Gerrit User Summit: Multimaster at Qualcomm

My name is Martin Fick and I work at Qualcomm as Gerrit Administrator and I have one of the key historical Gerrit Maintainers since its very beginning.

I am going to talk about my experience in introducing a truly Multi-Master Gerrit setup at Qualcomm and learn about the findings and the experiences we made to make that happen.

Let’s start with a question so that we can make this session very interactive from the start.

Q: Who would like to have multi-master in Gerrit Code Review? Why would you need it on your server? Failover? High-availability? Both?

A: All of that but also latency. We have developers all around the world, and we would have at least two datacentres, one in the States and another one in Europe, we were looking for a multi-site multi-master to descreen latency between the sites.

A: (Luca Milanesio – GerritForge). Scalability and elasticity that allows to grow and shrink the instances based on the incoming traffic. Additionally, we would like to have zero downtime and rolling upgrades without stopping the services when moving between minor versions of Gerrit.

Q: (Han-Wen Nienhuys – Google) Let me reverse the question: who is running multi-master and would like to have a single master?

A: (Dave Borowitz – Google) I would love if we had a single server that was powerful enough to serve all our traffic and no latency and we had not had to deal with slow database backends and replication lags between sites.

Q: Does anybody need multi-master? How are you surviving today if you needed it?

A: Right now we maintain a second master server for DR purposes, is a passive standby backup server. If our master went down, we would have to failover to it.

A: We would need multi-master soon. Currently, we are experimenting the HA-plugin as a fallback solution, but we are working towards achieving multi-master so that we can scale up, which is mainly our primary concern here.

Gerrit Multi-Master requirements

There are needs that I hear from the audience, high-availability, scalability. Not necessarily the same requirements and I believe that multi-master would address some of those. What’s important is that everyone is focussing on their needs and think about how they can approach those first instead of going to the “big everything” solution.

We have high-availability as an issue somewhere, so we want better availability. We have some scalability issues as well. We do have some site issue at times but we most generally deal with those through slaves all over the world, between 50 and 100 and we primarily have one data-center in San Diego USA. If that goes down, nobody is getting anything done even if Gerrit is up, that makes failing over to other sites not particularly useful right now.
Our current focus is on better availability and better scalability at one site.

Scalability problems at Qualcomm

We have a pretty hefty load, we have around 6k projects, but our load is enormous on a few large projects. They are not massive projects but they are significant from the standpoint of having many changes on them, and there are a lot of users using those projects. Mainly a few kernel projects are used, and even if there are many other little projects, most of the users are pushing to those single projects.

Screen Shot 2017-12-18 at 14.05.47.png

We are a single tenant server, so we are primarily dealing with that. Other people need other Gerrit servers, and they manage their own, but at the moment we don’t have to deal with those. However, if we had a more scalable system, then I believe multi-tenancy would become the next problem on the horizon. If I had a great scalable system why am I creating this instances here and there I would instead put all of them on the central system so that I can manage one system. But then I would have the problem that Gerrit doesn’t currently handle multi-tenancy well. Maybe is because we don’t have a multi-master system and then we are not ready to put everything on one server anyway. If we are starting having a multi-master solution, then multi-tenancy is then something that would become more interesting.

On one server we have around 6k projects, the main project has about 1/2M refs, and we have in total 2.3M changes on the entire server.

I like to break down the scaling topic into two different ideas: horizontal scaling and vertical scaling. In our area, we need to scale vertically more, while horizontally is where you have lots of projects and users and vertical they are more concentrated on a few projects.

Stairway to Gerrit Multi-Master

Here is what we decided to do and we decided to build it incrementally. These are the four phases in which we have approached it.

Screen Shot 2017-12-18 at 14.07.06.png

Step #1: Active/Passive

We have an active/passive standby, and we started doing regular failover. Let’s face it, if you have a failover machine that you probably have never failed over to, it would probably not work.
We have so much software around the Gerrit ecosystem, we have a lot of scripts, cronjobs and things that run on our server, and they use a lot of software, there are a lot of Python hooks and stuff like that.
All that software gets updated. If the failover machine is not updated through the same process and tested, there could be missing dependencies, wrong package versions, etc. Maybe some hacks or links on the filesystem that is pointing over here and not over there or maybe filesystem positions are different and things like that.

