Interface AsyncResultSet
- All Superinterfaces:
AutoCloseable
,ResultSet
,StructReader
- All Known Implementing Classes:
ForwardingAsyncResultSet
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Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic enum
static enum
Response code fromtryNext()
.static interface
Interface for receiving asynchronous callbacks when new data is ready. -
Method Summary
Modifier and TypeMethodDescriptionvoid
cancel()
Attempt to cancel this operation and free all resources.void
resume()
Resume callbacks from the cursor.com.google.api.core.ApiFuture<Void>
setCallback
(Executor exec, AsyncResultSet.ReadyCallback cb) Register a callback with the ResultSet to be made aware when more data is available, changing the usage pattern from sync to async.<T> List<T>
toList
(com.google.common.base.Function<StructReader, T> transformer) Transforms the row cursor into an immutable list using the given transformer function.<T> com.google.api.core.ApiFuture<List<T>>
toListAsync
(com.google.common.base.Function<StructReader, T> transformer, Executor executor) Transforms the row cursor into an immutable list using the given transformer function.tryNext()
Non-blocking call that attempts to step the cursor to the next position in the stream.Methods inherited from interface com.google.cloud.spanner.ResultSet
close, getCurrentRowAsStruct, getMetadata, getStats, next
Methods inherited from interface com.google.cloud.spanner.StructReader
getBigDecimal, getBigDecimal, getBigDecimalList, getBigDecimalList, getBoolean, getBoolean, getBooleanArray, getBooleanArray, getBooleanList, getBooleanList, getBytes, getBytes, getBytesList, getBytesList, getColumnCount, getColumnIndex, getColumnType, getColumnType, getDate, getDate, getDateList, getDateList, getDouble, getDouble, getDoubleArray, getDoubleArray, getDoubleList, getDoubleList, getJson, getJson, getJsonList, getJsonList, getLong, getLong, getLongArray, getLongArray, getLongList, getLongList, getPgJsonb, getPgJsonb, getPgJsonbList, getPgJsonbList, getProtoEnum, getProtoEnum, getProtoEnumList, getProtoEnumList, getProtoMessage, getProtoMessage, getProtoMessageList, getProtoMessageList, getString, getString, getStringList, getStringList, getStructList, getStructList, getTimestamp, getTimestamp, getTimestampList, getTimestampList, getType, getValue, getValue, isNull, isNull
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Method Details
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tryNext
Non-blocking call that attempts to step the cursor to the next position in the stream. The cursor may be inspected only if the cursor returnsCursorState.OK
.A caller will typically call tryNext in a loop inside the ReadyCallback, consuming all results available. For more information see
setCallback(Executor, ReadyCallback)
.Currently this method may only be called if a ReadyCallback has been registered. This is for safety purposes only, and may be relaxed in future.
- Returns:
- current cursor readiness state
- Throws:
SpannerException
- When an unrecoverable problem downstream occurs. Once this occurs you will get no further callbacks. You should return CallbackResponse.DONE back from callback.
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setCallback
Register a callback with the ResultSet to be made aware when more data is available, changing the usage pattern from sync to async. Details:- The callback will be called at least once.
- The callback is run each time more results are available, or when we discover that there will be no more results. (unless paused, see below). Spurious callbacks are possible, see below.
- Spanner guarantees that one callback is ever outstanding at a time. Also, future callbacks guarantee the "happens before" property with previous callbacks.
- A callback normally consumes all available data in the ResultSet, and then returns
AsyncResultSet.CallbackResponse.CONTINUE
. - If a callback returns
AsyncResultSet.CallbackResponse.CONTINUE
with data still in the ResultSet, the callback is invoked again immediately! - Once a callback has returned
AsyncResultSet.CallbackResponse.PAUSE
on the cursor no more callbacks will be run until a correspondingresume()
. - Callback will stop being called once any of the following occurs:
- Callback returns
AsyncResultSet.CallbackResponse.DONE
. ForwardingAsyncResultSet.tryNext()
returnsAsyncResultSet.CursorState.DONE
.ForwardingAsyncResultSet.tryNext()
throws an exception.
