it is of course discussable. I would look at the possibility to change the way how events are changing the entity state (fixing a bug, for example) and when you use events to reconstruct the entity state, you will be just fine, snapshots will stay the same and you'll need to apply correction events to fix all the snapshots.
I personally also prefer not to be tightly coupled to any infrastructure in my domain model. In fact, my domain models have zero dependencies on the infrastructure. By bringing the snapshotting logic to Kafka streams builder, I would be immediately coupled and from my point of view it is not the best solution.
Theoretically you can use Kafka for Event Store but as many people mentioned above that you will have several restrictions, biggest of those, only able to read event with the offset in the Kafka but no other criteria.
For this reason they are Frameworks there dealing with the Event Sourcing and CQRS part of the problem.
Kafka is only part of the toolchain which provides you the capability of replaying events and back pressure mechanism that are protecting you from overload.
If you want to see how all fits together, I have a blog about it
I'm new to akka http & streams and would like to figure out what the most idiomatic implementation is for a REST api. Let's say I need to implement a single endpoint that:
accepts a path parameter and several query string parameters
validates the params
constructs a db query based on the params
executes the query
sends response back to client
From my research I understand this is a good use case for akka http and modeling the flow of the request -> response seems to map well to using akka streams with several Flows but I'd like some clarification:
Does using the akka streams library make sense here?
Is that still true if the database driver making the call does not have an async api?
Do the stream back pressure semantics still hold true if there is a Flow making a blocking call?
How is parallelism handled with the akka streams implementation? Say for example the service experiences 500 concurrent connections, does the streams abstraction simply have a pool of actors under the covers to handle each of these connections?
Edit: This answers most of the questions I had: http://doc.akka.io/docs/akka-http/current/scala/http/handling-blocking-operations-in-akka-http-routes.html
I found Slick 3.0 introduced a new feature called streaming
http://slick.typesafe.com/doc/3.0.0-RC1/database.html#streaming
I'm not familiar with Akka. streaming seems a lazy or async value, but it is not very clear for me to understand why it is useful, and when will it be useful..
Does anyone have ideas about this?
So lets imagine the following use case:
A "slow" client wants to get a large dataset from the server. The client sends a request to the server which loads all the data from the database, stores it in memory and then passes it down to the client.
And here we're faced with problems: The client handles the data not so fast as we wanted => we can't release the memory => this may result in an out of memory error.
Reactive streams solve this problem by using backpressure. We can wrap Slick's publisher around the Akka source and then "feed" it to the client via Akka HTTP.
The thing is that this backpressure is propagated through TCP via Akka HTTP down to the publisher that represents the database query.
That means that we only read from the database as fast as the client can consume the data.
P.S This just a little aspect where reactive streams can be applied.
You can find more information here:
http://www.reactive-streams.org/
https://youtu.be/yyz7Keg1w9E
https://youtu.be/9S-4jMM1gqE
I am a new starter in Flink, I have a requirement to read data from Kafka, enrich those data conditionally (if a record belongs to category X) by using some API and write to S3.
I made a hello world Flink application with the above logic which works like a charm.
But, the API which I am using to enrich doesn't have 100% uptime SLA, so I need to design something with retry logic.
Following are the options that I found,
Option 1) Make an exponential retry until I get a response from API, but this will block the queue, so I don't like this
Option 2) Use one more topic (called topic-failure) and publish it to topic-failure if the API is down. In this way it won't block the actual main queue. I will need one more worker to process the data from the queue topic-failure. Again, this queue has to be used as a circular queue if the API is down for a long time. For example, read a message from queue topic-failure try to enrich if it fails to push to the same queue called topic-failure and consume the next message from the queue topic-failure.
I prefer option 2, but it looks like not an easy task to accomplish this. Is there is any standard Flink approach available to implement option 2?
This is a rather common problem that occurs when migrating away from microservices. The proper solution would be to have the lookup data also in Kafka or some DB that could be integrated in the same Flink application as an additional source.
If you cannot do it (for example, API is external or data cannot be mapped easily to a data storage), both approaches are viable and they have different advantages.
1) Will allow you to retain the order of input events. If your downstream application expects orderness, then you need to retry.
2) The common term is dead letter queue (although more often used on invalid records). There are two easy ways to integrate that in Flink, either have a separate source or use a topic pattern/list with one source.
Your topology would look like this:
Kafka Source -\ Async IO /-> Filter good -> S3 sink
+-> Union -> with timeout -+
Kafka Source dead -/ (for API call!) \-> Filter bad -> Kafka sink dead