MSc Mirko Köhler

Hochschulstr. 10
64289 Darmstadt, Germany

I work on programming abstractions for replicated data. Currently, I’m the main contributor to ConSysT, an object-oriented language for mixed consistency with replicated objects. My research interests include concurrency, mixed consistency protocols, and type systems.

Publications

ECOOP

A Direct-Style Effect Notation for Sequential and Parallel Programs

David Richter, Timon Böhler, Pascal Weisenburger, Mira Mezini

In Proceedings of the 37th European Conference on Object-Oriented Programming (ECOOP), Leibniz International Proceedings in Informatics (LIPIcs), 2023

Abstract PDF Supp

Modeling sequential and parallel composition of effectful computations has been investigated in a variety of languages for a long time. In particular, the popular do-notation provides a lightweight effect embedding for any instance of a monad. Idiom bracket notation, on the other hand, provides an embedding for applicatives. First, while monads force effects to be executed sequentially, ignoring potential for parallelism, applicatives do not support sequential effects. Composing sequential with parallel effects remains an open problem. This is even more of an issue as real programs consist of a combination of both sequential and parallel segments. Second, common notations do not support invoking effects in direct-style, instead forcing a rigid structure upon the code.
In this paper, we propose a mixed applicative/monadic notation that retains parallelism where possible, but allows sequentiality where necessary. We leverage a direct-style notation where sequentiality or parallelism is derived from the structure of the code. We provide a mechanisation of our effectful language in Coq and prove that our compilation approach retains the parallelism of the source program.
SPLASH Companion
ConSysT

Safe Combination of Data-Centric and Operation-Centric Consistency

Mirko Köhler, Guido Salvaneschi

In Companion Proceedings of the 2023 ACM SIGPLAN International Conference on Systems, Programming, Languages, and Applications: Software for Humanity, SPLASH Companion, 2023

Abstract PDF

Programming distributed systems requires maintaining consistency among data replicas. In recent years, various frameworks have proposed language-level abstractions for this, falling into two fundamental approaches: data-centric and operation-centric solutions. The former allow developers to explicitly assign consistency levels to data, the latter enable attaching consistency constraints to operations. In practice, developers may benefit from both in the same application: data-centric consistency harmonizes well with object-oriented programming, yet one may need the flexibility to access the same data with a different consistency level depending on the operation. Currently, there is no solution that integrates both: it is a conceptual challenge to unify these two models and design a type system capable of ensuring their correct interaction.
We present ConOpY, a programming language that integrates both data-centric and operation-centric consistency into the same design. The ConOpY type system guarantees the proper usage of consistency levels, preventing consistency violations resulting from an improper mix of consistency models. ConOpY is implemented as a Java extension based on annotations.
OOPSLA
ConSysT

Rethinking Safe Consistency in Distributed Object-Oriented Programming

Mirko Köhler, Nafise Eskandani, Pascal Weisenburger, Alessandro Margara, Guido Salvaneschi

Proceedings of the ACM on Programming Languages 4 (OOPSLA), 2020

Abstract PDF

Large scale distributed systems require to embrace the trade off between consistency and availability, accepting lower levels of consistency to guarantee higher availability. Existing programming languages are, however, agnostic to this compromise, resulting in consistency guarantees that are the same for the whole application and are implicitly adopted from the middleware or hardcoded in configuration files.
In this paper, we propose to integrate availability in the design of an object-oriented language, allowing developers to specify different consistency and isolation constraints in the same application at the granularity of single objects. We investigate how availability levels interact with object structure and define a type system that preserves correct program behavior. Our evaluation shows that our solution performs efficiently and improves the design of distributed applications.
FTfJP
ConSysT

ConSysT: Tunable, Safe Consistency Meets Object-Oriented Programming

Mirko Köhler, Nafise Eskandani, Alessandro Margara, Guido Salvaneschi

In Proceedings of the 22nd ACM SIGPLAN International Workshop on Formal Techniques for Java-Like Programs, FTfJP, 2020

Abstract

Data replication is essential in scenarios like geo-distributed datacenters, but poses challenges for data consistency. Developers adopt Strong consistency at the cost of performance or embrace Weak consistency and face a higher programming complexity. We argue that languages should associate consistency to data types. We present , a programming language and middleware that provides abstractions to specify consistency types, enabling mixing different consistency levels in the same application. Such mechanism is fully integrated with object-oriented programming and type system guarantees that different levels can only be mixed correctly.
ASE

Automated Refactoring to Reactive Programming

Mirko Köhler, Guido Salvaneschi

In Proceedings of the 34th IEEE/ACM International Conference on Automated Software Engineering, ASE, 2019

