Odbxx

A C++11 object-oriented interface to the ODB (online database) was introduced in May 2020. You can think of it like a "magic" map/dictionary that automatically sends changes you make to the ODB, and receives updates that others have made.

The header for this odbxx interface is at odbxx.h and example usage in [1].

You can find more details about the ODB on the ODB Access and Use page, which includes links to the command-line, javascript, python, and non-object C++ interfaces.

Basic usage

The simplest usage is like:

// Grab a bit of the ODB
midas::odb exp("/Experiment");

// Simple read
std::cout << "The current transition timeout is " << exp["Transition timeout"] << std::endl;

// Make a change. The new value is automatically sent to the ODB.
// Most C++ operators are supported (++, += etc), or you can do a simple
// re-assignment like `exp["Transition timeout"] = 12345;`.
exp["Transition timeout"] += 100;

// Read the new value
std::cout << "The transition timeout is now " << exp["Transition timeout"] << std::endl;

You can automatically cast to regular data types (int, double) etc if you want a copy of the value to work with:

int curr_timeout = exp["Transition timeout"];

Note: The ODB directory you connect to ("/Experiment" in the above example), has to start with a "/" to tell the midas::odb object to connect directly to the ODB. Otherwise, a simple local midas::odb string object gets created without any connection to the ODB.

Automatic refreshing

You may temporarily disable the automatic updating to/from the ODB using odb::set_auto_refresh_write(false) and odb::set_auto_refresh_read(false).

If auto-refresh is enabled (the default), your new values are sent to the ODB as soon as you touch the value in the midas::odb object. The ODB is queried for new values whenever you access the value. In the above example, the ODB is queried 4 times (during construction of exp, and each time exp["Transition timeout"] is mentioned), and written to 1 time (when exp["Transition timeout"] is assigned to).

See the #Callback functions section below for details on how to have a function called when a value changes.

Arrays/vectors

ODB arrays are represented by std vectors.

You can access/edit individual elements using []:

odb["Example"][1] = 1.2;

You can completely re-assign content using a std::vector or std::array:

std::vector<float> vec = std::vector<float>(10);
odb["Example"] = vec;

You can resize arrays using odb::resize(). If the existing array is longer, it will be truncated; if shorter it will be extended with default values (0 or an empty string).

odb["Example"].resize(5); // Now is 5 elements long

Note that arithmetic operators are supported for arrays, and will apply the operation to ALL ELEMENTS IN THE ARRAY:

// Create the vector
std::vector<float> vec = std::vector<float>(2);
vec[0] = 3;
vec[1] = 5;

// Assign in ODB
odb["Example"] = vec;

// Multiply ALL elements by 2
odb["Example"] *= 2;

// odb["Example"] now contains {6, 10}.

You can directly iterate over arrays/vectors:

// Iterating using standard begin/end.
for (auto it = o["Int Array"].begin(); it != o["Int Array"].end(); it++) {
   int val = *it;
   std::cout << val << std::endl;
}

// Iterating using C++11 range-based for loop.
for (int val : o["Int Array"]) {
   std::cout << val << std::endl;
}

Strings

Strings in the ODB are returned as std::string (unlike the midas.h db_get_value() family of functions, where strings are returned as char*). You may have vectors of strings.

Creating new bits of the ODB

You can automatically create bits of the ODB by passing a struct to the midas::odb constructor, then calling odb::connect(), like:

// Define the ODB structure
midas::odb o = {
   {"Int32 Key", 42},
   {"Bool Key", true},
   {"Subdir", {
      {"Float key", 1.2f},     // floats must be explicitly specified
   }},
   {"Int Array", {1, 2, 3}},
   {"Double Array", {1.2, 2.3, 3.4}},
   {"String Array", {"Hello1", "Hello2", "Hello3"}},
   {"Large Array", std::array<int, 10>{} },   // array with explicit size
   {"Large String", std::string(63, '\0') },  // string with explicit size
};

// Then sync the structure. This function
// - keeps the existing value of any keys that are in the ODB and your code
// - creates any keys that are in your code but not yet in the ODB
o.connect("/Test/Settings");

