Queries are an asynchronous means within a HELICS federation of asking for and receiving information from other federate components. A query provides the ability to evaluate the current state of a federation and typically addresses the configuration and architecture of the federation. Brokers, Federates, and Cores all have query functions. Federates are also able to define a callback for answering custom queries.

The general function looks like this:


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


query_result = h.helicsCreateQuery(traget_string, query_string)


Each query must define a “target”, the component in the federation that is being queried. The target is either specified in terms of the relationship to the querying federate (e.g. “broker”, “core”) or by name of the federation component (e.g. “dist_system_1_fed”). The table below lists the valid query targets; if a federate happens to be named one of the target names listed below, it will not be queries by that name. For example, naming one of your brokers “broker” will prevent it being a valid target of a query by name. Instead, any federate that queries “broker” will end up targeting their broker.

Query String

The queryStr is the specific data being requested; the tables below show the valid data provided by each queryable federation component. Invalid queries or targets return a json object “error” containing a code and an error message. Answers to queries can be (in C++ data types):

  • true/false [T/F]
  • a single quoted string "answer" [string]
  • a vector of quoted strings delimited by ',' ["answer1","answer"","answer3"] [sv] this is a JSON compliant string vector
  • a JSON string with an object [JSON]

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.

queryString Description
name the identifier of the federate [string]
exists Basic query if the federate exists in the Federation [T/F]
isinit If the federate has entered init mode [T/F]
state Current state of the federate as a string [string]
global_state Current state of the federate as a string [JSON]
publications current publications of a federate [sv]
subscriptions current subscriptions of a federate [sv]
inputs current inputs of a federate [sv]
endpoints current endpoints of a federate [sv]
dependencies list of the objects this federate depends on [sv]
dependents list of dependent objects [sv]
current_time the current time of the federate [JSON]
endpoint_filters data structure containing the filters on endpoints[JSON]
dependency_graph a graph of the dependencies in a federation [JSON]
data_flow_graph a structure with all the data connections [JSON]
queries list of available queries [sv]
version the version string of the helics library [string]

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, that is, the federate making the query. Federates may specify a callback function which allows arbitrary user defined Queries.

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]

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:

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.

error codes returned by the query follow http error codes for “Not Found (404)” or “Resource Not Available (400)” or “Server Failure (500)”.

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 omits the “target” specification and always queries the local federate.

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

There is also an asyncrhonous version (that is, non-blocking) that returns a query_id_t that can be use in queryComplete and isQueryComplete functions.

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

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 have the same valid targets and properties that can be queried but the construction of the query is slightly different. The basic operation is to create a query using helicsQueryCreate(target,query). Once created, the target or query string can be changed with helicsQuerySetTarget() and helicsQuerySetQueryString(), respectively.

This function returns a query object that can be used in one of the execute functions (helicsQueryExecute(), helicsQueryExecuteAsync(), helicsQueryBrokerExecute(), helicsQueryCoreExecute(), to perform the query and receive back results. The query 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 helicsQueryFree().


A full co-simulation example showing how queries can be used for dynamic configuration can can be found here (with the source code in the HELICS Examples repository).