Before even dreaming about multi-master you need to have a system to have your server systems consistent. If you don’t think that your organization can do that and make two systems precisely the same, you are not going to be anywhere near ready to start doing multi-master. If you are a large organization that has a lot of history, it’s a more significant challenge. If you are starting from scratch it is a lot easier, think about that from the start: how can I focus on repeatability and deploying the same stack of software to multiple servers, and then concentrate on failing over.

Sometimes this year around February/March, we started failing over, and now we are trying to fail over weekly. We have been doing that for getting the Team used to it, to ensure that our processes work.

Step #2: Active/Hot Standby

Then we moved to hot-standby. The difference here is that the passive node had all the software in place but was not necessarily running. So now we just started the other server, but only nobody was pointing to it. The active standby gives you the opportunity to test the software you have around your Gerrit master. If you have hooks or cronjobs, try to run them on the failover server, even if it is not the active master. Do you have the coordination needed? Do you have locks in place? If you are doing repository repacking, can you do it on both servers at the same time? Do they conflict with each other?

The assumption is that you have a shared repository on the backend such as an NFS and thus activities like repacking need that coordination. And you need that stuff to work before you can go to multi-master.

Step #3: Active/Active with Round-Robin DNS (low tech)

The third step which is where we are at is active-active. We chose this as a simple low-tech and easy to backup solution: round-robin DNS. It sucks from a load-balancing standpoint and for the failover recovery, but it gets us having traffic going to two servers. We have been running that for quite some time now, 15h!

Step #4: Active/Active with Load Balancer(s)

And the next step is of course to have a load balancer. We have a load-balancer setup that we use for slaves. It is much harder to back out because you have to set IPs to have DNS in place pointing to them, but we plan to transition to that.

What Gerrit data to share in a multi-master setup?

In a single-site Gerrit multi-master setup, you necessarily need to have shared Git repositories, you don’t have to share your H2 caches because you need them on each of them and then the ReviewDb needs to be shared until it goes away with Gerrit Ver. 2.15.

Screen Shot 2017-12-18 at 15.09.10.png

Sharing sessions

There is other stuff you need to share, like web sessions.
When you log in on with your web-browser, and you hit one machine, your browser keeps track that you are logged in with a cookie on your browser while the server keeps track of it on a shared persisted session. If your next hit goes to the other server, you don’t want that user to be kicked out, so you need to share the server session data across nodes.

In 2014 we developed the websession-flatfile plugin, and that helps taking care of sharing web sessions, and I know that a lot of people used that already for HA solutions. It is straightforward, just stores the sessions on the filesystem and if you are using Git repositories on NFS, just put them there also and share them.

Sharing events

The other thing that you require to share is the events data. Our events need persistence on the server side, and we share them on the filesystem too. That allows to share them across masters also. If you connect to one of our masters through SSH, you get events from both of the masters. It’s a poor man’s sharing, and it is a simple set of files that is there and is shared. One thing that we realized using multi-master is that possibly your events are going to be disrupted much more. Even if you think you have high-availability, the client can only connect to one server. So it doesn’t matter if the cluster has even ten servers, the client is still attached to just one of them.

Generally speaking, most of us want to do rolling upgrades, and that would bring one server down, and that is one of the reasons we want to go to multi-master. If you are doing that frequently, you are disrupting your SSH clients and for those who are permanently connected such as the stream events even more.

I have contributed a new stream events plugin, and I merged it upstream during the last hackathon so you can download it if you want to look at it that helps you to store stuff. You get the added benefit to add some flags to get IDs on your events, and then replay IDs. Since they are stored on the filesystem, when you connect you can tell what the last ID was you connected to, and you can start receiving all the events after that. It does it through the same interface of the old stream events: you drop it in, and it works. It respects Gerrit permissions, just like the core plugin does so that the user that is connecting will see only the events of the projects and branches he has access to.

Coordinating other processes on the Gerrit nodes

As processes on our Servers, we have the Gerrit JVM, but also we have a lot of hooks, some of them run for a very long time. You need to get those to run on both servers. We have a few cronjobs that do repack, we have others that are checking if users are active or inactive on LDAP and update their status on Gerrit and check their e-mails to make sure that they are really what they are saying. And last we have some various maintenance cronjobs.

We focussed on the background stuff first because you need to get that done anyway and you can use it as a peace mill, and we tried to coordinate those first.

