What is an UpdateGraphConflictException and how do I fix it?

My code is throwing an UpdateGraphConflictException, but I don't know what that means or why it's happening. Why does this happen and how can I fix it?

There are primarily two ways this exception can occur.

1. You are running the engine outside of a Deephaven server.

The Deephaven engine can be run outside of the context of a Deephaven server, such as in a standalone Java application. When run this way, some additional work is required to ensure that the current thread is aware of the necessary engine state when constructing a refreshing/ticking table. If you don't do this, you will likely encounter an UpdateGraphConflictException.

Consider a TablePublisher, which produces a blink table from added tables. This is a common way to construct a ticking table from a variety of streaming data sources. When run from a Deephaven server, the necessary engine state is set up automatically. However, run outside of the server, this work must be done manually.

The code below constructs an execution context in such a way that it is aware of the update graph. The update graph is passed into the TablePublisher constructor, which is necessary to ensure that the publisher can properly refresh the table when new data is available.

final UpdateGraph tableUpdateGraph = PeriodicUpdateGraph.newBuilder("symbolIndexGraph").build();

final ExecutionContext dhEngineContext = ExecutionContext.newBuilder()
    .setUpdateGraph(tableUpdateGraph)
    .markSystemic() // this tells the engine this context is "system-wide" instead of "user" specific
    .build();

// On any thread you want to perform DH table operations needs to have the execution context set; it's ThreadLocal state.
dhEngineContext.open();

try {
    // Should now be able to create a table publisher.
    final TablePublisher tablePublisherSymbolIndex = TablePublisher.of(
            TABLE_SYMBOL_INDEX,
            tableDefinition,
            tp -> new AtomicInteger().getAndIncrement(),
            () -> new MutableBoolean().setValue(true),
            tableUpdateGraph,
            2048
    );
} finally {
    dhEngineContext.close();
}

2. You try to combine ticking tables from separate update graphs.

Consider the following scenario, with two separate update graphs:

• Update graph A
  • Ticking table A
• Update graph B
  • Ticking table B

You want to combine these two ticking tables into a single table. Any attempt to do so will result in an UpdateGraphConflictException, because the two ticking tables are from different update graphs. The resultant table cannot possibly be consistent - the resultant table cannot live in either existing update graph, and it cannot be in a new update graph that is not the same as either of the existing graphs.

The only way to combine these tables is to take a snapshot of each to produce a static table, then combine the static tables.