Available languages: [Python](https://databento.com/docs/api-live/basics/slow-reader-behavior.python.md) · [C++](https://databento.com/docs/api-live/basics/slow-reader-behavior.cpp.md) · [Rust](https://databento.com/docs/api-live/basics/slow-reader-behavior.rust.md) · [Raw](https://databento.com/docs/api-live/basics/slow-reader-behavior.raw.md)

Source: https://databento.com/docs/api-live/basics/slow-reader-behavior


### Slow reader behavior

Occasionally, a client session's data processing may lag behind real-time data. This can happen due
to multiple reasons, such as a spike in messaging volume from the venue, network fluctuations
between the client and the server, or an inability of the client application to process messages
fast enough to keep up with the venue's message flow.

The live API allows clients to configure different behaviors when this happens via the
`slow_reader_behavior` parameter in the [Live constructor](https://databento.com/docs/api-live/client/live.md).
This parameter can be set to the values `skip` or `warn`, and defaults to `skip` when not set.
This parameter cannot be changed after the session is authenticated.

#### Skip

If the slow reader behavior is set to `skip`, whenever the gateway detects a client falling
behind real-time data, an `ErrorMsg` with error code `SKIPPED_RECORDS_AFTER_SLOW_READING`
will be sent to the client and the session will skip records until caught up with real-time
data.

This behavior only applies to subscriptions for the following schemas: MBP-1, MBP-10,
BBO-1s, BBO-1m, CMBP-1, CBBO-1s, and CBBO-1m. Subscriptions to other schemas will
always use the `warn` behavior.

#### Warn

If the slow reader behavior is set to `warn`, whenever the gateway detects a client falling
behind real-time data, a `SystemMsg` with system code `SLOW_READER_WARNING` will be sent
to the client. The session will not skip any records and will attempt to replay every
single record in the subscription.

In some cases, setting `warn` as the slow reader behavior can lead to increasing lag
between the session and real-time data.

#### Data aged out of retention

A replay can fall behind far enough that the data it was reading is removed, i.e. it now
exceeds the [supported replay window](https://databento.com/docs/api-live/basics/intraday-replay.md).
Sessions whose effective behavior is `skip` resume from the earliest retained records.
Sessions using `warn`, either by request or because the schema is incompatible with
skipping, receive a fatal `ErrorMsg` with error code `REPLAY_DATA_AGED_OUT` naming the
earliest available `start` time.