Abstract PDF

Reactive programming languages and libraries, such as ReactiveX, have been shown to significantly improve software design and have seen important industrial adoption over the last years. Asynchronous applications - which are notoriously error-prone to implement and to maintain - greatly benefit from reactive programming because they can be defined in a declarative style, which improves code clarity and extensibility.
In this paper, we tackle the problem of refactoring existing software that has been designed with traditional abstractions for asynchronous programming. We propose 2Rx, a refactoring approach to automatically convert asynchronous code to reactive programming. Our evaluation on top-starred GitHub projects shows that 2Rx is effective with common asynchronous constructs and it can provide a refactoring for 91.7% of their occurrences.
SPLASH Companion
ConSysT

Distributed Object-Oriented Programming with Multiple Consistency Levels in ConSysT

Nafise Eskandani, Mirko Köhler, Alessandro Margara, Guido Salvaneschi

In Companion Proceedings of the 2019 ACM SIGPLAN International Conference on Systems, Programming, Languages, and Applications: Software for Humanity, SPLASH Companion, 2019

Abstract

Data replication is essential in scenarios like geo-distributed datacenters and edge computing. Yet, it poses a challenge for data consistency. Developers either adopt high consistency at the detriment of performance or they embrace low consistency and face a much higher programming complexity. We argue that language abstractions should support associating the level of consistency to data types. We present ConSysT, a programming language and middleware that provides abstractions to specify consistency types, enabling mixing different consistency levels in the same application. Such mechanism is fully integrated with object-oriented programming and type system guarantees that different levels can be mixed only in a correct way.
OOPSLA

Language-Integrated Privacy-Aware Distributed Queries

Guido Salvaneschi, Mirko Köhler, Daniel Sokolowski, Philipp Haller, Sebastian Erdweg, Mira Mezini

Proceedings of the ACM on Programming Languages 3 (OOPSLA), 2019

Abstract PDF

Distributed query processing is an effective means for processing large amounts of data. To abstract from the technicalities of distributed systems, algorithms for operator placement automatically distribute sequential data queries over the available processing units. However, current algorithms for operator placement focus on performance and ignore privacy concerns that arise when handling sensitive data.
We present a new methodology for privacy-aware operator placement that both prevents leakage of sensitive information and improves performance. Crucially, our approach is based on an information-flow type system for data queries to reason about the sensitivity of query subcomputations. Our solution unfolds in two phases. First, placement space reduction generates deployment candidates based on privacy constraints using a syntax-directed transformation driven by the information-flow type system. Second, constraint solving selects the best placement among the candidates based on a cost model that maximizes performance. We verify that our algorithm preserves the sequential behavior of queries and prevents leakage of sensitive data. We implemented the type system and placement algorithm for a new query language SecQL and demonstrate significant performance improvements in benchmarks.
OOPSLA
ScalaLoci

Distributed System Development with ScalaLoci

Pascal Weisenburger, Mirko Köhler, Guido Salvaneschi

Proceedings of the ACM on Programming Languages 2 (OOPSLA), 2018

Abstract PDF Code Website

Distributed applications are traditionally developed as separate modules, often in different languages, which react to events, like user input, and in turn produce new events for the other modules. Separation into components requires time-consuming integration. Manual implementation of communication forces programmers to deal with low-level details. The combination of the two results in obscure distributed data flows scattered among multiple modules, hindering reasoning about the system as a whole. The ScalaLoci distributed programming language addresses these issues with a coherent model based on placement types that enables reasoning about distributed data flows, supporting multiple software architectures via dedicated language features and abstracting over low-level communication details and data conversions. As we show, ScalaLoci simplifies developing distributed systems, reduces error-prone communication code and favors early detection of bugs.
OOPSLA

i3QL: Language-Integrated Live Data Views

Ralf Mitschke, Sebastian Erdweg, Mirko Köhler, Mira Mezini, Guido Salvaneschi

In Proceedings of the 2014 ACM International Conference on Object-oriented Programming, Systems, Languages, and Applications, OOPSLA, 2014

Abstract PDF

An incremental computation updates its result based on a change to its input, which is often an order of magnitude faster than a recomputation from scratch. In particular, incrementalization can make expensive computations feasible for settings that require short feedback cycles, such as interactive systems, IDEs, or (soft) real-time systems.
This paper presents i3QL, a general-purpose programming language for specifying incremental computations. i3QL provides a declarative SQL-like syntax and is based on incremental versions of operators from relational algebra, enriched with support for general recursion. We integrated i3QL into Scala as a library, which enables programmers to use regular Scala code for non-incremental subcomputations of an i3QL query and to easily integrate incremental computations into larger software projects. To improve performance, i3QL optimizes user-defined queries by applying algebraic laws and partial evaluation. We describe the design and implementation of i3QL and its optimizations, demonstrate its applicability, and evaluate its performance.