DE4A Connector

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The DE4A Connector is a technical proxy that allows the final participants (DE or DO) to send requests for evidence or responses to other final participants over an eDelivery communication environment. In addition, to handle the message exchange process, the Connector is responsible for obtaining the message routing information, by exchanging information with external components such as the IDK, the SML/DNS or the SMP.

To do so, it provides a common interface to DEs and DOs, making the complexity of the system transparent to the final participants and integration easier.

The Connector component provides the AS4 Gateway functionality, so it can assume both the role of Data Requestor and Data Transferor. This first approach makes the Connector a stand-alone web application that can be deployed on any suitable application server. As an alternative, the Connector can make used of an external AS4 Gateway to provide the message exchange feature, in which case two components should be deployed: the DE4A Connector along with the chosen AS4 Gateway (i.e. Domibus).

The security and integrity of messages, as well as the unique identification of the participants involved, are the cornerstones of the Connector component.

Follow the link to the Installation and configuration guide of the DE4a Connector.

Conceptual schema of the target DE4A system for the first iteration

Functionalities provided

The main purpose of the DE4A Connector is sending and receiving of evidence requests and their responses. The message exchange process is described in the DE4A deliverables D2.4 Project Start Architecture and D5.3 Initial technical design of interfaces.

Routing information lookup

The Connector is responsible for obtaining the Data Provider information from the IDK. It exposes a REST API /lookupRoutingInformation to get information about the Data Owners that provide a specific Canonical Evidence Type and further related information. That information is used to construct a request message to be sent through the Connector. Thus, the Data Evaluator is the only consumer of the mentioned API method.

Dynamic discovery of Services

In order for the Connector to be able to send a message to the corresponding endpoint, the eDelivery dynamic discovery mode of operation is used. This operation mode is based on the use of the SML/DNS and SMP components of the eDelivery infrastructure. The main elements stored in the SML/DNS and SMPs for this purpose are the following:

  • ParticipantIdentifier: The Data Owner/Data Evaluator identifier who is publishing its AS4 communication point (of the Connector linked to it).
  • DocumentTypeIdentifier: Canonical evidence type.
  • ProcessIdentifier: Orchestration type (request/response).
  • AS4 endpoint: AS4 service endpoint URL.
  • Certificate: The X.509 certificate of the AS4 server, used to encrypt the transmitted data for this specific participant.

The information described above is managed by the SML/DNS and SMP components and is used by the Connector when working with the phase4 implementation of AS4. In cases where the Connector runs with external AS4 implementations such as Domibus, the end-to-end AS4 communication will be handled by the configuration of that external platform.

Focusing on the SML/DNS/SMP data retrieval, the process will take place according to the following features:

  • SSL/TLS communication is mandatory.
  • Response signature validation is mandatory.
  • The communication between SMP and SML requires the usage of a client certificate.

The service metadata lookup will be performed as a step prior to the AS4 message exchange. Therefore, the participant IDs and other related information must be known by the Connector in advance.

Supported interaction patterns

Interaction patterns define the flow of data through the Connector and the intercommunication between the different components. Each pattern exchanges certain types of messages, and the incoming/outgoing information will depend on the processes occurring in the external components [3].

The Connector currently supports two interaction patterns:

  • Intermediation (IM) pattern
    • ­Synchronous communication between the Connector and final participant (DE or DO).
    • Data Owner endpoint must be known by the Data Transferor.
    • Since the communication is synchronous, the Data Requestor does not need to know the identifier and endpoint of the Data Evaluator.
  • User supported intermediation (USI) pattern
    • Asynchronous communication between the Connector and final participant (DE or DO).
    • Data Owner endpoint must be known by the Data Transferor.
    • Since the communication is asynchronous, the Data Evaluator endpoint must be known by the Data Requestor. In addition, the Data Evaluator identifier is recovered from the request, since it is not sent in the response from the Data Transferor.

