Monetdbd is a MonetDB management daemon. We use it to: – Initialize a directory as a DBfarm. A DBfarm is a folder that can contain several MonetDB databases. – Monetdbd is used to set and read farm-wide daemon options that are shared between databases. – Monetdbd is used to start/stop the actual servers (Mserver5 process). – Users usually connect to the monetdbd daemon first, and then this daemon will redirect them to the selected database.
Monetdbd Informations
monetdbd -v monetdbd --version
This is how we can find version of the monetdbd daemon. We can use short or long command syntax.
This is how we can get help for monetdbd. monetdbd -h monetdbd --help
We can provide the name of a command after the "-h" option. That will give us help for that command.
Monetdbd and DBfarm
cd /home/fffovde/Desktop mkdir DBfarm monetdbd create DBfarm
We will create a folder with the name "DBfarm". With the command "monetdbd create", we will initialize this folder. Inside of it, a new file ".merovingian_properties" will appear. Now it is possible to create new databases inside of this folder.
This is the inside of the ".merovingian_properties" file.
Currently no one is listening to default MonetDB port 50.000. I will make DBfarm directory functional.
monetdbd start DBfarm
Now our daemon is listening to port 50000, and DBfarm is functional. ss -tlnp | grep 50000
After the first start of the DBfarm, three new files will appear in the DBfarm directory. ".merovingian_lock" file is empty.
"merovingian.pid" file has process ID for monetdbd process. We can confirm that with "pgrep monetdbd".
"merovingian.log" file is a file with a log.
We can stop DBfarm with monetdbd. monetdbd stop DBfarm
After closing the farm, file "merovingian.pid" will disappear. This file exists only when DBfarm is functional. Now, again, no one is listening to port 50.000. ss -tlnp | grep 50000
Getting and Setting DBfarm Settings
We can read setttings of the DBFarm.
monetdbd get all DBfarm
hostname FffOvdeKomp dbfarm DBfarm status no monetdbd is serving this dbfarm mserver unknown (monetdbd not running) logfile DBfarm/merovingian.log pidfile DBfarm/Merovingian.pid loglevel information sockdir /tmp listenaddr localhost port 50000 exittimeout 60 forward proxy discovery true discoveryttl 600 control no passphrase <unset> snapshotdir <unset> snapshotcompression .tar.lz4 keepalive 60 mapisock /tmp/.s.monetdb.50000 controlsock /tmp/.s.merovingian.50000
We can easily change any of these settings. monetdbd set loglevel=debug DBfarm
We can also change the settings while DBfarm is active, but some changes will become valid only after Monetdbd is restarted.
After we first change some property, a list of the properties and their values will appear in the ".merovingian_properties" file.
Not all of the properties that we can observer with the "get all" command will be listed here. Properties that do not appear here can not be changed with the "set" command. Those properties are informative and read only.
We should never make changes in this file manually. We should always use the "set" command.
Monetdbd Settings
We will take a look at settings that we can set with Monetdbd.
File and Log Settings
We know that in DBfarm folder we have files "merovingian.log" and "merovingian.pid". I will choose to use two other files instead of them.
I will change the settings for "log" and "pid" files: monetdbd set logfile=/home/fffovde/Desktop/logy.log DBfarm monetdbd set pidfile=/home/fffovde/Desktop/pidy.pid DBfarm I will start daemon again: monetdbd start DBfarm
Now, we have two new files on the desktop.
Inside of the "log" file, we have a log for monetdbd. Lines labeled with the MSG, are for information and warnings.
We can decide what will be logged in the log file. We can choose between modes "error, warning, information and debug".
error = only errors
warning = errors and warnings
information = errors, warnings and information
debug = the same as information
Error lines will be marked with ERR.
Default is "information". This is how we change the mode:
# monetdbd set loglevel=error DBfarm
Connection Settings
Settings "sockdir" and "forward" should almost never be changed.
Listendaddr is "localhost" by default. That means that our server is not available from the network.
If I set listenaddr to "0.0.0.0", our server will become available from the network. I will also change default port number to 50001.
monetdbd set listenaddr=0.0.0.0 DBfarm monetdbd set port=50001 DBfarm
This setting will become functional only after we restart monetdbd. monetdbd stop DBfarm monetdbd start DBfarm
Inside of the DBfarm, I will create and start one database: monetdb create DBfarmDB monetdb release DBfarmDB monetdb start DBfarmDB
I will now open the green server. I explained creation of this server in this blog post "link". Then I will try to connect to DBfarmDB from the green server. mclient -h 192.168.100.124 -p 50001 -u monetdb -d DBfarmDB
We can use any of these modes for "listenaddr". It is also possible to use "hostname", in that case Linux will resolve what will be used.
Loopback address:
External host:
only IPv4
127.0.0.1
0.0.0.0
only IPv6
::1
::
IPv4 and IPv6
localhost
all
I will change back the port number of the "voc" server to 50000: "monetdbd set port=50000 DBfarm".
