SCALA DOCUMENTATION PDF
Getting Started. Install Scala on your computer and start writing some Scala code! Overviews. In-depth documentation covering many of Scala's features. Version was released in November This document describes the second version of the language, which was released in March This document introduces Scala in an informal way, through a sequence of exam - ples. Chapters 2 and 3 highlight some of the features that.
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Scala Documentation, Release Introduction. Abstract Types. Annotations. Classes. Case Classes. Compound Types. Sequence. Akka has both a Scala Documentation and a java-api. Phi accrual failure detector (resourceone.info~defago/files/pdf/IS_RR__0. Scala is statically typed, enabling the compiler to find errors, so that you don't will also learn how to browse the Scaladoc documentation at a beginner's level.
If the resource is requested by the other participant, they try to get the other instance of the resource.
In the unfortunate case it might happen that the two participants bounce between the two resources, never acquiring it, but always yielding to the other. Race conditions often arise when multiple threads have a shared mutable state, and the operations of thread on the state might be interleaved causing unexpected behavior. While this is a common case, shared state is not necessary to have race conditions.
One example could be a client sending unordered packets e.
As the packets might potentially travel via different network routes, it is possible that the server receives P2 first and P1 afterwards. If the messages contain no information about their sending order it is impossible to determine by the server that they were sent in a different order. Depending on the meaning of the packets this can cause race conditions. Note: The only guarantee that Akka provides about messages sent between a given pair of actors is that their order is always preserved.
In the following sections we discuss various non-blocking properties with different strength. Wait-freedom A method is wait-free if every call is guaranteed to finish in a finite number of steps. If a method is bounded wait-free then the number of steps has a finite upper bound. From this definition it follows that wait-free methods are never blocking, therefore deadlock can not happen.
Additionally, as each participant can progress after a finite number of steps when the call finishes , wait-free methods are free of starvation. Lock-freedom Lock-freedom is a weaker property than wait-freedom. In the case of lock-free calls, infinitely often some method finishes in a finite number of steps.
This definition implies that no deadlock is possible for lock-free calls. On the other hand, the guarantee that some call finishes in a finite number of steps is not enough to guarantee that all of them eventually finish. In other words, lock-freedom is not enough to guarantee the lack of starvation.
Obstruction-freedom Obstruction-freedom is the weakest non-blocking guarantee discussed here. A method is called obstruction-free if there is a point in time after which it executes in isolation other threads make no steps, e.
All lock-free objects are obstruction-free, but the opposite is generally not true.
Optimistic concurrency control OCC methods are usually obstruction-free. The OCC approach is that every participant tries to execute its operation on the shared object, but if a participant detects conflicts from others, it rolls back the modifications, and tries again according to some schedule.
If there is a point in time, where one of the participants is the only one trying, the operation will succeed. Herlihy and N Shavit, Goetz, T. Peierls, J. Bloch, J. Bowbeer, D. Holmes and D. Lea, ISBN 2. In a sense, actors are the most stringent form of object-oriented programming, but it serves better to view them as persons: while modeling a solution with actors, envision a group of people and assign sub-tasks to them, arrange their functions into an organizational structure and think about how to escalate failure all with the benefit of not actually dealing with people, which means that we need not concern ourselves with their emotional state or moral issues.
The result can then serve as a mental scaffolding for building the software implementation. Note: An ActorSystem is a heavyweight structure that will allocate 1. N Threads, so create one per logical application. One actor, which is to oversee a certain function in the program might want to split up its task into smaller, more manageable pieces.
For this purpose it starts child actors which it supervises.
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While the details of supervision are explained here, we shall concentrate on the underlying concepts in this section. The only prerequisite is to know that each actor has exactly one supervisor, which is the actor that created it.
The quintessential feature of actor systems is that tasks are split up and delegated until they become small enough to be handled in one piece. In doing so, not only is the task itself clearly structured, but the resulting actors can be reasoned about in terms of which messages they should process, how they should react normally and how failure should be handled.
If one actor does not have the means for dealing with a certain situation, it sends a corresponding failure message to its supervisor, asking for help.
The recursive structure then allows to handle failure at the right level. Compare this to layered software design which easily devolves into defensive programming with the aim of not leaking any failure out: if the problem is communicated to the right person, a better solution can be found than if trying to keep everything under the carpet.
Now, the difficulty in designing such a system is how to decide who should supervise what. There is of course no single best solution, but there are a few guidelines which might be helpful: If one actor manages the work another actor is doing, e. The reason is that the manager knows which kind of failures are expected and how to handle them. If one actor carries very important data i.
Depending on the nature of the requests, it may be best to create a new child for each request, 2. This is known as the Error Kernel Pattern from Erlang. If one actor depends on another actor for carrying out its duty, it should watch that other actors liveness and act upon receiving a termination notice. This is different from supervision, as the watching party has no influence on the supervisor strategy, and it should be noted that a functional dependency alone is not a criterion for deciding where to place a certain child actor in the hierarchy.
There are of course always exceptions to these rules, but no matter whether you follow the rules or break them, you should always have a reason.
Several actor systems with different configuration may co-exist within the same JVM without problems, there is no global shared state within Akka itself. Couple this with the transparent communication between actor systemswithin one node or across a network connectionto see that actor systems themselves can be used as building blocks in a functional hierarchy.
Actors should be like nice co-workers: do their job efficiently without bothering everyone else needlessly and avoid hogging resources. Translated to programming this means to process events and generate re- sponses or more requests in an event-driven manner. Actors should not block i. Do not pass mutable objects between actors.
In order to ensure that, prefer immutable messages. If the encapsulation of actors is broken by exposing their mutable state to the outside, you are back in normal Java concurrency land with all the drawbacks. Actors are made to be containers for behavior and state, embracing this means to not routinely send behavior within messages which may be tempting using Scala closures.
One of the risks is to accidentally share mutable state between actors, and this violation of the actor model unfortunately breaks all the properties which make programming in actors such a nice experience.
Top-level actors are the innermost part of your Error Kernel, so create them sparingly and prefer truly hierarchical systems. Documentation Guidelines The Akka documentation uses reStructuredText as its markup language and is built using Sphinx. We use the following convention in the Akka documentation: For example: Akka Module This is the module documentation. Build the documentation First install Sphinx.
See below. Building For the html version of the docs: And have fun. Formatting The specifics of code formatting — so long as they are practical — are of little consequence. By definition style cannot be inherently good or bad and almost everybody differs in personal preference. However the consistent application of the same formatting rules will almost always enhance readability.
A reader already familiar with a particular style does not have to grasp yet another set of local conventions, or decipher yet another corner of the language grammar. This is of particular importance to Scala, as its grammar has a high degree of overlap.
Furthermore, the different styles of method invocations expose different ambiguities in its grammar! Surely the consistent application of a carefully chosen set of formatting rules will resolve a great deal of ambiguity for both man and machine.Values are declared using the val keyword and variables are declared using the var keyword.
In other words, lock-freedom is not enough to guarantee the lack of starvation. On the other hand, the guarantee that some call finishes in a finite number of steps is not enough to guarantee that all of them eventually finish. While this is a common case, shared state is not necessary to have race conditions.
Escalate the failure, thereby failing itself It is important to always view an actor as part of a supervision hierarchy, which explains the existence of the fourth choice as a supervisor also is subordinate to another supervisor higher up and has implications on the first three: resuming an actor resumes all its subordinates, restarting an actor entails restarting all its subordinates but see below for more details , similarly terminating an actor will also terminate all its subordinates.
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