Most of the specific behaviour of each interaction pattern is independent of the Connector itself, as the Connector component is just designed to exchange messages and the main differences between the patterns take place in the external components such as the Data Evaluator and the Data Owner.

Connector roles

A Connector instance can play two different roles:

  • Data Requestor (DR)
  • Data Transferor (DT)

No configuration is needed to differentiate the roles, it only depends on the usage, i.e., the behaviour will be according to the messages sent.

Also, in a functional scenario the SMP data will determine which Connector is playing the DR or DT role.

AS4 implementations

The message exchange is built on the AS4 protocol, which is an open standard for secure, payload-agnostic business-to-business document exchange via web services. Secure document exchange is governed by WS-Security aspects, including XML Encryption and XML Digital Signatures. There are many implementations of AS4, but the Connector currently implements phase4 and Domibus.


  • Main features
    • It is a generic AS4 solution.
    • It can be integrated in basically any Servlet-based application server.
    • It can easily be used with Peppol.
    • It can be used with other Dynamic Discovery solutions.
    • It supports a high degree of customization.
    • It is not limited to the CEF/e-SENS profile.
  • Implementation
    • The Connector implements phase4 as a gateway to perform the AS4 message exchange.

All necessary configuration parameters are provided by the properties file of the Connector.


  • Main features
    • ­It ensures interoperable, secure and reliable data exchange.
    • It is maintained by the European Commission.
    • It manages the participants’ information and the end-to-end communication parameters.
    • It is conceived as an eDelivery Access Point (external platform).
  • Implementation
    • The Connector implements Domibus as a gateway to send and receive messages to and from the Domibus access point.

Most of the configuration needed is on the Domibus service side.

Error handling

Since the Connector performs multiple communications between different external components and some data and structure validations are performed, the Connector needs to monitor all failure points and be able to identify them in order to build the corresponding messages and warnings to inform each external component. When an error happens, the corresponding component creates an error message with the information about the error to be sent back to the entity that sent the failed message.

Kafka messages

Within the data flow and message exchange, there are many key points where it is important to know how the data is being managed, as well as identifying intermediate errors and unhandled system states. In this respect, the Connector can send messages to a Kafka server to track the data flow and trace the state of the system at certain points.

This feature is an advantage from a technical and business point of view, as the Connector performs the message exchange transparently to the other components.

It should be noted that the messages sent to the Kafka server are hardcoded, it is not a parameterizable feature, so any enhancements must be hardcoded again and deployed. It has been developed in such a way because the necessity of a Kafka server is not expected outside the DE4A project. In a real scenario, an alternative way of collecting logs should be implemented.

Message types

The Connector implements a Kafka message producer through the de4a-kafka-client library of the de4a-commons package. This producer provides several types of messages or severity levels:

  • Success
  • Info
  • Warn
  • Error
  • Fatal error

Those message categories can be easily used to specify the severity level of the message sent to the Kafka server to track and identify them.

List of messages

The messages currently sent from the Connector to the Kafka server are as follows:

Services(Info level)

  • RequestTransferEvidenceIM message received - RequestId: {0}, CanonicalEvidenceType: {1}, DataEvaluator: {2}, DataOwner: {3}
  • RequestTransferEvidenceUSI message received - RequestId: {0}, CanonicalEvidenceType: {1}, DataEvaluator: {2}, DataOwner: {3}
  • RequestTransferEvidenceUSIDT message received - RequestId: {0}
  • RequestLookupRoutingInformation message received - CanonicalEvidenceType: {0}, CountryCode: {1}, DataOwnerId: {2}
  • Sending request to IDK - URL: {0}
  • ­Looking for Data Owner address - DataOwnerId: {0}

Client(Info level)

