Because it contains a sub-pipeline, it is a compound step.
因为它包含子管道，所以是一个复合步骤。
Variables: variables are used with compound steps and define XPath variables for use within a compound step sub-pipeline.
变量：变量和复合步骤一起使用，定义在复合步骤子管道中使用的Xpath变量。
Since the indexer is only one part of the pipeline a Tweet has to make it through, we needed the indexer itself to have a sub-second latency.
由于索引器本身只是推消息处理过程中的一个环节，所以光索引器本身的延时必须保证在亚秒级别。
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