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README
extensible-effects-concurrent
From Erlang to Haskell
This project is an attempt to implement core ideas learned from the Erlang/OTP
framework in Haskell using extensible-effects.
This library sketches my personal history of working on a large, real world Erlang application, trying to bring some ideas over to Haskell.
I know about cloud-haskell and transient, but I wanted something based on 'extensible-effects', and I also wanted to deepen my understanding of it.
Modeling an Application with Processes
The fundamental approach to modelling applications in Erlang is based on the concept of concurrent, communicating processes.
Example Code
module Main where
import Control.Eff
import Control.Eff.Concurrent
main :: IO ()
main = defaultMain example
example :: Eff Effects ()
example = do
person <- spawn "alice" alice
replyToMe <- self
sendMessage person replyToMe
personName <- receiveMessage
logInfo' ("I just met " ++ personName)
alice :: Eff Effects ()
alice = do
logInfo "I am waiting for someone to ask me..."
sender <- receiveMessage
sendMessage sender ("Alice" :: String)
logInfo' (show sender ++ " message received.")
This is taken from [example-4](./examples/example-4/Main.hs).
Running this example causes this output:
DEBUG no proc scheduler loop entered at ForkIOScheduler.hs:209
DEBUG init!1 enter process at ForkIOScheduler.hs:691
NOTICE init!1 ++++++++ main process started ++++++++ at ForkIOScheduler.hs:579
DEBUG alice!2 enter process at ForkIOScheduler.hs:691
INFO alice!2 I am waiting for someone to ask me... at Main.hs:19
INFO alice!2 !1 message received. at Main.hs:22
DEBUG alice!2 exit: Process finished successfully at ForkIOScheduler.hs:729
INFO init!1 I just met Alice at Main.hs:15
NOTICE init!1 ++++++++ main process returned ++++++++ at ForkIOScheduler.hs:581
DEBUG init!1 exit: Process finished successfully at ForkIOScheduler.hs:729
DEBUG no proc scheduler loop returned at ForkIOScheduler.hs:211
DEBUG no proc scheduler cleanup begin at ForkIOScheduler.hs:205
NOTICE no proc cancelling processes: [] at ForkIOScheduler.hs:222
NOTICE no proc all processes cancelled at ForkIOScheduler.hs:239
The mental model of the programming framework regards objects as processes with an isolated internal state.
Processes are at the center of that contraption. All actions happen in processes, and all interactions happen via messages sent between processes.
This is called Message Passing Concurrency;
in this library it is provided via the Process effect.
The Process effect itself is just an abstract interface.
There are two schedulers, that interpret the Process effect:
- A multi-threaded scheduler, based on the
async - A pure single-threaded scheduler, based on coroutines
Using the library
For convenience, it is enough to import one of three modules:
- Control.Eff.Concurrent for a multi threaded scheduler and
LoggingAndIo - Control.Eff.Concurrent.Pure, for a single threaded scheduler and pure log capturing and otherwise no IO
- Control.Eff.Concurrent.SingleThreaded, for a single threaded scheduler and totally impure logging via IO
Process Life-Cycles and Interprocess Links
All processes except the first process are spawned by existing
processes.
When a process spawns a new process they are independent apart from the fact that
the parent knows the process-id of the spawend child process.
Processes can monitor each other to be notified when a communication partner exits, potentially in unforseen ways.
Similarily processes may choose to mutually link each other.
That allows to model trees in which processes watch and start or restart each other.
Because processes never share memory, the internal - possibly broken - state of a process is gone, when a process exits; hence restarting a process will not be bothered by left-over, possibly inconsistent, state.
Timers
[The Timer module](src/Control/Eff/Concurrent/Process/Timer.hs) contains functions to send messages after a time has passed, and reiceive messages with timeouts.
More Type-Safety: [The Protocol Metaphor](./src/Control/Eff/Concurrent/Protocol.hs)
As the library carefully leaves the realm of untyped messages, it uses the concept of a protocol that governs the communication between concurrent processes, which are either protocol servers or clients as a metaphor.
The communication is initiated by the client.
The idea is to indicate such a protocol using a custom data type,
e.g. data TemperatureSensorReader or data SqlServer.
The library consists some tricks to restrict the kinds of messages that are acceptable when communicating with processes adhering to the protocol.
