Tuesday, December 17, 2013

TechTalk: Java Garbage Collection - Theory and Practice

Below are slide decks for open event held in Moscow Technology Center of Deutsche Bank.

Topic of event was garbage collection in JVM.

Part 1 by Alexey Ragozin

Part 2 by Alexander Ashitkin

Thursday, December 12, 2013

TechTalk: Virtualizing Java in Java

On 12th December, I was speaking at JUG in Saint-Petersburg, Russia.

It was a long talk about using NanoCloud.

Below is video

and slide deck from event

Thursday, December 5, 2013

Coherence SIG - Filtering 100M objects in cache

Today I was speaking on Coherence SIG event in London.

My topic was "Filtering 100M objects. What can go wrong?". It was a story of solving particular problem and obstacles we have encountered. One noticeable thing about this project - out team was using Performance Test Driven Development approach.

We have started with simplest solution, then were focusing on problem identified by testing.

Slide deck from presentation is below.

Thursday, November 14, 2013

Coherence 101 - Soothing the Guardian

Guardian was introduced in Oracle Coherence 3.5 as uniform and reliable mean to detect and report various stalls and hangs on data grid members. In addition to monitoring internal components of Coherence, Guardian has an API accessible for application developer.

While out-of-box Guardian does its job pretty well, there are few aspects you can improve.

There 3 techniques to work with Coherence Guardian. Your can choose to employ all of them or just few.

Guardian heartbeats

Guardian is using heartbeat mechanics to detect thread stalls. Internally Coherence code explicitly heartbeat in appropriate points in code. Application code could use similar technique if long execution time is expected. CacheStores are good example of this.

  • GuardSupport.heartbeat() – sends normal heartbeat
  • GuardSupport.heartbeat(long) – allows you to pass expected time till next heartbeat (e.i. if you expect that SQL query to take several minutes, you could prevent log warning by passing reasonably long timeout before execution SQL statement)

Implementing guardable

Normally the guardian would try to "recover" thread if no heartbeats were received during timeout (eigther specified in configuration or last heartbeat(...) call).
This behavior can be overridden though. Application can register own Guardable and temporary disable monitoring of current thread. Below is a code snippet which wraps cache loader operations in Guardable preventing thread interruption (default way to "recover" worker thread).

public static class GuardianAwareCacheLoader implements CacheLoader {

    private CacheLoader loader;

    public GuardianAwareCacheLoader(CacheLoader loader) {
        this.loader = loader;

    public Object load(Object key) {
        GuardContext ctx = GuardSupport.getThreadContext();
        if (ctx != null) {
            KeyLoaderGuard guard = new KeyLoaderGuard(Collections.singleton(key));
            GuardContext klg = ctx.getGuardian().guard(guard); 
        try {
            return loader.load(key);
        finally {
            if (ctx != null) {
                GuardContext klg = GuardSupport.getThreadContext();

    @SuppressWarnings({ "rawtypes", "unchecked" })
    public Map loadAll(Collection keys) {
        GuardContext ctx = GuardSupport.getThreadContext();
        if (ctx != null) {
            KeyLoaderGuard guard = new KeyLoaderGuard(keys);
            GuardContext klg = ctx.getGuardian().guard(guard); 
            // disable current context
        try {
            return loader.loadAll(keys);
        finally {
            if (ctx != null) {
                GuardContext klg = GuardSupport.getThreadContext();
                // reenable current context

public static class KeyLoaderGuard implements Guardable {

    Collection<Object> keys;
    GuardContext context;

    public KeyLoaderGuard(Collection<Object> keys) {
        this.keys = keys;

    public GuardContext getContext() {
        return context;

    public void setContext(GuardContext context) {
        this.context = context;

    public void recover() {
        System.out.println("got RECOVER signal");

    public void terminate() {
        System.out.println("got TERMINATE signal");

    public String toString() {
        return "KeyLoaderGuard:" + keys;

Using custom Guardable provides following advantages:

  • Additional context information is available and is logged for custom Guardable (e.g. SQL statement causing problems).
  • Custom code can choose how to react on timeout. You can choose to continue or try to cancel request somehow (e.g. closing JDBC connection).

Custom service failure policy

Service failure policy is responsible for reaction on guardian timeouts and critical service failures. Reaction is configurable, but for standalone Coherence processes I prefer to override this policy.

