Functionality, or simply NGSIPostgreSQLSink is a sink designed to persist NGSI-like context data events within a PostgreSQL server. Usually, such a context data is notified by a Orion Context Broker instance, but could be any other system speaking the NGSI language.

Independently of the data generator, NGSI context data is always transformed into internal NGSIEvent objects at Cygnus sources. In the end, the information within these events must be mapped into specific PostgreSQL data structures.

Next sections will explain this in detail.


Mapping NGSI events to NGSIEvent objects

Notified NGSI events (containing context data) are transformed into NGSIEvent objects (for each context element a NGSIEvent is created; such an event is a mix of certain headers and a ContextElement object), independently of the NGSI data generator or the final backend where it is persisted.

This is done at the cygnus-ngsi Http listeners (in Flume jergon, sources) thanks to NGSIRestHandler. Once translated, the data (now, as NGSIEvent objects) is put into the internal channels for future consumption (see next section).


Mapping NGSIEvents to PostgreSQL data structures

PostgreSQL organizes the data in schemas inside a database that contain tables of data rows. Such organization is exploited by NGSIPostgreSQLSink each time a NGSIEvent is going to be persisted.


PostgreSQL databases naming conventions

Previous to any operation with PostgreSQL you need to create the database to be used.

It must be said PostgreSQL only accepts alphanumeric characters and the underscore (_). This leads to certain encoding is applied depending on the enable_encoding configuration parameter.

PostgreSQL databases name length is limited to 63 characters.


PostgreSQL schemas naming conventions

A schema named as the notified fiware-service header value (or, in absence of such a header, the defaulted value for the FIWARE service) is created (if not existing yet).

It must be said PostgreSQL only accepts alphanumeric characters and the underscore (_). This leads to certain encoding is applied depending on the enable_encoding configuration parameter.

PostgreSQL schemas name length is limited to 63 characters.


PostgreSQL tables naming conventions

The name of these tables depends on the configured data model (see the Configuration section for more details):

  • Data model by service path (data_model=dm-by-service-path). As the data model name denotes, the notified FIWARE service path (or the configured one as default in NGSIRestHandler) is used as the name of the table. This allows the data about all the NGSI entities belonging to the same service path is stored in this unique table. The only constraint regarding this data model is the FIWARE service path cannot be the root one (/).
  • Data model by entity (data_model=dm-by-entity). For each entity, the notified/default FIWARE service path is concatenated to the notified entity ID and type in order to compose the table name. If the FIWARE service path is the root one (/) then only the entity ID and type are concatenated.

It must be said PostgreSQL only accepts alphanumeric characters and the underscore (_). This leads to certain encoding is applied depending on the enable_encoding configuration parameter.

PostgreSQL tables name length is limited to 63 characters.

The following table summarizes the table name composition (old encoding):

FIWARE service path dm-by-service-path dm-by-entity
/ N/A <entityId>_<entityType>
/<svcPath> <svcPath> <svcPath>_<entityId>_<entityType>

Using the new encoding:

FIWARE service path dm-by-service-path dm-by-entity
/ x002f x002fxffff<entityId>xffff<entityType>
/<svcPath> x002f<svcPath> x002f<svcPath>xffff<entityId>xffff<entityType>

Please observe the concatenation of entity ID and type is already given in the notified_entities/grouped_entities header values (depending on using or not the grouping rules, see the Configuration section for more details) within the NGSIEvent.


Row-like storing

Regarding the specific data stored within the above table, if attr_persistence parameter is set to row (default storing mode) then the notified data is stored attribute by attribute, composing an insert for each one of them. Each insert contains the following fields:

  • recvTimeTs: UTC timestamp expressed in miliseconds.
  • recvTime: UTC timestamp in human-redable format (ISO 8601).
  • fiwareServicePath: Notified fiware-servicePath, or the default configured one if not notified.
  • entityId: Notified entity identifier.
  • entityType: Notified entity type.
  • attrName: Notified attribute name.
  • attrType: Notified attribute type.
  • attrValue: In its simplest form, this value is just a string, but since Orion 0.11.0 it can be Json object or Json array.
  • attrMd: It contains a string serialization of the metadata array for the attribute in Json (if the attribute hasn't metadata, an empty array [] is inserted).


