rrdcached - Data caching daemon for rrdtool
rrdcached [-l/-L address] [-w timeout] [-z delay] [-f timeout] [-p pid_file] [-t write_threads] [-j journal_dir] [-F] [-g] [-b base_dir [-B]]
rrdcached is a daemon that receives updates to existing RRD files, accumulates them and, if enough have been received or a defined time has passed, writes the updates to the RRD file. A flush command may be used to force writing of values to disk, so that graphing facilities and similar can work with up-to-date data.
The daemon was written with big setups in mind. Those setups usually run into IO related problems sooner or later for reasons that are beyond the scope of this document. Check the wiki at the RRDTool homepage for details. Also check SECURITY CONSIDERATIONS below before using this daemon! A detailed description of how the daemon operates can be found in the HOW IT WORKS section below.
unix:
, everything following that prefix is
interpreted as the path to a UNIX domain socket. Otherwise the address or node
name are resolved using getaddrinfo.
For network sockets, a port may be specified by using the form
[address]:port
. If the address is an IPv4 address or a fully
qualified domain name (i. e. the address contains at least one dot
(.
)), the square brackets can be omitted, resulting in the (simpler)
address:port
pattern.. The default port is 42217/udp.
The following formats are accepted. Please note that the address of the UNIX domain socket must start with a slash in the second case!
unix:</path/to/unix.sock> /<path/to/unix.sock> <hostname-or-ip> [<hostname-or-ip>]:<port> <hostname-or-ipv4>:<port>
If the -l option is not specified the default address,
unix:/tmp/rrdcached.sock
, will be used.
$localststedir/run/rrdcached.pid
will be used.
On startup, the daemon will check for journal files in this directory. If found, all updates therein will be read into memory before the daemon starts accepting new connections.
The journal will be rotated with the same frequency as the flush timer given by -f.
When journaling is enabled, the daemon will use a fast shutdown procedure. Rather than flushing all files to disk, it will make sure the journal is properly written and exit immediately. Although the RRD data files are not fully up-to-date, no information is lost; all pending updates will be replayed from the journal next time the daemon starts up.
To disable fast shutdown, use the -F option.
/tmp
, will be
used.
+------------------------+------------------------+ ! Command line ! File updated ! +------------------------+------------------------+ ! foo.rrd ! /tmp/foo.rrd ! ! foo/bar.rrd ! /tmp/foo/bar.rrd ! ! /var/lib/rrd/foo.rrd ! /var/lib/rrd/foo.rrd ! +------------------------+------------------------+ Paths given on the command line and paths actually updated by the daemon, assuming the base directory "/tmp".
WARNING: The paths up to and including the base directory MUST NOT BE symbolic links. In other words, if the base directory is specified as:
-b /base/dir/somewhere
... then NONE of the following should be symbolic links:
/base /base/dir /base/dir/somewhere
../
will also be blocked.
The following commands may be made aware of the rrdcached using the command line argument --daemon or the environment variable RRDCACHED_ADDRESS:
The update command can send values to the daemon instead of writing them to the disk itself. All other commands can send a FLUSH command (see below) to the daemon before accessing the files, so they work with up-to-date data even if the cache timeout is large.
The daemon reports errors in one of two ways: During startup, error messages
are printed to STDERR
. One of the steps when starting up is to fork to the
background and closing STDERR
- after this writing directly to the user is
no longer possible. Once this has happened, the daemon will send log messages
to the system logging daemon using syslog(3). The facility used is
LOG_DAEMON
.
When receiving an update, rrdcached does not write to disk but looks for an entry for that file in its internal tree. If not found, an entry is created including the current time (called ``First'' in the diagram below). This time is not the time specified on the command line but the time the operating system considers to be ``now''. The value and time of the value (called ``Time'' in the diagram below) are appended to the tree node.
When appending a value to a tree node, it is checked whether it's time to write
the values to disk. Values are written to disk if
now() - First >= timeout
, where timeout
is the timeout specified
using the -w option, see OPTIONS. If the values are ``old enough'' they
will be enqueued in the ``update queue'', i. e. they will be appended to
the linked list shown below. Because the tree nodes and the elements of the
linked list are the same data structures in memory, any update to a file that
has already been enqueued will be written with the next write to the RRD file,
too.
A separate ``update thread'' constantly dequeues the first element in the update queue and writes all its values to the appropriate file. So as long as the update queue is not empty files are written at the highest possible rate.
Since the timeout of files is checked only when new values are added to the file, ``dead'' files, i. e. files that are not updated anymore, would never be written to disk. Therefore, every now and then, controlled by the -f option, the entire tree is walked and all ``old'' values are enqueued. Since this only affects ``dead'' files and walking the tree is relatively expensive, you should set the ``flush interval'' to a reasonably high value. The default is 3600 seconds (one hour).
The downside of caching values is that they won't show up in graphs generated from the RRD files. To get around this, the daemon provides the ``flush command'' to flush specific files. This means that the file is inserted at the head of the update queue or moved there if it is already enqueued. The flush command will return only after the file's pending updates have been written to disk.