Below is example of service failure policy, which I find more reasonable for dedicated Coherence nodes.

public class ServiceFailureHandler implements ServiceFailurePolicy {

    private final static Logger LOGGER = LogManager.getLogger(ServiceFailureHandler.class);

    public void onGuardableRecovery(Guardable guarable, Service service) {
        LOGGER.warn("Soft timeout detected. Service: " + service.getInfo().getServiceName() + " Task: " + guarable);

    public void onGuardableTerminate(Guardable guarable, Service service) {
        LOGGER.error("Hard timeout detected. Service: " + service.getInfo().getServiceName()
                     + " Task: " + guarable + ". Node will be terminated.");

    public void onServiceFailed(Cluster cluster) {
        LOGGER.error("Service failure detected. Node will be terminated.");

    private static void halt() {
        try {
        } finally {

Compared to standard policy it has following advantages:

  • In case of service failure processes would be terminated quickly (without waiting for shutdown hooks etc). In my case, process would be restarted by external watch dog immediately then.
  • "Soft timeouts" will not pollute log with thread dumps. The only thread dump will be logged just before termination of process (which is especially important in case of implementing custom Guardable).


Integrating you application with Coherence Guardian doesn't require too much code, but could make your logs more clear and troubleshooting less painful. While it will not make your application work faster, it could save hours of digging though logs.

Wednesday, November 6, 2013

HotSpot JVM garbage collection options cheat sheet (v3)

Two years ago I have published cheat sheet for garbage collection options in HotSpot JVM.

Recently I decided give that work some refreshing and today I'm publishing first HostSpot JVM options ref card covering generic GC options and CMS tuning. (G1 have got a plenty of tuning options during last two years so it will have dedicated ref card).

Content-wise GC log rotation options have been added and few esoteric CMS diagnostic options have been removed.

Two page PDF version

Single page PDF version

Tuesday, October 29, 2013

JVM deep dive at HighLoad++ 2013 (Moscow)

Today was speaking at HighLoad++ 2013 Moscow. I had two presentation covering deep internals of JVM. One about JIT compilation and other concerning pauseless garbage collection algorithms.

Slide decks are below (in Russian)

Tuesday, September 10, 2013

Coherence 101 - EntryProcessor traffic amplification

Oracle Coherence data grid has a powerful tool for inplace data manipulation - EntryProcessor. Using entry processor you can get reasonable atomicity guarantees without locks or transactions (and without drastic performance fees associated).

One good example of entry processor would be built-in ConditionalPut processor, which will verify certain condition before overriding value. This, in turn, could be used for implementing optimistic locking and other patterns.

ConditionalPut could accept only one value, but ConditionalPutAll processor is also available. ConditionalPutAll accepts a map of key/values. Using it, we can update multiple cache entries with single call to NamedCache API.

But there is one caveat.

We have placed values for all keys in single map instance inside of entry processor object. On the other side, in distributed cache keys are distributed across different processes.
How right values would be transferred to right keys?

Answer is simple - every node, owning at least one of keys to be updated, will receive a copy of whole map of values.
In other words, in mid size cluster (i.e. 20 nodes) you may actually transfer 20 times more data over network than really needed.

Modern networks are quite good and you may not notice this traffic amplification effect for some time (as long as you network bandwidth can handle it). But once traffic has reached network limit things are starting to break apart.

Coherence TCMP protocol is very aggressive at grabbing as much of network bandwidth as it can, so other communications protocols will likely perish first.
JDBC connections are likely victim of bandwidth shortage.
Coherence*Extend connection may also suffer (it is using TCP) and proxy nodes may start to fail in unusual ways (e.g. with OutOfMemoryError due transmission backlog overflow).

This problem may be hard to diagnose. TCP is much more vulnerable to bandwidth shortage and you will be kept distracted with TCP communication problems while root cause is excessive TCMP cluster traffic.

Monitoring TCMP statistics (available via MBean) could give you an insight about network bandwidth consumption by TCMP and network health and help to find root cause.

Isolating TCMP in separate switch is also a good practice, BTW

But how to fix it?

Manual data splitting

Simple solution is to split keys set by owning nodes, and then invoke entry processor for each subset individually. Coherence API allows you to find node owning particular key.
This approach is far from ideal though:

  • it will not work for Extend clients,
  • you either have to process all subset sequentially or use threads to do several parallel calls to Coherence API,
  • splitting of key set complicates application logic.

