MariaDB MaxScale plugin development guide
This document and the attached example code explain prospective plugin developers the MariaDB MaxScale plugin API and also present and explain some best practices and possible pitfalls in module development. We predict that filters and routers are the module types developers are most likely to work on, so the APIs of these two are discussed in detail.
Table of Contents
- Module categories
- Common definitions and headers
- Module information container
- Module API
- General module management
- Filter and Router
- Compiling, installing and running
- Hands-on example: RoundRobinRouter
- Summary and conclusion
MariaDB MaxScale is designed to be an extensible program. Much, if not most, of the actual processing is done by plugin modules. Plugins receive network data, process it and relay it to its destination. The MaxScale core loads plugins, manages client sessions and threads and, most importantly, offers a selection of functions for the plugins to call upon. This collection of functions is called the MaxScale Public Interface or just MPI for short.
The plugin modules are shared libraries (.so-files) implementing a set of interface functions, the plugin API. Different plugin types have different APIs, although there are similarities. The MPI is a set of C and C++ header files, from which the module code includes the ones required. MariaDB MaxScale is written in C/C++ and the plugin API is in pure C. Although it is possible to write plugins in any language capable of exposing a C interface and dynamically binding to the core program, in this document we assume plugin modules are written in C++.
The RoundRobinRouter is a practical example of a simple router plugin. The
RoundRobinRouter is compiled, installed and ran in section
5.1. The source for the router is located
This section lists all the module types and summarises their core tasks. The
modules are listed in the order a client packet would typically travel through.
For more information about a particular module type, see the corresponding
MaxScale/Documentation/, located in the main MariaDB MaxScale
Protocol modules implement I/O between clients and MaxScale, and between
MaxScale and backend servers. Protocol modules read and write to socket
descriptors using raw I/O functions provided by the MPI, and implement
protocol-specific I/O functions to be used through a common interface. The
Protocol module API is defined in
protocol.h. Currently, the only implemented
database protocol is MySQL. Other protocols currently in use include HTTPD
and maxscaled, which are used by the MaxInfo and MaxAdmin modules.
Authenticator modules retrieve user account information from the backend
databases, store it and use it to authenticate connecting clients. MariaDB
MaxScale includes authenticators for MySQL (normal and GSSApi). The
authenticator API is defined in
Filter modules process data from clients before routing. A data buffer may
travel through multiple filters before arriving in a router. For a data buffer
going from a backend to the client, the router receives it first and the
filters receive it in reverse order. MaxScale includes a healthly selection of
filters ranging from logging, overwriting query data and caching. The filter
API is defined in
Router modules route packets from the last filter in the filter chain to
backends and reply data from backends to the last filter. The routing decisions
may be based on a variety of conditions; typically packet contents and backend
status are the most significant factors. Routers are often used for load
balancing, dividing clients and even individual queries between backends.
Routers use protocol functions to write to backends, making them somewhat
protocol-agnostic. The router API is defined in
Monitor modules do not process data flowing through MariaDB MaxScale, but
support the other modules in their operation by updating the status of the
backend servers. Monitors are ran in their own threads to minimize
interference to the worker threads. They periodically connect to all their
assigned backends, query their status and write the results in global structs.
The monitor API is defined in
Common definitions and headers
Generally, most type definitions, macros and functions exposed by the MPI to be used by modules are prefixed with MXS. This should avoid name collisions in the case a module includes many symbols from the MPI.
Every compilation unit in a module should begin with
"<name>". This definition will be used by log macros for clarity, prepending
<name> to every log message. Next, the module should
#include <maxscale/cppdefs.h> (for C++) or
#include <maxscale/cdefs.h> (for
C). These headers contain compilation environment dependent definitions and
global constants, and include some generally useful headers. Including one of
them first in every source file enables later global redefinitions across all
MaxScale modules. If your module is composed of multiple source files, the above
should be placed to a common header file included in the beginning of the source
files. The file with the module API definition should also include the header
for the module type, e.g.
Other common MPI header files required by most modules are listed in the table below.
||Malloc, calloc etc. replacements|
||Packet buffer management|
||I/O using descriptor control blocks|
||Module information structure|
||Backend server information|
||Client session definition|
||Logging macros and functions|
Module information container
A module must implement the
MXS_CREATE_MODULE()-function, which returns a
pointer to a
MXS_MODULE-structure. This function is called by the module
loader during program startup.
