Github user bowenli86 commented on a diff in the pull request:
https://github.com/apache/flink/pull/5949#discussion_r185588020
--- Diff: docs/dev/event_time.md ---
@@ -35,30 +35,32 @@ Flink supports different notions of *time* in streaming
programs.
respective operation.
When a streaming program runs on processing time, all time-based
operations (like time windows) will
- use the system clock of the machines that run the respective operator.
For example, an hourly
+ use the system clock of the machines that run the respective operator.
An hourly
processing time window will include all records that arrived at a
specific operator between the
- times when the system clock indicated the full hour.
+ times when the system clock indicated the full hour. For example, if
an application
+ begins running at 9:15am, the first hourly processing time window will
include events
+ processed between 9:15am and 10:00am, the next window will include
events processed between 10:00am and 11:00am, and so on.
Processing time is the simplest notion of time and requires no
coordination between streams and machines.
It provides the best performance and the lowest latency. However, in
distributed and asynchronous
environments processing time does not provide determinism, because it
is susceptible to the speed at which
- records arrive in the system (for example from the message queue), and
to the speed at which the
- records flow between operators inside the system.
+ records arrive in the system (for example from the message queue), to
the speed at which the
+ records flow between operators inside the system, and to outages
(scheduled, or otherwise).
- **Event time:** Event time is the time that each individual event
occurred on its producing device.
- This time is typically embedded within the records before they enter
Flink and that *event timestamp*
- can be extracted from the record. An hourly event time window will
contain all records that carry an
- event timestamp that falls into that hour, regardless of when the
records arrive, and in what order
- they arrive.
+ This time is typically embedded within the records before they enter
Flink, and that *event timestamp*
+ can be extracted from each record. An hourly event time window will
contain all records that carry an
--- End diff --
better mention allowed lateness here. â...will contain all records, ...,
regardless of when the records arriveâ sounds too absolute, the guarantee can
only be achieved with lateness requirements
---