Another option is relocating your logic from entry processor to trigger and replacing invokeAll() by putAll() (putAll() does not suffer from traffic amplification). This solution is fairly good and fast, but has certain drawbacks too:

  • it is less transparent (put() is not just put() now),
  • trigger is configured once for all cache operations (not just one putAll() call),
  • you can only have one trigger and it should handle all your data update needs.
Synthetic data keys

Finally you can use DataSplittingProcessor from CohKit project. This utility class is using virtual cache keys to transfer data associated with keys, then it is using backing map API to access real entries.

This solution has its PROs and CONs too:

  • good drop-in replacement for ConditionalPutAll and alike,
  • prone to deadlocks if running concurrently with other bulk updates (it is partially mitigated by sorting keys before locking).

Choosing right solution

In practice I was using all three technique listed above.

Sometimes triggers fit overall cache design quite good.
Sometimes manual data split has its advantages.
And sometimes DataSplittingProcessor is just right remedy for existing entry processors.

Monday, September 9, 2013

SJK (JVM diagnostic/troubleshoting tools) is learning new tricks.

SJK is small command line tool implementing number of helpful commands for JMV troubleshooting. Internally SJK is using same diagnostic APIs as standard JDK tools (e.g. jps, jstack, jmap, jconsole).

Recently I've made few noteworthy additions to SJK package and would like to announce them here.

Memory allocation rates for Java threads

ttop command now displays memory allocation per thread and cumulative memory allocation for whole JVM process.
Memory allocation rate is key information for GC tuning, in past I was using GC log to derive these numbers. On contrast, per thread allocation counters give you more precise information in real time.
Process allocation rate is calculated by aggregating thread allocation rate.

more details about ttop

Support for remote JMX connections

Historically SJK were using PID to connect to JVM's MBean server. Using PID does not require you to explicitly enable JMX in JVM's command line and offers you OS level security.
Sometime you already have JMX port up and running (e.g. for other monitoring tools) and connection using host and port is more convenient.
Now all JVM based commands (ttop, gcrep, mx, mxdump) support socket based JMX connections (with optional user/password security).

Invoking arbitrary MBean operation

New command (mx) allows to get/set arbitrary MBean attributes and call arbitrary MBean operations.
This one is paralytically useful for scripting (I didn't find to invoke operation for custom MBean from command line, so I have added it to SJK).

more details about ttop

Code and binaries are available at GitHub

Sunday, July 28, 2013

Java GC in Numbers - Compressed OOPs

Compressed OOPs (OOP – ordinary object pointer) is a technique reducing size of Java object in 64 bit environments. HotSpot wiki has a good article explaining details. Downside of this technique is what address uncompressing is required before accessing memory referenced by compressed OOPs. Instruction set (e.g. x86) may support such addressing type directly, but still, additional arithmetic would affect processing pipeline of CPU.

Young GC involves a lot of reference walking, so its time is expected to be affected by OOPs compression.

In this article, I’m comparing young GC pause time for 64 bit HotSpot JVM with and without OOPs compression. Methodic from previous article is used and benchmark code is available at github. There is one caveat though. With compressed OOPs size of object is smaller and same amount of heap could accommodate more objects. Benchmark is autoscaling number of entries to fill heap based entry footprint and old space size, thus with fixed old space size experiments with compression enabled have to deal with slightly larger number of objects (entry footprints are 288 uncompressed and 246 compressed).

Chart below shows absolute young GC pause times.

As you can see, compressed case is consistently slower, which is not a surprise.

Another char is showing relative difference between two cases (compressed GC pause mean / uncompressed GC pause mean for same case).

Fluctuating line suggests that I should probably increase number of runs for each data points. But, let’s try to make some conclusion from what we have.

For heaps below 4GiB JVM is using special strategy (32 address could be used without uncompressing in this case). This difference is visible from chart (please note that point with 4GiB of old space, means that total heap size is above 4GiB and this optimization is inapplicable).

Above 4 GiB we see 10-30% increase in pause times. You should also not to forget that compressed case have to deal with 17% more data.


Using compressed OOPs affects young GC pause time which is not a surprise (especially taking increase amount of data). Using compression for heaps below 4GiB seems to be a total win, for larger heaps it seems to be reasonable price for increase capacity.

