It is possible to build a microservices-based architecture using SOAP. Like REST, SOAP uses HTTP, but it only uses POST messages to transfer data to a server. Ultimately, a SOAP call runs a method on a certain object on the server and is therefore an RPC mechanism (remote-procedure call).

SOAP lacks concepts such as HATEOAS that enable relationships between microservices to be handled flexibly. The interfaces have to be completely defined by the server and known on the client.

Flexible Transport

SOAP can convey messages using different transport mechanisms. For instance, it’s possible to receive a message via HTTP and to then send it on via JMS or as an email via SMTP/POP. SOAP-based technologies also support forwarding of requests. For example, the security standard WS-Security can encrypt or sign parts of a message. After this has been done, the parts can be sent on to different services without having to be decrypted. The sender can send a message in which some parts are encrypted. This message can be processed via different stations. Each station can process a part of the message or send it to other recipients. Finally, the encrypted parts will arrive at their final recipients—and only there do they have to be decrypted and processed.

SOAP has many extensions for special use contexts. For instance, the different extensions from the WS-*-environment cater for transactions and the coordination of web services. This enables a complex protocol stack to arise. The interoperability between the different services and solutions can suffer due to this complexity. Also, some technologies are not well suited for microservices. For example, a coordination of different microservices is problematic as this will result in a coordination layer, and modifications of a business process will probably impact the coordination of the microservices and also the microservices themselves. When the coordination layer consists of all microservices, a monolith is created that needs to be changed upon each modification. This contradicts the microservices concept of independent deployment. WS-* is better suited to concepts such as SOA.


Another communication option is Apache Thrift.[1] It uses very efficient binary encodings such as Protocol Buffer. Thrift can also forward requests from a process via the network to other processes. The interface is described in an interface definition specific to Thrift. Based on this definition different client and server technologies can communicate with each other.

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