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Version: Python

Interpret Barrage metrics

Barrage is an extension of Apache Arrow Flight, with a particular focus on incrementally updating data sets. This is a guide of what statistics Deephaven records on Barrage activity.

Metrics overview

Life of a Barrage update

  1. UpdateGraphProcessor Source Table Updates

Barrage listens to UpdateGraphProcessor (UGP) changes from the source table. Whenever the source table ticks, Barrage records enqueueMillis, the time it took to record relevant information.

///////////    ///////////    ///////////
// Delta // -> // Delta // -> // Delta //
/////////// /////////// ///////////
  1. UpdateGraphProcessor.targetCycleDurationMillis versus barrage.minUpdateInterval

The UGP ticks at an interval specified by the parameter -DUpdateGraphProcessor.targetCycleDurationMillis. Barrage ticks at an interval specified by the parameter -Dbarrage.minUpdateInterval (ms). Barrage records AggregateMillis, the time it took to coalesce all upstream updates into a single update.

/////////////////////
// Coalesced Delta //
/////////////////////
  1. Synchronizing Barrage state to the UGP update cycle

New subscriptions and subscription changes require initializing table state built from a series of snapshots. Barrage records SnapshotMillis, the time it took to retrieve a single snapshot. The generated snapshot occurs within the stream of updates. Due to raciness on table updates versus acquiring a snapshot. There might be coalesced deltas on either side (or both) of the snapshot.

/////////////////////    //////////////    /////////////////////
// Coalesced Delta // -> // Snapshot // -> // Coalesced Delta //
///////////////////// ////////////// /////////////////////
  1. Propagation to gRPC listeners

Each delta and snapshot is then propagated to subscribers. Barrage records PropagateMillis, the time it took to pass the message to the list of listeners.

note

Barrage records UpdateJobMillis, the aggregate time it took to coalesce deltas, fetch the snapshot, propagate, and housekeep.

  1. Writing to the OutputStream

Another thread writes the resulting bytes to the actual gRPC stream. Barrage records WriteMillis, the time it took to drain the subscriber-specific filtered view of the update (including coalesced and snapshots) to the OutputStream. Barrage records WriteMegabits, the number of megabits (Mb) that were written on a single message. The choice to record this in Mb is for easy comparison against the bandwidth allowed by the connected hardware.

  1. Receiver bundles gRPC messages into an update (coalesced or snapshot)

Barrage records DeserializationMillis, the time it took to read and parse the data from the InputStream to assemble an entire barrage message.

/////////////////////////    /////////////////////////    /////////////////////////
// Deserializied Delta // -> // Deserializied Delta // -> // Deserializied Delta //
///////////////////////// ///////////////////////// /////////////////////////
  1. Receiver bundles gRPC messages into an update (coalesced or snapshot)

Barrage records ProcessUpdateMillis, the amount of time it took to apply a single deserialized delta.

The Barrage Table refreshes once per the -DUpdateGraphProcessor.targetCycleDurationMillis interval. Potentially, many messages have arrived over the wire during this time. Barrage records RefreshMillis, the amount of time it took to coalesce deltas and to propagate the result to any receiver-side table listeners.

Life of a snapshot

In addition to subscribing to ticking data, we support fetching a full synchronized snapshot of a table. This enables functionality such as Arrow FlightService's DoGet.

  1. Request is received and queued

The snapshot request is queued for processing. Barrage records QueueMillis, the time the message queued waiting for an available thread to satisfy the snapshot request.

  1. The snapshot is constructed

The snapshot request is then fulfilled. The process typically can occur concurrently with the UGP, but larger snapshots may require holding the UGP exclusive lock. Barrage records SnapshotMillis, the time it took to construct the snapshot for the listener.

Similar to subscription requests, Barrage records WriteMillis and WriteMegabits, the time it took to write and how many Mb were written.

Identifying a table

Tables are identified by their TableId and TableKey. The TableId is determined by the source table's System.identityHashCode(). The TableKey defaults to Table#getDescription() but can be overridden by setting a table attribute.

from deephaven import empty_table

t = empty_table(0)
table = jpy.get_type("io.deephaven.engine.table.Table")
t.j_table.setAttribute(table.BARRAGE_PERFORMANCE_KEY_ATTRIBUTE, "MyTableKey")
note

The web client applies transformations to every table that it subscribes to. If a table is also subscribed by a non-web client then statistics for the original table and the transformed table will both appear in the metrics table. Their TableId will differ. Most transformations clear the TableKey attribute, such as when a column is sorted or filtered by the user through the GUI.

Extra configuration

Here are server side flags that change the behavior of Barrage metrics.

  • -DBarragePerformanceLog.enableAll: record metrics for tables that do not have an explicit TableKey (default: true)
  • -DBarragePerformanceLog.cycleDurationMillis: the interval to flush aggregated statistics (default: 60000 - once per minute)