But main conclusion is that experiment has not revealed any surprises neither bad nor good ones. This may be not very exciting but is useful information anyway.

Thursday, July 11, 2013

Coherence 101, Filters performance and indexing

In this post, I would like to share some knowledge about optimizing indexes in Oracle Coherence.

Normally you should not abuse queering features of your data grid and, hence, you are unlikely to ever need to tune indexing/queering (besides choosing which indexes to create). But sometimes, you really need to squeeze as much performance as you can from your filter based operations. If it is your case, then few tricks described below may be helpful.

Extractor used to add index, should be "equal" to extractor used in filter

You are probably aware of this fact, but it is of critical importance and repeating this one more time will not do any harm. All query planning in Coherence relies on matching (using equals() method) of extractors used in index and filter.

Typical mistakes you could do here:

  • Use semantically equivalent, but different types of extractors (e.g. ReflectionExtractor and ChainedExtractor may extract exactly same attribute, but they will not be equal in Java sense).
  • Use custom extractor classes without implementing equals() and hashCode().
  • Mixing reflection based and POF based extractors.

In all cases above, your code will work, but index will not be used.

Indexing attributes with low-cardinality

Sometimes your query may include criterion for low-cardinality attribute. Not indexing this attribute will cause deserialization of all candidate entries to check attribute value.

Deserialization is something you really want to avoid in Coherence cluster under heavy load. Besides being CPU consuming, deserialization will produce a lot of garbage, risking to bringing you GC out of balance.

Adding index may bring another risk though. If you put your predicates in wrong order, such index may only slow down query.

Below is result of simple benchmark. I was using 2 Coherence storage nodes and 1000000 as data set. Ticker predicate is matching 1000 objects, and side predicate matching 500000. EqualsFilter and AndFilter were used to build query. Execution time of count aggregator was measured.

Tests were run on my laptop, so absolute numbers are not important (and not statistically sound to be honest).

Without indexes
  • side & ticker -- 5780 ms
  • ticker & side -- 5687 ms
Index by ticker
  • side & ticker -- 66 ms
  • ticker & side -- 66 ms
Both ticker and side indexed
  • side & ticker -- 496 ms
  • ticker & side -- 10 ms

As you can see, if you are unlucky and your query is not in right order, adding index may actually harm query performance.

There is a trick to protect you in this case. NoIndexFilter is a filter wrapper, which disables inverted index lookup for nested index. Forward map of index remains accessible, so testing attribute value will not require desrialization.

Both ticker and side indexed
  • no_index(side) & ticker -- 17 ms
  • ticker & no_index(side) -- 18 ms

As you can see, it takes some toll on "good query", but negates effect of "wrong order of predicates". You can also see that it is still 3 times faster than case where "side" was not indexed.

Exploiting composite indexes

You can make query above even more faster if you really need to.

Normally, with Coherence, you do not use composite indexes (instead you are indexing attributes individually). Creation of composite index is possible, but you will have to use specially composed queries to exploit composite index.

Code to add composite index will look like

ValueExtractor[] ve = {
    new ReflectionExtractor("getTicker"),
    new ReflectionExtractor("getSide")          
MultiExtractor me = new MultiExtractor(ve);
cache.addIndex(me, false, null);

and filter exploiting it will look like

ValueExtractor[] ve = {
    new ReflectionExtractor("getTicker"),
    new ReflectionExtractor("getSide")          
MultiExtractor me = new MultiExtractor(ve);
EqualsFilter composite = new EqualsFilter(me, Arrays.asList(ticker, side));

Below are results compared with traditional index/query.

Without indexes
  • ticker & side -- 5687 ms
  • composite -- 5998 ms
All Indexes
  • ticker & side -- 11 ms
  • composite -- 3 ms

Composite index is awkward to use, but, if it matches your case, you can get significant performance gain.

Few more links

That is it, for this post. You can also take a look at my slide deck from one of London Coherence SIGs, it explains few more advanced topics about indexes in Oracle Coherence.

Friday, June 14, 2013

Java GC in Numbers – Parallel Young Collection

This is a first articles in series, where I would like to study effect of various HotSpot JVM options on duration of STW pauses associated with garbage collection.

