Synchronizing File Writes
Assuming we have a simple task to write events to a file, one event after another. Events can be written from multiple threads.
public interface EventLog {
public final class Event{
public final UUID id;
public final ZonedDateTime created;
public final String message;
public Event(UUID id, ZonedDateTime created, String message) {
this.id = id;
this.created = created;
this.message = message;
}
}
void recordEvent(Event event);
// Null Logger doing nothing to have a base line
EventLog NULL_LOGGER = new NullLogger();
class NullLogger implements EventLog {
@Override
public void recordEvent(Event event) {
// do nothing;
}
@Override
public void close() throws Exception {
}
}
}FileChannel Closes When Interrupted
Recently I worked on some threaded IO where a control thread interrupts the IO threads once in a while.
Periodically the app started to fail with a java.nio.channels.ClosedChannelException.
MVStore: Accessing Old Versions
A while back I blogged about MVStore basics. Let’s look at the way MVStore keeps old revisions around. First, MVStore maps have a .getVersion() method and a .openVersion() method pair. With that, you can access old version previous values. A new version of the map is created on (auto-) commit.
Bastel Proxy: HTTPS and reverse proxy for development environments
I tend to work on web applications and services which are interconnected. Different parts of the website and the services run as separate processes. In production something like NGINX does the HTTPS termination and unifies the services to a single domain. I found it useful to have HTTPS and a reverse proxy in my development environment too. Browsers complain about plain HTTPS sites and even disable certain features. Further, if your website is split across different processes and one part runs on http://localhost:8080 and other on http://localhost:8081, you can run into cross-domain issues. Also, I found it less error-prone to mirror the domain structure than having tons of localhost:{port} pointing to each other.
One option is to run a production reverse proxy like NGINX. However, it requires quite a bit of configuration and you have to provide it the HTTPS certificates etc. I wanted a solution specific for developments which tries to minimize setup time.
OpenJDK's Flight-Recorder and Mission Control
Flight Recorder is a low overhead profiler and diagnostics events recorder for the Hotspot JVM. Java Mission Control is
a software to visualize these events. It has been included for years in the Oracle JDK but used to be a commercial
feature. To use it you needed to add -XX:+UnlockCommercialFeatures -XX:+FlightRecorder flags and you were not allowed
to use it in production unless you paid a license.
Oracle donated the code to the OpenJDK project and it is now included in OpenJDK 11 or newer builds. However, if you look at your JDK there is no Mission Control, so where it is? It is part of the OpenJDK but is built separately. You can download binaries from AdoptOpenJDK or Azul, which rebranded it as Zulu Mission Control.