clarify need to carefully managing blocking, see #2616

akka-docs/rst/general/actor-systems.rst

@@ -89,10 +89,9 @@ Actor Best Practices
    bothering everyone else needlessly and avoid hogging resources. Translated
    to programming this means to process events and generate responses (or more
    requests) in an event-driven manner. Actors should not block (i.e. passively
-   wait while occupying a Thread) on some external entity, which might be a
+   wait while occupying a Thread) on some external entity—which might be a
-   lock, a network socket, etc. The blocking operations should be done in some
+   lock, a network socket, etc.—unless it is unavoidable; in the latter case
-   special-cased thread which sends messages to the actors which shall act on
+   see below.
-   them.
 
 #. Do not pass mutable objects between actors. In order to ensure that, prefer
    immutable messages. If the encapsulation of actors is broken by exposing
@@ -112,6 +111,53 @@ Actor Best Practices
    performance) and it also reduces the number of blocking calls made, since
    the creation of top-level actors involves synchronous messaging.
 
+Blocking Needs Careful Management
+---------------------------------
+
+In some cases it is unavoidable to do blocking operations, i.e. to put a thread
+to sleep for an indeterminate time, waiting for an external event to occur.
+Examples are legacy RDBMS drivers or messaging APIs, and the underlying reason
+in typically that (network) I/O occurs under the covers. When facing this, you
+may be tempted to just wrap the blocking call inside a :class:`Future` and work
+with that instead, but this strategy is too simple: you are quite likely to
+find bottle-necks or run out of memory or threads when the application runs
+under increased load.
+
+The non-exhaustive list of adequate solutions to the “blocking problem”
+includes the following suggestions:
+
+ - Do the blocking call within an actor (or a set of actors managed by a router
+   [:ref:`Java <routing-java>`, :ref:`Scala <routing-scala>`]), making sure to
+   configure a thread pool which is either dedicated for this purpose or
+   sufficiently sized.
+
+ - Do the blocking call within a :class:`Future`, ensuring an upper bound on
+   the number of such calls at any point in time (submitting an unbounded
+   number of tasks of this nature will exhaust your memory or thread limits).
+
+ - Do the blocking call within a :class:`Future`, providing a thread pool with
+   an upper limit on the number of threads which is appropriate for the
+   hardware on which the application runs.
+
+ - Dedicate a single thread to manage a set of blocking resources (e.g. a NIO
+   selector driving multiple channels) and dispatch events as they occur as
+   actor messages.
+
+The first possibility is especially well-suited for resources which are
+single-threaded in nature, like database handles which traditionally can only
+execute one outstanding query at a time and use internal synchronization to
+ensure this. A common pattern is to create a router for N actors, each of which
+wraps a single DB connection and handles queries as sent to the router. The
+number N must then be tuned for maximum throughput, which will vary depending
+on which DBMS is deployed on what hardware.
+
+.. note::
+
+   Configuring thread pools is a task best delegated to Akka, simply configure
+   in the ``application.conf`` and instantiate through an :class:`ActorSystem`
+   [:ref:`Java <dispatcher-lookup-java>`, :ref:`Scala
+   <dispatcher-lookup-scala>`]
+
 What you should not concern yourself with
 -----------------------------------------
 

akka-docs/rst/java/code/docs/dispatcher/DispatcherDocTestBase.java

@@ -5,10 +5,6 @@ package docs.dispatcher;
 
 //#imports
 import akka.actor.*;
-import akka.actor.ActorRef;
-import akka.actor.Props;
-import akka.actor.UntypedActor;
-import akka.actor.UntypedActorFactory;
 //#imports
 
 //#imports-prio
@@ -37,6 +33,7 @@ import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
 import scala.Option;
+import scala.concurrent.ExecutionContext;
 
 import com.typesafe.config.ConfigFactory;
 
@@ -75,6 +72,14 @@ public class DispatcherDocTestBase {
         .withDispatcher("my-pinned-dispatcher"));
     //#defining-pinned-dispatcher
   }
+  
+  public void compileLookup() {
+    //#lookup
+    // this is scala.concurrent.ExecutionContext
+    // for use with Futures, Scheduler, etc.
+    final ExecutionContext ex = system.dispatchers().lookup("my-dispatcher");
+    //#lookup
+  }
 
   @Test
   public void priorityDispatcher() throws Exception {

akka-docs/rst/java/dispatchers.rst

@@ -13,6 +13,15 @@ Default dispatcher
 Every ``ActorSystem`` will have a default dispatcher that will be used in case nothing else is configured for an ``Actor``.
 The default dispatcher can be configured, and is by default a ``Dispatcher`` with a "fork-join-executor", which gives excellent performance in most cases.
 
+.. _dispatcher-lookup-java:
+
+Looking up a Dispatcher
+-----------------------
+
+Dispatchers implement the :class:`ExecutionContext` interface and can thus be used to run :class:`Future` invocations etc.
+
+.. includecode:: code/docs/dispatcher/DispatcherDocTestBase.java#lookup
+
 Setting the dispatcher for an Actor
 -----------------------------------
 

akka-docs/rst/scala/code/docs/dispatcher/DispatcherDocSpec.scala

@@ -186,6 +186,13 @@ class DispatcherDocSpec extends AkkaSpec(DispatcherDocSpec.config) {
     //#defining-pinned-dispatcher
   }
 
+  "looking up a dispatcher" in {
+    //#lookup
+    // for use with Futures, Scheduler, etc.
+    implicit val executionContext = system.dispatchers.lookup("my-dispatcher")
+    //#lookup
+  }
+
   "defining priority dispatcher" in {
     //#prio-dispatcher
 

akka-docs/rst/scala/dispatchers.rst

@@ -13,6 +13,15 @@ Default dispatcher
 Every ``ActorSystem`` will have a default dispatcher that will be used in case nothing else is configured for an ``Actor``.
 The default dispatcher can be configured, and is by default a ``Dispatcher`` with a "fork-join-executor", which gives excellent performance in most cases.
 
+.. _dispatcher-lookup-scala:
+
+Looking up a Dispatcher
+-----------------------
+
+Dispatchers implement the :class:`ExecutionContext` interface and can thus be used to run :class:`Future` invocations etc.
+
+.. includecode:: code/docs/dispatcher/DispatcherDocSpec.scala#lookup
+
 Setting the dispatcher for an Actor
 -----------------------------------
 

akka-samples/akka-sample-remote/src/main/scala/sample/remote/calculator/CreationApplication.scala

@@ -40,7 +40,7 @@ class CreationActor extends Actor {
     case result: MathResult ⇒ result match {
       case MultiplicationResult(n1, n2, r) ⇒
         println("Mul result: %d * %d = %d".format(n1, n2, r))
-      case DivisionResult(n1, n2, r)       ⇒
+      case DivisionResult(n1, n2, r) ⇒
         println("Div result: %.0f / %d = %.2f".format(n1, n2, r))
     }
   }

akka-samples/akka-sample-remote/src/main/scala/sample/remote/calculator/LookupApplication.scala

@@ -40,7 +40,7 @@ class LookupActor extends Actor {
   def receive = {
     case (actor: ActorRef, op: MathOp) ⇒ actor ! op
     case result: MathResult ⇒ result match {
-      case AddResult(n1, n2, r)      ⇒
+      case AddResult(n1, n2, r) ⇒
         println("Add result: %d + %d = %d".format(n1, n2, r))
       case SubtractResult(n1, n2, r) ⇒
         println("Sub result: %d - %d = %d".format(n1, n2, r))