This article will study how number of parallel threads affects duration of young collection Stop-the-World pause. HotSpot JVM has several young GC algorithms. My experiments are covering following combinations:

  • Serial young (DefNew), Mark Sweep Compact old
  • Parallel young (ParNew), Mark Sweep Compact old
  • Serial young (DefNew), Concurrent Mark Sweep old
  • Parallel young (ParNew), Concurrent Mark Sweep old
  • There is also PSNew (Parallel Scavenge) algorithm similar to ParNew, but it cannot be used together with Concurrent Mark Sweep (CMS), so I have ignored it.

    In experiments, I was using synthetic benchmark producing evenly distributed load on memory subsystem. Size of young generation was same for all experiments (64MiB). Two versions of HotSpot JVM were used: JDK 6u43 (VM 20.14-b01) and JDK 7u15 (VM 23.7-b01).

    Test box was equipped with two 12 core x 2 hardware threads CPUs (totaling in 48 hardware threads).

    Mark Sweep Compact

    Mark Sweep Compact is prone to regular full GCs, so it is not a choice for pause sensitive applications. But it shares same young collection algorithms/code with concurrent collector and produces less noisy results, so I added to better understand concurrent case.

    Difference between single thread case and 48 thread case is significant so number are present in two graphics.

    Note worthy (not surprising though), that serial algorithm performs slightly better than parallel with one thread. Discrepancy between Java 6 and Java 7 is also interesting, but I have no ideas now to explain that.

    From graphics above you can get an idea that more threads is better, but it is not obvious how exactly better. Graphics below show effective parallelization (8 thread case is taken as base value, because smaller numbers of threads are producing fairly noisy results).

    You can see almost linear parallelization up to 16 threads. It is also worth to note, that 48 threads are considerably faster that 24 even though there are only 24 physical cores. Effect of parallelization is slightly better for larger heap sizes.

    Concurrent Mark Sweep

    Concurrent Mark Sweep is a collector used for pause sensitive applications and young collection pause time is something that you probably really care if you have consciously chosen CMS. Same hardware and same benchmark were used.
    Results are below.

    Compared to Mark Sweep Compact, concurrent algorithm is producing much noisy results (especially for small number of threads).

    Java 7 is systematically showing worse performance compared to Java 6, not too much though.

    Parallelization diagrams, show us same picture - linear scalability, which degrades with greater number of threads (experiment conditions is slightly different for CMS and MSC cases, so direct comparison of these diagrams is not correct).


    Tests have confirmed that parallel young collection algorithms in HotSpot JVM scales extremely well by number of CPU cores. Having a lot of CPU cores on server will help you greatly with JVM Stop-the-World pauses.

    Source code

    Source code used for benchmarking and its description is available at GitHub.

    Thursday, May 23, 2013

    Ad-hoc diagnostic tools for JVM

    There are bunch of graphical tools helping you to look inside of running JVM (VisualVm and JConsole are extremely helpful). Unfortunately, some times (almost always in my case), you may find yourself in SSH console on headless server side by side with your JVM process, trying to investigate problem.

    Why CLI is important?

    You can connect to remote JVM process via JMX in VisualVM and there are other interesting tools like CRaSH offering a lot of goodies for troubleshooting. But …

    • You may be behind firewall, using broker SSH relay as only way to access environment.
    • All remote tools require ahead of time setup on JVM side.
    • Remote connections should be secured properly – that is huge burden.

    CLI tools are leveraging OS security and will work in your SSH console, reliving you from all pains above.

    Stock tools

    Of cause there are CLI tools in your JDK package. Let me highlight few tools from Oracle’s stock JDK.


    This one could list you JVM process (instead of doing ps … | grep java). Similar to ps it could display command line arguments of process. It is useful to find PID of JVM you are interested in, which will be required for other tool.


    This little tool will allow you to take adhoc head dumps and calculate memory footprint histograms by classes. It also can be used to enfore full GC on target JVM. Be careful,  some of jmap operations could cause STW pauses in target JVM.


    Dump your thread stacks, look though your locks.


    JVM exposed a lot of internal details (e.g. memory size, compilation statistics, etc). jstat could report some of this data. Output is fairly cryptic (and machine oriented), but never less helpful.


    Introspect –XX options of running JVM. You could also change some of them for live JVM process (e.g. enable -XX:+PrintGCDetails to add GC details in application log).