We went for the low-tech solution, sharing as much as possible via the filesystem.
Years ago I created a lock implementation based on the filesystem. It is similar to an echo PID to a file but is a lot more robust and can recover deadlocks as long as he can contact the other server using SSH and does all of that automatically.

We use that as a primary method to coordinate things, so on top of that, I have built a scripting queue executor for hooks. Years ago before multi-master, we had issues with hooks that were running for too long. If you remember how Gerrit manages hooks, runs from the Java process with a queue that runs only one at a time. If your hooks take for instance three minutes, you may build up a backlog on your server. If your node goes down, you lose that backlog. What we decided years ago is to have the java process just to write the hook onto a file, and then we have a schedule that runs them in the background. The queue in Gerrit is then always at zero, it takes less than a second to write to the file and then it is done.

We made that multi-master friendly, but because that queue uses the locks, it just worked when we run them on more than one master. One server can write the hooks to a file, and the other could be the one that runs it, and thus it distributes the load in that way.

Gerrit cluster events

If you think about it, there are a lot of things that are hooked into the startup, so I just put a little hook framework that checks if the servers are running and check if the other nodes are running by ssh-ing into them. That allows identifying that if the two servers are down and one goes up, then your node is starting, but also your cluster is starting. And if the other just comes up you are not starting the cluster, but you are starting the node. Then if one node goes down, you haven’t stopped the cluster but you’ve stopped only the node, and if the other goes down you have now stopped the cluster.

I have created a little hooks framework so that you can plug into those events, and then you can do things based on that. For example, at the startup of any node, we are going to start the hooks just to make sure that they are running. But then when you stop a node we disable them only when all the cluster goes down. If one node goes down, the hooks need to be still running because they can contact the other server because they go to the domain name and not to the local server.

We plan to use the hooks framework for replication too. Currently, if you are familiar with the replication plugin, at startup try to replicate all the project to all the slaves to see if they are up to date. The theory behind it is that data could have been modified behind Gerrit back and possibly because when the server was shut down, it was in the middle of doing something and never finished it. In a cluster situation, if at any time something goes down, it could have been in the middle of replicating, and the other node won’t know. When any server goes down, you don’t want to replication, but there is no need to do replication at startup when any node starts but only when the cluster starts.
If there is at least one node up, when the other comes up you don’t want to do global replication. You could replicate it, but it would be a just extra load.

Caching across the cluster nodes

You can do some coordination better than the simple filesystem, for instance, the high-availability plugin does one to one connection, you need to configure the IP of the one master to connect to the other and they connect between each other. That works for two, but it is not a super-scalable system, it is an N-squared problem. Eventually, you want to move to a pub/sub system for things like that.

Something I haven’t mentioned yet it is what we had to do coordination wise it is caching. The high-availability plugin evicts caches, but we don’t do that yet, we consider it an optimization. The primary remedy is just to reduce the time on the caches, mainly decreasing to the values that we had on our slaves. Mostly you probably have lower cache time on your slaves, to make sure that you are checking your projects’ ACLs and your group memberships a little more often than your master. The Gerrit master knows when things get changed without the need to evict any cache, but the slaves do not know when they changed. We made the masters a little bit dumber like the slaves because they know when they changed something but they don’t know when the other masters did. As long as you are fine with the delay, let’s say 5 minutes, then it means that when you change an ACL, it may take up to 5 minutes to be seen by the other masters.

Demo: make your laptop a multi-master Gerrit server

As anybody tried to run two Gerrit instances on the same laptop?
The first problem that you will see is that if you point them to the same H2 caches it will just not start. But if you run init on two different directories and you don’t share any data on the two masters, then you need to go and modify your gerrit.config file to say to start sharing something.

Out of the box sharing abilities

You need to reconfigure the Database to be the same, and you should use a PostgreSQL or MySQL because the default H2 won’t allow any sharing. I am running 2.14 here, and I did not share the indexes, so they are going to be out of date for this demo. The user’s sessions are not going to be shared, so you need to configure them manually to make them shared.
If you use the websession-flatfile plugin, it will put the sessions on the filesystem and then you can share them on the filesystem.
Lastly, the stream events won’t be shared, but most of the rest will just work.

I am about to run stream events on both masters, on the left screen I am connecting to port 29418 and on the right screen on port 29419.
Now if I go to the WebUI, on port 8080 I have the same master of the one on the left, and on port 8081 I have the one on the right.

