diff options
Diffstat (limited to 'chapter/4')
| -rw-r--r-- | chapter/4/dist-langs.md | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/chapter/4/dist-langs.md b/chapter/4/dist-langs.md index 24c819e..eb7b7a4 100644 --- a/chapter/4/dist-langs.md +++ b/chapter/4/dist-langs.md @@ -249,6 +249,32 @@ It is worth noting that stack frame growth is still possible, but if the recursi #### Orleans (2011) +Orleans is a programming model for distributed computing based on actors. +An Orleans program can be conceptualized as a collection of actors. +Because Orleans is intended as a model for building cloud applications, actors do not spawn independent processes as they do in Emerald or Argus. +Rather, Orleans actors are designed to execute only when responding to requests. + +As in Argus, Orleans encapsulates requests (root function calls) within *transactions*. +When processing a transaction, function calls may span many actors. +To ensure consistency in the end result of a transaction, Orleans offers another abstraction, *activations*, to allow each transaction to operate on a consistent state of actors. +An activation is an instance of an actor. +Activations allow (1) consistent access to actors during concurrent transactions, and (2) high throughput when an actor becomes "hot." +Consistency is achieved by only allowing transactions to "touch" activations of actors that are not being used by another transaction. +High throughput is achieved by spawning many activations of an actor for handling concurrent requests. + +For example, suppose there is an actor that represents a specific YouTube video. +This actor will have data fields like `title`, `content`, `num_views`, etc. +Suppose their are concurrent requests (in turn, transactions) for viewing the video. +In the relevent transaction, the `num_views` field is incremented. +Therefore, in order to run the view requests concurrently, two activations (or copies) of the actor are created. + +Because there is concurrency within individual actors, Orleans also supports means of state reconciliation. +When concurrent transactions modify different activations of an actor, state must eventually be reconciled. +In the case of the above example, it may not be necessary to know immediately the exact view count of a video, but we would like to be able to know this value eventually. +To accomplish reconciliation, Orleans provides data structures that can be automatically merged. +As well, the developer can implement arbitrary logic for merging state. +In the case of the YouTube video, we would want logic to determine the delta of views since the start of the activation, and add that to the actors' sum. + ### Dataflow model (static and stream) The dataflow model has its roots in functional programming. |