// If you make the `write_defaults` argument true, then the function
// - overwrites the value of any keys that are in the ODB with the value in your code
// - creates any keys that are in your code but not yet in the ODB
o.connect("/Test/Settings", true);

// The `connect_and_fix_structure()` method acts like the old db_check_record() function, and
// - keeps the existing value of any keys that are in the ODB and your code
// - creates any keys that are in your code but not yet in the ODB
// - deletes any keys that are in the ODB but not your code
// - updates the order of keys in the ODB to match your code
o.connect_and_fix_structure("/Test/Settings");

Note that the ODB path in teh odb::connect() call must start with a '/'. The ODB root path like o.connect("/"); is not allowed in this call.

If you want to add new keys to existing ODB subdirectories, you can also just use the [] operator:

midas::odb existing_key("/MyExistingKey");
existing_key["MyNewSubKey"] = 1.23;

You can also create new keys by providing a default value when reading a value. If the key doesn't already exist, the default value will be used.

midas::odb existing_key("/MyExistingKey");
double val = existing_key["MyNewSubKey"](1.23);

Iterating over subkeys

You can iterate over subkeys using normal iterator functions.

// Iterating using standard begin/end.
midas::odb exp("/Experiment");

for (auto it = exp.begin(); it != exp.end(); it++) {
   midas::odb& subkey = *it;
   std::cout << subkey.get_name() << " = " << subkey << std::endl;
}

// Iterating using C++11 range-based for loop.
for (midas::odb& subkey : exp) {
   std::cout << subkey.get_name() << " = " << subkey << std::endl;
}

You can check whether a subkey exists using odb::is_subkey().

Callback functions

You may also set up callback functions that are called whenever a value changes, using the odb::watch() function. Note that you must call cm_yield() (from midas.h) periodically for this to work - deep down it is cm_yield() itself that calls your callback function.

The callback functions can either be a "normal" function, a C++ lambda, or a member function of a C++ class. In all cases it should accept one argument - a midas::odb object (passed by reference) that contains the new state.

// Example with a lambda:
midas::odb to_watch("/Experiment");
to_watch.watch([](midas::odb &arg) {
   std::cout << "Value of key \"" + arg.get_full_path() + "\" changed to " << arg << std::endl;
});

// Example with a "normal" function:
void my_function(midas::odb &arg) {
   std::cout << "Value of key \"" + arg.get_full_path() + "\" changed to " << arg << std::endl;
}

midas::odb to_watch("/Experiment");
to_watch.watch(my_function);

// Example with a member function of a class:
#include <functional>

void MyClass::my_function(midas::odb& arg) {
   std::cout << "Value of key \"" + arg.get_full_path() + "\" changed to " << arg << std::endl;
}

void MyClass::some_initialisation_code() {
  // Arguments to std::bind() are: "member function to call", "object to call that function on", "placeholder for midas::odb arg"
  std::function<void(midas::odb&)> callback = std::bind(&MyClass::my_function, this, std::placeholders::_1);

  midas::odb to_watch("/Experiment");
  to_watch.watch(callback);
}

Utility functions

There are various utility functions which can be used:

void odb::create(const char *name, int type)

Simple wrapper around db_create_key() to create a single key in the ODB. type is one of TID_xxx.

void odb::delete_key()

This member function of a midas::odb object deletes that object from the ODB:

midas::odb o("/Some/ODB/Path");
o.delete_key();

int odb::delete_key(const std::string &name)

This function deletes a key or a subtree in the ODB passed by its path in the name argument. It is a simple wrapper around the C function db_delete_key() and returns the status of that function.

bool odb::exists(const std::string *name)

This boolean function checks if a key given by its name exists in the ODB.

void odb::exists(const std::string *name)

This boolean function checks if a key given by its name exists in the ODB.

void odb::set_debug(bool flag) / bool odb::get_debug()

These functions set and retrieve the debug flag. If the debug flag is true all communication with the ODB is printed to the screen. This can be helpful in debugging some problems.

Example code

A full working example exploring most of the features can be found in odbxx/odbxx_test.cxx. The test executable will be compiled as build/odbxx/odbxx_test (it is not installed in the `bin` directory).