Monthly Archives: June 2021

Receiving a Message Using Azure.Messaging.ServiceBus

Azure.Messaging.ServiceBus is the latest SDK library that allows you to interface with Azure Service Bus.

In this post I wrote about receiving a message in Azure Service Bus using the Microsoft.Azure.ServiceBus library. Here, I’ll cover the method of receiving a message using Azure.Messaging.ServiceBus.

The first step is to create a ServiceBusClient instance:

_serviceBusClient = new ServiceBusClient(connectionString);

Once you’ve created this, the subsequent classes are created from there. This library draws a distinction between a message receiver and a message processor – the latter being event driven.

Receiving a Message

To receive a message:

            var messageReceiver = _serviceBusClient.CreateReceiver(QUEUE_NAME);            
            var message = await messageReceiver.ReceiveMessageAsync();

            //string messageBody = Encoding.UTF8.GetString(message.Body);
            string messageBody = message.Body.ToString();

It’s worth noting here that it is no longer necessary to decode the message body explicitly.

Processing a Message

This is the new version of registering a handler for the event, and it has a few additional features. Let’s see the code:

            var processor = _serviceBusClient.CreateProcessor(QUEUE_NAME);
            processor.ProcessMessageAsync += handleMessage;
            processor.ProcessErrorAsync += ExceptionHandler;

            await processor.StartProcessingAsync();                        

            await Task.Delay(2000);
            await processor.StopProcessingAsync();

We won’t worry too much about the events themselves for now, but the important events are StartProcessingAsync and StopProcessingAsync. Note that here we have a 2 second delay – this means that we will receive messages for two seconds, and then stop; obviously the start and stop don’t need to be in the same method.

References

https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-dotnet-get-started-with-queues

Using the Mongo DB Shell to remove all elements in a collection

In my recent experiments with using MongoDB, I came across a need to remove all the records from a collection – the equivalent of DELETE FROM MYTABLE in SQL.

It turns out that, in order to do this, you need to use the Mongo DB Shell. This comes with Compass, although you may miss it, tucked away at the bottom:

By default it’s collapsed, but once you un-collapse it, you can do a number of things. The first thing you should do is tell it which DB you would like to use (for the purpose of this, let’s assume the database is called aardvark):

> use aardvark
< 'switched to db aardvark'

You can see which collections you have in the database, like this:

> db.getCollectionNames()
< [ 'collection1', 'collection2' ]

You can also see some information about a specific collection; for example:

> db.collection1.exists()

This should return a JSON document that details collection1.

In my case, I wanted to clear everything from the collection:

> db.collection1.deleteMany({ })

You can also pass this a specific filter, but this allows you to delete everything.

References

https://docs.mongodb.com/mongodb-shell/run-commands/

https://www.mongodb.com/blog/post/introducing-the-new-shell

Azure Service Bus SDK Libraries

I’ve written pretty extensively on the Microsoft.Azure.ServiceBus SDK. In this post, I’m just covering the fact that this library is on its way to deprecation (don’t panic, its predecessor has been hanging around since 2011!)

Let’s see what these libraries are and some links.

WindowsAzure.ServiceBus

This library does look like it’s on its way to being deprecated. It supports .Net Framework only.

The NuGet package is here, but it’s closed source:

https://www.nuget.org/packages/WindowsAzure.ServiceBus

Microsoft.Azure.ServiceBus

This library was introduced to support .Net Core.

The NuGet package is here:

https://www.nuget.org/packages/Microsoft.Azure.ServiceBus

The code for this is open source:

https://github.com/Azure/azure-service-bus-dotnet

Azure.Messaging.ServiceBus

If you read Sean Feldman’s article here (which this was heavily based on), you’ll see that this seems to be due to some restructuring of teams. The code has changed, and MS say it’s more consistent (although what with, I’m unsure).

The NuGet Package is here:

https://www.nuget.org/packages/Azure.Messaging.ServiceBus

The source code for this is here:

https://github.com/Azure/azure-sdk-for-net/tree/master/sdk/servicebus

References

https://markheath.net/post/migrating-to-new-servicebus-sdk

https://weblogs.asp.net/sfeldman/the-future-of-asb-dotnet-sdk

Cloudflare Workers – Creating a worker to standardise the request URL

Imagine a situation where your web-site is being linked to, or referenced, such that the case of the URL differs; or, for example, where you wish to change a given part of the URL. Let’s use the following URL from my site as an example:

One possibility in Cloudflare is that you could set up a custom redirect rule (i.e. just create a new rule that redirects from the address above to the lower case version; for example:

But what if you have dozens of such examples, and they all differ very slightly. I’ve been playing a little with a new(ish) Cloudflare feature called Workers.