I am going to create a project using the master on the left: I hit create and you can see that the events appear on the stream events on 29418 but not on 29419.

Now let’s try to go to port 8081, and I have to log in again because the websessions are not shared yet. I am showing to you what it does out of the box. The events now came on the right screen but not on the left one.

The two masters are just pointing to the same Git data: I am trying to make them multi-master, but so far it is not working very well.

Adding plugins to share events and sessions

So, let’s step in and start deploying some new plugins. What we did with the plugins is to create a new one called “events” so that when you want to get the distributed events, you just call “events stream” where “events” is the name of the plugin and stream is the command.

What we did in the core is to remap the actual “gerrit stream-events” to “gerrit stream-events-core” so that you can still get to it if you want and this helps for debugging. And then we pointed the “gerrit stream-events” to “events stream” so that users will automatically get the new plugin events so that to them in invisible.

I am now on 8081 on the browser over here, create a new project called ‘Luca’ and here we go, events are generated on both 29418 and 29419 ports. You may have noticed that showed up on the right first and then to the left.

Screen Shot 2017-12-18 at 15.11.51.png

The events are on the filesystem, so the server just have to pull and here I am pulling every one second, but if something happened on this server, it will automatically catch-up on all the events generated on the other server as well.

The pulling is a backup: the events should come up even without a pulling mechanism. If we eventually had a pub/sub mechanism, I would still suggest keeping the pulling. Events are inherently unreliable and will get lost: you need a pulling mechanism in place if you want true reliability. Pulling is not great, but it is reliable. Events are effectively an optimisation for speed.

Let’s go back now to the other server at port 8080, and I did not have to login because the sessions are now shared and kept across masters. I create a ‘hugo’ project, and it came up pretty quick on both stream events on both masters.

So we have now shared web sessions and shared events. That’s in 2.14 but to make it a true multi-master you would need to share your indexes as well but as we are still on Gerrit v2.7 at Qualcomm, we don’t have an index and we don’t have to share it.

More exciting features in the events plugin

We’ve been using the events plugin in a single-master scenario since early this year, and it has been pretty reliable. I can show some other exciting features are in there. There are some new options here, IDs and resume after. If I give –ids and –resume-after 0, then it returns all the events since I started the instance, and I initialized it. Even if the server started and stopped a bunch of times, I still have all those events recorded. The IDs of the event have two parts of it: the left part is a UUID, and a right portion is a number. The idea being that this is your file store and this is your event store, a unique id associated with it. If someone deleted it on disk, it would create a new one. The idea is that if you had event 1M and someone has removed the file store, and you ask after event 1M, the new event store will restart at zero, and you get nothing. By having different Ids if you ask “give me all the events after ID 1M” then the UUID changed, and it realizes that you need everything. That allows giving some little extra safety. The plugin works on Gerrit v2.14, and I made some changes to make it work on v2.15 as well.

Screen Shot 2017-12-18 at 15.12.54.png

Q: If you use Hugo’s high-availability plugin for the index, you basically can probably do multi-master right now. Why then are you guys not running multi-master? Are you guys concerned about NFS and ref-updates?

A: (Hugo Ares – Gerrit Maintainer) We use it in failover mode for the only reason that if you just have a little bit of load, you are safe and consistency will be kept. But if you push it a bit more then sharing Git repositories from different machines in write mode doesn’t work. If you have two computers writing precisely to the same Git repository to the same branch to the same file at the same time, you are going to lose history, and we know because it happened for us. That’s why we don’t do multi-master right now.

We have been writing for several years to the same repositories from different machines using NFS and we haven’t found any issues because we do repack all over. Perhaps we have different NFS settings or just a different implementation.

A: It did not happen that often, but at least a couple of times and that’s why we don’t do it. We mainly use the high-availability plugin for evicting the caches so that we don’t need to lower the cache settings because they are removed automatically and takes care of the events in a slightly different way and trigger the reindex of the other copies. We can keep nodes down during the day, and the people would not notice any difference at all, no deal. We are just a little bit reluctant to write from both nodes, but that’s the plan.

So here you are already on a “multi-master ready” setup. We hear about your reports of the JGit problems, and we will be keeping an eye on it. I am confident we can fix any issues that could show up over here, understanding how Git works on NFS. There are some tricks that we can pull from our code base.