MXS_MODULE (type defined in
contains function pointers to further module entrypoints, miscellaneous
information about the module and the configuration parameters accepted by the
module. This function must be exported without C++ name mangling, so in C++ code
it should be defined
The information container describes the module in general and is constructed once during program excecution. A module may have multiple instances with different values for configuration parameters. For example, a filter module can be used with two different configurations in different services (or even in the same service). In this case the loader uses the same module information container for both but creates two module instances.
The MariaDB MaxScale configuration file
maxscale.cnf is parsed by the core.
The core also checks that all the defined parameters are of the correct type for
the module. For this, the
MXS_MODULE-structure includes a list of parameters
accepted by the module, defining parameter names, types and default values. In
the actual module code, parameter values should be extracted using functions
This section explains some general concepts encountered when implementing a module API. For more detailed information, see the module specific subsection, header files or the doxygen documentation.
Modules with configuration data define an INSTANCE object, which is created by
the module code in a
createInstance-function or equivalent. The instance
creation function is called during MaxScale startup, usually when creating
services. MaxScale core holds the module instance data in the
SERVICE-structure (or other higher level construct) and gives it as a
parameter when calling functions from the module in question. The instance
structure should contain all non-client-specific information required by the
functions of the module. The core does not know what the object contains (since
it is defined by the module itself), nor will it modify the pointer or the
referenced object in any way.
Modules dealing with client-specific data require a SESSION object for every
client. As with the instance data, the definition of the module session
structure is up to the module writer and MaxScale treats it as an opaque type.
Usually the session contains status indicators and any resources required by the
client. MaxScale core has its own
MXS_SESSION object, which tracks a variety
of client related information. The
MXS_SESSION is given as a parameter to
module-specific session creation functions and is required for several typical
operations such as connecting to backends.
Descriptor control blocks (
DCB), are generalized I/O descriptor types. DCBs
store the file descriptor, state, remote address, username, session, and other
data. DCBs are created whenever a new socket is created. Typically this happens
when a new client connects or MaxScale connects the client session to backend
servers. The module writer should use DCB handling functions provided by the MPI
to manage connections instead of calling general networking libraries. This
ensures that I/O is handled asynchronously by epoll. In general, module code
should avoid blocking I/O, sleep, yield or other potentially costly
operations, as the same thread is typically used for many client sessions.
Network data such as client queries and backend replies are held in a buffer
GWBUF. Multiple GWBUFs can form a linked list with type
information and properties in each GWBUF-node. Each node includes a pointer to a
reference counted shared buffer (
SHARED_BUF), which finally points to a slice
of the actual data. In effect, multiple GWBUF-chains can share some data while
keeping some parts private. This construction is meant to minimize the need for
data copying and makes it easy to append more data to partially received data
packets. Plugin writers should use the MPI to manipulate GWBUFs. For more
information on the GWBUF, see Filter and Router.
General module management
int process_init() void process_finish() int thread_init() void thread_finish()
These four functions are present in all
MXS_MODULE structs and are not part of
the API of any individual module type.
called by the module loader right after loading a module and just before
MaxScale terminates, respectively. Usually, these can be set to null in
MXS_MODULE unless the module needs some general initializations before
creating any instances.
thread_finish are thread-specific
void diagnostics(INSTANCE *instance, DCB *dcb)
A diagnostics printing routine is present in nearly all module types, although
with varying signatures. This entrypoint should print various statistics and
status information about the module instance
instance in string form. The
target of the printing is the given DCB, and printing should be implemented by
dcb_printf. The diagnostics function is used by the MaxInfo and
int32_t read(struct dcb *) int32_t write(struct dcb *, GWBUF *) int32_t write_ready(struct dcb *) int32_t error(struct dcb *) int32_t hangup(struct dcb *) int32_t accept(struct dcb *) int32_t connect(struct dcb *, struct server *, struct session *) int32_t close(struct dcb *) int32_t listen(struct dcb *, char *) int32_t auth(struct dcb *, struct server *, struct session *, GWBUF *) int32_t session(struct dcb *, void *) char auth_default() int32_t connlimit(struct dcb *, int limit)
Protocol modules are laborous to implement due to their low level nature. Each DCB maintains pointers to the correct protocol functions to be used with it, allowing the DCB to be used in a protocol-independent manner.
