How it works

What happens between Http.Get and the TResponse you get back.

A request, end to end

1. Build. Headers, query parameters, and the body are prepared. A User-Agent is added unless you set one.
2. Connect. https:// URLs load OpenSSL on first use. http:// URLs go through a proxy when one is set.
3. Follow redirects. Cookies are re-matched for each new URL, and Authorization is dropped if the redirect leaves the host.
4. Retry if it is safe. See below.
5. Return. Any HTTP status becomes a TResponse. A transport failure raises ETimeoutError, ENetworkError, or ERequestError, or fills TRequestResult for Try* methods.

Records with management operators free everything automatically: there is no client object to free, and Response.JSON is owned by the response.

Retries

• Only GET, HEAD, PUT, and DELETE are retried. These are safe to repeat; POST and PATCH could create or change something twice.
• Retried: network errors, 429 Too Many Requests, 503 Service Unavailable. Not retried: timeouts, other statuses.
• Waits double each time: 250 ms, then 500 ms by default.
• Retry-After replaces the wait. If it is longer than the timeout, the response is returned straight away so your code can decide.
• A download is not retried once bytes have been written.

Change it with Http.SetRetry(Count, FirstDelayMS); SetRetry(0, 0) turns it off.

Cookies and redirects in a session

• Set-Cookie headers on redirect responses are kept, so a login that answers with a redirect still logs you in.
• Each cookie remembers its Domain and Path and is only sent where they match. Secure cookies go over HTTPS only. Expired cookies are removed.
• When a redirect moves to another host, the session’s own cookies and the Authorization header stay behind.

Streaming downloads

• The body is written as it arrives, so a 4 GB file needs little memory.
• Bytes go to file.part. Only a complete 2xx response renames it to the real name, so an existing file is never half-overwritten.
• Error statuses leave the file alone and put the error body in Response.Text.
• A body shorter than Content-Length raises ENetworkError; FPC alone would accept it silently.
• Raising an exception in the progress callback cancels the download.

Uploads work the same way in reverse: PostMultipart reads each file from disk while sending.

Errors

Exception
└── ERequestError        any Request-FP failure
    └── ENetworkError    connection, DNS, socket, TLS, server closed early
        └── ETimeoutError connect timeout or I/O timeout

Catch the most specific class first. HTTP error statuses are not exceptions unless you call Response.RaiseForStatus.

See Technical details for FPC and OpenSSL internals.