The (in)famous NFS stale file handle bug

I know we fixed in the past a bug on JGit to handle NFS: when you delete a file, and someone else tries to access it from a different node gets a stale file handle error. That’s the main problem you are running into and we encountered and fixed some of those issues on JGit, and there are possibly a few more left here and there probably. The workaround to the stale file handle problem is just to make a copy or a hard link to it and keep it for a bit more time and remove it later, and then you are fine. Generally speaking is just the brief moment after the operation that would be a problem. It is what it would happen on a local file system anyway: if you delete a file that was still opened by other processes, the inode will stay there for a while until it gets unreferenced. That is the main issue we have been running into, and most of the other problems have been already fixed on JGit.

Distributed GC on a multi-master setup

We have a distributed Git GC and repacking relying on the SSH filesystem lock mechanism that I talked before. We basically have a list of repos to be GCed on the filesystem. All the nodes are saying: “I am going to this,” and another node says “Oh, good, then I am going to do this other one” and in this way, the GC load gets fairly well distributed and you get only a few hours to get through the whole thing. The lock allows preventing to step on each other toes when doing repacking. As far as the backup is concerned, we do a rsync and take database dumps, so that we don’t get inconsistent snapshots. You need to get your Database backup first and then the repositories next. If you are running an older version of the DataBase backup, you are generally safe because you don’t risk to have references to inexistent objects.


Q: I may have missed one point: the shared filesystem approach is assuming you are not doing multi-site, right? If you were doing multi-site, it would add extra latency that would introduce more problems and add more latency which was the problem you were trying to solve.

A: Yes, that’s correct. We are not interested in multi-site because there are a lot of downsides to it: it is a lot more complicated and makes your writes a lot slower while the reads would be faster. When you are pushing to you realize that is not the best experience. You need to go to a quorum to the majority of the people around the world to get the agreement of the majority of them to finalize a git push operation.
Single-site is the first initial step for us, multi-site requires a very different approach. You need to get the Git objects to be replicated and then having somewhere to store the refs that have to be shared without a system like Google’s database that assures that the refs are replicated around the world.
It is not impossible; we made some experiments on it. A few years ago Dave Borowitz uploaded a ZooKeeper implementation that does the refs sharing, but that only does the refs. Then you need to do something for the Git objects replication. You could use the regular Git replication to do that, but you need to come up with a bit of magical ref scheme that does the job behind the scene.

A: (Luca Milanesio – GerritForge) Last year at the Gerrit Hackathon in Mountain View, we presented a new implementation of the “missing bit” you were talking about. It an OpenSource project based on JGit and leverages Apache Cassandra for the Git objects, while uses the ZooKeeper implementation for the refs you mentioned. We are making a lot of progress on the project and it will soon be possibly THE solution for Gerrit Multi-Site.

Q: (Luca Milanesio – GerritForge) We found out and fixed recently in JGit a problem with the cache consistency: it was reported on NFS but had nothing to do with it. When JGit was not able to open a file for whatever reason, NFS latency, maximum opened files, whatever, then the packfile was removed from the list of packs because it was considered missing. Then JGit started to failing fetching objects from the repository, and the people were panicking thinking that their Git repository got corrupted. But then, why the other nodes accessing the same repository did not have the same problem? Then “magically” the problem disappeared when you restarted Gerrit. The problem has been fixed in recent versions of JGit but is still there for the version you are currently using as the basis of Gerrit v2.7. Did you find the same problem? How do you manage to keep up with the recent releases of JGit but staying on Gerrity v2.7?

A: We have an old version of JGit, but we have put our patches on it. I believe we discovered and fixed a bug like that years ago. We ported a series of patches upstream and we a set of performance improvements on our branch. However, it is hard sometimes to get stuff merged on JGit; there are not enough maintainers to even look at the thing.


Gerrit User Summit: Gerrit at Google

Starting from this week, we are going to share one video per week of the amazing talks that were presented at the Gerrit User Summit 2017 in London.

In addition to the YouTube recording, we are during the extraction of the text and publishing it together with the relevant pictures taken from the presenter’s slides, so that people can start digesting the content at small bites.

This week talk is Patrick Hiesel’s presentation on how Gerrit multi-tenant and multi-master setup has been implemented in Google.

Gerrit@Google – Patrick Hiesel, Google

My name is Patrick, and I am going to talk about the setup of Gerrit we are running at Google. I wanted to take you on a journey starting with Gerrit that you all know and making it the system we run at Google, step-by-step; and at the end will have a multi-master and multi-tenant system.


