CPL 2016, week 6. Asynchronous execution. Oleg Batrashev. March 14, Institute of Computer Science, Tartu, Estonia

Transcription

1 CPL 2016, week 6 Asynchronous execution Oleg Batrashev Institute of Computer Science, Tartu, Estonia March 14, 2016

2 Overview Studied so far: 1. Inter-thread visibility: JMM 2. Inter-thread synchronization: locks and monitors 3. Thread management: executors, tasks, cancelation 4. Inter-thread communication: confinements, queues, back pressure 5. Inter-thread collaboration: actors, inboxes, state diagrams Today: Asynchronous execution: callbacks, Pyramid of Doom, Java 8 promises. Next weeks: Performance considerations: asynchronous IO, Java 8 streams.

3 Asynchronous execution 118/145 Why asynchronous - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

4 Asynchronous execution 119/145 Why asynchronous - Synchronous vs asynchronous Synchronous execution algorithm executes sequentially, one step after another in single thread res1 = dosomething1 () res2 = dosomething2 ( res1 ) res3 = dosomething3 ( res1 ) print ( res2 + res3 ) dosomething3 executes after dosomething2 even with optimizations, think of the execution this way Asynchronous execution parts of the algorithm run separately, possibly in different threads execute dosomething2 and dosomething3 in separate threads and wait for their results solvable with Java Futures execute them in separate threads but release this thread until res2 and res3 are available, then execute printing not solvable with Java Futures

5 Asynchronous execution 120/145 Why asynchronous - Problems of synchronous execution parallelism in the algorithm is not utilized (solved with Java futures) current thread is not released even if nothing to do, consider res1 = dorequest1 () res2 = dorequest2 ( res1 ) print ( res2 ) if the thread is waiting for the result of request 1, it has nothing else to do the thread is not released, it may not be used elsewhere e.g. making network requests or waiting for data from web clients requires the same number of threads solution: release the thread between dorequest1() and dorequest2() how to do that?

6 Asynchronous execution 121/145 Why asynchronous - Authentication example: the algorithm Need for an algorithm with several steps that run over a network. Authentication with the salt and hash digest: 1. Client provides his name 2. Server checks if the name is ok and returns the salt the salt is just any random string 3. Client combines the salt and his password into single string, it then computes the hash for it. The hash is sent to the server. hash is a sequence of bytes 4. Server compares received hash with the locally computed hash. If they are equal, then the password was correct. The server sends resulting response to the client. password is in the server database 5. Client knows whether the authentication was successful Advantage: the password is never sent over the network.

7 Asynchronous execution 122/145 Why asynchronous - Authentication example: synchronous code // send user name conn. sendmessage ( new AuthComm. LoginRequest (" Oleg " )); // receive params or result response Message <Type > m1 = conn. readmessage (); if (m1. type == Type. RES_RESULT ) { ResultResponse m = ( ResultResponse ) m1; System. out. println (" Success "+m); return ; { // send password digest ParamsResponse m = ( ParamsResponse ) m1; MessageDigest digestalgo = MessageDigest. getinstance (" SHA -1"); String password = " mypassword "; byte [] digest = digestalgo. digest (( m. salt + password ). getbytes () conn. sendmessage ( new AuthComm. AuthRequest ( digest )); // receive result response Message <Type > m2 = conn. readmessage (); { ResultResponse m = ( ResultResponse ) m2; System. out. println (" Success "+m); return ;

8 Asynchronous execution 123/145 Callback hell - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

9 Asynchronous execution 124/145 Callback hell - Callback solution give the method another one which should be executed when the operations completes dorequest1 ( callback1 ); void callback1 ( res1 ) { dorequest2 ( res1, callback2 ); void callback2 ( res2 ) { print ( res2 ); put into inline callbacks results in Pyramid of Doom (not Java code!) dorequest1 ( void callback1 ( res1 ) { dorequest2 ( res1, void callback2 ( res2 ) { print ( res2 ); ); );

10 Asynchronous execution 125/145 Callback hell - Authentication example: callbacks response with the salt to the login request public interface ParamsCallback { void onsalt ( String salt ); public void login ( String username, ParamsCallback paramscallback, ResultCallback resultcallback ) response with result to the auth request public interface ResultCallback { void onsuccess (); void onfailure ( String error ); public void authenticate ( byte [] digest, ResultCallback callback ) login may result in failure through the result callback: if the username is invalid

