Ayende @ Rahien

Hi!
My name is Oren Eini
Founder of Hibernating Rhinos LTD and RavenDB.
You can reach me by email or phone:

ayende@ayende.com

+972 52-548-6969

, @ Q c

Posts: 6,524 | Comments: 47,985

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If you have a finalizer, watch your ctor

time to read 2 min | 289 words

We recently got into a very bad state with our build server. We had a few dozens failing tests in an area of the code that didn’t change in months. That was annoying, to say the least. I tried to run this on my machine, to see what was going on, and we got an even worse state, a process crash.

The good thing was that this is managed code, so you get stack traces, and we were able to figure out that the issue is with the SqlClient assembly. It looks like a recent change meant that loading it by reflection will give you the netstadnard 2.0 release, which is basically just a filler that always throw PlatformNotSupported. That is perfectly fine, we’ll change the way we do things to load the direct reference.

What killed us was this:

image

This will kill the process. How is that?

Well, DbConnection inherits from Component, and Component has a finalizer. Care to guess how what will happen when that is called?

image

So here is what will happen, the ctor throws, so no one can have an instance of this class. But the finalizer is already tracking it, and will call the finalizer. The finalizer will call the Dispose method, and that will end up throwing, and an exception during finalization is fatal to the entire process. Bye!

The general idea is that when you are working with a throwing ctor, you either make sure to call GC.SupprressFinalize or you cleanup in such a way that you can safely finalize things.

Carefully performing invalid operations to get the wrong error and the right result

time to read 2 min | 285 words

The RavenDB setup process happens in the browser, and the last part involves restarting the server and then redirecting you to the new server. Along the way you have also specified a certificate to use and the other configurations.

We got a bug report about that when the admin configured us with a self signed certificate. During the time the server restarts, the browser will ping the server periodically, waiting for it to come back up with the new configuration. That can be a problem when using self signed certificates, because the browser will reject them as untrusted.

From the point of view of the client side running in the browser, there is no way to tell the difference from a server that is down and a server that is using a self signed certificate. But we wanted to get the nice feature of showing you when you can move to the new URL. So how can you do that?

Remember that RavenDB have auto detection for invalid HTTP access when using HTTPS? And that this error is raised at the HTTP level?

That means that we can carefully construct an HTTP request such as “http://my.ravendb.sever:443” and check what the result is. If the server is up, the request will fail with a bad request error, and that means that we can distinguish between the server being down and the server being up (but maybe with bad cert).

In fact, once we know the server is up, we can check if the certificate is valid, and show something about it.

This is convoluted, requires us to do things at several places at once at very different levels of the stack. But it is quite amazing to see, it just works!

You are doing it REALLY wrong, the shortest code review ever

time to read 2 min | 394 words

We got a question from a customer that included the following code:

image

This was quite unrelated to the question being asked, but it was enough to trigger all sorts of alarms. Just based on this two lines, and the fact that we can assume that _session is a RavenDB session, we can tell quite a lot about the codebase.

To start with, it is broken, probably badly so.

Let us try to break down why. The fact that you are locking on the session means that you access it concurrently. And that is utterly forbidden. The session is not meant to be used concurrently, and Strange Stuff will happen if you try to.

Now, the session doesn’t have thread affinity, which means that if you are calling it serially, you can certainly move it between threads. In fact, with async sessions, that happens quite often. So one would expect that locking on the session would provide sufficient protection for the session from concurrent use.

However, nothing could be further from the truth. Look at the output of this method. It returns User instances, which are used outside the lock. However, if two threads will call this method concurrently, they will each run in turn, taking and releasing the lock as required. Which is great, but it is very likely that they will get the same User instance back (remember, the session implements Identity Map and ensure that within the scope of the session, the same document is always represented by the same object instance). That is when things start going boom, boom, BOOM!

Even if you are locking on the session, you aren’t locking on your entities, and that means that you have concurrent access to them. I haven’t actually reviewed the entity in question, but I’m 99.999999% certain that no attempt was made to make them safe for multi threading and 100% certain that it there was such an attempt, it would fail horribly in some manner quite easily.

Entities are not meant for concurrent usage, and even trying to reason about them effectively is going to be near impossible.

A session is meant to be a scope of work, in fact, look up the Unit of Work pattern, which is exactly how you are expected to use the session.

The best features are the ones you never knew were thereYou can’t do everything

time to read 2 min | 241 words

imageIn the previous posts in this series, I talked about the kind of features that we build into RavenDB. Things that you never even notice making your life easier.