In this post, I’ll cover getting started with Workers, how to use one to perform some custom logic on your URL, and link that to your domain.

Wrangler

The tool that you’d use to set these workers up is called Cloudflare Wrangler. To install Wrangler:

npm install -g @cloudflare/wrangler

Once installed, execute the following to ensure that you’ve correctly installed it:

wrangler --version

The next step is to log-in to your Cloudflare account from Wrangler:

wrangler login

That will ask to launch the browser, and get you to log-in:

Once you’ve authorised this, you can create your new Cloudflare Worker project:

wrangler generate wrangler-test-1   

This will generate a project folder in the current directory:

Once this is done, take the Account ID (shown above), and open the wrangler.toml file that has been created within your project. Set the account_id in there:

name = "wrangler-test-1"
type = "javascript"

account_id = "f8021e056d0d5e96c9b3ba4ad054bb2c"
workers_dev = true
route = ""
zone_id = ""

You should now be able to debug this locally by typing:

wrangler dev

However, if you run that at this point, you’ll most likely get a 500 error:

Error: HTTP status server error (500 Internal Server Error) for url (https://…

Set-up Workers

The reason you’ll get this is that you have yet to set-up Workers on your Cloudflare account.

(It’s worth bearing in mind that, whilst there is a free tier, Cloudflare workers can cost money after a given number of requests.)

In your Cloudflare account, select Workers and click Set up:

Now that Workers are set up, you should be able to resume:

wrangler dev

This looks like it’s running on localhost; which, in fact, it is; however, all the requests are being proxied through Cloudflare (and so the worker will execute on their servers).

You can also simply publish your worker, like so:

wrangler publish

As you can see, this gives you a specific Url to navigate to. The default example allows you to display “Hello World” based on the request.

Write the Worker

Up until now, we’ve just taken the default worker code; but what about something such as the following:

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})
/**
 * Respond with hello worker text
 * @param {Request} request
 */
async function handleRequest(request) {
  const destinationUrl = request.url;
  const newUrl = request.url.toLowerCase();
  if (destinationUrl == newUrl)  {    
    return await fetch(request)
  } else {
    return Response.redirect(newUrl, 301);
  }
}

All we’re doing here is checking is the URL is lower-case; if it’s not then we make it lower case. Obviously, if you execute this on its own domain, the worker will not respond, but you can then map this to a different domain; for example:

Now, all the requests coming into pmichaels.net will be manipulated, and then passed through. If we map this and re-execute the initial request:

It’s worth bearing in mind that this executes on all requests to your site. For example, say you did something like this:

async function handleRequest(request) {
  const destinationUrl = request.url;
  if (destinationUrl.includes('2021')) {
    return await fetch(request)
  } else {
    return Response.redirect("https://www.pmichaels.net", 301);
  }

This would work, but the CSS, for example, may fail, because the URL for the CSS may not have 2021 in the title, and so try to redirect the request away.

Git – What to do when things go wrong

I thought I’d detail some things that you can do when you get issues with git. Specifically, how you can deal with issues without having to resort to manually copying files around.

For this purpose of this post, I’ll assume that you have a git repository, and that some code has been pushed to it. I’ll be using git bash for the commands in this post.

Status and Log

Before we start working out how we can revert or fix issues that go wrong, let’s first see how we can reliably, and sensibly see what state we’re in.

Let’s start with having a look at the story so far. For this we can use the git log command:

git log --oneline

Our history has some commits in already – these have been pushed to the remote repository.

To demonstrate these commands, firstly, we’ll change a file:

$ echo "some test" >> test.txt

This file hasn’t been staged yet; we can prove this by using the git status command:

$ git status -s

This gives us a nice display of the state of our pending changes:

What git status -s gives you is a report of which files are staged, and which are committed in the following manner:

XY filename

Where X is a staged change, and Y is an unstaged, but tracked change. For example, if we stage the change to test.txt:

Of course, a file can have both staged, and unstaged changes:

Finally, let’s commit and push this change, and then view the log again:

git log --oneline

When things go wrong

Okay, so what we’ll cover here is how to revert changes that are in various states. The states that a file can exist in in git are:

1. Untracked
2. Tracked and unchanged
3. Tracked and changed

Let’s start with (1) – untracked. We’ll create a new file:

$ echo "new file 2" >> newfile2.txt
$ git status -s

This shows up like so:

The question mark here is telling us that git doesn’t know about this file – it’s untracked. In fact, if you just call git status it will tell you that the file is untracked.