  • Requesting to SMP - ParticipantId: {0}, DocumentTypeId: {1}, MessageType: {2}
  • Sending RequestForwardEvidence to the Data Evaluator - RequestId: {0}, DataEvaluatorId: {1}, DataOwnerId: {2}, Endpoint: {3}
  • Sending RequestExtractEvidence IM to the Data Owner - RequestId: {0}, CanonicalEvidenceType: {1}, DataEvaluatorId: {2}, DataOwnerId: {3}, Endpoint: {4}
  • ­Sending RequestExtractEvidence USI to the Data Owner - RequestId: {0}, CanonicalEvidenceType: {1}, DataEvaluatorId: {2}, DataOwnerId: {3}, Endpoint: {4}

AS4(Info level)

  • Sending request message via AS4 gateway - DataEvaluatorId: {0}, DataOwnerId: {1}, CanonicalEvidenceType: {2}
  • Sending response message via AS4 gateway - DataEvaluatorId: {0}, Message tag: {1}, CanonicalEvidenceType: {2}
  • Processing the request received via AS4 gateway - RequestId: {0}
  • ­Processing the response received via AS4 gateway - RequestId: {0}

Errors(Error level)

  • The corresponding request to the received response is not found on database - RequestId: {0}
  • RequestForwardEvidence has not been sent, unknown Data Evaluator endpoint - RequestId: {0}, DataEvaluatorId: {1}
  • RequestTransferEvidence not found on AS4 incoming message
  • Error processing incoming message from AS4 gateway
  • Error building URI from Data Evaluator endpoint: {0}
  • ­Error sending message to Data Requestor via AS4 gateway - RequestId: {0}
  • Data Owner address not found - DataOwnerId: {0}

[1] The {x} symbols are placeholders for dynamic text to be logged.

Data management

The Connector stores and manages certain information such as DE endpoints (not to be confused with SMP endpoints), DO endpoints, certain request records for asynchronous response matching, etc. All this data and where to find it is described in the following subsections.

Data Owner addresses

Once each Data Owner is publishing his Connector service information on the SMP, the request will arrive to that Connector service configured on the SMP (via the AS4 Gateway), and considering that one Connector could be serving to multiples DOs, this Connector has to know the addressing (base endpoint, without path name) information related to a specific participant identifier (e.g. iso6523-actorid-upis::9999:egov) to send forward the request to the corresponding Data Owner, so due to that, the Connector maintains a table, named  owner_addresses, with two columns:

  • AgentUrn: participant identifier in the DE4A format, e.g., iso6523-actorid-upis::9999:ess2833002e – see the “DE4A Policy for use of identifiers” for further information about the participant identifiers policy.
  • Endpoint: base endpoint URL of the Data Owner who is exposing the service /requestExtractEvidence

The information above will be used by a Connector playing the Transferor role when the RequestExtractEvidence message is being sent.


The message exchange performed by the Connector has been conceived to ensure different security aspects as:

  • Authenticity: ensures the identity of the participant entities and any common component involved in the message exchange. It is achieved through the participant identifiers definition, eDelivery specification and AS4 implementations.
  • Integrity: refers to the accuracy and completeness of data. Security controls focused on integrity are used to prevent data from being modified or misused by an unauthorized party, provided by the WS-security and XML message signature.
  • Confidentiality: data confidentiality at message level could be achieved by taking advantage of WS-Security encryption feature in User messages. This message encryption is performed via asymmetric keys: the sender of the message encrypts with the public key of the recipient, so the message can only be decrypted by the recipient, who owns the private key. Those keys should be configured either in the Connector in case of phase4 or in any external AS4 gateway.


The Connector creates a secure context to establish in/outgoing connections with external components.

The main elements for such communication securitisations are:

  • Private key: private part of private-public key pair used in the TLS handshake between your server and the connecting clients. This private key must be configured in the Connector properties in order to create the secure context as well in the web server, in the case of reverse proxy or any similar structure. The keys configured on each component are generated under the DE4A management.
  • Truststore: used for the storage of certificates from the trusted Certificate Authority (CA), which is used in the verification of the certificate provided by the server in an SSL connection. The Connector provides a default truststore which allows to trust on certificates generated under the main DE4A CA.

