3. Configuration

3.1. DDS Recorder Configuration

A DDS Recorder is configured by a .yaml configuration file. This .yaml file contains all the information regarding the DDS interface configuration, recording parameters, and DDS Recorder specifications. Thus, this file has four major configuration groups:

  • dds: configuration related to DDS communication.

  • recorder: configuration of data writing in the database.

  • remote-controller: configuration of the remote controller of the DDS Recorder.

  • specs: configuration of the internal operation of the DDS Recorder.

3.1.1. DDS Configuration

Configuration related to DDS communication. Topic Filtering

DDS Recorder includes a mechanism to automatically detect which topics are being used in a DDS network. By automatically detecting these topics, a DDS Recorder creates internal DDS Readers for each topic in order to record the data published on each discovered topic.


DDS Recorder entities are created with the QoS of the first Publisher/Subscriber found in this Topic, unless manually set in the built-in topics list.

DDS Recorder allows filtering of DDS Topics, that is, it allows to define the DDS Topics’ data that is going to be recorded by the application. This way, it is possible to define a set of rules in DDS Recorder to filter those data samples the user does not wish to save.

It is not mandatory to define such set of rules in the configuration file. In this case, a DDS Recorder will save all the data published under the topics that it automatically discovers within the DDS network to which it connects.

To define these data filtering rules based on the Topics to which they belong, the following lists are available:

  • Allowed topics list (allowlist)

  • Block topics list (blocklist)

These lists of topics stated above are defined by a tag in the YAML configuration file, which defines a YAML vector ([]). This vector contains the list of topics for each filtering rule. Each Topic is determined by its entries name and type, with only the first one being mandatory.

Topic entries

Data type

Default value







See Topic section for further information about the topic.


Placing quotation marks around values in a YAML file is generally optional. However, values containing wildcard characters must be enclosed by single or double quotation marks. Allow topic list

This is the list of topics that DDS Recorder will record, i.e. the data published under the topics matching the expressions in the allowlist will be saved by DDS Recorder.


If no allowlist is provided, data will be recorded for all topics (unless filtered out in blocklist). Block topic list

This is the list of topics that the DDS Recorder will block, that is, all data published under the topics matching the filters specified in the blocklist will be discarded by the DDS Recorder and therefore will not be recorded.

This list takes precedence over the allowlist. If a topic matches an expression both in the allowlist and in the blocklist, the blocklist takes precedence, causing the data under this topic to be discarded.

Example of usage - Allowlist and blocklist collision:

In the following example, the HelloWorldTopic topic is both in the allowlist and (implicitly) in the blocklist, so according to the blocklist preference rule this topic is blocked. Moreover, only the topics present in the allowlist are relayed, regardless of whether more topics are dynamically discovered in the DDS network. In this case the forwarded topics are AllowedTopic1 with data type Allowed and AllowedTopic2 regardless of its data type.

  - name: AllowedTopic1
    type: Allowed
  - name: AllowedTopic2
    type: "*"
  - name: HelloWorldTopic
    type: HelloWorld

  - name: "*"
    type: HelloWorld Built-in Topics

Apart from the dynamic DDS topics discovered in the network, the discovery phase can be accelerated by using the builtin topic list (builtin-topics). By defining topics in this list, the DDS Recorder will create the DataWriters and DataReaders in recorder initialization.

The builtin-topics list is defined in the same form as the allowlist and blocklist.

This feature also allows to manually force the QoS of a specific topic, so the entities created in such topic follows the specified QoS and not the one first discovered. Topic Quality of Service

For every topic contained in this list, both name and type must be specified and contain no wildcard characters. Apart from these values, the tag qos under each topic allows to configure the following values:

Quality of Service

Yaml tag

Data type

Default value

QoS set











History Depth



default value





Topic with / without partitions










Topic with / without key




default value

Downsampling factor

Max Reception Rate



default value

Maximum sample reception rate [Hz]

Example of usage:

  - name: HelloWorldTopic
    type: HelloWorld
      reliability: true       # Use QoS RELIABLE
      durability: true        # Use QoS TRANSIENT_LOCAL
      depth: 100              # Use History Depth 100
      partitions: true        # Topic with partitions
      ownership: false        # Use QoS SHARED_OWNERSHIP_QOS
      keyed: true             # Topic with key
      downsampling: 4         # Keep 1 of every 4 samples
      max-reception-rate: 10  # Discard messages if less than 100ms elapsed since the last sample was processed DDS Domain

Tag domain configures the Domain Id.