11 Asynchronous execution 126/145 Callback hell - Authentication example: Pyramid of Doom Java callbacks are 2 level, because they are in interfaces // send user name authproxy. login (" Oleg ", new ParamsCallback () { public void onsalt ( String salt ) { // send password digest String password = " mypassword "; byte [] digest = algo. digest (( salt + password ). getbytes ()); authproxy. authenticate ( digest, new ResultCallback () { public void onsuccess () { System. out. println (" Login was successful "); public void onfailure ( String error ) { System. out. println (" Login failed : "+ error ); );, null ); not pleasant to read, not reusable last null is the callback where error handling for login should happen, omitted here

12 Asynchronous execution 127/145 Callback hell - Named callbacks explicit classes: reusable code, but more difficult to follow public void run () { // send user name authproxy. login (" Oleg ", new ParamsCallbackImpl (), null ); class ParamsCallbackImpl implements ParamsCallback { public void onsalt ( String salt ) { // send password digest String password = " mypassword "; byte [] digest = algo. digest (( salt + password ). getbytes ()); authproxy. authenticate ( digest, new ResultCallbackImpl ()); class ResultCallbackImpl implements ResultCallback { public void onsuccess () { System. out. println (" Login was successful "); public void onfailure ( String error ) { System. out. println (" Login failed : " + error ); imagine if callbacks are split between different files

13 Asynchronous execution 128/145 Callback hell - Callback summary asynchronous execution requires code that will be executed later callbacks inline callbacks result in the Pyramid of Doom, which is unpleasant to read named callbacks split sequential logic into separate methods, which is even more difficult to follow Java callback as an object with method adds 2 levels to the Pyramid of Doom Java 8 solutions: Java 8 promises (CompletableFuture) solve the problem with unreadable and hard to follow code Java 8 lambdas solve the problem of 2-level callbacks

14 Asynchronous execution 129/145 Lambdas in Java 8 - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

15 Asynchronous execution 130/145 Lambdas in Java 8 - Java 8 lambdas Syntactic sugar for inline methods: (arg1,arg2, arg3 ) -> { code ;..; return res ; It is possible to write: Runnable r = ()-> System. out. println (" hello "); Callable < String > c1 = ()->" hello "; Callable < String > c2 = ()-> { System. out. println ("in c2"); return " hello "; ; r. run (); c1. call (); c2. call (); However, the following is invalid, because need an interface: Object o = ()-> System. out. println (" hello "); The target type of this expression must be a functional interface Java makes methods out of interface implementations functional interface[1, sec 1.3] must contain single method declaration

16 Asynchronous execution 131/145 Lambdas in Java 8 - Java 8 method references Java 8 provides nice way to reference methods as functional interfaces: obj::methodname this::methodname ClassName::staticMethodName java.util.function interfaces: Function < Integer, Integer > m1 = Math :: abs ; BiFunction < Random, Integer, Integer > m2 = Random :: nextint ; BiConsumer < PrintStream, char [] > m3 = PrintStream :: print ;

17 Asynchronous execution 132/145 Lambdas in Java 8 - Lambdas in Pyramid of Doom // send user name authproxy. login (" Oleg ", salt -> { // Java8 lambda // send password digest String password = " mypassword "; byte [] digest = algo. digest (( salt + password ). getbytes ()); authproxy. authenticate ( digest, new ResultCallback () { public void onsuccess () { System. out. println (" Login was successful "); public void onfailure ( String error ) { System. out. println (" Login failed : "+ error ); );, null ); problem with success/failure result, interface should contain single method failure handling at each call may pollute the code Java 8 provides error forwarding for promises

18 Asynchronous execution 133/145 Promises in Java 8 - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

19 Asynchronous execution 134/145 Promises in Java 8 - Problems of Java Future Java Future is the result of task execution it provides the result once execution is completed must submit a task before you can have its future what if want to return the future for the task executed third in the sequence of async steps Java Future does not provide fut.runthiscallback(callback) methods i.e. can only wait with fut.get() blocking the thread Java 8 CompletableFuture (promise): may be completed at any time by promise.complete(res) only first result is preserved and valid have a bunch of promise.thenapply(callback) methods all registered callbacks are executed when promise is completed

