Is there a way to slow down a Web Scraper so it wi

Question

I wrote a macro to go to WU to get historical data and for the most part, it works. However, I believe that the macro is running too fast for it to pick up the data from the website.

https://www.wunderground.com/history/daily/us/tx/el-paso/KELP/date/2017-1-3 Is the website and the table I want to get is tablesaw-sortable.

I have tried the following: DoEvents and Application.Wait (Now + TimeValue("00:00:01")) to try to slow down the process.

Sub BrowseToWU()

    Dim IE As New SHDocVw.InternetExplorer
    Dim HTMLDoc As MSHTML.HTMLDocument
    Dim RowAddress   As Integer
    Dim WebAddress As String
    Dim DateSheet As Date
    Dim WkDay As Integer
    Dim DateSheetName As String

    'Application.ScreenUpdating = False
    'Application.StatusBar = True
    RowAddress = 2
    IE.Visible = True
    Do Until RowAddress = 60

    WebAddress = Range("A" & RowAddress)
    DateSheet = Right(WebAddress, 8)
    DateSheetName = Right(WebAddress, 8)
    WkDay = Weekday(DateSheet, vbSunday)

    If WkDay < 3 Then
        RowAddress = RowAddress + 1

        ElseIf WkDay > 6 Then
            RowAddress = RowAddress + 1

        Else

        IE.Navigate WebAddress

            Do While IE.ReadyState <> READYSTATE_COMPLETE
            Loop


            Set HTMLDoc = IE.Document
            DoEvents

            Application.Wait (Now + TimeValue("00:00:05"))
            DoEvents

            ProcessHTMLPage HTMLDoc

            DateSheet = Right(WebAddress, 8)
            DoEvents
            Application.Wait (Now + TimeValue("00:00:01"))
            ActiveSheet.Name = DateSheetName

            DoEvents

            RowAddress = RowAddress + 1
            'IE.Quit

            Worksheets("Sheet1").Activate
        End If

    Loop

End Sub

Option Explicit

Sub ProcessHTMLPage(HTMLPage As MSHTML.HTMLDocument)

    Dim HTMLTable As MSHTML.IHTMLElement
    Dim HTMLTables As MSHTML.IHTMLElementCollection
    Dim HTMLRow As MSHTML.IHTMLElement
    Dim HTMLCell As MSHTML.IHTMLElement
    Dim RowNum As Long, ColNum As Integer
    'Dim IE As New SHDocVw.InternetExplorer
    'Dim Ws As Worksheet

    Set HTMLTables = HTMLPage.getElementsByClassName("tablesaw-sortable")
    'DoEvents

    For Each HTMLTable In HTMLTables

        Worksheets.Add
        DoEvents

        Range("A1").Value = HTMLTable.className
        Range("B1").Value = Now

        RowNum = 2

        For Each HTMLRow In HTMLTable.getElementsByTagName("tr")
            'Debug.Print vbTab & HTMLRow.innerText

            ColNum = 1
            For Each HTMLCell In HTMLRow.Children
                Cells(RowNum, ColNum) = HTMLCell.innerText
                ColNum = ColNum + 1


            Next HTMLCell
                RowNum = RowNum + 1
        Next HTMLRow
    Next HTMLTable
    DoEvents

    'IE.Quit

End Sub

1. The macro is supposed to run through sheet1 picking up the web address to the historical data if it satisfies the criteria of being a certain day of the week.

2. IE will open and then it will kick over to the next module that will take in the data.

3. A new worksheet is created and the data pasted into the new worksheet.

4. The worksheet is renamed to the date of the data.

5. The web address sheet is activated again and the process starts over again.

The error I get is that the data isn't taken from the website, so the For statement ends and the web address sheet is renamed and an error occurs.

Answer 1

One way around this is to call the API that the page is using to get that info.

The API returns json which you can parse with a json parser. I use jsonconverter.bas. After installing the code from that link in a standard module called JsonConverter, go to VBE > Tools > References > Add a reference to Microsoft Scripting Runtime.