HTTP Proxy mode

Sometimes, the environments have certain communication structure where the outgoing connections must be performed through an HTTP proxy server. The Connector provides de functionality and proper configuration to perform the communications via an HTTP proxy server.

Message cryptographic protection

The Connector messages exchange can be separated into:

REST APIs messages

Once the SSL connection is established, all the data exchanged through it, will be encrypted on transport level based on the SSL context configuration settled, either managed by a reverse proxy on the server environment or configuring the proper application properties.

AS4 messages

  • Phase4: internally the message is encrypted using a specific keystore configuration only used for this purpose; also the cross-gateways communications are additionally encrypted over the HTTPS secure protocol.
  • Domibus: the bridge between the Connector and Domibus service is securitized by encrypting the messages content as well as signing the message, in accordance with the AS4 policies and the transport protocol used. It implements a Spring web service gateway using WS-security at SOAP message exchange level. About the message exchange between Domibus instances, it is handled internally by the platform.

Message validation

Message validation is performed at different stages of the Connector data flow, from message structure compliance to content integrity.

Schema validation

All the messages produced and consumed by the Connector are performing an XML Schema validation once it is received or sent out. This validation is done by the object conversion library that keeps the schema model including all the structure and content constraints.

Signature validation

As well as the XML Schema validation, the Connector performs validations to ensure the content and reliability on certain phases of the message exchange process:

  • Services metadata querying
  • In the SMP data exchange a message signature validation is being performed to maintain the content reliability. This validation consists of verifying the embedded signature on the message against the trust certificate configured in the Connector for that purpose.

The signature of incoming AS4 messages is verified against the central AS4 Gateway CA globally used in DE4A project, internally validated by the implementation libraries that handle the reception and delivery.

Technology used

System core architecture

The Connector works as standalone application that runs different web services according to the RESTful API architecture principles. The application is built with the following tools:

  • Spring Framework version 5.x.
  • Java 11
  • Apache HTTP Client 4.x

In addition to the core architecture, the XML Schemas defined to model the exchanged information, data constraints, interfaces, etc. are also important. All this is part of the Connector core through the de4a-commons library, which contains the above-mentioned model as well as utilities and conversion tools.

Third party libraries

As part of the bunch of libraries used by the Connector there are some of the related with the core features and which represent a starting point for the functionalities provided by the Connector.

TOOP Connector

The TOOP Connector is a set of shared utility functions used in the Connector to perform common tasks that are required for a safe and interoperable data exchange. In the initial iteration the latest version of the TOOP Connector technical components were reused mainly for the usage of the built-in phase4 AS4 Gateway. Other elements of the TOOP Connector are currently ignored.

ph-oton libraries

Set of Java libraries to build Java web applications.

Peppol commons libraries

They include the SMP client library used by the Access Points to retrieve service metadata. This library supports the Peppol SMP specification, the OASIS BDXR SMP v1 and OASIS BDXR SMP v2 specification. This project uses Apache HTTP client to perform the REST lookups on foreign SMPs.

Data management

To manage the model and the data stored by the Connector the following technologies are used:

  • JPA: The Java Persistence API is a specification of Java. It is used to persist data between Java object and relational database. JPA acts as a bridge between object-oriented domain models and relational database systems. As JPA is just a specification, it does not perform any operation by itself.
  • Hibernate: An object–relational mapping tool for the Java programming language. It provides a framework for mapping an object-oriented domain model to a relational database. Hibernate handles object–relational impedance mismatch problems by replacing direct, persistent database accesses with high-level object handling functions.

Utilities libraries

The project uses several libraries and utilities to process and transform the data. They can be divided according to their nature:

  • Commercial libraries

To perform common and non-business operations on web and data exchange projects, the Connector uses different commercial solutions, also all the libraries embedded in the Java development kit.

  • In-house solutions

The Connector project includes some utilities that allow the data processing and internal tools to perform all the Connector tasks. Those utilities are within the Connector project as a module called de4a-commons.