LogCabin.appendEntry(1, "Hello, world!")

| raft logcabin

This is the first in a series of blog posts detailing the ongoing development of LogCabin. This first entry catches up on the developments from when I started working with Scale Computing in November, so it's longer than most of the future updates will be.

The theme of this entry is getting started in a new environment. Up until now, I'd done nearly all of the development of LogCabin on my laptop and on the RAMCloud cluster. Running it somewhere new uncovered a bunch of implicit assumptions baked-in about the environment, so it exposed a new set of issues and bugs. This is fairly inevitable when it comes to low-level systems code, and there's a lot of value in working through it. LogCabin is significantly easier to run now than it was before, and it should be easier for the rest of you to install on your systems, too.

Getting Started

At Scale, we've been working to integrate LogCabin into Scribe, the software that runs their private cloud products. We needed LogCabin to be installable and deployable as a normal system service with an RPM (Scale's software distribution is currently based on RHEL 6). So, I added an option for LogCabin to behave more like a daemon, reparenting to init and writing to a log file instead of stderr. Others at Scale wrote a basic init script and an RPC spec file, which we intend to merge into the upstream repo.

On the client side of things, I moved the Client.h header that clients need to include. Client code used to #include <Client/Client.h>, which is fairly confusing; now it's #include <LogCabin/Client.h>.

I also started removing the need for DNS in clients. Instead of requiring a DNS name that resolves to multiple addresses, the Cluster constructor now accepts a semicolon-delimited list of addresses and will randomly connect to hosts in that list. Left to do are simplifying the README and other scripts to take advantage of this, and finding a way for dynamic membership changes to work without DNS (maybe we should expose a way for running clients to reset the list of hosts?). For now, Scribe still uses a DNS hostname set up by /etc/hosts.

Client API

As I was adding the first lines of code to write to LogCabin from Scribe, I noticed that LogCabin had no way to do a conditional create. In other words, you could write to a key on the condition that it had a particular value, but you couldn't write to it on the condition that it had no value. Now, Tree::setCondition(path, "") will match not only "files" in the replicated state machine with 0-length values but also files that don't exist at all. In the future, we may want to extend the condition system to include matching on hashes of files for large files, or maybe switch to using version numbers.

Internal Improvements

I also fixed two crashes in the RPC system. One was a race condition, in which a socket that was disconnected and closed was still being used to attempt to send outbound messages. An additional mutex protecting access to that file descriptor fixed the problem.

The other bug in the RPC system was harder to track down. This crash happened to Nate when he reconfigured the cluster to include an address that was invalid, so that DNS resolution failed. Instead of retrying periodically, the non-leader servers would PANIC with messages like:

1418256261.050477 RPC/Address.cc:188 in refresh() WARNING[2:Peer(1)]: Unknown error from getaddrinfo("", "61023"): Temporary failure in name resolution
1418256261.050639 RPC/MessageSocket.cc:292 in read() ERROR[2:evloop]: Error while reading from socket: Transport endpoint is not connected Exiting...

But the MessageSocket never should have been instantiated with an invalid address! And strangely, I couldn't reproduce this problem on my laptop, only on Scale's servers.

It turns out that connect() behaves incorrectly on Scale's servers. As far as I understand from the POSIX standard, connect(fd, NULL, 0), with a 0-length sockaddr, should return -1 and set errno to EINVAL. That's what it does on my laptop, and that's what it does on Scale's servers when running through strace or valgrind. However, if I run that on Scale's servers as a normal program, it returns 0, that everything is ok, and then barfs later that the socket isn't connected! I couldn't find any other reference to this problem after a quick web search, but I suspect its a glibc issue. I worked around it by making sure not to call connect() with an empty sockaddr.

A couple additional improvements to the client library's internals:

Travis CI

Build Status

I also set up Travis CI to do automated builds for LogCabin. This started with wanting the code-level documentation (produced by Doxygen to be available on a web server. It's a simple idea, but the documentation changes as the code changes, so static hosting wouldn't quite work. On each commit, Travis CI will now check out the new version of the code, build it, run the unit tests, and build the documentation. Then, it'll push the docs to a GitHub static page, which GitHub then hosts at logcabin.github.io.

Travis CI runs these automated builds on fairly puny and/or overloaded VMs. I'm not blaming them (it's a free service), but this made some of the unit tests fail intermittently. Unfortunately, a few of the unit tests are just fundamentally sensitive to timing, like making sure that a condition variable waits for about the right amount of time. I renamed such tests to include "TimingSensitive" in the name, and, using a gtest filter, Travis CI will no longer fail the build for such tests.


Next time I'll discuss ongoing work to add timeouts to LogCabin's client library, including a battle against libstdc++ 4.4's broken support for clocks and time. Thanks to Scale Computing for supporting this work.