Low-latency geo-distributed applications currently face the barrier of cross-site coordination for ensuring state consistency. Existing mixed-consistency models leverage the existence of strongly- and weakly-consistent operations in a given application, to avoid coordination whenever possible. However, existing approaches are rather pessimistic, coordinating more than is necessary. In this paper, we introduce Semi-Linearizability (SL): a consistency model that executes application operations with linearizability guarantees only when strictly necessary, avoiding over-coordination. Specifically, we propose novel operation semantics that can encode ordering relationships between application operations and map them to coordination primitives. Our proposed semantics can be used to reason over latent, asymmetric dependencies between different operations and optimize their execution. We show how SL enables a new class of safe, uncoordinated operations that previous models would otherwise execute under globally strict order, while offering substantial performance gains without violating application invariants. To demonstrate the advantages of SL, we implemented DeMon, a system that achieves four orders of magnitude lower latency on the most frequent operation in the widely used RUBiS benchmark compared to state-of-the-art systems.

In this work, we evaluate autoscaling solutions for stream processing engines. Although autoscaling has become a mainstream subject of research in the last decade, the database research community has yet to evaluate different autoscaling techniques under a proper benchmarking setting and evaluation framework. As a result, every newly proposed autoscaling solution only performs a shallow performance evaluation and comparison against existing solutions. In this paper, we evaluate autoscaling solutions by employing two streaming queries and a dynamic workload that follows a cosinus pattern. Our experiments reveal that current autoscaling techniques fail to account for generated lag due to rescaling or underprovisioning and cannot efficiently handle practical scenarios of intensely dynamic workloads.

Serverless computing is currently the fastest-growing cloud services segment. The most prominent serverless offering is Function-as-a-Service (FaaS), where users write functions and the cloud automates deployment, maintenance, and scalability. Although FaaS is a good fit for executing stateless functions, it does not adequately support stateful constructs like microservices and scalable, low-latency cloud applications. Recently, there have been multiple attempts to add first-class support for state in FaaS systems, such as Microsoft Orleans, Azure Durable Functions, or Beldi. These approaches execute business code inside stateless functions, handing over their state to an external database. In contrast, approaches such as Apache Flink's StateFun follow a different design: a dataflow system such as Apache Flink handles all state management, messaging, and checkpointing by executing a stateful dataflow graph providing exactly-once state processing guarantees. This design relieves programmers from having to “pollute” their business logic with distributed systems error checking, management, and mitigation. Although programmers can easily develop applications without worrying about messaging and state management, executing transactions across stateful functions remains an open problem. In this paper, we introduce a programming model and implementation for transaction orchestration of stateful serverless functions. Our programming model supports serializable distributed transactions with two-phase commit, as well as eventually consistent workflows with Sagas. We design and implement our programming model on Apache Flink StateFun to leverage Flink's exactly-once processing and state management guarantees. Our experiments show that the approach of building transactional systems on top of dataflow graphs can achieve very high throughput, but with latency overhead due to checkpointing mechanism guaranteeing the exactly-once processing. We compare our approach to Beldi that implements two-phase commit on AWS lambda functions backed by DynamoDB for state management, as well as an implementation of a system that makes use of CockroachDB as its backend.

Serverless computing is currently the fastest-growing cloud services segment. The most prominent serverless offering is Function-as-a-Service (FaaS), where users write functions and the cloud automates deployment, maintenance, and scalability. Although FaaS is a good fit for executing stateless functions, it does not adequately support stateful constructs like microservices and scalable, low-latency cloud applications, mainly because it lacks proper state management support and the ability to perform function-to-function calls. Most importantly, executing transactions across stateful functions remains an open problem. In this paper, we introduce a programming model and implementation for transaction orchestration of stateful serverless functions. Our programming model supports serializable distributed transactions with two-phase commit, as well as relaxed transactional guarantees with Sagas. We design and implement our programming model on Apache Flink StateFun. We choose to build our solution on top of StateFun in order to leverage Flink's exactly-once processing and state management guarantees. We base our evaluation on the YCSB benchmark, which we extended with transactional operations and adapted for the SFaaS programming model. Our experiments show that our transactional orchestration adds 10% overhead to the original system and that Sagas can achieve up to 34% more transactions per second than two-phase commit transactions at a sub-200ms latency.