Throughout your experience with MonetDB, you might incur in bugs and issues that are not expected.
While we cannot give a solution to that right away, there are some tools that can help the debugging and tracing process to ease yoor experience in fixing issues.
Tracer is the general logging system for the MonetDB stack, modelled after other well-known logging schemes (e.g: Python). It provides a number of logging levels and options to increase or reduce the verbosity either of individual parts of the code or of the codebase as a whole. It allows users to focus on logging messages related to certain steps of execution, in order to aid debugging. The behaviour of Tracer can be controlled at runtime using a SQL API designed for that purpose.
Tracer supports the following logging levels with the following order:
MonetDB is built upon three different layers SQL, MAL and GDK. Each layer consists of certain components. For instance, SQL layer involves “SQL MVC”, “SQL optimizer” and many more. However, not all the components need to belong to a single layer. There are components that target the whole stack, such as memory allocation, I/O and threads or MonetDB modules like lidar, fits and bam, that fall under a more general category. Tracer introduces the concept of “component logging” where each component can be in a different logging level. The list of the available components is the following:
ACCELERATOR
ALGO
ALLOC
BAT_
CHECK_
DELTA
HEAP
IO_
PAR
PERF
TEM
THRD
GEOM
FITS
SHP
SQL_PARSER
SQL_TRANS
SQL_REWRITER
SQL_EXECUTION
SQL_STORE
MAL_WLC
MAL_REMOTE
MAL_MAPI
MAL_SERVER
MAL_OPTIMIZER
GDK
Tracer provides a way for
In the following list you can find the available layers that Tracer supports:
SQL_ALL Sets SQL layer
MAL_ALL Sets MAL layer
GDK_ALL Sets GDK layer
MDB_ALL Sets all the components (MonetDB codebase)
Tracer introduces the concept of adapters in MonetDB stack. Simply, an adapter determines where the logging messages will be output. MonetDB, for now, supports only one adapter, but more will come in the future. Below you can find the supported adapters along with a brief explanation.
BASIC: Writes all the logging messages produced to the log file “mdbtrace.log”. The file is created by default in the DB path directory, however, as it’s mentioned later on you have the option to specify the desired path on startup.
Tracer is compatible with logrotate utility. Sending a SIGHUP to “mserver5” will cause Tracer to reopen “mdbtrace.log” if the BASIC adapter is used.
Using directly mserver5: In order to use a different directory than the DB path in order to store the produced traces, you can simply start mserver with the dbtrace option mserver5 --dbtrace=<path>
Using merovingian (monetdbd): In order to use a different directory than the DB path in order to store the produced traces, you can use ‘monetdb’ program and set the variable ‘dbtrace’. For instance: monetdb set dbtrace /tmp/mydb_trace.log mydb
Tracer comes with a number of possible SQL calls at the user level. Using the mclient program, you can simply execute ‘CALL logging.*(…)’ in order to interact with the logging system.
logging.flush()
Tracer uses a buffer under the hood where it stores all the logs until reaching 64KB. After that point, a flush buffer operation is performed automatically. However, in case your messages remain in the buffer because e.g: you have too few logging messages, you can always perform this call in order to flush the buffer explicitly. The log messages are getting outputted to the active adapter at that moment.
logging.setcomplevel(str comp, str level)
Sets the log level for a specific component.
logging.resetcomplevel(str comp)
Resets the log level for a specific component back to the default (ERROR).
logging.setlayerlevel(str layer, str level)
Sets the log level for a specific layer
logging.resetlayerlevel(str layer)
Resets the log level for a specific layer back to the default (ERROR).
logging.setflushlevel(str level)
Sets the flush level. A flush level determines the importance of a certain logging level. When an important message comes to the logger which has the same logging level as the flush level, Tracer will be triggered to flush the buffer. Tracer by default will always decide to flush CRITICAL and ERROR messages no matter the flush level is set.
logging.resetflushlevel()
Resets the flush level back to the default (INFO).
logging.setadapter(str adapter)
Sets the adapter that Tracer is going to use in order to output the logging messages.
logging.resetadapter()
Resets the adapter back to the default (BASIC).
SELECT * FROM logging.compinfo();
Executing this query will trigger Tracer to dump all the available components along with their id and the logging level that has been set.
If you are a MonetDB developer/enthusiast and you would like to contribute in the logging system of MonetDB by either introducing new layers, components or logging levels, then ‘gdk/gdk_tracer.h’ and ‘gdk/gdk_tracer.c’ are the files you are looking for!
In most cases, the system produces informative error messages. However, there are situations where MonetDB enters an area not covered by the test suite or previous use and a segmentation fault occurs. These cases are hard to analyze outside the development lab. To isolate and resolve the issue we need at least the following information.
Sent us the result of the command
mserver5 --version --dbname=<databasename>
or the equivalent using
monetdb --version <databasename>
Is the error reproducible with a small (5-10 line) script/query/program? Trim your experiment to the minimal size that demonstrates the erroneous behavior. Such a script is the best we can hope for, because it will end up in the nightly testing.
In addition, follow the steps, assuming that you are logged onto the same Linux (!) machine as where the server will run:
monetdb or as a full command line mserver --dbpath="..."ps -al |grep mserver5gdb mserver5 <processid>where. It shows the context of the crash using the list command and the variables used in the context of the crash.thr app all bt.