LogCabin is a distributed system that provides a small amount of highly replicated, consistent storage. It is a reliable place for other distributed systems to store their core metadata and is helpful in solving cluster management issues. LogCabin uses the Raft consensus algorithm internally and is actually the very first implementation of Raft. It's released under the ISC license (equivalent to BSD).
External resources:
- Slide deck on LogCabin's usage, operations, and internals
- Code-level documentation built with Doxygen
- Recent updates on LogCabin's development on Diego's blog
Information about releases is in RELEASES.md.
This README will walk you through how to compile and run LogCabin.
The best place to ask questions about the LogCabin implementation is on the
logcabin-dev mailing
list. You might also try #logcabin on the freenode IRC network, although
there aren't always people around. Use GitHub Issues to report problems or
suggest features.
For questions and discussion about the Raft consensus algorithm, which LogCabin implements, use the raft-dev mailing list.
Pre-requisites:
- Linux x86-64 (v2.6.32 and up should work)
- git (v1.7 and up should work)
- scons (v2.0 and v2.3 are known to work)
- g++ (v4.4 through v4.9 and v5.1 are known to work) or clang (v3.4 through v3.7 are known to work with libstdc++ 4.9, and libc++ is also supported; see CLANG.md for more info)
- protobuf (v2.6.x suggested, v2.5.x should work, v2.3.x is not supported)
- crypto++ (v5.6.1 is known to work)
- doxygen (optional; v1.8.8 is known to work)
In short, RHEL/CentOS 6 should work, as well as anything more recent.
Get the source code:
git clone git://github.com/logcabin/logcabin.git
cd logcabin
git submodule update --init
Build the client library, server binary, and unit tests:
scons
For custom build environments, you can place your configuration variables in
Local.sc. For example, that file might look like:
BUILDTYPE='DEBUG'
CXXFLAGS=['-Wno-error']
To see which configuration parameters are available, run:
scons --help
It's a good idea to run the included unit tests before proceeding:
build/test/test
You can also run some system-wide tests. This first command runs the smoke tests against an in-memory database that is embedded into the LogCabin client (no servers are involved):
build/Examples/SmokeTest --mock && echo 'Smoke test completed successfully'
To run the same smoke test against a real LogCabin cluster will take some more setup.
This section shows you how to run the HelloWorld example program against a
three-server LogCabin cluster. We'll run all the servers on localhost for now:
- Server 1 will listen on 127.0.0.1:5254
- Server 2 will listen on 127.0.0.1:5255
- Server 3 will listen on 127.0.0.1:5256
Port 5254 is LogCabin's default port and is reserved by IANA for LogCabin. The other two belong to others and are hopefully not in use on your network.
We'll first need to create three configuration files. You can base yours off of sample.conf, or the following will work for now:
File logcabin-1.conf:
serverId = 1
listenAddresses = 127.0.0.1:5254
File logcabin-2.conf:
serverId = 2
listenAddresses = 127.0.0.1:5255
File logcabin-3.conf:
serverId = 3
listenAddresses = 127.0.0.1:5256
Now you're almost ready to start the servers. First, initialize one of the server's logs with a cluster membership configuration that contains just itself:
build/LogCabin --config logcabin-1.conf --bootstrap
The server with ID 1 will now have a valid cluster membership configuration in its log. At this point, there's only 1 server in the cluster, so only 1 vote is needed: it'll be able to elect itself leader and commit new entries. We can now start this server (leave it running):
build/LogCabin --config logcabin-1.conf
We don't want to stop here, though, because the cluster isn't fault-tolerant with just one server! We're going to start two more servers and then add them both to the first server's cluster.
Let's start up the second server in another terminal (leave it running):
build/LogCabin --config logcabin-2.conf
Note how this server is just idling, awaiting a cluster membership configuration. It's still not part of the cluster.