    Behind the scene

    Behind the scene JVM have several internal protocols which could be used by diagnostic tools (not to count JMX):

    • Attach API – lightweight protocol for connection to JVM processes.
    • Perf data – shared memory based protocol for JVM to expose its performance counters. Using shared memory makes it very lightweight.
    • JDI, JDWP, JVMTI – components of Java Platform Debugger Architecture

    Both Attach API and perf data are lightweight, fairly unintrusive for monitored application and could be easily used from Java code.

    Swiss Java Knife – CLI tools for ad hoc JVM diagnostic

    Some time ago I was blogging about few tools using Attach API and JMX – jtop and gcrep. Since then, few things have changed:

    • wrapper around attach API has been factored out in separate module org.gridkit.lab:jvm-attach-api:1.1
    • tools have been consolidated into single JAR and modularized (so it is easy to add new commands or build jar with specific set of commands)

    But most important new cool features have been added:

    java –jar sjk.jar jps

    Stock jps can print command line, but it is as far as it can help you. SJK’s version of jps allows you to choose which information about process you would like to be shown. E.g. you can add value of specific system property or –XX flag to the output. Another improvement is build-in filter option. You can filter java processes by command line (which includes main class name) and system properties.

    java –jar sjk.jar ttop

    Thread top command was also improved. Sort option has been added and you also can limit number of threads in output to top N. Additinally, filter option will help you to monitor only certain threads.

    java –jar sjk.jar mxdump

    This command will dump all MBeans from process to json format.

    java –jar sjk.jar hh

    “Heap histo” command is extended version of jmap –histo. Killer feature is --dead flag which will display histogram of garbage in heap (actually tool will take 2 histograms: all heap and live object – and show difference).

    You can download sjk jars here.
    Or build from sources https://github.com/aragozin/jvm-tools

    Tuesday, May 14, 2013

    TechTalk: Java GC - Theory and Practice

    I'm glad to announce upcoming tech talk in our user group at Moscow - "Java GC — Theory and Practice"

    Event will be held on 16 May at Moscow, online translation will be available (tech talk language - russian)

    Registration is open at http://aragozin.timepad.ru/event/60137/

    В программе:


    Алгоритмы сборки мусора. Слабая гипотеза о поколения. Механизмы барьеров записи. Математикечкая модель длительности пауз для Concurent Mark Sweep.

    Практика — HotSpot JVM — Concurrent Mark Sweep GC

    Принципы сайзинга памяти JVM. Тюнинг сборщика молодого поколения. Фрагментация. Специальные ссылки. Паузы не связанные со сборкой мусора.


    Работа с off-heap. Shareв read-only heap region. G1 и тренды развития HotSpot.

    Sunday, April 28, 2013

    Slides from JavaOne Russia

    Below are slidedecks (in russian) from two my presentation on JavaOne Russia this year.

    Борьба с GС паузами в JVM. Теория и практика


    Распределённый кэш или хранилище данных. Что выбрать?


    Thursday, April 18, 2013

    Tech Talk "Magic of VMs"

    Today, I was one of speaker at "Magic of VMs" tech talk in Moscow. Meet up was held under roof of Rambler, a lot thanks to them for helping us in this event.

    Topic of meetup was low level implementation details of  various runtimes for popular platforms.

    Below are link to slidedecks from presentation:

    Ruby MRI/YARV, CPython

    Dmitri Babaev

    Erlang BEAM internal

    Anton Lebedevich

    Devirtualization of method calls by JIT

    Alexey Ragozin

    Tuesday, March 19, 2013

    ChTest is out

    Writing automated tests for Coherence based application is quite challenging. Definitely, you can start single node cluster in your test JVM without too much hassle. Unfortunately, single node cluster will not allow you to replicate some important aspects of cluster (a thus, test wont be able to catch a number of nasty bugs).

    Running Coherence cluster with several nodes is a bit trickier (Coherence is using singletons). Idea to use classloader to keep multiple Coherence nodes is not new. I’m using this approach for few years already, and …
    Today I’m glad to announce availability of my test library for Coherence at Maven central repo.

