A sequence diagram is a type of UML interaction diagram that shows how objects, services, or actors exchange messages over time. Each participant gets a vertical lifeline, and ordered arrows between lifelines capture the exact order of calls, responses, and asynchronous events — making sequence diagrams the clearest way to document API calls, authentication flows, and microservice interactions.
Diagramming AI turns a plain-English description into an editable sequence diagram in seconds. Describe the flow you have in mind, pick Mermaid or PlantUML, and the AI generates ready-to-edit diagram code. You can then refine it conversationally — add an error path, insert a new participant, or split a step — without learning the full syntax first.
Sequence diagrams read top to bottom for time and left to right for participants. Solid arrows usually represent synchronous calls (the caller waits for a response), dashed arrows represent return messages, and open arrowheads represent asynchronous messages that do not block the sender. Activation bars along a lifeline show when a participant is actively processing a request.
Common building blocks include combined fragments such as alt (alternative branches), opt (optional steps), loop (repeated interactions), and par (parallel execution). These let a single diagram express conditional logic and concurrency without becoming a flowchart, which is why engineers reach for sequence diagrams when documenting request/response protocols and event-driven systems.
Start by describing the interaction in everyday language: name the participants (for example "browser", "API gateway", "auth service", "database") and the messages that pass between them, in order. Mention any conditional or error paths you care about. Diagramming AI converts that description into Mermaid or PlantUML sequence-diagram code.
Once the first version is generated, iterate with follow-up instructions in the chat — "add a retry loop around the database call", "show what happens when the token is expired", or "make the payment step asynchronous". Every change updates the editable code, and you can export the result or copy the syntax into your own docs and pull requests.
Scenario: A web client authenticates a user against an auth service, which validates credentials and issues a token. This is the canonical example for documenting login security.
Instruction text for AI
Create a sequence diagram for a user login flow: the browser sends credentials to the API, the API asks the auth service to validate them, the auth service checks the database, and on success a JWT is returned to the browser. Show the failure path when credentials are invalid.Generated Diagram (Mermaid)
sequenceDiagram
actor User
participant Browser
participant API as API Gateway
participant Auth as Auth Service
participant DB as Database
User->>Browser: Enter email and password
Browser->>API: POST /login (credentials)
API->>Auth: validateCredentials()
Auth->>DB: SELECT user by email
DB-->>Auth: user record
alt credentials valid
Auth->>Auth: issue JWT
Auth-->>API: 200 OK (token)
API-->>Browser: Set-Cookie: session
Browser-->>User: Redirect to dashboard
else credentials invalid
Auth-->>API: 401 Unauthorized
API-->>Browser: 401 Unauthorized
Browser-->>User: Show "Invalid login" error
endScenario: A client requests a resource through a service that checks a cache before falling back to the database, illustrating an opt fragment and a cache-miss path.
Instruction text for AI
Draw a sequence diagram where a client calls a product service. The service first checks Redis; on a cache hit it returns immediately, on a cache miss it reads from the database and then writes the result back to the cache before responding.Generated Diagram (Mermaid)
sequenceDiagram
participant Client
participant Service as Product Service
participant Cache as Redis
participant DB as Database
Client->>Service: GET /products/42
Service->>Cache: GET product:42
alt cache hit
Cache-->>Service: cached product
else cache miss
Cache-->>Service: (nil)
Service->>DB: SELECT * FROM products WHERE id = 42
DB-->>Service: product row
Service->>Cache: SET product:42 (ttl 300s)
end
Service-->>Client: 200 OK (product JSON)Scenario: An order service publishes an event that is consumed asynchronously by a worker, demonstrating async messages and a queue participant in PlantUML.
Instruction text for AI
Generate a PlantUML sequence diagram for asynchronous order processing: the order service publishes an OrderPlaced event to a message queue, a payment worker consumes it, charges the customer, and emits a PaymentCompleted event that the notification service consumes to email the customer.Generated Diagram (PlantUML)
@startuml
actor Customer
participant "Order Service" as Order
queue "Message Queue" as MQ
participant "Payment Worker" as Payment
participant "Notification Service" as Notify
Customer -> Order : Place order
Order -> MQ : publish OrderPlaced
Order --> Customer : 202 Accepted
MQ ->> Payment : consume OrderPlaced
Payment -> Payment : charge customer
Payment -> MQ : publish PaymentCompleted
MQ ->> Notify : consume PaymentCompleted
Notify -> Customer : Send confirmation email
@endumlSequence diagrams document how participants — such as users, services, and databases — exchange messages over time. They are most often used to specify API request/response flows, authentication and authorization logic, microservice interactions, and event-driven processes, because they make the order of calls and the handling of error paths explicit.
Yes. With Diagramming AI you describe the interaction in plain English — the participants and the messages between them — and the AI generates an editable Mermaid or PlantUML sequence diagram. You can then refine it through follow-up chat instructions and export the result.
Both are excellent. Mermaid is lightweight, renders well in Markdown and GitHub, and is ideal for quick documentation. PlantUML offers richer styling and more advanced sequence features for detailed UML modeling. Diagramming AI supports both, so you can pick whichever fits your toolchain and switch later.
A flowchart shows the steps of a single process and the decisions between them, regardless of who performs each step. A sequence diagram emphasizes the participants and the timed messages exchanged between them, which makes it better for documenting interactions between systems, services, or objects rather than a single linear procedure.
Continue with related guides, browse templates, or start building your own diagram in Diagramming AI.