Column-like storing

Regarding the specific data stored within the above table, if attr_persistence parameter is set to column then a single line is composed for the whole notified entity, containing the following fields:

  • recvTime: UTC timestamp in human-redable format (ISO 8601).
  • fiwareServicePath: The notified one or the default one.
  • entityId: Notified entity identifier.
  • entityType: Notified entity type.
  • For each notified attribute, a field named as the attribute is considered. This field will store the attribute values along the time.
  • For each notified attribute, a field named as the concatenation of the attribute name and _md is considered. This field will store the attribute's metadata values along the time.




Assuming the following NGSIEvent is created from a notified NGSI context data (the code below is an object representation, not any real data format):



Database, schema and table names

The PostgreSQL database name will be of the user's choice.

The PostgreSQL schema will always be vehicles.

The PostgreSQL table names will be, depending on the configured data model, the following ones (old encoding):

FIWARE service path dm-by-service-path dm-by-entity
/ N/A car1_car
/4wheels 4wheels 4wheels_car1_car

Using the new encoding:

FIWARE service path dm-by-service-path dm-by-entity
/ x002f x002fxffffcar1xffffcar
/4wheels x002f4wheels x002f4wheelsxffffcar1xffffcar


Row-like storing

Assuming attr_persistence=row as configuration parameters, then NGSIPostgreSQLSink will persist the data within the body as:

$ psql -U myuser
psql (9.5.0)
Type "help" for help.
postgres-# \c my-database

my-database# \dn
   List of schemas
   Name   |  Owner
 vehicles | postgres
 public   | postgres
(2 rows)

my-database=# \dt vehicles.*
               List of relations
  Schema  |       Name        | Type  |  Owner
 vehicles | 4wheels_car1_car  | table | postgres
(1 row)

postgresql> select * from vehicles.4wheels_car1_car;
| recvTimeTs | recvTime                   | fiwareServicePath | entityId | entityType | attrName    | attrType  | attrValue | attrMd |
| 1429535775 | 2015-04-20T12:13:22.41.124 | 4wheels           | car1     | car        |  speed      | float     | 112.9     | []     |
| 1429535775 | 2015-04-20T12:13:22.41.124 | 4wheels           | car1     | car        |  oil_level  | float     | 74.6      | []     |
2 row in set (0.00 sec)


Column-like storing

Coming soon.


Administration guide


NGSIPostgreSQLSink is configured through the following parameters:

Parameter Mandatory Default value Comments
type yes N/A Must be
channel yes N/A
enable_encoding no false true or false, true applies the new encoding, false applies the old encoding.
enable_grouping no false true or false. Check this link for more details.
enable_name_mappings no false true or false. Check this link for more details.
enable_lowercase no false true or false.
data_model no dm-by-entity dm-by-service-path or dm-by-entity. dm-by-service and are not currently supported.
postgresql_host no localhost FQDN/IP address where the PostgreSQL server runs.
postgresql_port no 5432
postgresql_database no postgres postgres is the default database that is created automatically when install
postgresql_username no postgres postgres is the default username that is created automatically when install
postgresql_password no N/A Empty value by default (No password is created when install)
attr_persistence no row row or column.
batch_size no 1 Number of events accumulated before persistence.
batch_timeout no 30 Number of seconds the batch will be building before it is persisted as it is.
batch_ttl no 10 Number of retries when a batch cannot be persisted. Use 0 for no retries, -1 for infinite retries. Please, consider an infinite TTL (even a very large one) may consume all the sink's channel capacity very quickly.
batch_retry_intervals no 5000 Comma-separated list of intervals (in miliseconds) at which the retries regarding not persisted batches will be done. First retry will be done as many miliseconds after as the first value, then the second retry will be done as many miliseconds after as second value, and so on. If the batch_ttl is greater than the number of intervals, the last interval is repeated.
backend.enable_cache no false true or false, true enables the creation of a Cache, false disables the creation of a Cache.