This protocol is not encoded in the users code, but rather something that the programmer keeps in his head.
In order to be appreciated by authors of real world applications, the protocol can be defined by giving an abstract message sum-type and code for spawning server processes.
It focusses on these questions:
- What messages does a process accept?
- When sending a certain message, should the sender wait for an answer?
Protocol Phantom Type
In this library, the key to a protocol is a single type, that could even be a so called phantom type, i.e. a type without any runtime values:
data UserRegistry -- look mom, no constructors!!
Such a type exists only for the type system.
It can only be used as a parameter to certain type constructors, and for defining type class and type family instances, e.g.
newtype Endpoint protocol = MkServer { _processId :: ProcessId }
data UserRegistry
startUserRegistry :: Eff e (Endpoint UserRegistry)
startUserRegistry =
error "just an example"
Here the Endpoint has a type parameter protocol but the type is not
used by the constructor to hold any values, hence we can use UserRegistry,
as a parameter, since UserRegistry has no value constructors.
Protocol Data Units
Messages that belong to a protocol are called protocol data units (PDU).
Protocol Servers Endpoints
The ProcessId of a process identifies the messages box that
receiveMessage will use, when waiting for an incoming message.
While it defines where the messages are collected, it does not restrict or inform about what data is handled by a process.
An Endpoint is a wrapper
around the ProcessId that takes a type parameter.
The type does not have to have any values, it can be a phantom type.
This type serves only the tag the process as a server accepting messages identified by the HasPdu type class.
Server/Protocol Composability
Usually a protocol consists of some really custom PDUs and some PDUs that more or less are found in many protocols, like event listener registration and event notification.
It is therefore helpful to be able to compose protocols.
The machinery in this library allows to list several PDU instances understood by endpoints of a given protocol phantom type.
Protocol Clients
Clients use a protocol by sending Pdus indexed by
some protocol phantom type to a server process.
Clients use Endpoints to address these servers, and the
functions defined in the corresponding module.
Most important are these two functions:
cast to send fire-and-forget messages
call to send RPC-style requests and wait (block) for the responses. Also, when the server is not running, or crashes in while waiting, the calling process is interrupted
Protocol Servers
This library offers an API for defining practically safe to use protocol servers:
- [EffectfulServer](./src/Control/Eff/Concurrent/Protocol/EffectfulServer.hs) This modules defines the framework of a process, that has a callback function that is repeatedly called when ever a message was received.
The callback may rely on any extra effects (extensible effects).
[StatefulServer](./src/Control/Eff/Concurrent/Protocol/StatefulServer.hs) A server based on the
EffectfulServerthat includes the definition of in internal state called Model, and some nice helper functions to access the model. These functions allow the use of lenses. Unlike the effect server, the effects that the callback functions can use are defined in this module.[CallbackServer](./src/Control/Eff/Concurrent/Protocol/CallbackServer.hs) A server based on the
EffectfulServerthat does not require a type class instance like the stateful and effect servers do. It can be used to define inline servers.
Events and Observers
A parameterized protocol for event handling is provided in the module:
- [Observer](./src/Control/Eff/Concurrent/Protocol/Observer.hs)
Brokers and Watchdogs
A key part of a robust system is monitoring and possibly restarting stuff that crashes, this is done in conjunction by two modules:
- [Broker](./src/Control/Eff/Concurrent/Protocol/Broker.hs)
- [Watchdog](./src/Control/Eff/Concurrent/Protocol/Watchdog.hs)
A client of a process that might be restarted cannot use the ProcessId
directly, but has to use an abstract ID and lookup the ProcessId from a process [broker](./src/Control/Eff/Concurrent/Protocol/Broker.hs), that manages the current ProcessId of protocol server
processes.
That way, when ever the server process registered at a broker crashes, a [watchdog](./src/Control/Eff/Concurrent/Protocol/Watchdog.hs) process can (re-)start the crashed server.
Additional services
Currently, a logging effect is also part of the code base.
Usage and Implementation
Should work with stack, cabal and nix.
Required GHC Extensions
In order to use the library you might need to activate some extension in order to fight some ambiguous types, stemming from the flexibility to choose different Scheduler implementations.
- AllowAmbiguousTypes
- TypeApplications
Planned Features
Stackage
Scheduler
ekgMonitoring