- Callback returns
- Callback may possibly be invoked after a call to
ForwardingAsyncResultSet.cancel()
call, but the subsequent call totryNext()
will yield a SpannerException. - Spurious callbacks are possible where cursors are not actually ready. Typically callback
should return
AsyncResultSet.CallbackResponse.CONTINUE
any time it seesAsyncResultSet.CursorState.NOT_READY
.
Flow Control
If no flow control is needed (say because result sizes are known in advance to be finite in size) then async processing is simple. The following is a code example that transfers work from the cursor to an upstream sink:
Flow control may be needed if for example the upstream system may not always be ready to handle more data. In this case the app developer has two main options:@Override public CallbackResponse cursorReady(ResultSet cursor) { try { while (true) { switch (cursor.tryNext()) { case OK: upstream.emit(cursor.getRow()); break; case DONE: upstream.done(); return CallbackResponse.DONE; case NOT_READY: return CallbackResponse.CONTINUE; } } } catch (SpannerException e) { upstream.doneWithError(e); return CallbackResponse.DONE; } }
- Semi-async: make
upstream.emit()
a blocking call. This will block the callback thread until progress is possible. When coding in this way the threads in the Executor provided to setCallback must be blockable without causing harm to progress in your system. - Full-async: call
cursor.pause()
and return from the callback with data still in the Cursor. Once in this state cursor waits until resume() is called before calling callback again.
- Parameters:
exec
- executor on which to run all callbacks. Typically use a threadpool. If the executor is one that runs the work on the submitting thread, you must be very careful not to throw RuntimeException up the stack, lest you do damage to calling components. For example, it may cause an event dispatcher thread to crash.cb
- ready callback- Returns:
- An
ApiFuture
that returnsnull
when the consumption of theAsyncResultSet
has finished successfully. No more calls to theAsyncResultSet.ReadyCallback
will follow and all resources used by theAsyncResultSet
have been cleaned up. TheApiFuture
throws anExecutionException
if the consumption of theAsyncResultSet
finished with an error.
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cancel
void cancel()Attempt to cancel this operation and free all resources. Non-blocking. This is a no-op for child row cursors and does not cancel the parent cursor. -
resume
void resume()Resume callbacks from the cursor. If there is more data available, a callback will be dispatched immediately. This can be called from any thread. -
toListAsync
<T> com.google.api.core.ApiFuture<List<T>> toListAsync(com.google.common.base.Function<StructReader, T> transformer, Executor executor) Transforms the row cursor into an immutable list using the given transformer function.transformer
will be called once per row, thus the returned list will contain one entry per row. The returned future will throw aSpannerException
if the row cursor encountered any error or if the transformer threw an exception on any row.The transformer will be run on the supplied executor. The implementation may batch multiple transformer invocations together into a single
Runnable
when possible to increase efficiency. At any point in time, there will be at most one invocation of the transformer in progress.WARNING: This will result in materializing the entire list so this should be used judiciously after considering the memory requirements of the returned list.
WARNING: The
RowBase
object passed to transformer function is not immutable and is not guaranteed to remain valid after the transformer function returns. The sameRowBase
object might be passed multiple times to the transformer with different underlying data each time. So *NEVER* keep a reference to theRowBase
outside of the transformer. Specifically do not useFunctions.identity()
function.- Parameters:
transformer
- function which will be used to transform the row. It should not return null.executor
- executor on which the transformer will be run. This should ideally not be an inline executor such asMoreExecutors.directExecutor()
; using such an executor may degrade the performance of the Spanner library.
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toList
<T> List<T> toList(com.google.common.base.Function<StructReader, T> transformer) throws SpannerExceptionTransforms the row cursor into an immutable list using the given transformer function.transformer
will be called once per row, thus the returned list will contain one entry per row. This method will block until all the rows have been yielded by the cursor.WARNING: This will result in consuming the entire list so this should be used judiciously after considering the memory requirements of the returned list.
WARNING: The
RowBase
object passed to transformer function is not immutable and is not guaranteed to remain valid after the transformer function returns. The sameRowBase
object might be passed multiple times to the transformer with different underlying data each time. So *NEVER* keep a reference to theRowBase
outside of the transformer. Specifically do not useFunctions.identity()
function.- Parameters:
transformer
- function which will be used to transform the row. It should not return null.- Throws:
SpannerException
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