Computer Laboratory

Technical reports

Kilim: A server framework with lightweight actors, isolation types and zero-copy messaging

Sriram Srinivasan

February 2010, 127 pages

This technical report is based on a dissertation submitted February 2010 by the author for the degree of Doctor of Philosophy to the University of Cambridge, King’s College.


Internet services are implemented as hierarchical aggregates of communicating components: networks of data centers, networks of clusters in a data center, connected servers in a cluster, and multiple virtual machines on a server machine, each containing several operating systems processes. This dissertation argues for extending this structure to the intra-process level, with networks of communicating actors. An actor is a single-threaded state machine with a private heap and a thread of its own. It communicates with other actors using well-defined and explicit messaging protocols. Actors must be light enough to comfortably match the inherent concurrency in the problem space, and to exploit all available parallelism. Our aims are two-fold: (a) to treat SMP systems as they really are: distributed systems with eventual consistency, and (b) recognize from the outset that a server is always part of a larger collection of communicating components, thus eliminating the mindset mismatch between concurrent programming and distributed programming.

Although the actor paradigm is by no means new, our design points are informed by drawing parallels between the macro and micro levels. As with components in a distributed system, we expect that actors must be isolatable in a number of ways: memory isolation, fault isolation, upgrade isolation, and execution isolation. The application should be able to have a say in actor placement and scheduling, and actors must be easily monitorable.

Our primary contribution is in showing that these requirements can be satisfied in a language and environment such as Java, without changes to the source language or to the virtual machine, and without leaving much of the idiomatic ambit of Java, with its mindset of pointers and mutable state. In other words, one does not have to move to a concurrency-oriented language or to an entirely immutable object paradigm.

We demonstrate an open-source toolkit called Kilim that provides (a) ultra-lightweight actors (faster and lighter than extant environments such as Erlang), (b) a type system that guarantees memory isolation between threads by separating internal objects from exportable messages and by enforcing ownership and structural constraints on the latter (linearity and tree-structure, respectively) and, (c) a library with I/O support and customizable synchronization constructs and schedulers.

We show that this solution is simpler to program than extant solutions, yet statically guaranteed to be free of low-level data races. It is also faster, more scalable and more stable (in increasing scale) in two industrial strength evaluations: interactive web services (comparing Kilim Web Server to Jetty) and databases (comparing Berkeley DB to a Kilim variant of it).

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BibTeX record

  author =	 {Srinivasan, Sriram},
  title = 	 {{Kilim: A server framework with lightweight actors,
         	   isolation types and zero-copy messaging}},
  year = 	 2010,
  month = 	 feb,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-769}