Queries

Queries are asynchronous means within HELICS of asking for and receiving information from other federate components. Brokers, Federates, and Cores all have query functions. Federates are also able to define a callback for answering custom queries.

The general function appears like

std::string query(const std::string& target, const std::string& queryStr,
                      helics_sequencing_mode mode = helics_sequencing_mode_fast)

Targets

A target is specified, and can be one of the following. A federate named one of the key words is valid for the federation, but cannot be queried using the name.

target	Description
broker	The first broker encountered in the hierarchy from the caller
root, federation, rootbroker	The root broker of the federation
global	Retrieve the data associated with a global variable
parent	The parent of the caller
core	The core of a federation, this is not a valid target if called from a broker
federate	A query to the local federate or the first federate of a core
<object name>	any named object in the federation can also be queried, brokers, cores, and federates

Query String

The queryStr is a specific data to request, there are a number of different things that can be queried from the system. Unrecognized queries or targets return #invalid Answers to queries can be

• “true”/”false” [T/F]

• a single string "answer" [string]

• a vector of strings delimited by ';' [answer1;answer2;answer3] [sv]

• a JSON string [JSON]

sequencing_mode

As of HELICS 2.7.0 Queries have an optional parameter to describe a sequencing mode. There are currently two modes, helics_sequencing_mode_fast which travels along priority channels and is identical to previous versions in which all queries traveled along those channels. The other mode is helics_sequencing_mode_ordered which travels along lower priority channels but is ordered with all other messages in the system. This can be useful in some situations where you want previous messages to be acknowledged as part of the federation before the query is run. The global_flush query is forced to run in ordered mode at least until after it gets to the specified target.

Federate Queries

The following queries are defined for federates. Federates may specify a callback function which allows arbitrary user defined Queries. The queries defined here are available inside of HELICS.

The global_time_debugging and global_flush queries are also acknowledged by federates but it is not usually recommended to run those queries on a particular federate as they are more useful at higher levels. See the Core and Broker queries for more description of them.

Local Federate Queries

The following queries are defined for federates but can only be queried on the local federate. Federates may specify a callback function which allows arbitrary user defined Queries. The queries defined here are available inside of HELICS.

queryString	Description
updated_input_indices	vector of number of the inputs that have been updated [sv]
updated_input_names	names or targets of inputs that have been updated [sv]
updates	values of all currently updated inputs [JSON]
values	current values of all inputs [JSON]
time	the current granted time [string]

Other strings may be defined for specific federates.

Core queries

The following queries will be answered by a core.

The version and version_all queries are valid but are not usually queried directly, but instead the same query is used on a broker and this query in the core is used as a building block.

Broker Queries

The Following queries will be answered by a broker.

queryString	Description
name	the identifier of the broker [string]
address	the network address of the broker [string]
isinit	If the broker has entered init mode [T/F]
isconnected	If the broker is connected to the network [T/F]
publications	current publications known to a broker [sv]
endpoints	current endpoints known to a broker [sv]
federates	current federates under the brokers hierarchy [sv]
brokers	current cores/brokers connected to a broker [sv]
dependson	list of the objects this broker depends on [sv]
dependencies	structure containing dependency information for the broker [JSON]
dependents	list of dependent objects [sv]
counts	a simple count of the number of brokers, federates, and handles [JSON]
current_state	a structure with the current known status of the brokers and federates [JSON]
global_state	a structure with the current state all system components [JSON]
status	a structure with the current known status (true if connected) of the broker [JSON]
current_time	if a time is computed locally that time sequence is returned, otherwise #na [string]
global_time	get a structure with the current time status of all the federates/cores [JSON]
federate_map	a Hierarchical map of the federates contained in a broker [JSON]
dependency_graph	a representation of the dependencies in the broker and all contained members [JSON]
data_flow_graph	a representation of the data connections from all interfaces in a federation [JSON]
queries	list of dependent objects [sv]
version_all	data structure with the version strings of all broker components [JSON]
version	the version string for the helics library [string]
counter	A single number with a code, changes indicate federation changes [string]
global_time_debugging	return detailed time debugging state [JSON]
global_flush	a query that just flushes the current system and returns the id’s [JSON]
global_status	an aggregate query that returns a combo of global_time and current_state [JSON]

federate_map, dependency_graph, global_time,global_state,global_time_debugging, and data_flow_graph when called with the root broker as a target will generate a JSON string containing the entire structure of the federation. This can take some time to assemble since all members must be queried. global_flush will also force the entire structure along the ordered path which can be quite a bit slower.

Usage Notes

Queries that must traverse the network travel along priority paths unless specified otherwise with a sequencing mode. The calls are blocking, but they do not wait for time advancement from any federate and take priority over regular communication.

The difference between current_state and global_state is that current_state is generated by information contained in the component so doesn’t generate secondary queries of other components. Whereas global_state will reach out to the other components to get up to date information on the state.

Application API

There are two basic calls in the application API as part of a federate object In addition to the call described above a second version without the target

std::string     query(const std::string& queryStr)

make the query of the current federate. an asynchronous version is also available.

query_id_t     queryAsync(const std::string& target, const std::string& queryStr)

This call returns a query_id_t that can be use in queryComplete and isQueryComplet functions.

In the header <helics\queryFunctions.hpp> a few helper functions are defined to vectorize query results and some utility functions to wait for a federate to enter init, or wait for a federate to join the federation.

C-api and interface API’s

Queries in the C api work similarly but the mechanics are different. The basic operation is to create a query using helicsQueryCreate(target,query)

This function returns a query object that can be used in one of the execute functions to generate results. It can be called asynchronously on a federate. The target field may be empty if the query is intended to be used on a local federate, in which case the target is assumed to be the federate itself. A query must be freed after use. The interface api’s (python, matlab, octave, Java, etc) will work similarly.

Timeouts

As long as timeouts are enabled in the library itself, queries have a timeout system so they don’t block forever if a federate fails or some other condition occurs. The current default is 15 seconds. It can be changed by using the command line option --querytimeout on cores or brokers (or in --coreinitstring on cores). In a later version an ability to set this and some other timeout values through properties will likely be added (HELICS 3.1). If the query times out a value of #timeout will be returned in the string.