这可能是一 jar 蠕虫，我会尽力描述这个问题。我们有一个长期运行的数据处理工作。我们的行动数据库会每晚添加一次，并且会处理未完成的行动。处理每晚的操作大约需要15分钟。在Vapor 2中，我们利用了很多原始查询来创建PostgreSQL游标并循环遍历它，直到它为空。

目前，我们通过命令行参数运行该处理。将来，我们希望它作为主服务器的一部分运行，以便在执行处理时可以检查进度。

func run(using context: CommandContext) throws -> Future<Void> {
    let table = "\"RecRegAction\""
    let cursorName = "\"action_cursor\""
    let chunkSize = 10_000


    return context.container.withNewConnection(to: .psql) { connection in
        return PostgreSQLDatabase.transactionExecute({ connection -> Future<Int> in

            return connection.simpleQuery("DECLARE \(cursorName) CURSOR FOR SELECT * FROM \(table)").map { result in
                var totalResults = 0
                var finished : Bool = false

                while !finished {
                    let results = try connection.raw("FETCH \(chunkSize) FROM \(cursorName)").all(decoding: RecRegAction.self).wait()
                    if results.count > 0 {
                        totalResults += results.count
                        print(totalResults)
                        // Obviously we do our processing here
                    }
                    else {
                        finished = true
                    }
                }

                return totalResults
            }
        }, on: connection)
    }.transform(to: ())
}

最佳答案

好的，正如您所知道的，问题出在以下几行:

while ... {
    ...
    try connection.raw("...").all(decoding: RecRegAction.self).wait()
    ...
}

/// delivers partial results (integers) and `nil` if no further elements are available
func deliverPartialResult() -> EventLoopFuture<Int?> {
    ...
}

func deliverFullResult() -> EventLoopFuture<[Int]>

func accumulateResults(eventLoop: EventLoop,
                       partialResultsSoFar: [Int],
                       getPartial: @escaping () -> EventLoopFuture<Int?>) -> EventLoopFuture<[Int]> {
    // let's run getPartial once
    return getPartial().then { partialResult in
        // we got a partial result, let's check what it is
        if let partialResult = partialResult {
            // another intermediate results, let's accumulate and call getPartial again
            return accumulateResults(eventLoop: eventLoop,
                                     partialResultsSoFar: partialResultsSoFar + [partialResult],
                                     getPartial: getPartial)
        } else {
            // we've got all the partial results, yay, let's fulfill the overall future
            return eventLoop.newSucceededFuture(result: partialResultsSoFar)
        }
    }
}

func deliverFullResult() -> EventLoopFuture<[Int]> {
    return accumulateResults(eventLoop: myCurrentEventLoop,
                             partialResultsSoFar: [],
                             getPartial: deliverPartialResult)
}

accumulateResults(eventLoop: el, partialResultsSoFar: []) {
    connection.raw("FETCH \(chunkSize) FROM \(cursorName)")
              .all(decoding: RecRegAction.self)
              .map { results -> Int? in
        if results.count > 0 {
            return results.count
        } else {
            return nil
        }
   }
}.map { allResults in
    return allResults.reduce(0, +)
}

func accumulateResults<T>(eventLoop: EventLoop,
                          getPartial: @escaping () -> EventLoopFuture<T?>) -> EventLoopFuture<[T]> {
    // this is an inner function just to hide it from the outside which carries the accumulator
    func accumulateResults<T>(eventLoop: EventLoop,
                              partialResultsSoFar: [T] /* our accumulator */,
                              getPartial: @escaping () -> EventLoopFuture<T?>) -> EventLoopFuture<[T]> {
        // let's run getPartial once
        return getPartial().then { partialResult in
            // we got a partial result, let's check what it is
            if let partialResult = partialResult {
                // another intermediate results, let's accumulate and call getPartial again
                return accumulateResults(eventLoop: eventLoop,
                                         partialResultsSoFar: partialResultsSoFar + [partialResult],
                                         getPartial: getPartial)
            } else {
                // we've got all the partial results, yay, let's fulfill the overall future
                return eventLoop.newSucceededFuture(result: partialResultsSoFar)
            }
        }
    }
    return accumulateResults(eventLoop: eventLoop, partialResultsSoFar: [], getPartial: getPartial)
}

func processPartialResults<T, V>(eventLoop: EventLoop,
                                 process: @escaping (T) -> EventLoopFuture<V>,
                                 getPartial: @escaping () -> EventLoopFuture<T?>) -> EventLoopFuture<V?> {
    func processPartialResults<T, V>(eventLoop: EventLoop,
                                     soFar: V?,
                                     process: @escaping (T) -> EventLoopFuture<V>,
                                     getPartial: @escaping () -> EventLoopFuture<T?>) -> EventLoopFuture<V?> {
        // let's run getPartial once
        return getPartial().then { partialResult in
            // we got a partial result, let's check what it is
            if let partialResult = partialResult {
                // another intermediate results, let's call the process function and move on
                return process(partialResult).then { v in
                    return processPartialResults(eventLoop: eventLoop, soFar: v, process: process, getPartial: getPartial)
                }
            } else {
                // we've got all the partial results, yay, let's fulfill the overall future
                return eventLoop.newSucceededFuture(result: soFar)
            }
        }
    }
    return processPartialResults(eventLoop: eventLoop, soFar: nil, process: process, getPartial: getPartial)
}

关于swift - 使用Vapor 3创建和使用游标，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/51625050/