came back online? • How long do we have to wait before we decide that we have seen all events? ## Watermarks ## Stream progress  confident that no more delayed events will arrive. • Watermarks provide a logical clock which informs the system about the current event time. Watermarks (in Flink) flow along dataflow edges. They are special upstream stages • minimum of output watermarks of all upstream tasks • The output watermark captures the progress of the stage itself • minimum of input watermarks and event-times of non-late data ##

对于使用事件时间语义的应用程序来说，watermarks随着时间的推移而变化是非常重要的。watermarks的时间t表名框架再也不应该期望接收到时间戳比t早的事件了，相反，那些时间戳小于t的operations将会被触发的触发。例如，当watermarks超过30时，结束于t=30的事件时间窗口将被关闭并计算。 因此，您应该在应用程序中对事件时间敏感的operators(如流程函数和窗口)上监控watermarks。如果当前处理时间与被称为even-time n-time skew的watermarks之间的差异非常高，那么它通常意味着可能会出现两种情况。首先，它可能意味着您只是在处理旧的事件，例如在停机后的追赶期间，或者当您的工作无法继续，而事件正在排队时。其次，它可能意味着单个上游子任务很长时间没有发送watermarks(例如因为它没有收到任何基于watermarks的事件)，这也阻止了下游操作符中的watermarks的进展。 ### 4.6

t.getExecutionEnvironment(); env.setStreamTimeCharacteristic(TimeCharacteristic.EventTime); ## Watermarks 23 19 22 24 21 14 17 13 12 15 9 11 7 2 4 → • Data may arrive out of order • Sorting data is expensive and may not always be required • Watermark is a good heuristic to bound out of orderness ## Watermarks DataStream stream = ... DataStream withTimestampsAndWatermarks = stream.as

#34;), "1 hour", "5 minutes")) ▶ Spark streaming uses watermarks to measure progress in event time. ▶ Watermarks flow as part of the data stream and carry a timestamp t. ▶ A $ W(t) $

windows and transformations might not be suitable: • they provide access to record timestamps and watermarks • they can register timers that trigger at a specific time in the future ProcessFunction, KeyedProcessFunction

from upstream and processes them in parallel with the primary but it doesn't output results • Watermarks are used to identify duplicate output tuples and trim the secondary's output queue • Negligible

aggregations, distinct... • State is commonly partitioned by key - State can be cleared based on watermarks or punctuations • window fires, post becomes inactive ## Example: Apache Flink DataStream API

aggregations, distinct... - State is commonly partitioned by key - State can be cleared based on watermarks or punctuations • window fires, post becomes inactive ## Operator selectivity • The number of

• How can we perform iterative computation in a streaming dataflow engine? How can we propagate watermarks? • Do we need to run the computation from scratch for every new edge? - Can we use graph synopses

document.pdf") # save the document with a new filename ## Note: Taking it further Adding watermarks is essentially as simple as adding an image at the base of each page. You should ensure that the background or foreground image for the page, like a copyright or a watermark. Please remember, that watermarks require a transparent image if put in foreground. 4. The image may be inserted uncompressed, e separate output page (see posterize.py). • include PDF-based vector images like company logos, watermarks, etc., see $ svg-logo.py $ , which puts an SVG-based logo on each page (requires additional packages