Multi-tenant is the ability to serve multiple hosts from the same single Java process. Imagine like the same JVM task serving and

Multi-master is the ability to have multiple Gerrit servers all over the world. You can contact any one of them for reads and writes.
Most systems have read replicas, which is straightforward, but write replicas is where the juicy meat is.


We have, based on OpenSource Gerrit that you can download right now and have it running hopefully under ten minutes. That is core-Gerrit, and it depends on three things:

  1. JGit: for all the Git stuff
  2. Multiple indexes for the accounts, changes, and other stuff
  3. Caches

All these three components are based on the filesystem in one way or the other.

Now you have a friend that is accessing, what are you going to do?
The most natural solution is to start another Gerrit instance for it. You can have all of them on one machine, you can give them different ports, very easy, and in the end, they’ll be all based on the filesystem.
All those Gerrit instances do not need to talk to each other; they can just be separate instances operating on separate ports. This is not a multi-tenant system, but only different Gerrit instances on the same host.

Screen Shot 2017-10-10 at 15.35.08.png

You can add another layer on top of it: a servlet engine which receives all the traffic, check which host the traffic is for, and just delegate to the individual host.
To take one step further, have that selection filter doing that for you. Gerrit has a daemon that runs all the functionalities. You can integrate that daemon into the incoming servlet filter. When you can get a request for and I do not have where to allocate it, you can just launch it, instantiate all the objects and then run the traffic from there. Also, unload instances would work in the same way.
The Gerrit server engine and the selection filter can run in a single JVM.

How Gerrit can conquer the world

So you have a master here in Europe, and you have got one friend on the west coast in the US. He says: “Oh your Gerrit is so slow I have no idea why and I wish I could move to GitHub.” You say: “Hold on, I can do better than that!” and so you put a new master for the person on West coast.

So the key to that is the replication and comes in two sets:

  • Objects that you have to replicate related to all the Git data. JGit is putting objects into the disk, and these are the data you need to replicate correctly and fast.
  • Other stuff that should replicate and fast to provide a pleasant user experience but it really can be best-effort. That is the indexes and the caches.

If it is okay for your master having a 200/600 msec additional latency, then do not replicate the caches. You can have a cold cache in Singapore or the US, and you can reread them without problems.

Screen Shot 2017-10-10 at 15.37.31.png

For the index, replication can be best-effort, but you should make an effort to replicate them. It is still nonetheless a mandatory requirement. One way to achieve that is to use ElasticSearch, but other index implementations that give indexes replication can be used as well.

Multi-tenant and Multi-master together

We talked about a multi-tenant system and then replicate them globally, so we have now a multi-tenant and multi-master system, actually pretty close to what we run at Google.

That is the stack that we run in total. We have a selection filter and two other filters to decide what the traffic is directed. We are also based on JGit, no magic there, we have index and caches we replicate, all our systems are based on filesystem and BigTable.

Some “magic” happens at the Git layer at Google because that is where all the majority consensus across all the cells. When you are pushing anything that is Git and, with NoteDb, anything that is a review is in the repository as well, the system tries to reach the majority of the cells and write the objects to them. When that is acknowledged, you get a green light on the push.
Majority consensus also means that you have it only in so many cells, but don’t have it on all the cells all the times. Some of the replication is happening in the long tail, by replication events eventually get acknowledged by the cells, and then they get written to all the masters.

Our indexes and caches are also replicated, but some of them are just in-memory and a component that gets replicated on top of BigTable.

Redundancy everywhere.

We run five data-centers across three continents (Americas, Europe, Asia-Pacific), with precisely the stack we just saw which gives us a good latency for most of our users worldwide.

Let’s talk about load balancing. We have a system that is multi-master and multi-tenant, and any of the tasks can serve any requests, but just because it can, doesn’t mean that it should.
Maybe it has in cold memory caches, or it is in Singapore, and you are in the US; so the question is what if the biggest machine is not big enough and we want to optimize it?

Screen Shot 2017-10-10 at 15.39.02.png

The idea is that we want to reduce latency that comes out of cold caches and minimize the time the site takes to load.
So you have a request for, and your instance has a cold cache, and you need to read from disk to memory to serve the request.
You have a fleet of 300 tasks available but you want to serve from only just five of them. If you serve 300 requests from 300 tasks in a round-robin manner, you pay the latency to load data from disk to memory for every single request. And the second motivation is that you want to distribute the load.