    ChTest is a third generation of my test framework for Oracle Coherence. Besides ability to run multiple Coherence nodes in different classloader, ChTest offers a bunch of extra features.
    To name few:

  • System property isolation between “virtualized” nodes.
  • Console output of each “virtualized” node is prefixed by its name (helps to make sense out of logs).
  • Very convenient way to execute code in “virtualized” node context.
  • Option to start “virtualized” node as separate JVM (JVM start option could also be configured).
  • Coherence MBeans could be enabled in “virtualized” node; they will be exposed in separate domains and accessible via JConsole or VisualVM.
  • Specific classes could be shared between classloaders (useful to use static variable to share state between nodes).
  • Classpath could be tweaked per “virtualized” node (e.i. exclude some jar from cluster nodes).
  • Coherence “node” crash simulation (I’m using it to test disaster recover cases).
  • Utility to choose Coherence jar per “virtualized” node.
  • ... and this list is not full ?

    Below is slide deck outlining features of ChTest.

    Add following Maven dependency and you would be able to try ChTest yourself.


    Thursday, March 14, 2013

    Coherence user meet up in Moscow

    First meet up of Oracle Coherence users in Moscow was held on 14 Mar.

    Many thanks to Gene Gleyzer (Oracle), who was key speaker on this event (joining us remotely).

    Below are slides from event:

    Tuesday, March 12, 2013

    Upcomming tech meetups in Moscow

    Here is announcing of upcoming tech meetups in Moscow.
    Уже больше года я занимаюсь организацией встреч посвящённых IT тематике. Отрадно видеть, что и мест готовых предоставить нам крышу, так и докладчиков, становится больше.
    Ниже список ближайших запланированных встреч.

    • Март 14 – Встреча пользователей Oracle Coherence – один из создателей Coherence удалённо выступит для нас с докладом.
    • Март 28 – И снова деплоймент – обсудим проблему на примере развёртывания OpenStack.
    • Апрель 18 – Магия виртуальных машин – Python, Erlang, Ruby, JavaScript  - что мы знаем про виртуальные машины для этих языков?
    • Май 16 – Сборка мусора в Java - возвращаемся к вечной теме, как изменилось положение дел за последний год? Так же будет организована видео трансляция. Дата предварительная.

    Так же, приглашаю вас, присоединится к нашей группе на meetup.com

    Friday, February 1, 2013

    How to simulate Coherence node failure in JUnit test

    Recently, I was working on renewed version of Coherence data loss listener. New version provides simple utility to attach partition loader to any cache. Such partition loader is guaranteed to be called for every partition of newly created cache or after partition has been lost. Unlike CacheStore, partition loader will be called on the node there it has been added. This way you could have dedicated set loader processes, which are not involved at storing a data. Also it is guaranties that

  • only one instance of partition loader are executed for particular partition in cluster,
  • if there is at least one live node with registered partition loader it would be invoked for empty partition.
  • But let’s get back to a topic of post. I’m actively using JUnit and my test-utils library to automate testing of my Coherence stuff. Test-utils is using class loader isolation to run multiple Coherence nodes in single JVM.

    But, unlike many other tests, here I need to test disaster case. Coherence should think that one of its nodes has died. Normally, I’m using CacheFactory.shutdown() to kill virtualized Coherence node, but this way it would be a graceful shutdown.

    For data loss listener, I really want to test disaster case.

    How Coherence track node liveness?

    Naïve approach using timeout is not working well with data grid prioritizing resilience and performance such as Coherence.

    What is problem with timeout?

    If you let it be too short, there will be too many false positives making grid unstable (JVM may do a GC, OS may start swapping, etc).

    If you let it be too long, time of recovery from disaster would be too long.

    How this can be improved? Let’s see that kind of disaster could possibly happen with your cluster:

  • JVM process could be killed, crushed or just exited without shutdown.
  • Sever could crush or become unreachable via network.
  • Death of process could be easily tracked if you keep open TCP connection open. OS will close all TCP connections for dead process, so you could make very good assumption that remote process is dead.

    Coherence is using so called TCP ring for that purpose. Each cluster node keeps two open TCP connections to other cluster nodes (forming a ring). If cluster detects that both TCP connections have been closed, it has very good reason to disconnect node right now and start recovery procedure.

    In case of server/network failure, TCP connection will not be closed immediately. In addition to TCP ring, Coherence is using IP monitor to track reachability of IP addresses. If IP address cannot be reached by rest of nodes, cluster will not hesitate to disconnect all nodes from that IP.

    This two tricks allow Coherence to detect real failure very fast, yet to be very tolerant to long GC pauses and other non fatal slowdowns.

