How to capture the history of ticking tables
This guide will show you how to capture the history of ticking tables.
Append-only tables are very simple. New rows are added at the bottom of the table, and rows are never deleted or modified. This makes examining the history of append-only tables very easy. If a table is not append-only, rows are added, deleted, and modified, which makes examining the history more complex. By using snapshotHistory, you can capture the history of a table, even if it is not append-only.
Syntax#
The snapshotHistory operation produces an in-memory table containing the history of the source table. Values are added to the history every time the trigger table ticks.
history = trigger.snapshotHistory(source)note
The trigger table is often a time table, a special type of table that adds new rows at a regular, user-defined interval. The sole column of a time table is Timestamp.
caution
Columns from the trigger table appear in the result table. If the trigger and source tables have columns with the same name, an error will be raised. To avoid this problem, rename conflicting columns.
caution
Because snapshotHistory stores a copy of the source table for every trigger event, large source tables or rapidly changing trigger tables can result in large memory usage.
Include a history#
In this example, there are two input tables. The source table updates every 0.01 seconds with new data. The trigger table updates every second, triggering a new snapshot of the source table to be added to the result table. This design pattern is useful for examining the history of a table.
- Python
- Groovy
from deephaven.TableTools import timeTableimport random
source = timeTable("00:00:00.01").update("X = i%2 == 0 ? `A` : `B`", "Y = (int) random.randint(0, 100)", "Z = sqrt(Y)").lastBy("X")
trigger = timeTable("00:00:01").renameColumns("TriggerTimestamp = Timestamp")
result = trigger.snapshotHistory(source)source = timeTable("00:00:00.01").update("X = i%2 == 0 ? `A` : `B`", "Y = new Random().nextInt(100)", "Z = sqrt(Y)").lastBy("X")
trigger = timeTable("00:00:01").renameColumns("TriggerTimestamp = Timestamp")
result = trigger.snapshotHistory(source)