---

Finding the API:

If you press F12 to open developer tools and go to the Network tab and then press F5 to refresh any url of interest you will see the recorded web traffic. You can find the API call there.

See my answer here on how to search the network traffic using a specific observation value you expect to see in the response - this will filter the list of network traffic to those items containing the value of interest. Be sensible in selecting the value - you want something unlikely to occur elsewhere. You can also filter the network traffic to XHR only.

---

The API response:

The API returns json. More specifically, it returns a dictionary containing 2 keys. The second key, "observations", can be used to return a collection (denoted by []) of dictionaries (denoted by {}). Each dictionary represents a row of the table (daily observations). You can loop this collection, and then loop the inner dictionaries, to access the table row values and reconstruct the table by populating an array. Explore example json response here.

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Explanation of json structure:

_{click here to enlarge}

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Explanation of code:

The code is broken down into a number of helper subs and functions, allocating certains tasks to each, to make code easier to debug and follow, as well as better align with Object Oriented Programming Principles.

Overall the process is:

1. Gather urls for Worksheet("Sheet1"). Helper function GetAllUrls.
2. Process those urls and only retain the dates which correspond with Tue-Thur. These are kept as strings formatted as "yyyymmdd" so can be passed to API later. This is handled by helper functions GetOnlyQualifyingUrlsDates and IncludeThisDate. IncludeThisDate performs the check for whether to include; GetOnlyQualifyingUrlsDates handles the looping and formatting of results.
3. Issue xmlhttp requests by looping over qualifying url dates and concatenating those into the url for the API call, then issuing the request. This is performed by the main sub GetTables.
4. Sheet creation, for output, is handled by helper function CreateWorksheet. This function calls another helper function, SheetExists, to ensure sheets are only created if they don't already exist, otherwise, the existing sheet by that name is used.
5. The resultant json response, from step 3, is passed to a helper sub WriteOutResults which accepts the json variable and the output sheet object as arguments. It extracts all the info from the json response; essentially reconstructing the table. It adds the table and headers to the appropriate sheet. It calls helper function Epoch2Date, which handles the unix timestamp to datetime conversion for the two unix fields in the json object.

---

TODO:

1. The API key may be time limited. Add a helper function which returns the current valid key.
2. The API accepts start date and end date parameters in the url construct. It would be far better to issue one request for the entire range if possible, or chunked ranges e.g. months, to reduce the number of requests made. This would also reduce the likelihood of being blocked. This would mean some additional code would need to be written, before writing out results, to ensure only dates of interest are being written to sheets. Though you could write out all then simply loop all sheets and delete those that aren't wanted (perfectly doable if we are talking about 365 dates total). Personally, I would handle the include date part in the construction of the table from a single request (if possible) that has the min and max dates for entire urls listed passed as start and end date parameters. I would then write a single flat table out to one sheet as this will be much easier for later data analysis.

---

VBA:

Option Explicit

Public Sub GetTables()
    'VBE > Tools > References > Microsoft Scripting Runtime
    Dim json As Object, qualifyingUrlsDates(), urls(), url As String
    Dim ws As Worksheet, wsOutput As Worksheet, i As Long, startDate As String, endDate As String

    Set ws = ThisWorkbook.Worksheets("Sheet1")
    urls = GetAllUrls(2, ws, "A")
    qualifyingUrlsDates = GetOnlyQualifyingUrlsDates(urls)