    Steps to kill node in JUnit test

    In JUnit test all nodes in cluster are sharing same JVM. I cannot really kill a process. To simulate node death, I’m calling Thread.suspend() on all threads related to victim node (a feature of test-utils). This is making node totally unresponsive.

    Two mechanisms above should be turned off in Coherence operational configuration. Disconnect timeout also should be set to smaller value (otherwise each test will take too long).

    That is it, now I can test disaster cases for Coherence using JUnit.

    Below is snippet of actual test:

    @Test public void verify_parallel_init_crash_case() throws InterruptedException { final int partitions = 2000; final int timeout = 15000; CacheTemplate.usePartitionedServicePartitionCount(cluster, partitions); CohHelper.setTCMPTimeout(server(0), timeout); CohHelper.disableTcpRing(server(0)); CohHelper.setTCMPTimeout(client(0), timeout); CohHelper.disableTcpRing(client(0)); ... server(0).getCache("a-cache1"); statics().initPartitionCounter(partitions); // init Coherence nodes client(0).getCache("a-cache1"); client(1).getCache("a-cache1"); ... // attaching test partition loader attachTouchMonitor(0, 20, "a-cache1"); attachTouchMonitor(1, 20, "a-cache1"); ... Thread.sleep(500); System.out.println("Simulating crash for 2,3,4,5"); // simulating client crash, verify lock revocation client(2).suspend(); client(3).suspend(); client(4).suspend(); client(5).suspend(); // waiting for all test listeners to finish statics().waitAllLatches(); System.out.println("Latches are open"); Thread.sleep(200); // checking cache state assertAllCanaries(0, "a-cache1"); }

    Here is a link to full java file in SVN.

    Monday, January 28, 2013

    Remote code execution in Java made trivial

    SSH offers a very convenient way to execute shell scripts remotely. Code like

    ssh myserver.acme.com << EOC
    cd $APP_HOME

    are fairly easy to write and read.

    But while remote execution itself is easy, writing actual distributed code is total mess.

    I’m a Java guy. I wish, I could run Java code remotely as easy as I can do it with shell.
    And now, finally, I can.

    @Test public void remote_hello_world() throws InterruptedException { ViManager cloud = CloudFactory.createSimpleSshCloud(); cloud.node("myserver.acme.com"); cloud.node("**").exec(new Callable<Void>() { @Override public Void call() throws Exception { String localHost = InetAddress.getLocalHost().toString(); System.out.println("Hi! I'm running on " + localHost); return null; } }); // Console output is transfered asynchronously, // so it is better to wait few seconds for it to arrive Thread.sleep(1000); }

    And that snippet will work for you too, only two requirements are:

  • You should be able to SSH to target host without password (you have SSH key-pairs set up).
  • Remote host should have JDK installed, and java command on PATH.
  • That is it, there is no need to install anything special to target servers.

    You can find more details in tutorial at GridKit site.

    How it work?

    A lot of black magic is happening behind the scene. In particular:

  • Classpath of current Java processes is replicated and cached at remote host using SFTP.
  • SSH is used to start remote Java process.
  • All communications between master and slave are tunneled in stdIn and stdOut (thus you can care less about NAT and firewall).
  • Modified version of RMI/serialization is used to allow anonymous classes to be run remotely.
  • By despite all that internal complexity, it just works. I’m using it to run agents on Linux and Solaris. I was using that stuff on Amazon EC2 and I cloud have master process running on my Windows desktop while slave scattered across Unixes.
    It was long road. A lot of issues such as firewalls, bugs in JSch (java SSH client), subtle SSH limitation, etc has been solved along that path.
    But now, I believe, it will “just work” for you too.

    Java vs shell

    But what is the need for such library in a first place? Below are just few of my reasons:

  • I’m a Java guy, I do not want to invest much in shell scripting skills.
  • Java is "really" cross platform, heck I can even debug stuff on Windows then run them on Linux.
  • I have to do a lot to distributed orchestration (starting/stopping processes in right order etc), it is so much easier to do in java.
  • It is hard to write reusable shell scripts, but easy for java (heck, I can even unit test pieces of deployment logic).
  • Remotting is just a remotting

    Ability to effortlessly run remote java code is not much by itself. But it enables you to reach new levels of day-to-day automation (which was prohibitively expensive before).

    But that would be topic for another post.