DDS11 — (R#126) Correction to DataWriter blocking behavior

The DDS spec currently states that the max_blocking_time parameter of the RELIABILITY QoS only applies for data writers that are RELIABLE and have HISTORY QoS of KEEP_ALL.

These assertions are not true. Depending on the RESOURCE_LIMITS QoS, even a KEEP_LAST writer may eventually need to block.

Proposed Resolution:

The specification needs to be updated in the table of QoS in Section 2.1.3 and in the DataWriter section 2.1.2.4.2.10 for write to account for the case in which (max_samples < max_instances * HISTORY depth). In this case, the writer may attempt to write a new value for an existing instance whose history is not full and fail because it exceeds the max_samples limit. Therefore, if (max_samples < max_instances * HISTORY depth), then in the situation where the max_samples resource limit is exhausted the middleware is allowed to discard samples of some other instance as long as at least one sample remains for that instance. If it is still not possible to make space available for the new sample, the writer is allowed to block.

The behavior in the case where max_samples < max_instances must also be described. In that case the writer is allowed to block.

Proposed Revised Text:

In the QoS table in 2.1.3, change the first sentence of the "Meaning" cell of the RELIABILITY max_blocking_time row to: "This setting applies only to the case where kind=RELIABLE."

In section 2.1.2.4.2.10, replace the final paragraph with the following:

If the RELIABILITY kind is set to RELIABLE, the write operation on the DataWriter may block if the modification would cause data to be lost or else cause one of the limits specified in the RESOURCE_LIMITS to be exceeded. Under these circumstances, the RELIABILITY max_blocking_time configures the maximum time the write operation may block waiting for space to become available. If max_blocking_time elapses before the DataWriter is able to store the modification without exceeding the limits, the write operation will fail and return TIMEOUT.

Specifically, the DataWriter may block in the following situations (although the list may not be exhaustive), even if its HISTORY kind is KEEP_LAST.

· If (RESOURCE_LIMITS max_samples < RESOURCE_LIMITS max_instances * HISTORY depth), then in the situation where the max_samples resource limit is exhausted the Service is allowed to discard samples of some other instance as long as at least one sample remains for such an instance. If it is still not possible to make space available to store the modification, the writer is allowed to block.

· If (RESOURCE_LIMITS max_samples < RESOURCE_LIMITS max_instances), then the DataWriter may block regardless of the HISTORY depth.

In section 2.1.3.13 RELIABILITY, the second paragraph currently states:

The setting of this policy has a dependency on the setting of the HISTORY and RESOURCE_LIMITS policies. In case the RELIABILITY kind is set to RELIABLE and the HISTORY kind set to KEEP_ALL the write operation on the DataWriter may block if the modification would cause data to be lost or else cause one of the limits specified in the RESOURCE_LIMITS to be exceeded.

The above text should be rewritten as follows:

The setting of this policy has a dependency on the RESOURCE_LIMITS policy. In case the RELIABILITY kind is set to RELIABLE the write operation on the DataWriter may block if the modification would cause data to be lost or else cause one of the limits specified in the RESOURCE_LIMITS to be exceeded.

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