One feature we don’t have is doing HTTPS to HTTP downgrade. What do I mean by that? Assume that you have a RavenDB instance that is running using HTTP, and a client attempts to connect to it using HTTPS. Remember that we are assuming that the access it made on the same port. So the client wrote "https://my.raven.database:8080” instead of “http://my.raven.database:8080”.

If the other thing would happen, we would detect that and give a clear error to the user. But the other way around? We don’t do that, but why?

Well, the reasoning is very simple. If you connect to an HTTP endpoint using HTTPS, the first packet on the wire wants to do SSL negotiation. However, we don’t have a certificate that we can use here, so we can’t even start the negotiation process.

We could try generating a self signed certificate on the fly and answer the request with an error. But at this point, the client will likely already error at a low level because of the self signed certificate not being trusted.

Another point against implementing this feature is that HTTP endpoints typically become HTTPS, but rarely the other way around.

NHibernate Profiler 5.0 Alpha has been released

time to read 1 min | 181 words

imageThe next generation of our profiler is starting to shift into higher gear, with today’s release of the NHibernate Profiler 5.0 Alpha.

The new bits are available directly from NuGet.

The new release brings:

  • Support for NHibernate 5.0.0 and NHibernate 5.0.1
  • The profiler appender now require .NET Standard 2.0 (older appenders, supporting older runtimes will still work with the new release).
  • Fixes for various bugs, aches and pains.
  • Faster serialization format over the wire.
  • Reduce the cost of profiling your application significantly by reducing contention.
  • Better stack trace and profiling support for .NET standard 2.0 apps.

This is just the beginning, getting the major stuff that need to be there for the next release before we get down and start implementing the cool features. But these changes (and especially the support for NHibernate 5.x)  are major enough that we would like to have users test them out in the field.

Licensing wise, for the alpha period any 4.x license for the profiler will work on the 5.0 alpha.

You can download it here.

RavenDB SetupHow the automatic setup works

time to read 8 min | 1456 words

imageOne of the coolest features in the RC2 release for RavenDB is the automatic setup, in particular, how we managed to get a completely automated secured setup with minimal amount of fuss on the user’s end.

You can watch the whole thing from start to finish, it takes about 3 minutes to go through the process (if you aren’t also explaining what you are doing) and you have a fully secured cluster talking to each other over secured TLS 1.2 channels.  This was made harder because we are actually running with trusted certificates. This was a hard requirement, because we use the RavenDB Studio to manage the server, and that is a web application hosted on RavenDB itself. As such, it is subject to all the usual rules of browser based applications, including scary warnings and inability to act if the certificate isn’t valid and trusted.

In many cases, this lead people to chose to use HTTP. Because at least with that model, you don’t have to deal with all the hassle. Consider the problem. Unlike a website, that has (at least conceptually) a single deployment, RavenDB is actually deployed on customer sites and is running on anything from local developer machines to cloud servers. In many cases, it is hidden behind multiple layers of firewalls, routers and internal networks. Users may chose to run it in any number of strange and wonderful configurations, and it is our job to support all of them.

In such a situation, defaulting to HTTP only make things easy. Mostly because things work. Using HTTPS require that we’ll use a certificate. We can obviously use a self signed certificate, and have the following shown to the user on the first access to the website:

image

As you can imagine, this is not going to inspire confidence with users. In fact, I can think of few other ways to ensure the shortest “download to recycle bin” path. Now, we could ask the administrator to generate a certificate an ensure that this certificate is trusted. And that would work, if we could assume that there is an administrator. I think that asking a developer that isn’t well versed in security practices to do that is likely to result in an even shorter “this is waste of my time” reaction than the unsecured warning option.

We considered the option of installing a (locally generated) root certificate and generating a certificate from that. This would work, but only on the local machine, and RavenDB is, by nature, a distributed database. So that would make for a great demo, but it would cause a great deal of hardships down the line. Exactly the kind of feature and behavior that we don’t want. And even if we generate the root certificate locally and throw it away immediately afterward, the idea still bothered me greatly, so that was something that we considered only in times of great depression.

So, to sum it all up, we need a way to generate a valid certificate for a random server, likely running in a protected network, inaccessible from the outside (as  in, pretty much all corporate / home networks these days). We need to do without requiring the user to do things like setup dynamic DNS, port forwarding in router or generating their own certificates. We also need to to be fast enough that we can do that as part of the setup process. Anything that would require a few hours / days is out of the question.

We looked into what it would take to generate our own trusted SSL certificates. This is actually easily possible, but the cost is prohibitive, given that we wanted to allow this for free users as well, and all the options we got always had a per generated certificate cost associated with it.

Let’s Encrypt is the answer for HTTPS certificate generation on the public web, but the vast majority all of our deployments are likely to be inside the firewall, so we can’t verify a certificate using Let’s Encrypt. Furthermore, doing so will require users to define and manage DNS settings as part of the deployment of RavenDB. That is something that we wanted to avoid.

