Kestrel is based on Blaine Cook's "starling" simple, distributed message queue, with added features and bulletproofing, as well as the scalability offered by actors and the JVM.
Each server handles a set of reliable, ordered message queues. When you put a
cluster of these servers together, with no cross communication, and pick a
server at random whenever you do a
get, you end up with a reliable,
loosely ordered message queue.
In many situations, loose ordering is sufficient. Dropping the requirement on cross communication makes it horizontally scale to infinity and beyond: no multicast, no clustering, no "elections", no coordination at all. No talking! Shhh!
For more information about what it is and how to use it, check out the included guide.
Kestrel has a mailing list here: firstname.lastname@example.org
Author's address: Robey Pointer <email@example.com>
It runs on the JVM so it can take advantage of the hard work people have put into java performance.
Queues are stored in memory for speed, but logged into a journal on disk so that servers can be shutdown or moved without losing any data.
A client can ask to "tentatively" fetch an item from a queue, and if that client disconnects from kestrel before confirming ownership of the item, the item is handed to another client. In this way, crashing clients don't cause lost messages.
Kestrel is not:
While each queue is strongly ordered on each machine, a cluster will appear "loosely ordered" because clients pick a machine at random for each operation. The end result should be "mostly fair".
This is not a database. Item ownership is transferred with acknowledgement, so no jobs will be lost, but kestrel does not support grouping multiple operations into an atomic unit.
The latest release is always on the homepage here:
Or the latest development versions & branches are on github:
Kestrel requires java 6 and sbt 0.7.4. On OS X 10.5, you may have to hard-code
JAVA_HOME to use java 6:
$ export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home
Building from source is easy:
$ sbt clean update package-dist
Scala libraries and dependencies will be downloaded from maven repositories
the first time you do a build. The finished distribution will be in
You can run kestrel by hand, in development mode, via:
Like all ostrich-based servers, it uses the "stage" property to determine
which config file to load, so
When running it as a server, a startup script is provided in
dist/kestrel-VERSION/scripts/kestrel.sh. The script assumes you have
daemon, a standard daemonizer for Linux, but also available
here for all common unix platforms.
The created archive
kestrel-VERSION.tar.bz2 can be expanded into a place
/usr/local (or wherever you like) and executed within its own folder as
a self-contained package. All dependent jars are included, and the startup
script loads things from relative paths.
The default configuration puts logfiles into
/var/log/kestrel/ and queue
journal files into
The startup script logs extensive GC information to a file named
the log folder. If kestrel has problems starting up (before it can initialize
logging), it will usually appear in
error in the same folder.
Queue configuration is described in detail in
docs/guide.md (an operational
guide). Scala docs for the config variables are
Performance / Load
Several performance/load tests are included. To run them, first start up a kestrel instance locally.
$ sbt clean update package-dist $ ./dist/kestrel-VERSION/scripts/devel.sh
All of the tests can be run from scripts in
scripts/load/, and all of them
respond to "
This test just spams a kestrel server with "put" operations, to see how quickly it can absorb and journal them.
A sample run (on a Mac laptop):
$ ./dist/kestrel/scripts/load/put-many -n 100000 Flushing queues first. Put 100000 items of 1024 bytes in bursts of 1 to localhost:22133 in 1 queues named spam using 100 clients. Finished in 4410 msec (44.1 usec/put throughput). Distribution in usec: min=0 max=3858 p50=1 p75=1 p90=1 p95=2 p99=10 p999=316 p9999=472
This test has one producer that trickles out one item at a time, and a pile of consumers fighting for each item. It usually takes exactly as long as the number of items times the delay, but is useful as a validation test to make sure kestrel works as advertised without blowing up.
A sample run:
$ ./dist/kestrel/scripts/load/many-clients many-clients: 100 items to localhost using 100 clients, kill rate 0%, at 100 msec/item Flushing queues first. Received 100 items in 12138 msec.
This test always takes over 10 seconds -- it's a load test instead of a speed test.
This test starts up one producer and one consumer, and just floods items through kestrel as fast as it can.
A sample run:
$ ./dist/kestrel/scripts/load/flood Flushing queues first. flood: producers=1 consumers=1 each sending 10000 items of 1kB through spam Finished in 884 msec (88.4 usec/put throughput). Consumer(s) spun 6 times in misses.
This test starts up one producer and one consumer, seeds the queue with a bunch of items to cause it to fall behind, then does cycles of flooding items through the queue, separated by pauses. It's meant to test kestrel's behavior with a queue that's fallen behind and stays behind indefinitely, to make sure the journal files are cleaned up periodically without affecting performance too badly.
(In the past, journals were "packed" as they fell behind. These days, the old journal files are just erased as their items are read, so the name of this test is historic and a bit misleading.)
A sample run:
$ ./dist/kestrel/scripts/load/packing -c 10 -q small packing: 25000 items of 1kB with 1 second pauses Flushing queues first. Wrote 25000 items starting at 0. cycle: 1 Wrote 25000 items starting at 25000. Read 25000 items in 2862 msec. Consumer spun 0 times in misses. cycle: 2 Wrote 25000 items starting at 50000. Read 25000 items in 2426 msec. Consumer spun 0 times in misses. ... cycle: 10 Wrote 25000 items starting at 250000. Read 25000 items in 2464 msec. Consumer spun 0 times in misses. Read 25000 items in 2181 msec. Consumer spun 0 times in misses.
You can see the journals being built and erased in the kestrel log. Like "many-clients", this test is a load test instead of a speed test.
This test starts a producer and several consumers, with the consumers occasionally "forgetting" to acknowledge an item that they've read. It verifies that the un-acknowledged items are eventually handed off to another consmer.
A sample run:
$ ./dist/kestrel/scripts/load/leaky-reader -n 100000 -t 10 leaky-reader: 10 threads each sending 100000 items through spam Flushing queues first. 1000 2000 100000 Finished in 40220 msec (40.2 usec/put throughput). Completed all reads
Like "many-clients", it's just a load test.