We want a system that can dynamically scale with changing load patterns. We want a system that can optimise the caches, to send a request for a site/repo to the few number of servers and tasks based on two conditions:

  1. serve from one machine as long as it fits on it
  2. server from more machines if you really have to

Level-1 load balancer

In the stack at Google, you saw two load balancing levels. What you see down in the picture is the Gerrit tasks, that contains all the software layers we talked about. We have a user that triggers JSON calls from the browser, with PolyGerrit. The first thing that the JSON request is hitting is the L1 load balancer. The primary routing of your request is by geographic proximity. We have five datacentres at Google; the L1 load balancer picks the one with the lowest latency. When the request goes into the data-center, we have another load balancer which is the one I’ll talk about more, because this is the one where the Gerrit specifics happen.

One thing that L1 is doing as well is managing the spillover of traffic. When a datacentre says “I can handle up to 100 QPS” the L1 load balancer starts redirecting traffic to other datacentres should that threshold be reached.

Level 2 load balancer

Let’s dive into L2 load balancer, we want to know how much traffic we are getting into each Gerrit task, and we want to know in the load balancer where the single request should go, and we want to know that fast!

We added three new components to the architecture:

  • An element to redirect tasks and provide functionality and can report the load we are handling right now. When I mean load it can be anything: QPS (Queries Per Second), metrics, we just want to know from the tasks: what is your current load? We have a system called slicer, which I am going to talk about in a second and it’s added there in the picture.
  • A second component we are adding to the load balancer, with a query interface that responds to the following question: “we have a request for, where should it go?”. All of that should be done in memory and should be regularly updated with the new elements in the background so that we don’t add another component of latency by having another RPC.
  • A third part is coordinating everything and is called the assigner, and it takes all the load metrics that we reported generates new assignments and gives them to the query interface.

Introducing the slicer

We have a system that is called slicer. There is a very nice paper that I can recommend, published last fall, that talks about that. It is a load balancer that works on custom keys and can do automatic re-sharding based on new traffic patterns. When your nodes receive more traffic, the slicer will automatically distribute the load or re-shard the whole system. That is a suitable method for local sharding that happens within the data center; we do not use it for inter-datacentre because that is all done via geographic proximity.

Screen Shot 2017-10-10 at 15.40.13.png

The system works with 64-bit keys and gives you a lot of combinations. You can slice the keyspace, for instance, in 400 slices. That gives you 400 ranges, and you can take any of them and assign to one or more tasks. The hostname is my key for instance, and then you hash it, and you end up in the first slice that gets assigned to a single task with an index zero.

What can we do if the load changes? Let’s say that you have key zero that gets assigned to the first range and then the traffic changes. We have two options.

  1. The first option is to assign more tasks, let’s say task 6, and then you round-robin between task 0 and task 6.
  2. The second option is splitting into 600 or 800 slides to get a better grip on each of the keys.

Screen Shot 2017-10-10 at 15.41.20

We can also do that, and then we factor our the load for and, and we put them into different hosts.

We do that for Gerrit, and one of the things we want for Gerrit is when we have to split per-host traffic with the affinity on the repository. Caches are based on the project, and because is a massive host served from a lot of tasks, we don’t want all these tasks just to serve all general traffic for android. We want tasks serving android/project1 from here and android/project2 from there so that we optimise the second layer of caches.

What we do is to mangle these keys together based on both host and project. Before, all these chromium keyspace was served from a single host; when the load increases we just split the keys into Chromium source and the rest of metadata. This is the graph that we obtained after we implemented the load balancer. The load we have on each of the tasks in a single data center is represented by a line with a different color. What you can see is that are all nicely aligned, so that each task is serving precisely the same amount of traffic which is what you want.

Screen Shot 2017-10-10 at 15.42.21.png

What if one project is 100 times the size of the others and we are optimizing on queries per second? The system will just burn resources fast. We had that situation in May, we saw the graphs, and we said “all good, looks nice”; however, people were sending e-mails and raising bugs wondering if the system were serving any traffic at all or if it were down completely.
It turned out that Android had a lot of large repositories, regarding the number of references, and the objects. We were just optimizing the queries per second, but some of the tasks were doing just CPU intensive work, where others were happy with it. Some of them were burning CPU in flames, and others were fine.