This might require some explanation. The setup process that I’m talking about is not just to setup a production instance. We consider any installation of RavenDB to be worth a production grade setup. This is a lesson from the database ransomware tales. I see no reason why we should learn this lesson again on the backs of our users, so a high priority was given to making sure that the default install mode is also the secure and proper one.

All the options that are ruled out in this post (provide your own certificate, setup DNS, etc) are entirely possible (and quite easily) with RavenDB, if an admin so chose, and we expect that many will want to setup RavenDB in a manner that fits their organization policies. But here we are talkingh about the base line (yes, dear) install and we want to make it as simple and straightforward as we possibly can.


There is another problem with Let’s Encrypt for our situation, we need to generate a lot of certificates, significantly more than the default rate limit that Let’s Encrypt provides. Luckily, they provide a way to request an extension to this rate limit, which is exactly what we did. Once this was granted, we were almost there.

imageThe way RavenDB generates certificates as part of the setup process is a bit involved. We can’t just generate any old hostname, we need to provide proof to Let’s Encrypt that we own the hostname in question. For that matter, who is the we in question? I don’t want to be exposed to all the certificates that are generated for the RavenDB instances out there. That is not a good way to handle security.

The key for the whole operation is the following domain name: dbs.local.ravendb.net

During setup, the user will register a subdomain under that, such as arava.dbs.local.ravendb.net. We ensure that only a single user can claim each domain. Once they have done that, they let RavenDB what IP address they want to run on. This can be a public IP, exposed on the internet, a private one (such as 192.168.0.28) or even a loopback device (127.0.0.1).

The local server, running on the user’s machine then initiates a challenge to Let’s Encrypt for the hostname in question. With the answer to the challenge, the local server then call to api.ravendb.net. This is our own service, running on the cloud. The purpose of this service is to validate that the user “owns” the domain in question and to update the DNS records to match the Let’s Encrypt challenge.

The local server can then go to Let’s Encrypt and ask them to complete the process and generate the certificate for the server. At no point do we need to have the certificate go through our own servers, it is all handled on the client machine. There is another thing that is happening here. Alongside the DNS challenge, we also update the domain the user chose to point to the IP they are going to be hosted at. This means that the global DNS network will point to your database. This is important, because we need the hostname that you’ll use to talk to RavenDB to match the hostname on the certificate.

Obviously, RavenDB will also make sure to refresh the Let’s Encrypt certificate on a timely basis.

The entire process is seamless and quite amazing when you see it. Especially because even developers might not realize just how much goes on under the cover and how much pain was taken away from them.

We run into a few issues along the way and Let’s Encrypt support has been quite wonderful in this regard, including deploying a code fix that allowed us to make the time for RC2 with the full feature in place.

There are still issues if you are running on a completely isolated network, and some DNS configurations can cause issues, but we typically detect and give a good warning about that (allowing you to switch to 8.8.8.8 as a good workaround for most such issues). The important thing is that we achieve the main goal, seamless and easy setup with the highest level of security.

Looking for a security review

time to read 1 min | 115 words

I’m reaching out through my contacts, and I thought that this might help me reach the right people.

We need to complete a security review on RavenDB 4.0, and I would really like that to hire an external party for this, since they will likely have a fresh look at things. This involves analyzing our authentication and authorization systems, verifying the use of SSL and X509 certificates, etc.

We probably don’t need to look at things like XSS or SQL Injection, though Smile. If you know someone who can do that, I would appreciate the referral.

If they are located at a similar time zone (GMT+2), that would be great, but not required.

RavenDB Setupa secured cluster in 10 minutes or less

time to read 1 min | 86 words

One of the major features of the RC2 release for RavenDB has been the setup process. In particular, we worked on making sure that the default and easiest manner to install RavenDB will be the one with the highest level of security.

I’m excited enough by this feature that I recorded myself setting up a full blown cluster, including everything you need for production deployment in under 10 minutes, with a lot of my explanations in the middle. Take a look.

The best features are the ones you never knew were thereUnsecured SSL/TLS

time to read 3 min | 598 words

imageI wish I would have been sufficient to use HTTPS for security. With RavenDB 4.0’s move toward TLS as the security mechanism for encryption of data over the wire and authentication using x509 we had to learn way too much about how Transport Layer Security works.