Start the third server also (LogCabin checks to make sure all the servers in your new configuration are available before committing to switch to it, just to keep you from doing anything stupid):
build/LogCabin --config logcabin-3.conf
Now use the reconfiguration command to add the second and third servers to the cluster:
ALLSERVERS=127.0.0.1:5254,127.0.0.1:5255,127.0.0.1:5256
build/Examples/Reconfigure --cluster=$ALLSERVERS set 127.0.0.1:5254 127.0.0.1:5255 127.0.0.1:5256
This Reconfigure command is a special LogCabin client. It first queries each
of the servers given in its positional command line arguments (space-delimited
after the command "set") to retrieve their server IDs and listening addresses
(as set in their configuration files). Then, it connects to the cluster given
by the --cluster option (comma-delimited) and asks the leader to set the
cluster membership to consist of those servers. Note that the existing cluster
members should be included in the positional arguments if they are to remain in
the cluster; otherwise, they will be evicted from the cluster.
If this succeeded, you should see that the first server has added the others to the cluster, and the second and third servers are now participating. It should have output something like:
Current configuration:
Configuration 1:
- 1: 127.0.0.1:5254
Attempting to change cluster membership to the following:
1: 127.0.0.1:5254 (given as 127.0.0.1:5254)
2: 127.0.0.1:5255 (given as 127.0.0.1:5255)
3: 127.0.0.1:5256 (given as 127.0.0.1:5256)
Membership change result: OK
Current configuration:
Configuration 4:
- 1: 127.0.0.1:5254
- 2: 127.0.0.1:5255
- 3: 127.0.0.1:5256
Note: If you're sharing a single magnetic disk under heavy load for all the servers, the cluster may have trouble maintaining a leader. See issue 57 for more details on symptoms and a workaround.
Finally, you can run a LogCabin client to exercise the cluster:
build/Examples/HelloWorld --cluster=$ALLSERVERS
That program doesn't do anything very interesting. Another tool called TreeOps exposes LogCabin's data structure on the command line:
echo -n hello | build/Examples/TreeOps --cluster=$ALLSERVERS write /world
build/Examples/TreeOps --cluster=$ALLSERVERS dump
See the --help for a complete listing of the available commands.
You should be able to kill one server at a time and maintain availability, or kill more and restart them and maintain safety (with an availability hiccup).
If you find it annoying to pass --cluster=$ALLSERVERS everywhere, you can also use a DNS name to return all the IP addresses. However, you will need distinct IP addresses for each server, not just distinct ports.
If you have your own application, you can link it against
build/liblogcabin.a. You'll also need to link against the following
libraries:
- pthread
- protobuf
- cryptopp
The procedure described above for running a cluster is fairly tedious when you
just want to run some tests and tear everything down again. Thus,
scripts/smoketest.py automates it. Create a file called scripts/localconfig.py
to override the smokehosts and hosts variables found in scripts/config.py:
smokehosts = hosts = [
('192.168.2.1', '192.168.2.1', 1),
('192.168.2.2', '192.168.2.2', 2),
('192.168.2.3', '192.168.2.3', 3),
]
The scripts use this file to when launching servers using SSH. Each tuple in the (smoke)hosts list represents one server, containing:
- the address to use for SSH,
- the address to use for LogCabin TCP connections, and
- a unique ID.
Each of these servers should be accessible over SSH without a password and should have the LogCabin directory available in the same filesystem location. The script currently assumes this directory to be on a shared filesystem, such as an NFS mount or localhost.
You may optionally create a smoketest.conf file, which can define
various options that apply to all the servers. The servers' listen addresses
will be merged with your smoketest.conf automatically.
Now you're ready to run:
scripts/smoketest.py && echo 'Smoke test completed successfully'
This script can also be hijacked/included to run other test programs.
To build the documentation from the source code, run:
scons docs
The resulting HTML files will be placed in docs/doxygen.
You can also find this documentation at https://logcabin.github.io.
To install a bunch of things on your filesystem, run:
scons install
Along with the binaries, this installs a RHEL 6-compatible init script.
If you don't want these files to pollute your filesystem, you can install the files
to any given directory as follows (replace pathtoinstallprefix in both places
with wherever you'd like the files to go):
scons --install-sandbox=pathtoinstallprefix pathtoinstallprefix
Finally, you can build a binary RPM as follows:
scons rpm
This creates a file called build/logcabin-0.0.1-0.1.alpha.0.x86_64.rpm or
similar that you can then install using RPM, with the same effect as scons install.
Please use GitHub to report issues and send pull requests.
All commits should pass the pre-commit hooks. Enable them to run before each commit:
ln -s ../../hooks/pre-commit .git/hooks/pre-commit