So we moved out of the per-request affinity, and we modified the per-repository sharding to optimise all of this.

Warm vs. Cold Cache

There is an extra in the system that is pre-warming caches. What the load balancer can do for you is to tell you that traffic is changing and I need to reconsider how to split the load on the system. For each of the tasks is going to tell “I’m going to give you traffic for” with a notice of 30 seconds. That time you can use for pre-warm caches.

That is especially nice if you restart your tasks because all the associated in-memory cache gets flushed. The load balancer tells you “oh, this is the list of the tasks I need” and then you can get them all and pre-warm their caches. This graph shows the impact of the cache warmer on our system, on the 99.9% requests latency, really on the long tail of requests latency. That looks nice because we brought the latency down by a third.

Screen Shot 2017-10-10 at 15.43.15.png

What is a task start dying during peak traffic? Imagine that the load balancer is saying “You’re going to handle this” and two seconds later says “I have to reconsider, you’re going to handle that instead”. Again you’re going to watch your system burning on fire, because you’re serving peak traffic and then you’re running close to 100% CPU. That situation causes the load balancer loading and unloading tasks all the time, which is inconvenient. The way we work around this is to make this cache warming a best-effort activity. You can do it if you’re below 50% CPU when you have time to do fancy things, but when you receive peak traffic, you just handle peak traffic without any optimisation made.

Multi-master and multi-tenant outside of Google

The question is: how do we do that in a non-Google setup?
There are plenty of options.

With the new Gerrit release in 2.15, we introduced to a new URL scheme, which includes the project name in the URL. Previously you had and there was no way to directly know which project this is for and no way to do that load balancing that we just saw.

What we did in 2.15 is just add the project before that, so that extraction for both host and project can be made in a simpler way. You could do the same even before v2.15 but you needed a secondary index lookup, which most opensource load-balancers such as HAProxy or NGINX did not support. And of course, there are lots of products like Google Cloud load balancer, and others that you can use to achieve the same thing.


We went through a journey where we took OpenSource Gerrit, we added sites selection and got a multi-tenant Gerrit.
Then we took this multi-tenant Gerrit, added replication and obtained multi-master Gerrit.
And then we took that with load balancing and lots of failures and lots of fixes, and we got pretty much the Gerrit that we run at Google, which brings me to the end of this talk 🙂


Q: How strategic is Gerrit@Google? Do you have any other code-review systems? If yes, how is used Gerrit vs. the others?

We have another code-review system for internal use only, and Gerrit is used whenever we are doing OpenSource stuff, so for GoLang, Chromium, Android, Gerrit, and whenever the Google Team wants to collaborate with other OpenSource users, or in general with users that are not sitting at Google.

Historically the source at Google was developed in Perforce, and we ported from that to a home-based system called Piper. Around that, we have a tooling ecosystem which is internal. In parallel to that, Google started to do a lot of projects that have nothing to do with the internal search engine and available outside. What we see is that a lot of projects started at Google from scratch were thinking about “what system should we use?”. Many people said: “well, we’re going just to use Git because that’s what we know and we like, ” and when they needed code-review for Git they ended up with us. Gerrit and Git are very popular inside Google.

Q. You have two levels of load balancer. The first one is the location, and the second one is to decide what to do inside the data-center. What about if a location is off? Maybe is not fully off-line but has big problems, or has a very low-percentage of consensus, and some of the locations have not the “latest and greatest” of the repo. Possibly a location that should be “inconvenient for me” actually has the data I want.

You’re talking about replication layer where you have the objects in one location but not in the other. Our replication latency is in the order of seconds, but it may happen that one location is just really slow in getting the objects. That happens from time to time, and we have metrics that says what the replication lag is accounted for. When it exceeds a threshold we just shut the data-center off, which means cut-off the traffic, the data-center will not receive user-traffic anymore but it will still be able to get the replication done, and when the decrease the objects we need to replicate we can send the traffic again.

Cutting off the traffic is happing at the L1 load balancer where we said “don’t send anything there”.

Q. Do all the tasks have the same setup? Or do have a sort of micro-service architecture inside where some of the tasks are more dedicated to this type of operations and other for another type. Serving data from memory in one thing, but calculating diff change is a different type of task.

Not in general. All of our tasks are the same, except for checking access control permissions. We do not go through the whole Gerrit stack but we have only this little task that knows how the project config works and is going to tell us yes or no.