    'API key may be not be valid over time so look at obtaining by prior request

    With CreateObject("MSXML2.XMLHTTP")          'issue xmlhttp request for each valid date (this would be better done using start and enddate to specify entire range _
                                                 of batches e.g. months within total range to cut down on requests
        For i = LBound(qualifyingUrlsDates) To UBound(qualifyingUrlsDates)
            startDate = qualifyingUrlsDates(i)
            endDate = startDate                 ' a little verbose but useful for explaining
            url = "https://api.weather.com/v1/geocode/31.76/-106.49/observations/historical.json?apiKey=6532d6454b8aa370768e63d6ba5a832e&startDate=" & startDate & "&endDate=" & endDate & "&units=e"
            .Open "GET", url, False
            .send
            Set json = JsonConverter.ParseJson(.responseText)("observations")
            Set wsOutput = CreateWorksheet(qualifyingUrlsDates(i))
            WriteOutResults wsOutput, json
        Next
    End With
End Sub

Public Sub WriteOutResults(ByVal wsOutput As Worksheet, ByVal json As Object)
'json is a collection of dictionaries. Each dictionary is a time period reading from the day i.e. one row in output
    Dim results(), item As Object, headers(), r As Long, c As Long, key As Variant
    headers = json.item(1).keys 'get the headers which are the keys of each dictionary
    ReDim results(1 To json.Count, 1 To UBound(headers) + 1)
    For Each item In json
        r = r + 1: c = 0 'increase row in results array to store results for table row
        For Each key In item.keys
            c = c + 1 'increase column number in results array for writing out results
            Select Case key
            Case "valid_time_gmt", "expire_time_gmt" 'convert unix timestamp fields to datetime
                results(r, c) = Epoch2Date(item(key))
            Case Else
                results(r, c) = item(key)
            End Select
        Next
    Next
    With wsOutput
        .Cells(1, 1).Resize(1, UBound(headers) + 1) = headers
        .Cells(2, 1).Resize(UBound(results, 1), UBound(results, 2)) = results
    End With
End Sub

Public Function GetOnlyQualifyingUrlsDates(ByVal urls As Variant) As Variant
    Dim i As Long, output(), counter As Long
    ReDim output(1 To UBound(urls))

    For i = LBound(urls) To UBound(urls)
        If IncludeThisDate(urls(i)) Then 'check if weekday is to be included
            counter = counter + 1
            output(counter) = Format$(Right$(urls(i), 8), "yyyymmdd") 'if to include then add to output array of urls of interest
        End If
    Next
    ReDim Preserve output(1 To counter)
    GetOnlyQualifyingUrlsDates = output
End Function

Public Function IncludeThisDate(ByVal url As String) As Boolean
    'tue, wed, thurs are valid
    IncludeThisDate = Not IsError(Application.Match(Weekday(Right$(url, 8), vbSunday), Array(3, 4, 5)))
End Function

Public Function SheetExists(ByVal sheetName As String) As Boolean '<==  function by @Rory
    SheetExists = Evaluate("ISREF('" & sheetName & "'!A1)")
End Function

Public Function GetAllUrls(ByVal startRow As Long, ByVal ws As Worksheet, ByVal columnName As String) As Variant
    'transpose used based on premise no more than a couple of years of dates
    'startRow is start row for urls, ws is sheet where urls found, columnName is string representation of column for urls e.g. "A"
    With ws
        GetAllUrls = Application.Transpose(ws.Range("A" & startRow & ":A" & .Cells(.rows.Count, columnName).End(xlUp).Row).Value)
    End With
End Function

Public Function CreateWorksheet(ByVal sheetName As String) As Worksheet
    Dim ws As Worksheet
    If SheetExists(sheetName) Then
        Set ws = ThisWorkbook.Worksheets(sheetName)
        'do something.... clear it? Then add new data to it?
    Else
        Set ws = ThisWorkbook.Worksheets.Add
        ws.Name = sheetName
    End If
    Set CreateWorksheet = ws
End Function

Public Function Epoch2Date(ByVal E As Currency, Optional msFrac) As Date '@ Schmidt http://www.vbforums.com/showthread.php?805245-EPOCH-to-Date-and-vice-versa
    Const Estart As Double = #1/1/1970#
    msFrac = 0
    If E > 10000000000@ Then E = E * 0.001: msFrac = E - Int(E)
    Epoch2Date = Estart + (E - msFrac) / 86400
End Function