Session parking

Keep a Realtime session open without holding compute capacity during long stretches of silence

How session parking works

Speechmatics detects when a session's audio rate has stayed significantly below real time, eg less than 50%, for a while and parks the session automatically.

Speechmatics acknowledges audio sent while parked, so client-side flow control keeps working. When the audio stream returns to close to real time, the session resumes on its own.

A parked session's transcript timeline (timestamps and speaker labels) either continues across the pause or restarts, depending on preserve_timeline, covered in Pausing and resuming explicitly.

Pausing and resuming explicitly

You can also pause and resume a session explicitly with PauseRecognition and ResumeRecognition. This is the best fix for an integration that needs connections prewarmed and ready: pause instead of holding the connection open at a low rate.

Resuming a paused session is faster than closing the connection and opening a new one. A new session pays for a fresh WebSocket connection — DNS lookup, TCP handshake, TLS handshake — followed by a StartRecognition / RecognitionStarted round trip; a large custom dictionary in StartRecognition can add hundreds of milliseconds to that round trip on its own.

ResumeRecognition skips all of that: the connection is already open, so resuming is a single message on it. Depending on network distance and throughput, that can save on the order of 1.5 seconds compared to opening a new session.

The following table shows messages involved in explict session parking:

Client sends	Server replies
{"message": "PauseRecognition"}	{"message": "RecognitionPaused"}
{"message": "PauseRecognition", "preserve_timeline": true}	{"message": "RecognitionPaused"}
{"message": "ResumeRecognition"}	{"message": "RecognitionResumed"}

messagerequired

Constant value: PauseRecognition

preserve_timelineboolean

Keep the transcript timeline (timestamps, speaker labels) continuous across this pause. Optional; without it, the session resumes as a new one when it comes back: timestamps restart from zero and speaker labels are not carried over.

messagerequired

Constant value: ResumeRecognition

RecognitionPaused is sent once the transcript for any audio already sent has arrived and the session's compute has been released. RecognitionResumed is sent once the session is transcribing again.

Send audio immediately after ResumeRecognition, without waiting for RecognitionResumed. Waiting for the acknowledgement adds a full round trip to time-to-first-final, depending on network conditions even 200-500ms, resumed session accepts and queues that audio correctly.

preserve_timeline is optional and applies to that pause only:

• Omitted, or false — the session resumes as a new one. Timestamps restart from zero and speaker labels are not carried over.
• true — the transcript timeline continues: timestamps and speaker labels carry across the pause.

A session that hasn't sent any audio yet, or that can't preserve its timeline at all, always resumes as a new session regardless of preserve_timeline.

If you send PauseRecognition on a session that can't be parked, the server replies with a Warning of type not_allowed and the session continues normally: pausing was refused, not the connection.

Session limits while parked

A parked session still counts toward your account's concurrent session limit — parking releases compute, not the connection.

The account's session limits continue to apply, but which ones depends on how the session was parked:

• Parked automatically, because of extremely low audio rate — both the idle timeout and the max duration limit can end the session.
• Parked explicitly with PauseRecognition — exempt from the idle timeout for as long as it stays parked. The max duration limit still applies.

Speechmatics caps how much audio it buffers while a session is parked. If the cap is reached, Speechmatics drops the oldest buffered audio to make room for new audio — this can only happen with preserve_timeline: true, since audio isn't buffered for replay otherwise.

Where session parking is available

Session parking is available on SaaS on Cloud and when RT SaaS helm chart is used for on Prem.