Snapshot Settings
In the blog post "link", we have saw how to create a snapshot of our database. We have used stored procedure for that. The same is possible from the command line. Settings for command line snapshots are in monetdbd properties file.
"snapshotdir" is a directory where our snapshot will be made. If this is not set, then we will not be able to make a snapshot.
monetdbd set snapshotdir=/home/fffovde/Desktop DBfarm
"snapshotcompression" is a level of compression. We can choose between "tar.lz4,tar.bz2,tar.gz,tar.xz". This is optional setting.
monetdbd set snapshotcompression=.tar.lz4 DBfarm There is a dot before "tar".
I will set these changes, and then we will create a snapshot. We will create snapshot with the "monetdb" console program.
monetdb snapshot create DBfarmDB
This will create ".tar.lz4" snapshot on our Desktop. Inside of it we will get "DBfarmDB" database.
We can unset a property by providing an empty value:
monetdbd set snapshotdir= DBfarm
Remote Control Settings
We can call and control "monetdbd" on remote computer. I will allow remote control for "monetdbd" on the "voc" server.
monetdbd set control=true DBfarm
We also have to provide SHA512 password.
monetdbd set passphrase=zebra DBfarm
Our password will be written to ".merovingian_properties" file. It will be written as 128-digits hexadecimal number with the prefix "{SHA512}".
If we only have hashed password, then we can provide it directly.
monetdbd set passphrase={SHA512}9df34…24fec DBfarm
We have to restart monetdbd for these settings to become functional. We will also start DBfarmDB database.
Monetdb can "shout" its existence on the local network by UDP broadcast. In regular intervals Monetdb server will tell other computers on the LAN about its existence. This is how we can discover all of the monetdb servers in our network.
I will tell "voc" server to broadcast its existence every 300 seconds. It will use default port 50000.
monetdbd set discovery=true DBfarm monetdbd set discoveryttl=300 DBfarm
I will restart DBfarm.
monetdbd stop DBfarm monetdbd start DBfarm
On the green server I will run this command. monetdb discover We can see here that on the "voc" server (hostname fffOvdeKomp), we have database "DBfarmDB". We also see local green database. So, we get a list of all of the available databases.
Start and Stop Settings
monetdbd stop DBfarm monetdbd start -n DBfarm
I will stop "DBfarm", and then I will start it with a "-n" switch. This will leave "monetdbd" daemon in the foreground and will not return control to the shell (we will not get prompt in the next line). This is useful for debugging. We can close "monetdbd" using "ctrl+C" and then Enter.
There is "monetdbd" setting "exittimeout" with the default value 60.
This means that "monetdbd" will ask "mserver" to shutdown politely with the SIGTERM command ( "mserver" can refuse ). If the "mserver" is not closed after 60+5 seconds, then "monetdbd" will send SIGKILL signal, and "mserver" will be violently shut down.
There are three possible values for "exittimeout".
POSITIVE VALUE: "Mserver", if alive, will be killed 5 seconds after the "exittimeout".
NEGATIVE VALUE: "Mserver" will only receive SIGTERM signal. It will never be killed.
ZERO: "Monetdbd" will close, but "Mservers" will not. We should not use this.
When we wait for a really long query to finish, our client will not communicate with the server. Firewall can interpret that like an inactive connection, so it will close the connection. We want to avoid this, so we can use probe signal to keep connection alive.
This is done with the "keepalive" setting, with the default value of 60 seconds. # monetdbd set keepalive=60 DBfarm This probe signal will be sent exactly 127 times. If "keepalive" time is 60 seconds, that means that are connection will be at least (127 x 60 seconds) ≈ 2 hours alive.
Monetdbd Return Value
Monetdbd will return "0" for successful action, and "1" for unsuccessful. If we try to start already started daemon, we will get an error. monetdbd start DBfarm; echo $?
Monetdbd Process Signals
I will start the monetdbd again, and then I will read process ID of our daemon. monetdbd start DBfarm cat ./pidy.pid
Let's assume that I want to use some other file, instead of "logy.log", for logging, but I do not want to restart monetdbd. I will do that in the two steps.
monetdbd set logfile=/home/fffovde/Desktop/DBfarm/merovingian.log DBfarm kill -HUP "$(cat ./pidy.pid)"
I am changing the file for my log. I am sending SIGHUP signal to monetdbd.
Now the daemon will stop using "logy.log" file, and will use "merovingian.log" file. We didn't have to restart the daemon.
We can kill monetdbd with a signal. All of the signals bellow work the same. kill -TERM "$(cat "pidy.pid")" kill -INT "$(cat "pidy.pid")" kill -QUIT "$(cat "pidy.pid")"
They will ask our "mserver5" process to gently end (it can refuse). Signals: SIGTERM SIGINT SIGQUIT