In particular, it can be quite annoying when you realize that just because you use SSL (or more accurately, TLS) that isn’t sufficient. You need to use the proper version, and there are interoperability issues. Many of RavenDB’s users run it in an environments that are subject to strict scrutiny and high level of regulation and oversight. That means that we need to make sure that we are able to operate in such environment. One option would be to use something like a FIPS configuration. We have a “normal”configuration and one that is aimed at people that need stricter standards. For many reasons, this is a really bad idea. Not least of all is the problem that even if you don’t have to meet FIMS mandate, you still want to be secured. Amusingly enough, many FIPS certified stacks are actually less secured (because they can’t get patches to the certified binaries).

So the two options mode was rejected. That meant that we should run in a mode that is can be match the requirements of the most common deployment regulations. In particular interest to us is PCI compliance, since we are often deployed in situations that involve money and payment processing.

That can be a bit of a problem. PCI requires that your communication will use TLS, obviously. But it also requires it to use TLS 1.2. That is great and with .NET it is easily supported. However, not all the tools are aware of this. This put us back in the same state as with HTTP vs. HTTPS. If your client does not support TLS 1.2 and your server require TLS 1.2, you end up in with a with a connection error.

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Such a thing can be maddening for the user.

Therefor, RavenDB will actually allow Tls and Tls11 connections, but instead of processing the request, it will give you an error that give you something to work with.

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Updated: I forgot to actually read the message. The reason you are getting the error about no certificate is because there isn’t a certificate here. In order for this to work, we need to actually pass the certificate, in which case we’ll get the appropriate error. I apologize for the error handling, but PowerShell:
image

Armed with this information, you can now do a simple web search and realize that you actually need to do this:

image

And that saves us a lot of TCP level debugging. It took a bit of time to set this (and the other) errors properly, and they are exactly the kind of things that will save you hours or days of frustration, but you’ll never realize that they were there even if you run into them unless you know the amount of effort that went into setting this up.

The bare minimum a distributed system developer should know aboutBinding to IP addresses

time to read 3 min | 543 words

It is easy to think about a service that listen to the network as just that, it listens to the network. In practice, this is often quite a bit more complex than that.

For example, what happens when I’m doing something like this?

image

In this case, we are setting up a web server with binding to the local machine name. But that isn’t actually how it works.

At the TCP level, there is no such thing as machine name. So how can this even work?

Here is what is going on. When we specify a server URL in this manner, we are actually doing something like this:

image

And then the server is going to bind to each and every one of them. Here is an interesting tidbit:

image

What this means is that this service doesn’t have a single entry point, you can reach it through multiple distinct IP addresses.

But why would my machine have so may IP addresses? Well, let us take a look. It looks like this machine has quite a few network adapters:

image

I got a bunch of virtual ones for Docker and VMs, and then the Wi-Fi (writing on my laptop) and wired network.

Each one of these represent a way to bind to the network. In fact, there are also over 16 million additional IP addresses that I’m not showing, the entire 127.x.x.x range. (You probably know that 127.0.0.1 is loopback, right? But so it 127.127.127.127, etc.).

All of this is not really that interesting, until you realize that this has real world implications for you. Consider a server that has multiple network cards, such as this one:

image

What we have here is a server that has two totally separate network cards. One to talk to the outside world and one to talk to the internal network.

When is this useful? In pretty much every single cloud provider you’ll have very different networks. On Amazon, the internal network gives you effectively free bandwidth, while you pay for the external one. And that is leaving aside the security implications

It is also common to have different things bound to different interfaces. Your admin API endpoint isn’t even listening to the public internet, for example, it will only process packets coming from the internal network. That adds a bit more security and isolation (you still need encryption, authentication, etc of course).

Another deployment mode (which has gone out of fashion) was to hook both network cards to the public internet, using different routes. This way, if one went down, you could still respond to requests, and usually you could also handle more traffic. This was in the days where the network was often the bottleneck, but nowadays I think we have enough network bandwidth that program efficiency is of more importance and this practice somewhat fell out of favor.

FUTURE POSTS

  1. Queries++ in RavenDB: I suggest you can do better - 6 hours from now
  2. Queries++ in RavenDB: Spatial searches - 3 days from now
  3. PR Review: The simple stuff will trip you - 4 days from now
  4. The married couple component design pattern - 5 days from now
  5. Distributed computing fallacies: There is one administrator - 6 days from now

There are posts all the way to Dec 21, 2017

RECENT SERIES

  1. PR Review (9):
    08 Nov 2017 - Encapsulation stops at the assembly boundary
  2. Queries++ in RavenDB (4):
    11 Dec 2017 - Gimme more like this
  3. Production postmortem (21):
    06 Dec 2017 - data corruption, a view from INSIDE the sausage
  4. API Design (9):
    04 Dec 2017 - The lack of a method was intentional forethought
  5. The best features are the ones you never knew were there (5):
    27 Nov 2017 - You can’t do everything
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