Remove Untracked files

Okay, so by default, git will not remove newfile2.txt for you – you’re working away and you type the wrong command, you don’t really want git to go and wipe your files; however, sometimes, you do.

Because we aren’t tracking the new file, we’ll need the command:

git clean

However, if you run this, you’ll get the error:

fatal: clean.requireForce defaults to true and neither -i, -n, nor -f given; refusing to clean

The reason here is that, by default, git tries to prevent you from breaking things – and removing all the files in a directory that git doesn’t know about it quite drastic. We’ll therefore need the -f parameter to force the issue:

git clean -f

This will remove any untracked files that it finds; however, it will leave directories, and also anything that you’ve explicitly told it to ignore. To remove absolutely everything, we’ll include two extra parameters:

git clean -f -d -x

That will remove anything that git doesn’t know about, whether or not you’ve told it to ignore the files. Obviously, use this with caution – it WILL delete files and you WILL NOT be able to restore them – however, it will ONLY remove local files – any remote repository will be unaffected.

Let’s now consider a situation where you have made a change to a file, but you wish to revert to the unchanged state.

Reverting a tracked change

There is more than one category of tracked change; they are:

1. Changes that are unstaged
2. Changes that are staged
3. Changes that are committed
4. Changes are committed and pushed to a remote repository

All four of these can represent different changes in the same file. Let’s change a file so that it’s different in all four stages:

echo "change type 4" >> test.txt
git add .
git commit -m "change type 4"
git push
echo "change type 3" >> test.txt
git add .
git commit -m "change type 3"
echo "change type 2" >> test.txt
git add .
echo "change type 1" >> test.txt

We can see the status of the local repository by issuing a status command:

git status -s

This will show that the file has both stages and unstaged changes:

And of the remote by viewing the tree:

git log --all --graph --decorate --oneline

So, we can see that each category of change is represented. Let’s start by reverting the unstaged change.

Unstaged

This is perhaps the easiest to revert, as there’s a command designed specifically for it:

git restore .

This will leave staged changes, but will discard any unstaged changes to the state of HEAD. If you wish the keep these changes for later, then stash might be a better answer.

Staged

One way of unstaging changes is to use the git restore command with the parameter staged:

git restore --staged .

Doing this will unstage the change that you’ve made; for example:

Similarly, you can issue a reset command for the same purpose:

git reset HEAD

Which has the same effect.

However, if you wish to completely discard the staged change, you can issue a reset with the hard parameter; for example:

None of these commands will affect any of the commits made:

Let’s look at how you can revert a local commit.

Undoing a local commit

If we look at the result of git log above, we get a clue as to how this may be possible, commit 335cf90 points to the local master branch, but origin/master points to a remote master branch. So we can just point our local HEAD back to the remote. There’s a few ways that this can be done; they are quite subtly different.

git revert

To move to the state of a previous commit, you can issue a git revert. However, this does not actually revert the commit, it simply creates a new commit which is the reverse of the one that’s already there.

This can result in merge conflicts, depending on the state of the local and remote branch.

Once you’ve started a git revert, you can issue a git revert --abort to abandon the revert.

git reset

A quick way to discard a local commit is to simply reset HEAD to the origin/master:

$ git reset --hard origin/master
HEAD is now at 00f3c8e change type 4

Undoing a remote change

Because you’re now manipulating the remote repository, this carries a very real risk of causing some serious damage.

The first step here is to set your HEAD, as before, to a previous commit; for example:

$ git reset --hard origin/master~1

This will set your HEAD to the commit 1 before the current remote master (to go back two, you would use master~2

If you now look at your log, you’ll see that it now looks a little strange:

There’s a couple of things to note here – the first is that you now have a detached HEAD – this means that your HEAD no longer refers to a branch. Secondly, your branch and the remote branch are now different. To complete this process, you can push the change; however, if you just call git push you’ll get something like the following error:

$ git push
fatal: You are not currently on a branch.
To push the history leading to the current (detached HEAD)
state now, use

    git push origin HEAD:<name-of-remote-branch>

What’s needed here is a bit of force:

$ git push -f origin master

Looking at the history now, we can see that the latest commit is gone:

It’s worth bearing in mind that, one of the great things about git is that, nothing is ever really gone. If we use git reflog we’ll see that the “lost” commit is, in fact, safe and well – just orphaned:

git reflog

You can even see this in the tree structure:

$ git log --graph --all --oneline --reflog

If we now add a new commit, we can use this command to see where we went back to and forked: