e6043981f5
AOF fsync=always, and certain Redis Cluster bus operations, require to
fsync data on disk before replying with an acknowledge.
In such case, in order to implement Group Commits, we want to be sure
that queries that are read in a given cycle of the event loop, are never
served to clients in the same event loop iteration. This way, by using
the event loop "before sleep" callback, we can fsync the information
just one time before returning into the event loop for the next cycle.
This is much more efficient compared to calling fsync() multiple times.
Unfortunately because of a bug, this was not always guaranteed: the
actual way the events are installed was the sole thing that could
control. Normally this problem is hard to trigger when AOF is enabled
with fsync=always, because we try to flush the output buffers to the
socekt directly in the beforeSleep() function of Redis. However if the
output buffers are full, we actually install a write event, and in such
a case, this bug could happen.
This change to ae.c modifies the event loop implementation to make this
concept explicit. Write events that are registered with:
AE_WRITABLE|AE_BARRIER
Are guaranteed to never fire after the readable event was fired for the
same file descriptor. In this way we are sure that data is persisted to
disk before the client performing the operation receives an
acknowledged.
However note that this semantics does not provide all the guarantees
that one may believe are automatically provided. Take the example of the
blocking list operations in Redis.
With AOF and fsync=always we could have:
Client A doing: BLPOP myqueue 0
Client B doing: RPUSH myqueue a b c
In this scenario, Client A will get the "a" elements immediately after
the Client B RPUSH will be executed, even before the operation is persisted.
However when Client B will get the acknowledge, it can be sure that
"b,c" are already safe on disk inside the list.
What to note here is that it cannot be assumed that Client A receiving
the element is a guaranteed that the operation succeeded from the point
of view of Client B.
This is due to the fact that the barrier exists within the same socket,
and not between different sockets. However in the case above, the
element "a" was not going to be persisted regardless, so it is a pretty
synthetic argument.
This README is just a fast quick start document. You can find more detailed documentation at http://redis.io.
What is Redis?
Redis is often referred as a data structures server. What this means is that Redis provides access to mutable data structures via a set of commands, which are sent using a server-client model with TCP sockets and a simple protocol. So different processes can query and modify the same data structures in a shared way.
Data structures implemented into Redis have a few special properties:
- Redis cares to store them on disk, even if they are always served and modified into the server memory. This means that Redis is fast, but that is also non-volatile.
- Implementation of data structures stress on memory efficiency, so data structures inside Redis will likely use less memory compared to the same data structure modeled using an high level programming language.
- Redis offers a number of features that are natural to find in a database, like replication, tunable levels of durability, cluster, high availability.
Another good example is to think of Redis as a more complex version of memcached, where the operations are not just SETs and GETs, but operations to work with complex data types like Lists, Sets, ordered data structures, and so forth.
If you want to know more, this is a list of selected starting points:
- Introduction to Redis data types. http://redis.io/topics/data-types-intro
- Try Redis directly inside your browser. http://try.redis.io
- The full list of Redis commands. http://redis.io/commands
- There is much more inside the Redis official documentation. http://redis.io/documentation
Building Redis
Redis can be compiled and used on Linux, OSX, OpenBSD, NetBSD, FreeBSD. We support big endian and little endian architectures, and both 32 bit and 64 bit systems.
It may compile on Solaris derived systems (for instance SmartOS) but our support for this platform is best effort and Redis is not guaranteed to work as well as in Linux, OSX, and *BSD there.
It is as simple as:
% make
You can run a 32 bit Redis binary using:
% make 32bit
After building Redis is a good idea to test it, using:
% make test
Fixing build problems with dependencies or cached build options
Redis has some dependencies which are included into the deps directory.
make does not rebuild dependencies automatically, even if something in the
source code of dependencies is changed.
When you update the source code with git pull or when code inside the
dependencies tree is modified in any other way, make sure to use the following
command in order to really clean everything and rebuild from scratch:
make distclean
This will clean: jemalloc, lua, hiredis, linenoise.
Also if you force certain build options like 32bit target, no C compiler
optimizations (for debugging purposes), and other similar build time options,
those options are cached indefinitely until you issue a make distclean
command.
Fixing problems building 32 bit binaries
If after building Redis with a 32 bit target you need to rebuild it
with a 64 bit target, or the other way around, you need to perform a
make distclean in the root directory of the Redis distribution.
In case of build errors when trying to build a 32 bit binary of Redis, try the following steps:
- Install the packages libc6-dev-i386 (also try g++-multilib).
- Try using the following command line instead of
make 32bit:make CFLAGS="-m32 -march=native" LDFLAGS="-m32"
Allocator
Selecting a non-default memory allocator when building Redis is done by setting
the MALLOC environment variable. Redis is compiled and linked against libc
malloc by default, with the exception of jemalloc being the default on Linux
systems. This default was picked because jemalloc has proven to have fewer
fragmentation problems than libc malloc.
To force compiling against libc malloc, use:
% make MALLOC=libc
To compile against jemalloc on Mac OS X systems, use:
% make MALLOC=jemalloc
Verbose build
Redis will build with a user friendly colorized output by default. If you want to see a more verbose output use the following:
% make V=1
Running Redis
To run Redis with the default configuration just type:
% cd src
% ./redis-server
If you want to provide your redis.conf, you have to run it using an additional parameter (the path of the configuration file):
% cd src
% ./redis-server /path/to/redis.conf
It is possible to alter the Redis configuration passing parameters directly as options using the command line. Examples:
% ./redis-server --port 9999 --slaveof 127.0.0.1 6379
% ./redis-server /etc/redis/6379.conf --loglevel debug
All the options in redis.conf are also supported as options using the command line, with exactly the same name.
Playing with Redis
You can use redis-cli to play with Redis. Start a redis-server instance, then in another terminal try the following:
% cd src
% ./redis-cli
redis> ping
PONG
redis> set foo bar
OK
redis> get foo
"bar"
redis> incr mycounter
(integer) 1
redis> incr mycounter
(integer) 2
redis>
You can find the list of all the available commands at http://redis.io/commands.
Installing Redis
In order to install Redis binaries into /usr/local/bin just use:
% make install
You can use make PREFIX=/some/other/directory install if you wish to use a
different destination.
Make install will just install binaries in your system, but will not configure init scripts and configuration files in the appropriate place. This is not needed if you want just to play a bit with Redis, but if you are installing it the proper way for a production system, we have a script doing this for Ubuntu and Debian systems:
% cd utils
% ./install_server.sh
The script will ask you a few questions and will setup everything you need to run Redis properly as a background daemon that will start again on system reboots.
You'll be able to stop and start Redis using the script named
/etc/init.d/redis_<portnumber>, for instance /etc/init.d/redis_6379.
Code contributions
Note: by contributing code to the Redis project in any form, including sending a pull request via Github, a code fragment or patch via private email or public discussion groups, you agree to release your code under the terms of the BSD license that you can find in the COPYING file included in the Redis source distribution.
Please see the CONTRIBUTING file in this source distribution for more information.
Enjoy!