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layout	title	nav_order	parent	grand_parent
default	Remote segment backpressure	10	Remote-backed storage	Availability and recovery

Remote segment backpressure

This is an experimental feature and is not recommended for use in a production environment. For updates on the progress of the feature or if you want to leave feedback, see the associated GitHub issue. {: .warning}

Remote segment backpressure is a shard-level rejection mechanism that dynamically rejects indexing requests when the remote segment store falls behind the local committed segments on the primary shard. With remote segment backpressure, you can prevent the lag between the remote store and the local primary store. The lag can be caused by slow or failed remote store interaction, remote store throttling, long garbage collection pauses, or high CPU utilization.

Thresholds

Remote segment backpressure is activated if any of the following thresholds is breached:

Consecutive failure: The backpressure is activated if there are N or more consecutive failures. The value of N is configurable in remote_store.segment.pressure.consecutive_failures.limit.
Bytes lag: The bytes lag is computed by adding the size of all files that are present in local committed segments but not in the remote store. The backpressure is activated if the bytes lag is greater than K multiplied by the moving average of the size (in bytes) of files uploaded after each refresh. The variance factor K is configurable in remote_store.segment.pressure.bytes_lag.variance_factor. The moving window size is configurable in remote_store.segment.pressure.upload_bytes_moving_average_window_size.
Time lag: The time lag is computed by comparing the timestamps of the most recent local refresh and the most recent remote store segment upload. The backpressure is activated if the time lag is greater than K multiplied by the moving average of the time taken to upload new segments and metadata file after each refresh. The variance factor K is configurable in remote_store.segment.pressure.time_lag.variance_factor. The moving window size is configurable in remote_store.segment.pressure.upload_time_moving_average_window_size.

Handling segment merges

At every segment merge, a corresponding refresh is initiated. Because this refresh has new merged segments, the bytes lag instantly spikes. To compensate for this spike, the bytes lag and time lag are evaluated only if the remote store is behind the local primary store by more than one refresh. However, backpressure induced by consecutive failures activates regardless of refresh lag (the number of refreshes by which the remote store is lagging behind the local store).