20 Asynchronous execution 135/145 Promises in Java 8 - Java 8 CompletableFuture java.util.concurrent.completablefuture<result> like Java Future completed once but no specific task required use complete(), completeexceptionally(), cancel() like java Future may be waited with get() or get(t,unit) unlike Java Futures continuations may be added to it CF <Void > thenrun ( Runnable action ) CF <Void > thenaccept ( Consumer <? super T> action ) CF <U> thenapply ( Function <? super T,? extends U> fn) CF <U> thencompose ( Function <? super T,? extends CompletionStage <U>> fn) CF <U> handle ( BiFunction <? super T, Throwable,? extends U> fn) CF <T> whencomplete ( BiConsumer <? super T,? super Throwable > action ) the callbacks are executed after the promise is completed create the promise completedfuture(), runasync(), supplyasync()

21 Asynchronous execution 136/145 Promises in Java 8 - Apply vs compose After a promise has completed run a function with the result: apply callback returns (? extends U) compose callback returns (? extends CompletionStage<U>) both take value of type T from the source promise Imagine we have CompletableFuture<String> is a username we get asynchronously (e.g. receive from network) Want to check the validity of a username: 1. Boolean checklocally(string username) only validates that characters are ok, may be done synchronously 2. CF<Boolean> checkremotely(string username) validates that username exists in a database, the result comes asynchronously Need to use apply in the first case and compose in the second: 1. unamepromise.thenapply(this::checklocally) 2. unamepromise.thencompose(this::checkremotely) Otherwise, compose returns CF<CF<Boolean> > need to flatten

22 Asynchronous execution 137/145 Promises in Java 8 - Handle vs whencomplete whencomplete() returns the old future i.e. it invokes the callback, but does not enhance the callback chain handle() does return the new future this may be used to run actions when async action defined in the handle method completes

23 Asynchronous execution 138/145 Promises in Java 8 - Authentication example: promise chaining use thencompose and handle to chain promises error in the chain is propagated to the last promise omitting all subsequent callbacks in the chain // send user name CompletableFuture < String > saltpromise = authproxy. login (" Oleg "); CompletableFuture <Void > resultpromise = saltpromise. thencompose ( salt -> { // send password digest String password = " mypassword "; byte [] digest = algo. digest (( salt + password ). getbytes ()); return authproxy. authenticate ( digest ); ); // handle all errors or final success resultpromise. handle (( v, err ) -> { if ( err == null ) System. out. println (" Login was successful "); else System. out. println (" Login failed : "+ err ); return null ; );

24 Asynchronous execution 139/145 Promises in Java 8 - Promise chaining with separate methods easier to see the overall process public void doauth () throws IOException { authproxy. connect (" localhost ", 5000); CompletableFuture. completedfuture (" Oleg "). thencompose ( authproxy :: login ). thencompose ( this :: sendhashfromsaltandpassword ). handle ( this :: printresult ); CompletableFuture <Void > sendhashfromsaltandpassword ( String salt ) { // send password digest String password = " mypassword "; byte [] digest = digestalgo. digest (( salt + password ). getbytes ()); return authproxy. authenticate ( digest ); Void printresult ( Void v, Throwable err ) { if ( err == null ) System. out. println (" Login was successful "); else System. out. println (" Login failed : "+ err ); return null ;

25 Asynchronous execution 140/145 Promises within actors - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

26 Asynchronous execution 141/145 Promises within actors - Promise executors Almost all methods provide 3 flavors, e.g. for run: CF <Void > thenrun ( Runnable action ); CF <Void > thenrunasync ( Runnable action ); CF <Void > thenrunasync ( Runnable action, Executor executor ); action may be executed in the thread that has completed the promise action is executed by the ForkJoinPool.commonPool() action is executed by the provided executor We must think about the consequences of synchronization or the lack of it!

27 Asynchronous execution 142/145 Promises within actors - Promises within actors actor is inherently asynchronous its logic is split between message handling code pieces often it is more convenient to program sequence of actions, like with authentication use promise chains inside an actor Pay attention: callbacks must be executed in the actor thread, current state is not automatically checked when the callback is executed, consider promise (chain) cancellation if done from the actor thread, the rest of the promise chain won t be executed

28 Asynchronous execution 143/145 Promise libraries - Outline Asynchronous execution Why asynchronous Callback hell Lambdas in Java 8 Promises in Java 8 Promises within actors Promise libraries

29 Asynchronous execution 144/145 Promise libraries - Libraries Akka futures RxJava ReactiveX

30 Asynchronous execution 145/145 Promise libraries - Summary asynchronous execution is necessary to utilize more resources and release threads when not needed callbacks allow to continue execution, but it results in Pyramid of Doom or very scattered callback code Java futures help with resource utilization but not thread release Java 8 completable future (promise) solves the problem allow to add callbacks through the then*() methods callbacks are called in other threads, which is especially critical for actors