A configuration example could be:

cygnus-ngsi.sinks = postgresql-sink
cygnus-ngsi.channels = postgresql-channel
cygnus-ngsi.sinks.postgresql-sink.type = = postgresql-channel
cygnus-ngsi.sinks.postgresql-sink.enable_encoding = false
cygnus-ngsi.sinks.postgresql-sink.enable_grouping = false
cygnus-ngsi.sinks.postgresql-sink.enable_lowercase = false
cygnus-ngsi.sinks.postgresql-sink.enable_name_mappings = false
cygnus-ngsi.sinks.postgresql-sink.data_model = dm-by-entity
cygnus-ngsi.sinks.postgresql-sink.postgresql_host =
cygnus-ngsi.sinks.postgresql-sink.postgresql_port = 5432
cygnus-ngsi.sinks.postgresql-sink.postgresql_database = mydatabase
cygnus-ngsi.sinks.postgresql-sink.postgresql_username = myuser
cygnus-ngsi.sinks.postgresql-sink.postgresql_password = mypassword
cygnus-ngsi.sinks.postgresql-sink.attr_persistence = row
cygnus-ngsi.sinks.postgresql-sink.batch_size = 100
cygnus-ngsi.sinks.postgresql-sink.batch_timeout = 30
cygnus-ngsi.sinks.postgresql-sink.batch_ttl = 10
cygnus-ngsi.sinks.postgresql-sink.batch_retry_intervals = 5000
cygnus-ngsi.sinks.postgresql.backend.enable_cache = false


Use cases

Use NGSIPostgreSQLSink if you are looking for a big database with several tenants. PostgreSQL is bad at having several databases, but very good at having different schemas.


Important notes

About the table type and its relation with the grouping rules

The table type configuration parameter, as seen, is a method for direct aggregation of data: by default destination (i.e. all the notifications about the same entity will be stored within the same PostgreSQL table) or by default service-path (i.e. all the notifications about the same service-path will be stored within the same PostgreSQL table).

The Grouping feature is another aggregation mechanism, but an inderect one. This means the grouping feature does not really aggregates the data into a single table, that's something the sink will done based on the configured table type (see above), but modifies the default destination or service-path, causing the data is finally aggregated (or not) depending on the table type.

For instance, if the chosen table type is by destination and the grouping feature is not enabled then two different entities data, car1 and car2 both of type car will be persisted in two different PostgreSQL tables, according to their default destination, i.e. car1_car and car2_car, respectively. However, if a grouping rule saying "all cars of type car will have a modified destination named cars" is enabled then both entities data will be persisted in a single table named cars. In this example, the direct aggregation is determined by the table type (by destination), but indirectly we have been deciding the aggregation as well through a grouping rule.


About the persistence mode

Please observe not always the same number of attributes is notified; this depends on the subscription made to the NGSI-like sender. This is not a problem for the row persistence mode, since fixed 8-fields data rows are inserted for each notified attribute. Nevertheless, the column mode may be affected by several data rows of different lengths (in term of fields). Thus, the column mode is only recommended if your subscription is designed for always sending the same attributes, event if they were not updated since the last notification.

In addition, when running in column mode, due to the number of notified attributes (and therefore the number of fields to be written within the Datastore) is unknown by Cygnus, the table can not be automatically created, and must be provisioned previously to the Cygnus execution. That's not the case of the row mode since the number of fields to be written is always constant, independently of the number of notified attributes.


About batching

As explained in the programmers guide, NGSIPostgreSQLSink extends NGSISink, which provides a built-in mechanism for collecting events from the internal Flume channel. This mechanism allows extending classes have only to deal with the persistence details of such a batch of events in the final backend.

