Error Log Download Applications
ELMAH allows the entire error log to be downloaded CSV format, which can be useful for occasional and ad-hoc analysis. For example, you can filter and chart using Microsoft Excel and/or Microsoft Log Parser. The download feature also supports an additional format dubbed in ELMAH as HTML-JSONP. This format enables interesting, dynamic and asynchronous reporting pages to be developed on top of the error log.
In the HTML-JSONP format, ELMAH produces a single HTML document with JSONP callbacks wrapped in script tags. Each callback delivers a single page of hundred or so errors. For best results, the HTML document is designed to be hosted in an iframe in a parent document with JSONP callbacks being delivered to it. This has two benefits. First, the iframed HTML document loads asynchronously while the parent document can remain interactive. Second, ELMAH streams down the document in chunks so that the browser can parse and evaluate each script tag as it arrives and make the JSONP callbacks on the parent. This enables the parent or host document to process a page of errors as soon as it is delivered, meaning the user does not have to wait for the entire log to download before beginning to receive some feedback or reporting. This can be especially important when the log grows large or if you happen to be browsing on a mobile device such as a phone or a tablet and the network latency is high.
The HTML-JSONP format is requested by appending a query to the log download URL with the following named fields or parameters:
| Name | Required | Meaning |
|---|---|---|
format |
Yes | Must be html-jsonp |
callback |
Yes | Client-side JavaScript method to call |
limit |
No | Maximum number of most recent errors to return from the log (entire log is returned if omitted) |
Assuming that ELMAH is deployed at http://www.example.com/elmah.axd and the download URL is http://www.example.com/elmah.axd/download, you would request HTML-JSONP format using http://www.example.com/elmah.axd/download?format=html-json&callback=parent.onerrors. The callback parameter identifies the JavaScript function to call back and here it is assumed that you have an onerrors function defined on the parent window. In the top HTML document, you would have an iframe like this:
<iframe width="0" height="0"
src="http://www.example.com/elmah.axd/download?format=html-json&callback=parent.onerrors"></iframe>The callback function receives a single object value each time with two properties, namely total and errors. total is always a number representing the total errors recorded in the log whereas errors is an array of objects representing the actual error entries. Below is an example of one such entry in JSON:
{
"application":"/LM/W3SVC/3/ROOT",
"host":"EXAMPLEHOST",
"type":"System.FormatException",
"message":"Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx).",
"source":"mscorlib",
"time":"2010-01-05T12:13:03.280Z",
"hrefs":[
{
"type":"text/html",
"href":"http://www.example.com/elmah.axd/detail?id=165fcfa3-ad18-41b0-9240-4357e29d4db3"
},
{
"type":"aplication/json",
"href":"http://www.example.com/elmah.axd/json?id=165fcfa3-ad18-41b0-9240-4357e29d4db3"
},
{
"type":"application/xml",
"href":"http://www.example.com/elmah.axd/xml?id=165fcfa3-ad18-41b0-9240-4357e29d4db3"
}
]
}Note the the hrefs array can be used to obtain further details about the error in various formats (HTML, JSON and XML). To tell the callback function that the entire log has been delivered, ELMAH issues a last callback where the errors array is empty.
The callback function should be designed to handle duplicate entires and a changing total number of errors. Both can happen, for example, when new errors get logged while the log is being downloaded. This is the reason ELMAH delivers the total count of error with each callback and page of errors.
Elvue is an open source and a working example of a single HTML page that produces a tabular and graphical report of the types of errors logged in an ELMAH error log (see the screenshot below). You are encouraged to study the source and also contribute ideas.
