Total sample cap set below the per-instance cap

Rule 2 · applies to publishers and subscribers · Back to all rules

Won't connect. The two limits contradict each other, and Cyclone DDS rejects the endpoint at creation.

If you set Resource Limits max_samples = N together with max_samples_per_instance greater than N

Won't connect

• Settings involved: Resource Limits and Resource Limits
• What QoS Guard checks: [max_samples < max_samples_per_instance]

Example

max_samples 5 with max_samples_per_instance 10 is impossible, because a single instance alone could exceed the total budget.

How to fix it

Set max_samples greater than or equal to max_samples_per_instance.

Why this rule is flagged

What the DDS specification says

OMG DDS §	QoS policy	Standard statement
§2.2.3.19	RESOURCE_LIMITS	The setting of RESOURCE_LIMITS max_samples must be consistent with the max_samples_per_instance. For these two values to be consistent they must verify that max_samples >= max_samples_per_instance.

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What the engine source code shows

Fast DDS checks RESOURCE_LIMITS internal consistency in check_allocation_consistency. When max_samples < max_instances * max_samples_per_instance, creation returns RETCODE_INCONSISTENT_POLICY.

Fast DDS implementation evidence

// rule 2: check_allocation_consistency
if ((qos.resource_limits().max_samples > 0) &&
        (qos.resource_limits().max_samples <
        (qos.resource_limits().max_instances * qos.resource_limits().max_samples_per_instance)))
{
    EPROSIMA_LOG_ERROR(DDS_QOS_CHECK,
        "max_samples should be greater than max_instances * max_samples_per_instance");
    return ReturnCode_t::RETCODE_INCONSISTENT_POLICY;
}

Source: DataReaderImpl.cpp (lines 1523–1552).

Cyclone DDS rejected the violating RESOURCE_LIMITS profile at creation time in the tested cases. Fast DDS accepted the same violating profiles in the tested versions.

Item	Value
Dataset	Download CSV
Fixed QoS setting	RESLIM.mpi = 4
Tested variable	RESLIM.max_samples
Tested values	{2, 3, 4, 5}
Valid / boundary cases	{4, 5}
Violating cases	{2, 3}
Tested engines / versions	Fast DDS 2.6.11 (Humble), Fast DDS 2.14.6 (Jazzy), Cyclone DDS 0.10.5
Network setting	RTT = 1 ms, loss = 0%, PP = 50 ms, message size = 1024 B

Engine	Tested setting	Observed behavior
Fast DDS 2.6.11 (Humble)	max_samples < RESLIM.mpi	Profile accepted at creation time
Fast DDS 2.14.6 (Jazzy)	max_samples < RESLIM.mpi	Profile accepted at creation time
Cyclone DDS 0.10.5	max_samples < RESLIM.mpi	Profile rejected at creation time

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What you'll see when you run it

QoS Guard flags this as a structural conflict:

[STRUCTURAL] [state_sub] [SUB] max_samples(5) <-> max_samples_per_instance(10)

At runtime the two limits contradict each other, because a single instance is allowed 10 samples while the whole reader is capped at 5. Cyclone DDS rejects the endpoint at creation, and Fast DDS returns an inconsistent-policy error from its own consistency check. Either way the reader never starts, so no data is delivered.