hangup are epoll handlers for their
write implements writing and is usually called in a router
accept is a listener socker handler.
connect is used during session
creation when connecting to backend servers.
listen creates a listener socket.
close closes a DCB created by
In the ideal case modules other than the protocol modules themselves should not be protocol-specific. This is currently difficult to achieve, since many actions in the modules are dependent on protocol-speficic details. In the future, protocol modules may be expanded to implement a generic query parsing and information API, allowing filters and routers to be used with different SQL variants.
void* initialize(char **options) void* create(void* instance) int extract(struct dcb *, GWBUF *) bool connectssl(struct dcb *) int authenticate(struct dcb *) void free(struct dcb *) void destroy(void *) int loadusers(struct servlistener *) void diagnostic(struct dcb*, struct servlistener *) int reauthenticate(struct dcb *, const char *user, uint8_t *token, size_t token_len, uint8_t *scramble, size_t scramble_len, uint8_t *output, size_t output_len);
Authenticators must communicate with the client or the backends and implement authentication. The authenticators can be divided to client and backend modules, although the two types are linked and must be used together. Authenticators are also dependent on the protocol modules.
Filter and Router
Filter and router APIs are nearly identical and are presented together. Since these are the modules most likely to be implemented by plugin developers, their APIs are discussed in more detail.
INSTANCE* createInstance(SERVICE* service, char** options) void destroyInstance(INSTANCE* instance)
createInstance should read the
options and initialize an instance object for
service. Often, simply saving the configuration values to fields is
destroyInstance is called when the service using the module is
deallocated. It should free any resources claimed by the instance. All sessions
created by this instance should be closed before calling the destructor.
SESSION* newSession(INSTANCE* instance, MXS_SESSION* mxs_session) void closeSession(INSTANCE* instance, SESSION* session) void freeSession(INSTANCE* instance, SESSION* session)
These functions manage sessions.
newSession should allocate a router or filter
session attached to the client session represented by
will pass the returned pointer to all the API entrypoints that process user data
for the particular client.
closeSession should close connections the session
has opened and release any resources specific to the served client. The
SESSION structure allocated in
newSession should not be deallocated by
closeSession but in
freeSession. These two are called in succession
by the core.
int routeQuery(INSTANCE *instance, SESSION session, GWBUF* queue) void clientReply(INSTANCE* instance, SESSION session, GWBUF* queue, DCB *backend_dcb) uint64_t getCapabilities(INSTANCE* instance)
routeQuery is called for client requests which should be routed to backends,
clientReply for backend reply packets which should be routed to the
client. For some modules, MaxScale itself is the backend. For filters, these can
be NULL, in which case the filter will be skipped for that packet type.
routeQuery is often the most complicated function in a router, as it
implements the routing logic. It typically considers the client request
the router settings in
instance and the session state in
session when making
a routing decision. For filters aswell,
routeQuery typically implements the
main logic, although the routing target is constant. For router modules,
routeQuery should send data forward with
dcb->func.write(). Filters should
routeQuery for the next filter or router in the chain.
clientReply processes data flowing from backend back to client. For routers,
this function is often much simpler than
routeQuery, since there is only one
client to route to. Depending on the router, some packets may not be routed to
the client. For example, if a client query was routed to multiple backends,
MaxScale will receive multiple replies while the client only expects one.
Routers should pass the reply packet to the last filter in the chain (reversed
order) using the macro
MXS_SESSION_ROUTE_REPLY. Filters should call the
clientReply of the previous filter in the chain. There is no need for filters
to worry about being the first filter in the chain, as this is handled
transparently by the session creation routine.
Application data is not always received in complete packets from the network
stack. How partial packets are handled by the receiving protocol module depends
on the attached filters and the router, communicated by their
getCapabilities should return a bitfield
resulting from ORring the individual capabilities.
routing.h lists the allowed
If a router or filter sets no capabilities,
clientReply may be
called to route partial packets. If the routing logic does not require any
information on the contents of the packets or even tracking the number of
packets, this may be fine. For many cases though, receiving a data packet in a
complete GWBUF chain or in one contiguos GWBUF is required. The former can be
getCapabilities returning RCAP_TYPE_STMT, the latter by
RCAP_TYPE_CONTIGUOUS. Separate settings exist for queries and replies. For
replies, an additional value, RCAP_TYPE_RESULTSET_OUTPUT is defined. This
requests the protocol module to gather partial results into one result set.
