sequenceDiagram
  autonumber
  participant App as Application
  participant Builder as Sender.Builder
  participant LHS as AbstractLineHttpSender
  participant HC as HttpClient
  participant S1 as Server A
  participant S2 as Server B

  App->>Builder: address(host1), port(p1), address(host2), port(p2)
  Builder-->>App: build() -> HTTP sender (multi-host)

  App->>LHS: table(...).symbol(...).flush()
  alt No protocol/version yet
    LHS->>HC: GET /settings on host[current]
    HC-->>LHS: 200 JSON { protocol, accepting.writes }
    LHS-->>App: proceed
  end

  LHS->>HC: POST /imp (lines) to host[current]
  alt 204 No Content
    HC-->>LHS: 204
    LHS-->>App: ok
  else 421 Misdirected/Not accepting
    HC-->>LHS: 421
    LHS->>LHS: rotate to next host
    LHS->>HC: retry POST on next host
    HC-->>LHS: 204
    LHS-->>App: ok
  else 5xx retriable
    HC-->>LHS: 5xx
    LHS-->>App: throw LineSenderException(retryable=true)
  end

sequenceDiagram
  autonumber
  participant Client as HTTP Client
  participant Req as Request
  participant Net as Network
  participant TLS as TLS
  Client->>Req: send(host, port, timeout)
  Req->>Net: connect(host, port)
  alt connect error
    Net-->>Req: errno
    Req-->>Client: HttpClientException(errno)
  else connect ok
    Req->>TLS: start if enabled
    alt TLS error
      TLS-->>Req: errno
      Req-->>Client: HttpClientException(errno)
    else TLS ok
      Req->>Net: write request, read response
      Net-->>Req: status/headers/body
      Req-->>Client: ResponseHeaders
    end
  end

sequenceDiagram
  autonumber
  participant Sender as Line HTTP Sender
  participant Server as HTTP Server
  participant Proc as LineHttpProcessor
  participant Engine as Cairo Engine

  Sender->>Server: POST /imp (lines)
  Server->>Proc: onHeadersReady()
  Proc->>Engine: config.isAcceptingWrites()
  alt Not accepting
    Engine-->>Proc: false
    Proc-->>Server: Status NOT_ACCEPTING_WRITES (421)
    Server-->>Sender: 421
  else Accepting
    Engine-->>Proc: true
    Proc-->>Server: proceed to ingest
    Server-->>Sender: 204
  end