What is important regarding the batch mechanism is it largely increases the performance of the sink, because the number of writes is dramatically reduced. Let's see an example, let's assume a batch of 100 NGSIEvents. In the best case, all these events regard to the same entity, which means all the data within them will be persisted in the same PostgreSQL table. If processing the events one by one, we would need 100 inserts into PostgreSQL; nevertheless, in this example only one insert is required. Obviously, not all the events will always regard to the same unique entity, and many entities may be involved within a batch. But that's not a problem, since several sub-batches of events are created within a batch, one sub-batch per final destination PostgreSQL table. In the worst case, the whole 100 entities will be about 100 different entities (100 different PostgreSQL tables), but that will not be the usual scenario. Thus, assuming a realistic number of 10-15 sub-batches per batch, we are replacing the 100 inserts of the event by event approach with only 10-15 inserts.

The batch mechanism adds an accumulation timeout to prevent the sink stays in an eternal state of batch building when no new data arrives. If such a timeout is reached, then the batch is persisted as it is.

Regarding the retries of not persisted batches, a couple of parameters is used. On the one hand, a Time-To-Live (TTL) is used, specifing the number of retries Cygnus will do before definitely dropping the event. On the other hand, a list of retry intervals can be configured. Such a list defines the first retry interval, then se second retry interval, and so on; if the TTL is greater than the length of the list, then the last retry interval is repeated as many times as necessary.

By default, NGSIPostgreSQLSink has a configured batch size and batch accumulation timeout of 1 and 30 seconds, respectively. Nevertheless, as explained above, it is highly recommended to increase at least the batch size for performance purposes. Which are the optimal values? The size of the batch it is closely related to the transaction size of the channel the events are got from (it has no sense the first one is greater then the second one), and it depends on the number of estimated sub-batches as well. The accumulation timeout will depend on how often you want to see new data in the final storage. A deeper discussion on the batches of events and their appropriate sizing may be found in the performance document.


Time zone information

Time zone information is not added in PostgreSQL timestamps since PostgreSQL stores that information as a environment variable. PostgreSQL timestamps are stored in UTC time.


About the encoding

Until version 1.2.0 (included), Cygnus applied a very simple encoding:

  • All non alphanumeric characters were replaced by underscore, _.
  • The underscore was used as concatenator character as well.
  • The slash, /, in the FIWARE service paths is ignored.

From version 1.3.0 (included), Cygnus applies this specific encoding tailored to PostgreSQL data structures:

  • Lowercase alphanumeric characters are not encoded.
  • Upercase alphanumeric characters are encoded.
  • Numeric characters are not encoded.
  • Underscore character, _, is not encoded.
  • Equals character, =, is encoded as xffff.
  • All other characters, including the slash in the FIWARE service paths, are encoded as a x character followed by the Unicode of the character.
  • User defined strings composed of a x character and a Unicode are encoded as xx followed by the Unicode.
  • xffff is used as concatenator character.

Despite the old encoding will be deprecated in the future, it is possible to switch the encoding type through the enable_encoding parameter as explained in the configuration section.


Programmers guide

NGSIPostgreSQLSink class

As any other NGSI-like sink, NGSIPostgreSQLSink extends the base NGSISink. The methods that are extended are:

void persistBatch(Batch batch) throws Exception;

A Batch contains a set of NGSIEvent objects, which are the result of parsing the notified context data events. Data within the batch is classified by destination, and in the end, a destination specifies the PostgreSQL table where the data is going to be persisted. Thus, each destination is iterated in order to compose a per-destination data string to be persisted thanks to any PostgreSQLBackend implementation.

public void start();

An implementation of PostgreSQLBackend is created. This must be done at the start() method and not in the constructor since the invoking sequence is NGSIPostgreSQLSink() (contructor), configure() and start().

public void configure(Context);

A complete configuration as the described above is read from the given Context instance.


Authentication and authorization

Current implementation of NGSIPostgreSQLSink relies on the database, username and password credentials created at the PostgreSQL endpoint.