Enforcing complete packets will delay processing, since the protocol module will
have to wait for the entire data packet to arrive before sending it down the
void handleError(INSTANCE* instance,SESSION* session, GWBUF* errmsgbuf, DCB* problem_dcb, mxs_error_action_t action, bool* succp);
This router-only entrypoint is called if
routeQuery returns an error value or
if an error occurs in one of the connections listened to by the session. The
steps an error handler typically takes depend on the nature of the
and the error encountered. If
problem_dcb is a client socket, then the session
is lost and should be closed. The error handler should not do this by itself and
just report the failure by setting
succp to false. If
problem_dcb is a
backend socket, then the error handler should try to connect to another backend
if the routing logic allows this. If the error is simply a failed authentication
on the backend, then it is usually best to send the message directly to the
client and close the session.
MONITOR* startMonitor(MXS_MONITOR *monitor, const MXS_CONFIG_PARAMETER *params) void stopMonitor(MXS_MONITOR *monitor) void diagnostics(DCB *, const MXS_MONITOR *)
Monitor modules typically run a repeated monitor routine with a used defined
MXS_MONITOR is a standard monitor definition used for all
monitors and contains a void pointer for storing module specific data.
startMonitor should create a new thread for itself using functions in the MPI
and have it regularly run a monitor loop. In the beginning of every monitor
loop, the monitor should lock the
SERVER-structures of its servers. This
prevents any administrative action from interfering with the monitor during its
Compiling, installing and running
The requirements for compiling a module are: The public headers (MPI) A compatible compiler, typically GCC * Libraries required by the public headers
Some of the public header files themselves include headers from other libraries.
These libraries need to be installed and it may be required to point out their
location to gcc. Some of the more commonly required libraries are:
* MySQL Connector-C, used by the MySQL protocol module
* pcre2 regular expressions (libpcre2-dev), used for example by the header
After all dependencies are accounted for, the module should compile with a command similar to
gcc -I /usr/local/include/mariadb -shared -fPIC -g -o libmymodule.so mymodule.cpp
Large modules composed of several source files and using additional libraries may require a more complicated compilation scheme, but that is outside the scope of this document. The result of compiling a plugin should be a single shared library file.
The compiled .so-file needs to be copied to the MaxScale library folder, which
/usr/local/lib/maxscale by default. MaxScale expects the filename to be
<name> must match the module name given in the
Hands-on example: RoundRobinRouter
In this example, the RoundRobinRouter is compiled, installed and tested. The software environment this section was written and tested is listed below. Any recent Linux setup should be applicaple.
- Linux Mint 18
- gcc 5.4.0, glibc 2.23
- MariaDB MaxScale 2.1.0 debug build (binaries in
usr/local/maxscale, modules in
- MariaDB Connector-C 2.3.2 (installed to
/usr/local/lib/mariadb, headers in
roundrobinrouter.cppin the current directory
- MaxScale plugin development headers (in
Step 1 Compile RoundRobinRouter with
$gcc -I /usr/local/include/mariadb
-shared -fPIC -g -o libroundrobinrouter.so roundrobinrouter.cpp.
Assuming all headers were found, the shared library
Step 2 Copy the compiled module to the MaxScale module directory:
Step 3 Modify the MaxScale configuration file to use the RoundRobinRouter as a router. Example service and listener definitions are below. The servers and write_backend-lines should be configured according to the actual backend configuration.
[RR Service] type=service router=roundrobinrouter servers=LocalMaster1,LocalSlave1,LocalSlave2 user=maxscale passwd=maxscale filters=MyLogFilter1 max_backends=10 write_backend=LocalMaster1 print_on_routing=true dummy_setting=two [RR Listener] type=listener service=RR Service protocol=MySQLClient port=4009
Step 4 Start MaxScale:
$ maxscale -d. Output:
MariaDB Corporation MaxScale 2.1.0 Mon Feb 20 17:22:18 2017 ------------------------------------------------------ Info : MaxScale will be run in the terminal process. See the log from the following log files : Configuration file : /etc/maxscale.cnf Log directory : /var/log/maxscale Data directory : /var/lib/maxscale Module directory : /usr/local/lib/maxscale Service cache : /var/cache/maxscale
Step 5 Test with a MySQL client. The RoundRobinRouter has been tested with both a command line and a GUI client. With
DEBUG_RRROUTER defined and
print_on_routing enabled, the
/var/log/maxscale/maxscale.log file will report nearly every action taken by the router.