domain: 101 Ignore Participant Flags

A set of discovery traffic filters can be defined in order to add an extra level of isolation. This configuration option can be set through the ignore-participant-flags tag:

ignore-participant-flags: no_filter                          # No filter (default)
# or
ignore-participant-flags: filter_different_host              # Discovery traffic from another host is discarded
# or
ignore-participant-flags: filter_different_process           # Discovery traffic from another process on same host is discarded
# or
ignore-participant-flags: filter_same_process                # Discovery traffic from own process is discarded
# or
ignore-participant-flags: filter_different_and_same_process  # Discovery traffic from own host is discarded

See Ignore Participant Flags for more information. Custom Transport Descriptors

By default, DDS Recorder internal participants are created with enabled UDP and Shared Memory transport descriptors. The use of one or the other for communication will depend on the specific scenario, and whenever both are viable candidates, the most efficient one (Shared Memory Transport) is automatically selected. However, a user may desire to force the use of one of the two, which can be accomplished via the transport configuration tag.

transport: builtin    # UDP & SHM (default)
# or
transport: udp        # UDP only
# or
transport: shm        # SHM only


When configured with transport: shm, DDS Recorder will only communicate with applications using Shared Memory Transport exclusively (with disabled UDP transport). Interface Whitelist

Optional tag whitelist-interfaces allows to limit the network interfaces used by UDP and TCP transport. This may be useful to only allow communication within the host (note: same can be done with Ignore Participant Flags). Example:

  - ""    # Localhost only

See Interface Whitelist for more information.


When providing an interface whitelist, external participants with which communication is desired must also be configured with interface whitelisting.

3.1.2. Recorder Configuration

Configuration of data writing in the database. Output File

The recorder output file does support the following configuration settings under the output tag:




Data type

Default value

File path


Configure the path to save the output file.



File name


Configure the name of the output file.



Timestamp format


Configure the format of the output file
timestamp (as in std::put_time).



Local timestamp


Whether to use a local or global (GMT)



When DDS Recorder application is launched (or when remotely controlled, every time a start/pause command is received while in SUSPENDED/STOPPED state), a temporary file with filename name (+timestamp prefix) and .mcap.tmp~ extension is created in path. This file is not readable until the application is terminated (or a suspend/stop/close command is received). On such event, the temporal file is renamed to have .mcap extension in the same location, and is then ready to be processed. Buffer size

buffer-size indicates the number of samples to be stored in the process memory before the dump to disk. This avoids disk access each time a sample is received. By default, its value is set to 100. Event Window

DDS Recorder can be configured to continue saving data when it is in paused mode. Thus, when an event is triggered from the remote controller, samples received in the last event-window seconds are stored in the database.

In other words, the event-window acts as a sliding time window that allows to save the collected samples in this time window only when the remote controller event is received. By default, its value is set to 20 seconds. Log Publish Time

By default (log-publish-time: false) received messages are stored in the MCAP file with logTime value equals to the reception timestamp. Additionally, the timestamp corresponding to when messages were initially published (publishTime) is also included in the information dumped to MCAP files. In some applications, it may be required to use the publishTime as logTime, which can be achieved by providing the log-publish-time: true configuration option. Max Reception Rate

Limits the frequency [Hz] at which samples are processed, by discarding messages received before 1/max-reception-rate seconds have elapsed since the last processed message was received. When specified, max-reception-rate is set for all topics without distinction, but a different value can also set for a particular topic under the qos configuration tag within the builtin-topics list. This parameter only accepts non-negative values, and its default value is 0 (no limit). Downsampling

Reduces the sampling rate of the received data by keeping 1 out of every n samples received (per topic), where n is the value specified in downsampling. If max-reception-rate is also set, downsampling applies to messages that already managed to pass this filter. When specified, this downsampling factor is set for all topics without distinction, but a different value can also set for a particular topic under the qos configuration tag within the builtin-topics list. This parameter only accepts positive integer values, and its default value is 1 (no downsampling). Only With Type

By default, all (allowed) received messages are recorded regardless of whether their associated type information has been received. However, a user can enforce that only samples whose type is received are recorded by setting only-with-type: true. Compression

Compression settings for writing to an MCAP file can be specified under the compression configuration tag. The supported compression options are:




Data type

Default value

Possible values

Compression Algorithm


Compression algorithm to
use when writing Chunks.




Compression Level


Compression level to use
when writing Chunks.




Force Compression


Force compression on all
Chunks (even for those
that do not benefit from



false Record Types

By default, all type information received during execution is stored in a dedicated MCAP file section. This information is then leveraged by DDS Replayer on playback, publishing recorded types in addition to data samples, which may be required for receiver applications relying on Dynamic Types (see Replay Types). However, a user may choose to disable this feature by setting record-types: false.

3.1.3. Remote Controller

Configuration of the DDS remote control system. Please refer to Remote Control for further information on how to use DDS Recorder remotely. The supported configurations are:




Data type

Default value

Possible values



Enable DDS remote
control system.




