Status
Current state: [ UNDER DISCUSSION ]
Discussion thread: Add in-memory system
JIRA: SAMZA-1395
Released:
Problem
With 0.13 release, the rich high level APIs allows users to chain complex processing logic as one coherent and fluent application. With so much power, there is a need for inherent support for ease of testing. Currently, the users will have get their hands dirty and understand some implementation details of Samza to write exhaustive integration tests. We want to tackle this problem in steps and this SEP, will take us one step closer towards the goal by introducing an in-memory system in Samza.
Terminologies
- IME - Incoming Message Envelope
- EOS - End of Stream
Motivation
With in-memory system, we will alleviate the following pain points.
Dependency on Kafka for intermediate streams for testing
Running time for tests (time spent on setting up and tearing down)
Ease of testing
Lack of collection based input for testing (this SEP addresses this problem partly)
Assumptions
In-memory system is applicable only for jobs in local execution environment. Remote execution environment isn’t supported.
The scope of in-memory system and the data it handles are limited to a container. I.e. there is no support for process to process interaction or sharing.
Checkpointing is not supported and consumers always start from the beginning in case of restart.
In-memory system doesn’t support persistence and is not the source of truth for the data. The data in the queue is lost when the job restarts or shutdowns unexpectedly.
Design
Data Source & Sink
The input data source for the in-memory system can be broadly classified as bounded and unbounded data. We are limiting the scope of this SEP to only bounded data source that is immutable as the input source. It simplifies the view of the data and also the initialization step for the consumers. However, in-memory system for intermediate streams supports both bounded and unbounded data. The sink a.k.a output source is modeled to be mutable.
Data Type
Samza has a pluggable system design allowing users to implement their own system consumers. Typically, consumers consume raw message and wrap them using IME. However, it is possible for some systems to introduce subclass of IME and pass them to tasks. For this reason, we need to support for different data types within in-memory collection.
- Raw messages: In-memory system will behave like a typical consumer and wrap the raw message using IME. The offset and key fields for the message are populated by the in-memory system. Note, the offset is defined as the position of the data in the collection and the key is the hash code of the raw message.
- Type of IME: In-memory system acts as a pass through system consumer, passing the actual message envelope to the task without any wrapping.
Data Partitioning
Samza is a distributed stream processing framework that achieves parallelism with partitioned data. With a bounded data source, we need to think about how the data is going to be partitioned and how do we map data to SystemStreamPartition in Samza. Partitioning is only interesting in the case when the input source is raw message. With IME, partitioning data is already part of it and in-memory system will respect the partition information within the IME.
Single Partition
We can use a trivial and simpler approach of associating all of our data source to a single partition. It is not a bad strategy since the primary use case for in-memory systems is testing and the volume of data is negligible that we can barely notice the effects of parallelism. Although it does come w/ a downside that it constraints the users to only test their job with only one task. It might not be a desirable and exhaustive testing strategy from a user’s perspective.
Multiple Partition
In order to exploit the parallelism that the Samza framework offers and to enable users test their job with multiple tasks, we need to support multiple partitioning.
Partitioning at source
In this approach, we push the partitioning to the source. For e.g. we can read of a `Collection<Collection<T>>` and have each collection within the collection assigned to one partition. This is surprisingly simple yet powerful since it eliminates the need for repartitioning phase and allows the user to group the data at his/her whim. The downside w/ this approach is the input collections can be skewed and Samza don’t control the evenness in the distribution of the data. Since the primary use case is testing, the skew should have negligible impact.
End of Stream
In-memory system will leverage the EOS feature introduced in SEP-6 to mark the end of stream for bounded sources.
Proposed Changes
Architecture
Implementation
- Approach A - Use existing BlockingEnvelopeMap and have one common class that shares the responsibility of consumer as well as producer. The class will be responsible for handling both producing and consuming messages off the same queue.
- Approach B - Have separate producer and consumer. Tie up the consumer with the producer so that producer has hooks to produce to the same underlying `BlockingEnvelopeMap` that consumer uses.
- Approach C - Have separate consumer and producer. Introduce a custom queue that are shared between consumer and producer. The queue lifecycle is managed by the SystemAdmin.
Test Plan
High level application
High level application with durable state
Low level application
Low level application with durable state
Low level application with manual checkpoint
Samza SQL application
Users should be able to leverage in-memory collection based system to test Samza SQL application.
Details: TBD