2017-02-21 10:37:23 notice : [RoundRobinRouter] Creating instance. 2017-02-21 10:37:23 notice : [RoundRobinRouter] Settings read: 2017-02-21 10:37:23 notice : [RoundRobinRouter] 'max_backends': 10 2017-02-21 10:37:23 notice : [RoundRobinRouter] 'write_backend': 0xf0ce70 2017-02-21 10:37:23 notice : [RoundRobinRouter] 'print_on_routing': 1 2017-02-21 10:37:23 notice : [RoundRobinRouter] 'dummy_setting': 2 . . . 2017-02-21 10:37:37 notice : [RoundRobinRouter] Session with 4 connections created. 2017-02-21 10:37:37 notice : [RoundRobinRouter] QUERY: SHOW VARIABLES WHERE Variable_name in ('max_allowed_packet', 'system_time_zone', 'time_zone', 'sql_mode') 2017-02-21 10:37:37 notice : [RoundRobinRouter] Routing statement of length 110u to backend 'LocalMaster1'. 2017-02-21 10:37:37 notice : [RoundRobinRouter] Replied to client. 2017-02-21 10:37:37 notice : [RoundRobinRouter] QUERY: set session autocommit=1,sql_mode='NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES' 2017-02-21 10:37:37 notice : [RoundRobinRouter] Routing statement of length 103u to 4 backends. 2017-02-21 10:37:37 notice : [RoundRobinRouter] Replied to client. 2017-02-21 10:37:37 notice : [RoundRobinRouter] QUERY: SET @ApplicationName='DBeaver 3.8.5 - Main' 2017-02-21 10:37:37 notice : [RoundRobinRouter] Routing statement of length 48u to 4 backends. 2017-02-21 10:37:37 notice : [RoundRobinRouter] Replied to client. 2017-02-21 10:37:37 notice : [RoundRobinRouter] QUERY: select @@lower_case_table_names 2017-02-21 10:37:37 notice : [RoundRobinRouter] Routing statement of length 36u to backend 'LocalSlave1'. 2017-02-21 10:37:37 notice : [RoundRobinRouter] Replied to client.
Step 5 Connect with MaxAdmin, print diagnostics and call a custom command.
$sudo maxadmin MaxScale> show service "RR Service" Service: RR Service Router: roundrobinrouter State: Started Queries routed successfully: 37 Failed routing attempts: 0 Client replies routed: 38 Started: Tue Feb 21 11:52:08 2017 Root user access: Disabled Filter chain: MyLogFilter1 Backend databases: 127.0.0.1:3001 Protocol: MySQLBackend Name: LocalMaster1 127.0.0.1:3002 Protocol: MySQLBackend Name: LocalSlave1 127.0.0.1:3003 Protocol: MySQLBackend Name: LocalSlave2 Total connections: 2 Currently connected: 2 MaxScale> call command rrrouter test_command "one" 0
The result of the
test_command "one" 0 is printed to the terminal MaxScale is
RoundRobinRouter wishes the Admin a good day. The module got 2 arguments. Argument 0: type 'string' value 'one' Argument 1: type 'boolean' value 'false'
Summary and conclusion
Plugins offer a way to extend MariaDB MaxScale whenever the standard modules are found insufficient. The plugins need only implement a set API, can be independently compiled and installation is simply a file copy with some configuration file modifications.
Out of the different plugin types, filters are the easiest to implement. They work independently and have few requirements. Protocol and authenticator modules require indepth knowledge of the database protocol they implement. Router module complexity depends on the routing logic requirements.
The provided RoundRobinRouter example code should serve as a valid starting point for both filters and routers. Studying the MaxScale Public Interface headers to get a general idea of what services the core provides for plugins, is also highly recommeded.
Lastly, MariaDB MaxScale is an open-source project, so code contributions can be accepted if they fulfill the requirements.