+------+ +------+ +------+ ! head ! ! root ! ! tail ! +---+--+ +---+--+ +---+--+ ! /\ ! ! / \ ! ! /\ /\ ! ! /\/\ \ `----------------- ... --------, ! V / `-------, ! V +---+----+---+ +------+-----+ +---+----+---+ ! File: foo ! ! File: bar ! ! File: qux ! ! First: 101 ! ! First: 119 ! ! First: 180 ! ! Next:&bar -+--->! Next:&... -+---> ... --->! Next:NULL ! | Prev:NULL !<---+-Prev:&foo !<--- ... ----+-Prev: &... ! +============+ +============+ +============+ ! Time: 100 ! ! Time: 120 ! ! Time: 180 ! ! Value: 10 ! ! Value: 0.1 ! ! Value: 2,2 ! +------------+ +------------+ +------------+ ! Time: 110 ! ! Time: 130 ! ! Time: 190 ! ! Value: 26 ! ! Value: 0.1 ! ! Value: 7,3 ! +------------+ +------------+ +------------+ : : : : : : +------------+ +------------+ +------------+ ! Time: 230 ! ! Time: 250 ! ! Time: 310 ! ! Value: 42 ! ! Value: 0.2 ! ! Value: 1,2 ! +------------+ +------------+ +------------+
The above diagram demonstrates:
The client/server protocol does not have any authentication or authorization mechanism. Therefore, take care to restrict which users can connect to the daemon.
Control sockets are divided into high-privilege (-l) and low-privilege (-L) sockets. High-privilege sockets accept all commands, whereas low-privilege sockets accept only FLUSH, STATS, and HELP.
For a multi-user environment where only certain users require read/write access, the recommended configuration uses two sockets as follows:
If you (want to) use the network capability, i. e. let the daemon bind to an IPv4 or IPv6 socket, it is your job to install a packet filter or similar mechanism to prevent unauthorized connections. Unless you have a dedicated VLAN or VPN for this, using the network option is probably a bad idea!
The daemon will blindly write to any file it gets told, so you really should create a separate user just for this daemon. Also it does not do any sanity checks, so if it gets told to write values for a time far in the future, your files will be messed up good!
You have been warned.
The daemon communicates with clients using a line based ASCII protocol which is easy to read and easy to type. This makes it easy for scripts to implement the protocol and possible for users to use telnet to connect to the daemon and test stuff ``by hand''.
The protocol is line based, this means that each record consists of one or more
lines. A line is terminated by the line feed character 0x0A
, commonly
written as \n
. In the examples below, this character will be written as
<LF>
(``line feed'').
After the connection has been established, the client is expected to send a ``command''. A command consists of the command keyword, possibly some arguments, and a terminating newline character. For a list of commands, see Valid Commands below.
Example:
FLUSH /tmp/foo.rrd<LF>
The daemon answers with a line consisting of a status code and a short status message, separated by one or more space characters. A negative status code signals an error, a positive status code or zero signal success. If the status code is greater than zero, it indicates the number of lines that follow the status line.
Examples:
0 Success<LF>
2 Two lines follow<LF> This is the first line<LF> And this is the second line<LF>
The following commands are understood by the daemon:
<num_vals> <file>
The format in which the values are returned is similar to many other line based protocols: Each value is printed on a separate line, each consisting of the name of the value, a colon, one or more spaces and the actual value.
Example:
9 Statistics follow QueueLength: 0 UpdatesReceived: 30 FlushesReceived: 2 UpdatesWritten: 13 DataSetsWritten: 390 TreeNodesNumber: 13 TreeDepth: 4 JournalBytes: 190 JournalRotate: 0
Note that rrdcached only accepts absolute timestamps in the update values. Updates strings like ``N:1:2:3'' are automatically converted to absolute time by the RRD client library before sending to rrdcached.
read()
and write().
All commands are executed just as they would be if given individually, except for output to the user. Messages indicating success are suppressed, and error messages are delayed until the client is finished.
Command processing is finished when the client sends a dot (``.'') on its own line. After the client has finished, the server responds with an error count and the list of error messages (if any). Each error messages indicates the number of the command to which it corresponds, and the error message itself. The first user command after BATCH is command number one.
client: BATCH server: 0 Go ahead. End with dot '.' on its own line. client: UPDATE x.rrd 1223661439:1:2:3 <--- command #1 client: UPDATE y.rrd 1223661440:3:4:5 <--- command #2 client: and so on... client: . server: 2 Errors server: 1 message for command 1 server: 12 message for command 12
The following counters are returned by the STATS command:
rrd_update_r
, since the
daemon was started.
1223661439:123:456
is one data set with two
values. The term ``data set'' is used to prevent confusion whether
individual values or groups of values are counted.
No known bugs at the moment.
the rrdtool manpage, the rrdgraph manpage
rrdcached and this manual page have been written by Florian Forster <octo at verplant.org>.
kevin brintnall <kbrint@rufus.net>