Swimlanes.io is a free webapp for making sequence diagrams. You simply edit the text on the left and the diagram is updated in real time. You can download your sequence diagrams as images or distribute with a link.
Title: TMS API /introduce Client -> Server: /introduce Server -> Server: Request validation Server --> Client: Error (400) Server -> Introduce Service: Introduce Service -> Introduce Service: loops through array. (also adds the swaped version) Introduce Service -> Database: save to db Database --> Client: Error 500 Database -> Introduce Service: Introduce Service --> Client: Success 200