DDS Domain


DDS Domain of the
DDS remote control


DDS domain being

From 0 to 255

Initial state


Initial state of
DDS Recorder.




Command Topic Name


Name of Controller
Command DDS Topic.



Status Topic Name


Name of Controller
Status DDS Topic.



3.1.4. Specs Configuration

The internals of a DDS Recorder can be configured using the specs optional tag that contains certain options related with the overall configuration of the DDS Recorder instance to run. The values available to configure are: Number of Threads

specs supports a threads optional value that allows the user to set a maximum number of threads for the internal ThreadPool. This ThreadPool allows to limit the number of threads spawned by the application. This improves the performance of the internal data communications.

This value should be set by each user depending on each system characteristics. In case this value is not set, the default number of threads used is 12. Maximum Number of Pending Samples

It is possible that a DDS Recorder starts receiving data from a topic that it has not yet registered, i.e. a topic for which it does not know the data type. In this case, messages are kept in an internal circular buffer until their associated type information is received, event on which they are written to disk.

However, the recorder execution might end before this event ever occurs. Depending on configuration (see Only With Type), messages kept in the pending samples buffer will be stored or not on closure. Hence, note that memory consumption would continuously grow whenever a sample with unknown type information is received.

To avoid the exhaustion of memory resources in such scenarios, a configuration option is provided which lets the user set a limit on memory usage. The max-pending-samples parameter allows to configure the size of the aforementioned circular buffers for each topic that is discovered. The default value is equal to 5000 samples, with -1 meaning no limit, and 0 no pending samples.

Depending on the combination of this configuration option and the value of only-with-type, the following situations may arise when a message with unknown type is received:

  • If max-pending-samples is -1, or if it is greater than 0 and the circular buffer is not full, the sample is added to the collection.

  • If max-pending-samples is greater than 0 and the circular buffer reaches its maximum capacity, the oldest sample with same type as the received one is popped, and either written without type (only-with-type: false) or discarded (only-with-type: true).

  • If max-pending-samples is 0, the message is written without type if only-with-type: false, and discarded otherwise. Cleanup Period

As explained in Event Window, a DDS Recorder in paused mode awaits for an event command to write in disk all samples received in the last event-window seconds. To accomplish this, received samples are stored in memory until the aforementioned event is triggered and, in order to limit memory consumption, outdated (received more than event-window seconds ago) samples are removed from this buffer every cleanup-period seconds. By default, its value is equal to twice the event-window.

3.1.5. General Example

A complete example of all the configurations described on this page can be found below.


This example can be used as a quick reference, but it may not be correct due to incompatibility or exclusive properties. Do not take it as a working example.

  domain: 0

    - name: "topic_name"
      type: "topic_type"

    - name: "topic_name"
      type: "topic_type"

    - name: "HelloWorldTopic"
      type: "HelloWorld"
        reliability: true
        durability: true
        keyed: false
        partitions: true
        ownership: false
        downsampling: 4
        max-reception-rate: 10

  ignore-participant-flags: no_filter
  transport: builtin
    - ""

    filename: "output"
    path: "."
    timestamp-format: "%Y-%m-%d_%H-%M-%S_%Z"
    local-timestamp: false

  buffer-size: 50
  event-window: 60
  log-publish-time: false
  downsampling: 3
  max-reception-rate: 20
  only-with-type: false
    algorithm: lz4
    level: slowest
    force: true
  record-types: true

  enable: true
  domain: 10
  initial-state: "PAUSED"
  command-topic-name: "/ddsrecorder/command"
  status-topic-name: "/ddsrecorder/status"

  threads: 8
  max-pending-samples: 10
  cleanup-period: 90

3.2. Fast DDS Configuration

As explained in this section, a DDS Recorder instance stores (by default) all data regardless of whether their associated data type is received or not. Some applications rely on this information being recorded and written in the resulting MCAP file, which requires that the user application is configured to send the necessary type information. However, Fast DDS does not send the data type information by default, it must be configured to do so.

First of all, when generating the topic types using eProsima Fast DDS Gen, the option -typeobject must be added in order to generate the needed code to fill the TypeObject data.

For native types (data types that does not rely in other data types) this is enough, as Fast DDS will send the TypeObject by default. However, for more complex types, it is required to use TypeInformation mechanism. In the Fast DDS DomainParticipant set the following QoS in order to send this information:

DomainParticipantQos pqos;
pqos.wire_protocol().builtin.typelookup_config.use_server = true;

Feel free to review this section, where it is explained in detail how to configure a Fast DDS Publisher/Subscriber leveraging Dynamic Types.